11: Meiosis and Sexual Reproduction

Question 1

What are chiasmata?

Answer 1

The structure that forms at the crossover points after genetic material is exchanged (singular = chiasma).

Question 2

What is cohesin?

Answer 2

Proteins that form a complex that seals sister chromatids together at their centromeres until anaphase II of meiosis.

Question 3

What is crossover?

Answer 3

The exchange of genetic material between non-sister chromatids resulting in chromosomes that incorporate genes from both parents of the organism.

Question 4

What is fertilization?

Answer 4

The union of two haploid cells from two individual organisms.

Question 5

What is interkinesis?

Answer 5

A brief period of rest between meiosis I and meiosis II. AKA interphase II.

Question 6

What is meiosis?

Answer 6

A nuclear division process that results in four haploid cells.

Question 7

What is meiosis I?

Answer 7

The first round of meiotic cell division; referred to as reduction division because the ploidy level is reduced from diploid to haploid.

Question 8

What is meiosis II?

Answer 8

The second round of meiotic cell division following meiosis I; sister chromatids are separated into individual chromosomes, and the result is four unique haploid cells.

Question 9

What are recombination nodules?

Answer 9

Protein assemblies formed on the synaptonemal complex that marks the point of crossover events and mediate the multistep process of genetic recombination between non-sister chromatids.

Question 10

What is reduction division?

Answer 10

Nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction division.

Question 11

What are somatic cells?

Answer 11

All the cells of a multicellular organism except the gametes or reproductive cells.

Question 12

What is a spore?

Answer 12

A haploid cell that can produce a haploid multicellular organism or can fuse with another spore to form a diploid cell.

Question 13

What is synapsis?

Answer 13

The formation of a close association between homologous chromosomes during prophase I.

Question 14

What is a synaptonemal complex?

Answer 14

A protein lattice that forms between homologous chromosomes during prophase I, supporting crossover.

Question 15

What is a tetrad?

Answer 15

Two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I.

Question 16

What is an organism’s ploidy level?

Answer 16

The number of sets of chromosomes in a cell is called its ploidy level.

Question 17

What is the difference between mitosis and meiosis?

Answer 17

Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.”

Question 18

What happens during interphase preceding meiosis?

Answer 18

Meiosis is preceded by an interphase consisting of the G₁, S, and G₂ phases, which are nearly identical to the phases preceding mitosis. The G₁ phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. Finally, the G₂ phase, also called the second gap phase, is the third and final phase of interphase; in this phase, the cell undergoes the final preparations for meiosis.

Question 19

What happens during the S phase preceding meiosis?

Answer 19

During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids together until anaphase II. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. This prepares the cell to enter prophase I, the first meiotic phase.

Question 20

What happens during prophase I?

Answer 20

Early in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair. The synaptonemal complex first forms at specific locations and then spreads to cover the entire length of the chromosomes. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata.

Question 21

How does a synaptonemal complex form between sex chromosomes?

Answer 21

In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology.

Question 22

What do recombination nodules do?

Answer 22

Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules. These assemblies mark the points of later chiasmata and mediate the multistep process of crossover—or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids.

Question 23

What do chiasmata do?

Answer 23

As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

Question 24

What do crossover events do?

Answer 24

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.

Question 25

What happens during prometaphase I?

Answer 25

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes’ kinetochores. With each member of the homologous pair attached to opposite poles of the cell, in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

Question 26

What happens during metaphase I?

Answer 26

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator.

Question 27

What is the significance of the randomness of homologous chromosome alignment at the metaphase plate during metaphase I?

Answer 27

This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate; the possible number of alignments therefore equals 2ⁿ, where n is the number of chromosomes per set.

Question 28

What is the probability that two human haploid cells produced by meiosis will have the same genetic makeup?

Answer 28

Humans have 23 chromosome pairs, which results in over eight million (2²³) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.

Question 29

What happens during anaphase I?

Answer 29

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.

Question 30

What happens during telophase I and cytokinesis?

Answer 30

In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

Question 31

What is the product of meiosis I?

Answer 31

Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over).

Question 32

What happens between meiosis I and II?

Answer 32

In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated.

Question 33

What happens during meiosis II?

Answer 33

The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.

Question 34

What happens during prophase II?

Answer 34

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.

Question 35

What happens during prometaphase II?

Answer 35

The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

Question 36

What happens during metaphase II?

Answer 36

The sister chromatids are maximally condensed and aligned at the equator of the cell.

Question 37

What happens during anaphase II?

Answer 37

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

Question 38

What happens during telophase II and cytokinesis?

Answer 38

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.

Question 39

How might meiosis have evolved from mitosis?

Answer 39

Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that the first step is the hardest and most important, and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis.

Question 40

Why are protists studied in order to determine how meiosis may have evolved?

Answer 40

There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Some appear to be simpler or more “primitive” forms of meiosis. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and colleagues compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.

Question 41

What is alternation of generations?

Answer 41

A life-cycle type in which the diploid and haploid stages alternate.

Question 42

What is diploid-dominant?

Answer 42

A life-cycle type in which the multicellular diploid stage is prevalent.

Question 43

What is haploid-dominant?

Answer 43

Life-cycle type in which the multicellular haploid stage is prevalent.

Question 44

What is a gametophyte?

Answer 44

A multicellular haploid life-cycle stage that produces gametes.

Question 45

What are germ cells?

Answer 45

Specialized cell line that produces gametes, such as eggs or sperm.

Question 46

What is a life cycle?

Answer 46

The sequence of events in the development of an organism and the production of cells that produce offspring.

Question 47

What is a sporophyte?

Answer 47

A multicellular diploid life-cycle stage or organism that produces haploid spores by meiosis.

Question 48

What are some advantages of asexual reproduction over sexual reproduction?

Answer 48

Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. It appears to have been very successful because most eukaryotes are able to reproduce sexually, and in many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, creating offspring that are genetic clones of the parent appears to be a better system. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring whenever circumstances are favorable by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so in theory an asexual population could grow twice as fast.

Question 49

Why is sexuality (and meiosis) so common?

Answer 49

This is one of the important unanswered questions in biology and has been the focus of much research beginning in the latter half of the twentieth century. There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms, but in addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by crossovers during prophase I and random assortment at metaphase I.

Question 50

What is the Red Queen hypothesis?

Answer 50

A hypothesis first proposed by Leigh Van Valen in 1973 to explain the benefits of sexual reproduction and ongoing genetic variation. Named after the Red Queen and her race in Lewis Carroll’s book Through the Looking Glass. Basically, no species can progress too far ahead of others because genetic variation among progeny provides all species with a mechanism to improve rapidly. Species that cannot compete become extinct. “It takes all the running you can do to stay in the same place.”

Question 51

What are the main categories of life cycles in multicellular organisms?

Answer 51

There are three main categories: diploid-dominant, haploid-dominant, and alternation of generations.

Question 52

How does reproduction occur in diploid-dominant species?

Answer 52

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state.

Question 53

How does reproduction occur in haploid-dominant species?

Answer 53

Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals, designated the (+) and (−) mating types, join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although haploid like the “parents,” these spores contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are conducive, the spores form multicellular haploid structures by many rounds of mitosis.

Question 54

How does reproduction occur during alternation of generations?

Answer 54

The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes, because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case, because the organism that produces the gametes is already a haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes.

Question 55

What are some examples of differences between sporophytes and gametophytes in plants?

Answer 55

Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte organism is the free-living plant, and the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living; however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.