2: Mitosis and Meiosis Flashcards by Secret acc

Genetic continuity between generations of cells and between generations of sexually reproducing organisms is maintained through the processes of ______ and ______, respectively.

mitosis, meiosis

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______ eukaryotic cells contain their genetic information in pairs of ______, with one member of each pair being derived from the ______ parent and one from the ______ parent.

Diploid, homologous chromosomes, maternal, paternal

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______ provides a mechanism by which chromosomes, having been duplicated, are distributed into progeny cells during cell reproduction.

Mitosis

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______ converts a diploid cell into two diploid daughter cells.

Mitosis

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The process of ______ distributes one member of each homologous pair of chromosomes into each ______ or ______, thus reducing the diploid chromosome number to the ______ chromosome number.

meiosis, gamete, spore, haploid

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______ generates genetic variability by distributing various combinations of maternal and paternal members of each homologous pair of chromosomes into ______ or ______.

Meiosis, gametes, spores

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During the stages of mitosis and meiosis, the genetic material is condensed into discrete structures called ______.

chromosomes

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Every living thing contains a substance described as the ______. Except in certain viruses, this material is composed of the ______.

genetic material, nucleic acid DNA

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DNA has an underlying linear structure possessing segments called ______, the products of which direct the metabolic activities of cells.

genes

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An organism’s DNA, with its arrays of ______, is organized into structures called ______, which serve as vehicles for transmitting genetic information. The manner in which chromosomes are transmitted from one generation of cells to the next and from organisms to their descendants must be exceedingly precise.

genes, chromosomes

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Two major processes are involved in the genetic continuity of nucleated cells: ______ and ______. Although the mechanisms of the two processes are similar in many ways, the outcomes are quite different.

mitosis, meiosis

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______ leads to the production of two cells, each with the same number of chromosomes as the parent cell.

Mitosis

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In contrast, ______ reduces the genetic content and the number of chromosomes by precisely half. This reduction is essential if ______ is to occur without doubling the amount of genetic material in each new generation.

meiosis, sexual reproduction

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Strictly speaking, ______ is that portion of the cell cycle during which the hereditary components are equally partitioned into daughter cells.

mitosis

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______ is part of a special type of cell division that leads to the production of sex cells: ______ or ______. This process is an essential step in the transmission of genetic information from an organism to its offspring.

Meiosis, gametes, spores

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Normally, ______ are visible only during mitosis and meiosis.

chromosomes

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When cells are not undergoing division, the genetic material making up chromosomes unfolds and uncoils into a diffuse network within the nucleus, generally referred to as ______.

chromatin

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Before 1940, our knowledge of cell structure was limited to what we could see with the ______.

light microscope

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Around 1940, the ______ was in its early stages of development, and by ______, many details of cell ultrastructure had emerged.

transmission electron microscope, 1950

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Under the ______, cells were seen as highly varied, highly organized structures whose form and function are dependent on specific genetic expression by each cell type. A new world of whorled membranes, organ- elles, microtubules, granules, and filaments was revealed. These discoveries revolutionized thinking in the entire field of biology.

electron microscope

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Many cell components, such as the ______, ______, and ______, are involved directly or indirectly with genetic processes.

nucleolus, ribosome, centriole

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Other components—the ______ and ______—contain their own unique genetic information.

mitochondria, chloroplasts

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All cells are surrounded by a ______, an outer covering that defines the cell boundary and delimits the cell from its immediate external environment. This membrane is not ______ but instead actively controls the movement of materials into and out of the cell.

plasma membrane, passive

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In addition to this membrane, plant cells have an outer covering called the ______ whose major component is a polysaccharide called ______.

cell wall, cellulose

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Many, if not most, animal cells have a covering over the plasma membrane, referred to as the ______, or ______. Consisting of ______ and ______, this covering has a chemical composition that differs from comparable structures in either plants or bacteria.

glycocalyx, cell coat, glycoproteins, polysaccharides

The ______, among other functions, provides biochemical identity at the surface of cells, and the components of the coat that establish cellular identity are under genetic control. For example, various cell-identity markers that you may have heard of—the ______, ______, and ______ antigens—are found on the surface of ______, among other cell types. On the surface of other cells, ______ antigens, which elicit an immune response during tissue and organ transplants, are present.

glycocalyx, AB, Rh, MN, red blood cells, histocompatibility

Various ______ are also found on the surfaces of cells. These molecules act as recognition sites that transfer specific chemical signals across the cell membrane into the cell.

receptor molecules

Living organisms are categorized into two major groups depending on whether or not their cells contain a ______.

nucleus

The presence of a nucleus and other membranous organelles is the defining characteristic of ______ organisms.

eukaryotic

The ______ in eukaryotic cells is a membrane-bound structure that houses the genetic material, ______, which is complexed with an array of acidic and basic ______ into thin fibers. During nondivisional phases of the cell cycle, the fibers are ______ and ______ into ______ (as mentioned above). During mitosis and meiosis, chromatin fibers ______ and ______ into ______.

nucleus, DNA, proteins, uncoiled, dispersed, chromatin, coil, condense, chromosomes

Also present in the nucleus is the ______, an amorphous component where ribosomal RNA (rRNA) is synthesized and where the initial stages of ribosomal assembly occur.

nucleolus

The portions of DNA that encode rRNA are collectively referred to as the ______, or the ______.

nucleolus organizer region, NOR

______ organisms, of which there are two major groups, lack a nuclear envelope and membranous organelles. For the purpose of our brief discussion here, we will consider the eubacteria, the other group being the more ancient bacteria referred to as archaea.

