Biology-Cell Division Flashcards
How many phases does cell division consist of?
It consists of two phases, nuclear division followed by cytokinesis
What does nuclear division divide?
It divides the genetic material in the nucleus
What does the cytokinesis divide?
It divides the cytoplasm
What are the two kinds of nuclear division?
Mitosis and Meiosis
Mitosis
divides the nucleus so that both daughter cells are genetically identical
Meiosis
reduction division, which produces genetically variable daughter cells that contain half the genetic info of the parent cell
What’s the first step in either mitosis or meiosis?
The first step begins w/ the condensation of the genetic material, chromatin, into tightly coiled bodies called chromosomes
Chromosomes is made up of two identical halves called _______________, which are joined at the _____________.
Chromosomes are made up of two identical halves called SISTER CHROMATIDS, which are joined at the CENTROMERE
Each chromatid consists of a single, tightly coiled molecule of ________, the genetic material of the cell.
Each chromatid consists of a single, tightly coiled molecule of DNA, the genetic material of the cell.
Describe a homologous pair of chromosomes
one homologue comes from the maternal parent, the other from the paternal parent
How many chromosomes do humans have?
46 chromosomes, 23 homologous pairs, consisting of a total of 92 chromatids
cell cycle
describes the sequence of events that occurs during the life of most eukaryotic cells
interphase
the period during which the cell is not dividing; the chromatin is enclosed within a clearly defined nuclear envelope
Within the nucleus, one or more nucleoli are visible. Outside the nucleus, two microtubule organizing centers lie adjacent to one another.
In animals, each MTOC contains a pair of centrioles.
What are the four phases of mitosis?
prophase, metaphase, anaphase, and telophase, followed by cytokinesis
(Mitosis) Prophase
3 activities occur simultaneously:
First, the nucleoli disappear and the chromatin condense into chromosomes
Second, the nuclear envelope breaks down
Third, the mitotic spindle is assembled
Fourth, microtubules from each MTOC connect to a specialized region in the centromere called a kinetochore
(mitosis) Metaphase
begins when the chromosomes are distributed across the metaphase plate. Metaphase ends when the microtubules, still attached to the kinetochores, pull each chromosome apart into two chromatids. Once separated from its sister chromatid, each chromatid is called a chromosome.
(Mitosis) Anaphase
the microtubules connected to the chromatids (now chromosomes) shorten, effectively pulling the chromosomes to opposite poles. At the end of anaphase, each pole has a complete set of chromosomes, the same number of chromosomes as the original cell (Since they consist of only one chromatid, each chromosome contains only a single copy of the DNA molecule).
(Mitosis) Telophase
concludes the nuclear division. During this phase, a nuclear envelope develops around each pole, forming two nuclei. The chromosomes within each of these nuclei disperse into chromatin, and the nucleoli reappear. Cytokinesis divides the cytoplasm to form two cells.
How are plant and animal cell cytokinesis different?
Cell Plate- In plants, vesicles originating from Golgi bodies migrate to the plane between the two newly forming nuclei. The vesicles fuse to form a cell plate, which subsequently becomes the plasma membranes for the 2 daughter cells. Cell walls develop between the membranes.
Cleavage furrow- In animals, microfilaments form a ring inside the plasma membrane between the two newly forming nuclei. As microfilaments shorten, they act like purse strings to pull the plasma membrane into the center, dividing the cell into 2 daughter cells. The groove that forms as the purse strings are tightened is called a cleavage furrow.
What happens when mitosis and cytokinesis are completed and interphase begins?
the cell begins a period of growth that is divided into 3 phases: G1, S, and G2.
Does growth occur in all phases: G1, S, and G2?
Yes; although you can associate G1 and G2 w/ growth and S w/ synthesis
What happens in the S phase?
the second DNA molecule for each chromosome is synthesized. As a result of this DNA replication, each chromosome that appears at the beginning of the next mitotic division will appear as two sister chromatids
What happens in the G2 phase?
materials for the next mitotic division are prepared
Cell cycle
the time span through mitosis and cytokinesis, together referred to as the M phase, through G1, S, and G2
diploid cell
a cell w/ 2 copies of every chromosome (designated by 2n)
Meiosis
similar to mitosis, but meiosis consists of two groups of divisions, meiosis I and meiosis II
Meiosis I
homologous chromosomes pair at the metaphase plate, and then the homologues migrate to opposite poles
Meiosis II
chromosomes spread across the metaphase plate and sister chromatids separate and migrate to opposite poles.
(Meiosis) Prophase I
the nucleolus disappears, chromatin condenses into chromosomes, the nuclear envelope breaks down, and the spindle apparatus develops. MT’s begin attaching to kinetochores.