Prokaryotic

In eubacteria, such as ______, the genetic material is present as a ______, ______ DNA molecule that is compacted into an unenclosed region called the ______. Part of the DNA may be attached to the ______, but in general the ______ extends through a large part of the cell. Although the DNA is compacted, it does not undergo the extensive ______ characteristic of the stages of mitosis, during which the chromosomes of eukaryotes become visible. Nor is the DNA associated as extensively with proteins as is eukaryotic DNA. Figure 2.2, which shows two bacteria forming by cell division, illustrates the nucleoid regions containing the bacterial chromosomes.

Escherichia coli, long, circular, nucleoid, cell membrane, nucleoid, coiling

Prokaryotic cells do not have a distinct ______ but do contain genes that specify ______ molecules.

nucleolus, rRNA

The remainder of the eukaryotic cell within the plasma membrane, excluding the nucleus, is referred to as ______ and includes a variety of extranuclear cellular organelles.

cytoplasm

In the cytoplasm, a nonparticulate, colloidal material referred to as the ______ surrounds and encompasses the cellular organelles.

cytosol

The cytoplasm also includes an extensive system of tubules and filaments, comprising the ______, which provides a lattice of support structures within the cell. Consisting primarily of ______, which are made of the protein ______, and ______, which derive from the protein ______, this structural framework maintains cell shape, facilitates cell mobility, and anchors the various organelles.

cytoskeleton, microtubules, tubulin, microfilaments, actin

One organelle, the membranous ______, compartmentalizes the cytoplasm, greatly increasing the surface area available for biochemical synthesis.

endoplasmic reticulum (ER)

The ER appears smooth in places where it serves as the site for synthesizing ______ and ______; in other places, it appears rough because it is studded with ______.

fatty acids, phospholipids, ribosomes

______ serve as sites where genetic information contained in messenger RNA (mRNA) is translated into proteins.

Ribosomes

Three other cytoplasmic structures are very important in the eukaryotic cell’s activities: ______, ______, and ______.

mitochondria, chloroplasts, centrioles

______ are found in most eukaryotes, including both animal and plant cells, and are the sites of the oxidative phases of cell respiration. These chemical reactions generate large amounts of the energy-rich molecule ______.

Mitochondria, adenosine triphosphate (ATP)

______, which are found in plants, algae, and some protozoans, are associated with ______, the major energy-trapping process on Earth.

Chloroplasts, photosynthesis

Both mitochondria and chloroplasts contain ______ in a form distinct from that found in the ______. They are able to ______ themselves and ______ and ______ their own genetic information.

DNA, nucleus, duplicate, transcribe, translate

Animal cells and some plant cells also contain a pair of complex structures called ______. These cytoplasmic bodies, each located in a specialized region called the ______, are associated with the organization of spindle fibers that function in mitosis and meiosis.

centrioles, centrosome

In some organisms, the centriole is derived from another structure, the ______, which is associated with the formation of ______ and ______ (hair-like and whip-like structures for propelling cells or moving materials).

basal body, cilia, flagella

The organization of ______ by the centrioles occurs during the ______ phases of mitosis and meiosis. These fibers play an important role in the movement of ______ as they separate during cell division. They are composed of arrays of microtubules consisting of polymers of the protein ______.

spindle fibers, early, chromosomes, tubulin

Chromosomes are most easily visualized during ______. When they are examined carefully, distinctive ______ and ______ are apparent.

mitosis, lengths, shapes

Each chromosome contains a constricted region called the ______, whose location establishes the general appearance of each chromosome.

centromere

Figure 2.3 shows chromosomes with centromere placements at different distances along their length. Extending from either side of the centromere are the ______ of the chromosome. Depending on the position of the centromere, different ______ are produced.

arms, arm ratios

As Figure 2.3 illustrates, chromosomes are classified as ______, ______, ______, or ______ on the basis of the centromere location.

metacentric, submetacentric, acrocentric, telocentric

The shorter arm, by convention, is shown above the centromere and is called the ______ (p, for “______”).

p arm, petite

The longer arm is shown below the centromere and is called the ______ (q because it is the next letter in the alphabet).

q arm

In the study of mitosis, several other observations are of particular relevance. First, all ______ cells derived from members of the same species contain an identical number of chromosomes. In most cases, this represents what is referred to as the ______ (2n). When the lengths and centromere placements of all such chromosomes are examined, a second general feature is apparent. With the exception of ______ chromosomes, they exist in pairs with regard to these two properties, and the members of each pair are called ______. So, for each chromosome exhibiting a specific length and centromere placement, another exists with ______ features.

somatic, diploid number, sex, homologous chromosomes, identical

There are exceptions to this rule. Many ______ and ______ have but one chromosome, and organisms such as ______ and ______, and certain plants such as ______ (mosses), spend the predominant phase of their life cycle in the haploid stage. That is, they contain only ______ member of each ______ of chromosomes during most of their lives.

bacteria, viruses, yeasts, molds, bryophytes, one, homologous pair

Centromere locations and the chromosome designations that are based on them. Middle ______

Metacentric

Centromere locations and the chromosome designations that are based on them. Between middle and end ______

Submetacentric

Centromere locations and the chromosome designations that are based on them. Close to end ______

Acrocentric

Centromere locations and the chromosome designations that are based on them. At end ______

Telocentric

Figure 2.4 illustrates the physical appearance of different pairs of homologous chromosomes. There, the human mitotic chromosomes have been photographed, cut out of the print, and matched up, creating a display called a ______.

karyotype

As you can see, humans have a ______ number of 46 chromosomes, which on close examination exhibit a diversity of sizes and centromere placements. Note also that each of the 46 chromosomes in this karyotype is clearly a double structure consisting of two parallel ______ connected by a common centromere. Had these chromosomes been allowed to continue dividing, the ______, which are replicas of one another, would have separated into the two new cells as division continued.

2n, sister chromatids, sister chromatids

The ______ (n) of chromosomes is equal to one-half the diploid number. Collectively, the genetic information contained in a haploid set of chromosomes constitutes the ______ of the species. This, of course, includes copies of all genes as well as a large amount of noncoding DNA. The examples listed in Table 2.1 demonstrate the wide range of n values found in plants and animals.

haploid number, genome

______ have important genetic similarities. They contain identical gene sites along their lengths; each site is called a ______ (pl. ______). Thus, they are identical in the traits that they influence and in their genetic potential. In sexually reproducing organisms, one member of each pair is derived from the maternal parent (through the ______) and the other member is derived from the paternal parent (through the ______). Therefore, each diploid organism contains ______ copies of each gene as a consequence of ______, inheritance from two parents.

Homologous chromosomes, locus, loci, ovum, sperm, two, biparental inheritance

As we shall see during our discussion of transmission genetics (Chapters 3 and 4), the members of each pair of genes, while influencing the same characteristic or trait, need not be identical. In a population of members of the same species, many different alternative forms of the same gene, called ______, can exist.

alleles

The concepts of haploid number, diploid number, and homologous chromosomes are important for understanding the process of meiosis. During the formation of ______ or ______, meiosis converts the diploid number of chromosomes to the ______ number. As a result, haploid gametes or spores contain precisely one member of each ______ of chromosomes—that is, one complete haploid set. Following fusion of two gametes at ______, the ______ number is reestablished; that is, the zygote contains two complete ______ sets of chromosomes. The constancy of genetic material is thus maintained from generation to generation.

gametes, spores, haploid, homologous pair, fertilization, diploid, haploid

There is one important exception to the concept of homologous pairs of chromosomes. In many species, one pair, consisting of the ______, is often not homologous in size, centromere placement, arm ratio, or genetic content.

sex-determining chromosomes

For example, in humans, while ______ carry two homologous X chromosomes, ______ carry one Y chromosome in addition to one X chromosome. These X and Y chromosomes are not strictly ______. The Y is considerably ______ and lacks most of the gene ______ contained on the X. Nevertheless, they contain homologous regions and behave as homologs in ______ so that gametes produced by males receive either one X or one Y chromosome.

females, males, homologous, smaller, loci, meiosis

The process of mitosis is critical to all eukaryotic organisms. In some single-celled organisms, such as ______ and some ______ and ______, mitosis (as a part of cell division) provides the basis for ______.

protozoans, fungi, algae, asexual reproduction

Multicellular diploid organisms begin life as single-celled fertilized eggs called ______.

zygotes

The mitotic activity of the zygote and the subsequent daughter cells is the foundation for the development and growth of the organism. In adult organisms, mitotic activity is the basis for ______ and other forms of ______ in certain tissues. For example, the ______ of the skin and the ______ of humans are continuously sloughed off and replaced.

wound healing, cell replacement, epidermal cells, intestinal lining

Cell division also results in the continuous production of ______ that eventually shed their ______ and replenish the supply of ______ in vertebrates. In abnormal situations, somatic cells may lose control of cell division, and form a ______.

reticulocytes, nuclei, tumor

The genetic material is partitioned into daughter cells during ______, or ______. This process is quite complex and requires great precision. The chromosomes must first be exactly replicated and then accurately partitioned. The end result is the production of ______, each with a chromosome composition ______ to that of the parent cell.

nuclear division, karyokinesis, two daughter nuclei, identical

Karyokinesis is followed by ______, or ______. This less complex process requires a mechanism that partitions the volume into two parts and then encloses each new cell in a distinct plasma membrane. As the cytoplasm is reconstituted, organelles replicate themselves, arise from existing membrane structures, or are synthesized ______ (anew) in each cell.

cytoplasmic division, cytokinesis, de novo

Following cell division, the initial size of each new daughter cell is approximately ______ the size of the parent cell. However, the nucleus of each new cell is not appreciably ______ than the nucleus of the original cell. Quantitative measurements of DNA confirm that there is an amount of genetic material in the daughter nuclei ______ to that in the parent cell.

one-half, smaller, equivalent

Interphase and the Cell Cycle Many cells undergo a continuous alternation between ______ and ______. The events that occur from the completion of one division until the completion of the next division constitute the ______.

division, nondivision, cell cycle

Interphase and the Cell Cycle We will consider ______, the initial stage of the cell cycle, as the interval between divisions.

interphase

Interphase and the Cell Cycle It was once thought that the biochemical activity during interphase was devoted solely to the cell’s growth and its normal function. However, we now know that another biochemical step critical to the ensuing mitosis occurs during interphase: the ______ of the DNA of each chromosome. This period, during which DNA is ______, occurs before the cell enters mitosis and is called the ______. The initiation and completion of synthesis can be detected by monitoring the incorporation of ______ into DNA.

replication, synthesized, S phase, radioactive precursors

Interphase and the Cell Cycle Investigations of this nature demonstrate two periods during interphase when no ______ occurs, one before and one after the ______. These are designated ______ and ______, respectively. During both of these intervals, as well as during S, intensive ______, ______, and ______ are evident.