Once the chromosomes are condensed, homologous chromosomes pair up (synapsis). These pairs of homologous chromosomes are variously referred to as tetrads (a group of four chromatids) or bivalents. During synapsis, corresponding regions along nonsister chromatids form close associations called chiasmata
chiasmata
sites where genetic material is exchanged between nonsister homologous chromatids, a process called crossing over
synaptonemal complex
protein structure that temporarily forms between homologous chromosomes: gives rise to a tetrad w/ chiasmata and crossover events
(Meiosis) Metaphase I
homologous pairs are spread across the metaphase plate. Microtubules attached to the kinetochore of one member of each homologous pair. Microtubules from the other pole are connected to the second member of each homologous pair.
(Meiosis) Anaphase I
begins when homologues within tetrads uncouple as they are pulled to opposite poles (disjunction)
(Meiosis) Telophase I
nuclear membrane develops. Since daughter nuclei will have half the number of chromosomes, cells that they eventually form will be haploid.
Beginning in telophase I, the cells of many species begin cytokinesis and form cleavage furrows or cell plates, In other species, cytokinesis is delayed until after meiosis II. Also, a short interphase II may begin. In any case, no replication of chromosomes occurs during this period. Instead, part II of meiosis begins in both daughter nuclei.
(Meiosis) Prophase II
the nuclear envelope disappears and the spindle develops. No chiasmata and no crossing over of genetic material as in prophase I
(Meiosis) Metaphase II
the chromosomes align on the metaphase plate, but now with half number of chromosomes
(Meiosis) Anaphase II
begins as each chromosome is pulled apart into two chromatids by the microtubules of the spindle apparatus. The chromatids (now chromosomes) migrate to their respective poles. Again, this is exactly what happens in mitosis except that now there is only half the # of chromosomes
(Meiosis) Telophase II
the nuclear envelope reappears at each pole and cytokinesis occurs. The end result of meiosis is four haploid cells (chromosome makeup of each daughter cell designated by n).
Each cell contains half the number of chromosomes, and each chromosome consists of only one chromatid.
Later in interphase, a second chromatid in each chromosome is replicated, but the cell will still have only half the # of chromosomes.
What is mitosis responsible for?
it occurs during growth and development of multicellular organisms and for repair of existing cells. Mitosis is also responsible for asexual reproduction, common among plants and single-celled organisms. Mitosis occurs in somatic cells, body cells.
What does mitosis end with?
mitosis ends w/ 2 diploid daughter cells
What does meiosis end with?
meiosis ends w/ 4 haploid daughter cells
What is meiosis responsible for?
meiosis is responsible for producing gametes
fertilization
fusing of an egg and sperm, gives rise to a diploid cell called a zygote
What’s the chromosome number in a parent cell before division begins in mitosis and meiosis?
Mitosis- 46
Meiosis I- 46
Meiosis II-46
What’s the chromatid number in a parent cell before division begins?
Mitosis-92
Meiosis I-92
Meiosis II- 92
Does crossing over occur at prophase for mitosis and meiosis?
Mitosis- no
Meiosis I- yes
Meiosis II- no
Does chromosome arrangement occur on metaphase plate?
Mitosis- Chromosomes line up
Meiosis I-Homologues pair
Meiosis II- Chromosomes line up
What’s the number of chromosomes in each daughter nucleus?
Mitosis-46
Meiosis I-23
Meiosis II-23
of chromatids in each daughter nucleus?
Mitosis-46
Meiosis I-46
Meiosis II-23
of daughter cells at end of division?
Mitosis-2
Meiosis I-2
Meiosis II-4
Chromosome notation for daughter cells?
Mitosis-2n
Meiosis I-n
Meiosis II-n
Genome notation for daughter cells?
Mitosis-diploid
Meiosis I-haploid
Meiosis II-haploid
Purpose of division?
Mitosis-cell replacement, organism growth, asexual reproduction
Meiosis I- sexual reproduction
Meiosis II- sexual reproduction
genetics of daughter cells?
Mitosis-genetically identical (clones)
Meiosis I- genetically variable
Meiosis II- genetically variable
type of cells where division occurs?
Mitosis- somatic cells
Meiosis I- reproductive cells (ovaries, testes, anthers)
Meiosis II- reproductive cells (ovaries, testes, anthers)
type of cells produced?