DNA synthesis, S phase, G1 (gap I), G2 (gap II), metabolic activity, cell growth, cell differentiation

Interphase and the Cell Cycle By the end of G2, the volume of the cell has roughly ______, DNA has been ______, and ______ is initiated. Following mitosis, continuously dividing cells then repeat this cycle (______, ______, ______, ______) over and over, as shown in Figure 2.5.

doubled, replicated, mitosis (M), G1, S, G2, M

Interphase and the Cell Cycle Much is known about the cell cycle based on ______ (literally, “in glass”) studies. When grown in culture, many cell types in different organisms traverse the complete cycle in about ______. The actual process of mitosis occupies only a small part of the overall cycle, often less than an ______.

in vitro, 16 hours, hour

Interphase and the Cell Cycle The lengths of the ______ and ______ phases of interphase are fairly consistent in different cell types. Most variation is seen in the length of time spent in the ______ stage. Figure 2.6 shows the relative length of these intervals as well as the length of the stages of mitosis in a human cell in culture.

S, G2, G1

Interphase and the Cell Cycle ______ is of great interest in the study of cell proliferation and its control. At a point during G1, all cells follow one of two paths. They either ______ from the cycle, become ______, and enter the ______ stage, or they become committed to proceed through ______, initiating ______, and completing the cycle.

G1, withdraw, quiescent, G0, G1, DNA synthesis

Interphase and the Cell Cycle Cells that enter G0 remain ______ and metabolically ______ but are not ______. ______ cells apparently avoid entering G0 or pass through it very quickly. Other cells enter G0 and never reenter the cell cycle. Still other cells in G0 can be stimulated to return to G1 and thereby reenter the cell cycle.

viable, active, proliferative, Cancer

Interphase and the Cell Cycle Cytologically, interphase is characterized by the absence of ______. Instead, the nucleus is filled with ______ that are formed as the chromosomes ______ and ______ after the previous mitosis.

visible chromosomes, chromatin fibers, uncoil, disperse

Interphase and the Cell Cycle Once G1, S, and G2 are completed, ______ is initiated. Mitosis is a dynamic period of vigorous and continual activity. For discussion purposes, the entire process is subdivided into discrete stages, and specific events are assigned to each one. These stages, in order of occurrence, are ______, ______, ______, ______, and ______.

mitosis, prophase, prometaphase, metaphase, anaphase, telophase

Prophase Often, over half of mitosis is spent in ______, a stage characterized by several significant occurrences.

prophase

Prophase One of the early events in prophase of all animal cells is the migration of ______ pairs of ______ to opposite ends of the cell. These structures are found just outside the nuclear envelope in an area of differentiated cytoplasm called the ______.

two, centrioles, centrosome

Prophase It is believed that each pair of centrioles consists of one ______ unit and a ______, newly formed daughter centriole.

mature, smaller

Prophase The centrioles migrate and establish poles at ______ ends of the cell. After migration, the ______, in which the centrioles are localized, are responsible for organizing cytoplasmic microtubules into the ______ that run between these poles, creating an axis along which chromosomal separation occurs. Interestingly, the cells of most plants (there are a few exceptions), fungi, and certain algae seem to lack ______. ______ are nevertheless apparent during mitosis.

opposite, centrosomes, spindle fibers, centrioles, Spindle fibers

Prophase As the centrioles migrate, the ______ begins to break down and gradually disappears. In a similar fashion, the ______ disintegrates within the nucleus. While these events are taking place, the diffuse ______ have begun to condense, until distinct thread-like structures, the ______, become visible. It becomes apparent near the end of prophase that each chromosome is actually a ______ structure split longitudinally except at a single point of constriction, the ______.

nuclear envelope, nucleolus, chromatin fibers, chromosomes, double, centromere

Prophase The two parts of each chromosome are called ______ because the DNA contained in each of them is genetically ______, having formed from a single replicative event.

sister chromatids, identical

Prophase Sister chromatids are held together by a multi-subunit protein complex called ______. This molecular complex is originally formed between them during the ______ of the cell cycle when the ______ of each chromosome is replicated.

cohesin, S phase, DNA

Prophase Thus, even though we cannot see chromatids in ______ because the chromatin is ______ and ______ in the nucleus, the chromosomes are already ______ structures, which becomes apparent in ______ prophase.

interphase, uncoiled, dispersed, double, late

Prophase In humans, with a diploid number of ______, a cytological preparation of ______ prophase reveals ______ chromosomes randomly distributed in the area formerly occupied by the ______.

46, late, 46, nucleus

Prometaphase and Metaphase The distinguishing event of the two ensuing stages is the migration of every chromosome, led by its ______, to the ______.

centromeric region, equatorial plane

Prometaphase and Metaphase The ______, also referred to as the ______, is the midline region of the cell, a plane that lies perpendicular to the axis established by the spindle fibers.

equatorial plane, metaphase plate

Prometaphase and Metaphase In some descriptions, the term ______ refers to the period of chromosome movement, and the term ______ is applied strictly to the chromosome configuration following migration.

prometaphase, metaphase

Prometaphase and Metaphase Migration is made possible by the binding of spindle fibers to the chromosome’s ______, an assembly of multilayered plates of proteins associated with the centromere. This structure forms on opposite sides of each paired ______, in intimate association with the two sister chromatids.

kinetochore, centromere

Prometaphase and Metaphase Once properly attached to the spindle fibers, cohesin is degraded by an enzyme, appropriately named ______, and the sister chromatid arms disjoin, except at the ______.

separase, centromere region

Prometaphase and Metaphase A unique protein family called ______ (from the Japanese meaning “______”) protects cohesin from being degraded by separase at the centromeric regions.

shugoshin, guardian spirit

Prometaphase and Metaphase We know a great deal about the molecular interactions involved in kinetechore assembly along the centromere. This is of great interest because of the consequences when ______ alter the proteins that make up the ______.

mutations, kinetechore complex

Prometaphase and Metaphase Altered kinetechore function potentially leads to errors during chromosome ______, altering the ______ content of daughter cells.

migration, diploid

Prometaphase and Metaphase We also know a great deal about spindle fibers and the mechanism responsible for their attachment to the ______.

kinetechore

Prometaphase and Metaphase ______ consist of microtubules, which themselves consist of molecular subunits of the protein ______.

Spindle fibers, tubulin

Prometaphase and Metaphase Microtubules seem to ______ and “______” out of the two ______ regions at opposite poles of the cell. They are dynamic structures that ______ and ______ as a result of the ______ or ______ of polarized tubulin subunits.

originate, grow, centrosome, lengthen, shorten, addition, loss

Prometaphase and Metaphase The microtubules most directly responsible for chromosome migration make contact with, and adhere to, ______ as they grow from the centrosome region. They are referred to as ______ and have one end near the ______ (at one of the poles of the cell) and the other end anchored to the ______.

kinetochores, kinetochore microtubules, centrosome region, kinetochore

Prometaphase and Metaphase The number of microtubules that bind to the kinetochore varies greatly between organisms. ______ has only a single microtubule bound to each plate-like structure of the kinetochore. Mitotic cells of mammals, at the other extreme, reveal ______ to ______ microtubules bound to each portion of the kinetochore.

Yeast (Saccharomyces), 30, 40

Prometaphase and Metaphase At the completion of metaphase, each centromere is aligned at the ______ with the chromosome arms extending ______ in a ______ array.

metaphase plate, outward, random

Anaphase Events critical to chromosome distribution during mitosis occur during ______, the shortest stage of mitosis. During this phase, ______ of each chromosome, held together only at their ______, disjoin (separate) from one another—an event described as ______—and are pulled to opposite ends of the cell.

anaphase, sister chromatids, centromere regions, disjunction

Anaphase For complete disjunction to occur: (1) ______ must be degraded, reversing its protective role; (2) the ______ holding the centromere region of each sister chromosome is then cleaved by ______; and (3) ______ of each chromosome are pulled toward the opposite poles of the cell. As these events proceed, each migrating chromatid is now referred to as a ______.

shugoshin, cohesin complex, separase, sister chromatids, daughter chromosome

Anaphase Movement of daughter chromosomes to the opposite poles of the cell is dependent on the ______ attachment.

kinetechore–spindle fiber

Anaphase Recent investigations reveal that chromosome migration results from the activity of a series of specific molecules called ______ found at several locations within the dividing cell. These proteins, described as ______, use the energy generated by the hydrolysis of ATP. Their effect on the activity of microtubules serves ultimately to ______ the spindle fibers, drawing the chromosomes to opposite ends of the cell.

motor proteins, molecular motors, shorten

Anaphase The ______ of each chromosome appear to lead the way during migration, with the ______ trailing behind. Several models have been proposed to account for the shortening of ______. They share in common the selective removal of ______ subunits at the ends of the spindle fibers. The removal process is accomplished by the ______ described above.

centromeres, chromosome arms, spindle fibers, tubulin, molecular motor proteins

Anaphase The location of the centromere determines the ______ of the chromosome during separation, as you saw in Figure 2.3. The steps that occur during anaphase are critical in providing each subsequent daughter cell with an ______ set of chromosomes. In human cells, there would now be ______ chromosomes at each pole, one from each original sister pair.

shape, identical, 46

Telophase ______ is the final stage of mitosis and is depicted in Figure 2.7(f). At its beginning, ______ complete sets of chromosomes are present, one set at each pole.

Telophase, two

Telophase The most significant event of this stage is ______, the division or partitioning of the cytoplasm.

cytokinesis

Telophase ______ is essential if two new cells are to be produced from one cell.

Cytokinesis

Telophase The mechanism of cytokinesis differs greatly in plant and animal cells, but the end result is the same: ______ new cells are produced.

two

Telophase In plant cells, a ______ is synthesized and laid down across the region of the metaphase plate.

cell plate

Telophase Animal cells, however, undergo a constriction of the ______, much as a loop of string might be tightened around the middle of a balloon.

cytoplasm

Telophase It is not surprising that the process of cytokinesis varies in different organisms. ______, which are more regularly shaped and structurally rigid, require a mechanism for depositing new cell wall material around the plasma membrane. The cell plate laid down during telophase becomes a structure called the ______. Subsequently, the primary and secondary layers of the ______ are deposited between the ______ and ______ in each of the resulting daughter cells.