Mitosis- somatic cells
Meiosis I- gametes: eggs, sperm, pollen
Meiosis II- gametes: eggs, sperm, pollen
Human cells contain ___ chromosomes (___ homologous pairs)
46 chromosomes, 23 homologous pairs
Thus, 2n= 46
For human gametes, n=23
What produces spores? (plants)
meiosis
spores (plants)
haploid cells that divide by mitosis to become a multicellular haploid structure, the gametophyte
sporophyte
gametes produced by the gametophyte by mitosis, fuse and produce a diploid cell that grows by mitosis
alternation of generations
when both the gametophyte and sporophyte stages are multicellular; alternation of haploid and diploid stages
genetic recombination
reassortment of genetic material that originates from: crossing over during prophase I, independent assortment of homologues during metaphase I (which chromosome goes into which cell), and random joining of gametes aka germ cells ( which sperm fertilizes which egg - genetic composition of gamete affects this)
crossing over
during prophase I, nonsister chromatids of homologous chromosomes exchange pieces of genetic material. As a result each homologue no longer entirely represents a single parent
Independent assortment of homologues
during metaphase I, tetrads of homologous chromosomes separate into chromosomes that go to opposite poles. Which chromosome goes to which pole depends upon the orientation of a tetrad at the metaphase plate.
random joining of gametes
which sperm fertilizes which egg is to a large degree a random event. In many cases, however, this event may be affected by the genetic composition of a gamete.
For example, some sperm may be faster swimmers and have a better chance of fertilizing the egg
What are two functional limitations for cell size that limit growth or influence the start of a new cell division?
- Surface-to-volume ratio (S/V) volume gets much larger when cells grow (4/3pi r^3) vs. SA (4pi r^2). When S/V is large, exchange becomes much easier. When S/V is small, exchange is hard, leads to cell death or cell division to increase SA
- Genome-to-volume ratio (G/V)- genome size remains constant throughout life; as cell grows, only volume increase. G/V will be small and thus exceed the ability of its genome to produce sufficient amts of regulator of activities. Some large cells (paramecium, human skeletal muscle) are multinucleated to deal with this.
What influences the onset of cell division?
- Checkpoints
- Cyclin-dependent kinases (Cdk’s).
- Growth factors
- Density-dependent inhibition
- Anchorage dependence
Checkpoints
at specific points during the cell cycle, the cell evaluates internal and external conditions to determine whether or not to continue through the cell cycle.
The G1 checkpoint
The G2 checkpoint
The M checkpoint
The G1 checkpoint
occurs near the end of G1 phase. If conditions are not appropriate or if the cell is genetically programmed not to divide, the cell proceeds no further through the cell cycle, remaining in a an extended G1 phase (or G0 phase), never beginning the S or G2 phases (unless some internal or external cue initiates a resumption of the cell cycle)
aka restriction point, the most important one. At the end of G1 phase, if cell is not ready to divide it may arrest here (G0 phase- nerve and muscle cells remain here, rarely divide after maturing) and never procedd or wait until it is ready
the G2 checkpoint
occurring at the end of the G2 phase of the cell cycle, evaluates the accuracy of DNA replication and signals whether or not to begin mitosis.
checks for sufficient mitosis promoting factor (MPF) levels to proceed
the M checkpoint
occurring during metaphase, ensures that microtubules are properly attached to all kinetochores at the metaphase plate before division continues w/ anaphase.
Cyclin-dependent kinases (Cdk’s)
Cdk enzymes activate proteins that regulate the cell cycle by attaching a phosphate group to them (phosphorylation). Cdk’s are themselves activated by the protein cyclin, a protein whose presence varies during the different phases of the cell cycle
Growth factors
the plasma membranes of cells have receptors for external molecules, or growth factors, that stimulate a cell to divide. One such growth factor is produced by damaged cells, stimulating other cells to divide. More than 50 different growth factors are known
Density-dependent inhibition
cells stop dividing when the surrounding cell density reaches a certain maximum
anchorage dependence
most cells only divide when they are attached to an external surface, such as the flat surface of a neighboring cell (or the side of a culture dish)
cancer
characterized by uncontrolled cell growth and division
transformed cells
cells that have become cancerous, proliferate w/o regard to cell cycle checkpoints, density-dependent inhibition, anchorage dependence, and other regulatory mechanism.
In what stage of mitosis does karyotyping occurs?
Metaphase
Is more time spent in interphase or mitosis?
Interphase (>90%)
How many steps does prophase I have?
- leptotene (chromosomes start condensing)
- zygotene (synapsis begins; synaptonemal complex forming)
- pachytene (synapsis complete, crossing over)
- diplotene (synatopnemal complex disappears, chiasma still present)
- diakinesis (nuclear envelope fragments, chromosomes complete condensing, tetrads ready for metaphase)
fertilization/syngamy
fusion of two haploid gametes = diploid zygote
Plant meiosis and mitosis
meiosis in sporangia produces spores (haploid); spores undergo mitosis to become multicellular (gametophyte) which are haploid (n) isnce psores are already haploid. The gametes fuse and produce a diploid cell (zygote 2n) that grows by mitosis to become sporophyte. Cells in sporophyte (sporangia) undergo meiosis to produce haploid spores which germinate and repeat life cycle.