Plant cells, middle lamella, cell wall, cell membrane, middle lamella

Telophase

Telophase In animals, complete constriction of the cell membrane produces the ______ characteristic of newly divided cells.

cell furrow

Telophase Other events necessary for the transition from mitosis to interphase are initiated during ______ telophase. They generally constitute a reversal of events that occurred during ______. In each new cell, the chromosomes begin to ______ and become ______ once again, while the ______ reforms around them, the ______ disappear, and the ______ gradually reforms and becomes visible in the nucleus during early ______. At the completion of telophase, the cell enters ______.

late, prophase, uncoil, diffuse chromatin, nuclear envelope, spindle fibers, nucleolus, interphase, interphase

Cell-Cycle Regulation and Checkpoints The cell cycle, culminating in ______, is fundamentally the same in all eukaryotic organisms. This similarity in many diverse organisms suggests that the cell cycle is governed by a genetically regulated program that has been conserved throughout evolution. Because disruption of this regulation may underlie the uncontrolled cell division characterizing ______, interest in how genes regulate the cell cycle is particularly strong.

mitosis, malignancy

Cell-Cycle Regulation and Checkpoints A mammoth research effort over the past ______ years has paid high dividends, and we now have knowledge of many genes involved in the control of the cell cycle. This work was recognized by the awarding of the ______ Nobel Prize in Medicine or Physiology to ______, ______, and ______.

20, 2001, Lee Hartwell, Paul Nurse, Tim Hunt

Cell-Cycle Regulation and Checkpoints As with other studies of genetic control over essential biological processes, investigation has focused on the discovery of mutations that interrupt the ______ and on the effects of those mutations.

cell cycle

Cell-Cycle Regulation and Checkpoints Many mutations are now known that exert an effect at one or another stage of the cell cycle. First discovered in ______, but now evident in all organisms, including humans, such mutations were originally designated as ______ mutations.

yeast, cell division cycle (cdc)

Cell-Cycle Regulation and Checkpoints The normal products of many of the mutated genes are enzymes called ______ that can add ______ to other proteins. They serve as “______” molecules functioning in conjunction with proteins called ______.

kinases, phosphates, master control, cyclins

Cell-Cycle Regulation and Checkpoints Cyclins bind to these kinases (creating ______ kinases), activating them at appropriate times during the cell cycle. Activated kinases then ______ other target proteins that regulate the progress of the cell cycle.

cyclin-dependent, phosphorylate

Cell-Cycle Regulation and Checkpoints The study of cdc mutations has established that the cell cycle contains at least three ______, where the processes culminating in normal mitosis are monitored, or “checked,” by these master control molecules before the next stage of the cycle is allowed to commence.

cell-cycle checkpoints

Cell-Cycle Regulation and Checkpoints The importance of cell-cycle control and these checkpoints can be demonstrated by considering what happens when this regulatory system is impaired. Let’s assume, for example, that the DNA of a cell has incurred damage leading to one or more mutations impairing cell-cycle control. If allowed to proceed through the cell cycle, this genetically altered cell would divide uncontrollably—a key step in the development of a ______. If, instead, the cell cycle is arrested at one of the checkpoints, the cell can repair the DNA damage or permanently stop the cell from dividing, thereby preventing its potential ______.

cancer cell, malignancy

Whereas in diploid organisms, mitosis produces two daughter cells with full diploid complements, ______ produces gametes or spores that are characterized by only one haploid set of chromosomes.

meiosis

During sexual reproduction, ______ gametes then combine at ______ to reconstitute the diploid complement found in parental cells. Figure 2.9 compares the two processes by following two pairs of homologous chromosomes.

haploid, fertilization

Meiosis must be highly ______ since, by definition, haploid gametes or spores must contain precisely one member of each ______ of chromosomes.

specific, homologous pair

When successfully completed, ______ provides the basis for maintaining genetic continuity from generation to generation.

meiosis

Another major accomplishment of meiosis is to ensure that during ______ an enormous amount of genetic variation is produced among members of a species.

sexual reproduction

Such variation occurs in two forms. First, meiosis produces gametes with many unique combinations of ______ and ______ derived chromosomes among the haploid complement, thus ensuring that following ______, a large number of unique chromosome combinations are possible. As we will see (Chapter 3), this process is the underlying basis of Mendel’s principles of ______ and independent assortment.

maternally, paternally, fertilization, segregation, segregation

The second source of variation is created by the meiotic event referred to as ______, which results in genetic exchange between members of each homologous pair of chromosomes prior to one or the other finding its way into a haploid gamete or spore. This creates intact chromosomes that are mosaics of the ______ and ______ homologs from which they arise, further enhancing genetic variation. ______ therefore significantly reshuffles the genetic material, producing highly diverse offspring.

crossing over, maternal, paternal, Sexual reproduction

Meiosis: Prophase I As in mitosis, the process in meiosis begins with a ______ cell duplicating its genetic material in the ______ stage preceding chromosome division. To achieve haploidy, ______ divisions are thus required.

diploid, interphase, two

Meiosis: Prophase I The meiotic achievements, as described above, are largely dependent on the behavior of chromosomes during the initial stage of the first division, called ______.

prophase I

Meiosis: Prophase I Recall that in mitosis the paternally and maternally derived members of each homologous pair of chromosomes behave ______ during division. Each chromosome is duplicated, creating genetically identical ______, and subsequently, one chromatid of each pair is distributed to each new cell.

autonomously, sister chromatids

Meiosis: Prophase I The major difference in meiosis is that once the chromatin characterizing interphase has condensed into visible structures, the homologous chromosomes are not ______ but are instead seen to be ______, having undergone the process called ______. Figure 2.10 illustrates this process as well as the ensuing events of prophase I.

autonomous, paired up, synapsis

Meiosis: Prophase I Each synapsed pair of homologs is initially called a ______, and the number of bivalents is equal to the ______ number. In Figure 2.10, we have depicted two homologous pairs of chromosomes and thus ______ bivalents.

bivalent, haploid, two

Meiosis: Prophase I As the homologs condense and shorten, each bivalent gives rise to a unit called a ______, consisting of ______ pairs of ______, each of which is joined at a common ______. Remember that one pair of sister chromatids is ______ derived and the other pair is ______ derived.

tetrad, two, sister chromatids, centromere, maternally, paternally

Meiosis: Prophase I The presence of tetrads is visible evidence that both homologs have, in fact, ______. As prophase I progresses, each pair of sister chromatids within a tetrad is seen to pull ______. However, one or more areas remain in contact where chromatids are ______. Each such area, called a ______, is thought to represent a point where ______ (one paternal and one maternal chromatid) have undergone genetic exchange through the process of ______.

duplicated, apart, intertwined, chiasma (pl., chiasmata), nonsister chromatids, crossing over

Meiosis: Prophase I Since crossing over is thought to occur one or more times in each tetrad, ______ are routinely created during every meiotic event. During the final period of prophase I, the ______ and ______ break down, and the two ______ of each tetrad attach to the recently formed ______.

mosaic chromosomes, nucleolus, nuclear envelope, centromeres, spindle fibers

Metaphase, Anaphase, and Telophase I The remainder of the meiotic process is depicted in Figure 2.11. After meiotic prophase I, stages similar to those of ______ occur. In the first division, ______, the chromosomes have maximally shortened and thickened.

mitosis, metaphase I

Metaphase, Anaphase, and Telophase I The terminal ______ of each tetrad are visible and appear to be the major factor holding the nonsister chromatids together.

chiasmata

Metaphase, Anaphase, and Telophase I Each tetrad interacts with ______, facilitating its movement to the ______.

spindle fibers, metaphase plate

Metaphase, Anaphase, and Telophase I The alignment of each tetrad prior to the first anaphase is random: ______ of the tetrad (one of the dyads) will subsequently be pulled by ______ to one or the other pole, and the other ______ will be pulled to the ______ pole.

Half, spindle fibers, half, opposite

Metaphase, Anaphase, and Telophase I During the stages of meiosis I, a single ______ holds each pair of sister chromatids together. It appears as a single unit, and a ______ forms around each one.

centromeric region, kinetechore

Metaphase, Anaphase, and Telophase I As in our discussion of mitosis, ______ plays the major role in keeping sister chromatids together.

cohesin

Metaphase, Anaphase, and Telophase I At anaphase I, ______ is degraded between sister chromatids, except at the ______, which, as in mitosis, is protected by a ______ complex. Then, one-half of each tetrad (a ______) is pulled toward each pole of the dividing cell. Because this process effectively reduces the number of centromeres by half, it is referred to as a ______. This separation process is the physical basis of ______, the separation of homologous chromosomes from one another.

cohesin, centromere region, shugoshin, dyad, reductional division, disjunction

Metaphase, Anaphase, and Telophase I Occasionally, errors in meiosis occur and separation is not achieved. The term ______ describes such an error. At the completion of the normal anaphase I, a series of ______ equal to the ______ number is present at each pole.

nondisjunction, dyads, haploid

Metaphase, Anaphase, and Telophase I If ______ had not occurred in the first meiotic prophase, each dyad at each pole would consist solely of either paternal or maternal chromatids. However, the exchanges produced by crossing over create ______ chromatids of paternal and maternal origin.

crossing over, mosaic

Metaphase, Anaphase, and Telophase I In many organisms, telophase I reveals a ______ forming around the dyads. In this case, the ______ next enters into a short interphase period. If interphase occurs, the chromosomes do not ______ because they already consist of two chromatids.

nuclear membrane, nucleus, replicate

Metaphase, Anaphase, and Telophase I In other organisms, the cells go directly from anaphase I to ______. In general, meiotic ______ is much shorter than the corresponding stage in mitosis.

meiosis II, telophase

The Second Meiotic Division A second division, referred to as ______, is essential if each gamete or spore is to receive only one chromatid from each original tetrad. The stages characterizing meiosis II are shown on the right side of Figure 2.11.

meiosis II

The Second Meiotic Division During ______, each dyad is composed of one pair of sister chromatids attached by the common centromeric region.

prophase II

The Second Meiotic Division During ______, the centromeres are positioned on the equatorial plate.

metaphase II

The Second Meiotic Division When the shugoshin complex is degraded, the centromeres sepa- rate, ______ is initiated, and the sister chromatids of each dyad are pulled to opposite poles.

anaphase II

The Second Meiotic Division Because the number of dyads is equal to the haploid number, ______ reveals one member of each pair of homologous chromosomes present at each pole. Each chromosome is now a ______. Because the number of centromeres is not reduced in number in the two resulting cells, the process is referred to as an ______.

telophase II, monad, equational division

The Second Meiotic Division Following cytokinesis in telophase II, ______ haploid gametes may result from a single meiotic event. At the conclusion of meiosis II, not only has the haploid state been achieved, but if crossing over has occurred, each monad contains a combination of maternal and paternal genetic information. As a result, the offspring produced by any gamete will receive a mixture of genetic information originally present in his or her grandparents. Meiosis thus significantly increases the level of ______ in each ensuing generation.

four, genetic variation

Although events that occur during the meiotic divisions are similar in all cells participating in gametogenesis in most animal species, there are certain differences between the production of a male gamete (______) and a female gamete (______). Figure 2.12 summarizes these processes.

spermatogenesis, oogenesis

______ takes place in the testes, the male reproductive organs. The process begins with the enlargement of an undifferentiated diploid germ cell called a ______. This cell grows to become a ______, which undergoes the first meiotic division. The products of this division, called ______, contain a haploid number of dyads. The secondary spermatocytes then undergo meiosis II, and each of these cells produces two haploid ______. Spermatids go through a series of developmental changes, ______, to become highly specialized, motile ______, or ______. All sperm cells produced during spermatogenesis contain the haploid number of chromosomes and equal amounts of cytoplasm.

Spermatogenesis, spermatogonium, primary spermatocyte, secondary spermatocytes, spermatids, spermiogenesis, spermatozoa, sperm

Spermatogenesis may be ______ or may occur ______ in mature male animals; its onset is determined by the species’ reproductive cycles. Animals that reproduce year-round produce sperm continuously, whereas those whose breeding period is confined to a particular season produce sperm only during that time.

continuous, periodically

In animal ______, the formation of ______, or eggs, occurs in the ovaries, the female reproductive organs. The daughter cells resulting from the two meiotic divisions of this process receive equal amounts of genetic material, but they do not receive equal amounts of ______. Instead, during each division, almost all the cytoplasm of the ______, itself derived from the ______, is concentrated in one of the two daughter cells. The concentration of cytoplasm is necessary because a major function of the mature ovum is to nourish the developing embryo following fertilization.

oogenesis, ova (sing. ovum), cytoplasm, primary oocyte, oogonium

During ______ in oogenesis, the tetrads of the primary oocyte separate, and the dyads move toward opposite poles. During ______, the dyads at one pole are pinched off with very little surrounding cytoplasm to form the ______. The first polar body may or may not divide again to produce two small haploid cells. The other daughter cell produced by this first meiotic division contains most of the cytoplasm and is called the ______. The mature ovum will be produced from the secondary oocyte during the second meiotic division. During this division, the cytoplasm of the secondary oocyte again divides unequally, producing an ______ and a ______. The ootid then differentiates into the ______.

anaphase I, telophase I, first polar body, secondary oocyte, ootid, second polar body, mature ovum

Unlike the divisions of spermatogenesis, the two meiotic divisions of oogenesis may not be ______. In some animal species, the second division may directly follow the first. In others, including humans, the first division of all oocytes begins in the ______ but arrests in ______. Many years later, meiosis resumes in each oocyte just prior to its ovulation. The second division is completed only after fertilization.

continuous, embryonic ovary, prophase I

The process of meiosis is critical to the successful ______ of all diploid organisms. It is the mechanism by which the diploid amount of genetic information is reduced to the ______ amount. In animals, meiosis leads to the formation of ______, whereas in plants haploid ______ are produced, which in turn lead to the formation of haploid gametes.

sexual reproduction, haploid, gametes, spores

Each diploid organism stores its genetic information in the form of ______ pairs of chromosomes. Each pair consists of one member derived from the maternal parent and one from the paternal parent. Following meiosis, ______ cells potentially contain either the paternal or the maternal representative of every homologous pair of chromosomes. However, the process of ______, which occurs in the first meiotic prophase, further reshuffles the alleles between the maternal and paternal members of each homologous pair, which then segregate and assort independently into gametes. These events result in the great amount of genetic variation present in gametes.

homologous, haploid, crossing over

It is important to touch briefly on the significant role that meiosis plays in the life cycles of fungi and plants. In many fungi, the predominant stage of the life cycle consists of ______ vegetative cells. They arise through ______ and proliferate by ______ cell division.

haploid, meiosis, mitotic

In multicellular plants, the life cycle alternates between the diploid ______ stage and the haploid ______ stage. While one or the other predominates in different plant groups during this “______,” the processes of meiosis and fertilization constitute the “______” between the sporophyte and gametophyte stages. Therefore, meiosis is an essential component of the life cycle of plants.

sporophyte, gametophyte, alternation of generations, bridges

An interesting question is why chromosomes are invisible during ______ but visible during the various stages of mitosis and meiosis.

interphase

Recall that, during interphase, only dispersed ______ are present in the nucleus. Once mitosis begins, however, the fibers ______ and ______, condensing into typical mitotic chromosomes. If the fibers comprising a mitotic chromosome are ______, the areas of greatest spreading reveal individual fibers similar to those seen in interphase chromatin. Very few fiber ends seem to be present, and in some cases, none can be seen. Instead, individual fibers always seem to loop back into the interior. Such fibers are obviously ______ and ______ around one another, forming the regular pattern of folding in the mitotic chromosome.

chromatin fibers, coil, fold, loosened, twisted, coiled

Starting in late ______ of mitosis and continuing during ______ of interphase, chromosomes unwind to form the long fibers characteristic of ______, which consist of DNA and associated proteins, particularly proteins called ______. It is in this physical arrangement that DNA can most efficiently function during transcription and replication.

telophase, G1, chromatin, histones

Electron microscopic observations of metaphase chromosomes in varying degrees of coiling led Ernest DuPraw to postulate the ______, shown in Figure 2.14(c).

folded-fiber model

During metaphase, each chromosome consists of two ______ joined at the ______. Each arm of the chromatid appears to be a single fiber wound much like a skein of ______. The fiber is composed of tightly coiled double-stranded ______ and ______. An orderly coiling–twisting–condensing process appears to facilitate the transition of the interphase chromatin into the more condensed mitotic chromosomes.

sister chromatids, centromeric region, yarn, DNA, protein

Geneticists believe that during the transition from interphase to prophase, a ______-fold compaction occurs in the length of DNA within the chromatin fiber! This process must be extremely precise given the highly ordered and consistent appearance of mitotic chromosomes in all eukaryotes. Note particularly in the micrographs the clear distinction between the sister chromatids constituting each chromosome. They are joined only by the common ____44 that they share prior to anaphase.

5000, centromere