Recall that, in mitosis, homologous chromosomes do not pair together. 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 , a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis.
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. Figure 1. Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.
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. 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.
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 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 Figure 2 and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Figure 2. Crossover occurs between non-sister chromatids of homologous chromosomes.
The result is an exchange of genetic material between homologous chromosomes. 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. The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres.
Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. 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. 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.
Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis at How Cells Divide.
Sexual reproduction requires that organisms produce cells that can fuse during fertilization to produce offspring. In most animals, meiosis is used to produce haploid eggs and sperm from diploid parent cells so that the fusion of an egg and sperm produces a diploid zygote.
As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle so that each chromosome becomes a pair of sister chromatids. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first division separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes.
Meiosis generates variation in the daughter nuclei during crossover in prophase I as well as during the random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique. Meiosis and mitosis share similar processes, but have distinct outcomes.
Mitotic divisions are single nuclear divisions that produce genetically identical daughter nuclei i. In contrast, meiotic divisions include two nuclear divisions that ultimately produce four genetically different daughter nuclei that have only one chromosome set instead of the two sets of chromosomes in the parent cell.
The main differences between the two nuclear division processes take place during the first division of meiosis: homologous chromosomes pair, crossover, and exchange homologous nonsister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover and chromosome recombination in prophase I.
Which of the following is not true in regard to crossover? If a muscle cell of a typical organism has 32 chromosomes, how many chromosomes will be in a gamete of that same organism? Which statement best describes the genetic content of the two daughter cells in prophase II of meiosis? Assuming no crossing over events occur, how many unique gametes could one pea plant produce?
During the meiotic interphase, each chromosome is duplicated. The sister chromatids that are formed during synthesis are held together at the centromere region by cohesin proteins. All chromosomes are attached to the nuclear envelope by their tips.
As the cell enters prophase I, the nuclear envelope begins to fragment and the proteins holding homologous chromosomes locate each other. The four sister chromatids align lengthwise, and a protein lattice called the synaptonemal complex is formed between them to bind them together. The synaptonemal complex facilitates crossover between nonsister chromatids, which is observed as chiasmata along the length of the chromosome.
As prophase I progresses, the synaptonemal complex breaks down and the sister chromatids become free, except where they are attached by chiasmata. At this stage, the four chromatids are visible in each homologous pairing and are called a tetrad. Explain how the random alignment of homologous chromosomes during metaphase I contributes to the variation in gametes produced by meiosis. Random alignment leads to new combinations of traits.
The chromosomes that were originally inherited by the gamete-producing individual came equally from the egg and the sperm. In metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the cell. The orientation of each tetrad is random. There is an equal chance that the maternally derived chromosomes will be facing either pole. The same is true of the paternally derived chromosomes.
The alignment should occur differently in almost every meiosis. As the homologous chromosomes are pulled apart in anaphase I, any combination of maternal and paternal chromosomes will move toward each pole. Each gamete is unique. What is the function of the fused kinetochore found on sister chromatids in prometaphase I?
In metaphase I, the homologous chromosomes line up at the metaphase plate. In anaphase I, the homologous chromosomes are pulled apart and move to opposite poles. Sister chromatids are not separated until meiosis II. The fused kinetochore formed during meiosis I ensures that each spindle microtubule that binds to the tetrad will attach to both sister chromatids.
In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosis? All of the stages of meiosis I, except possibly telophase I, are unique because homologous chromosomes are separated, not sister chromatids. In some species, the chromosomes do not decondense and the nuclear envelopes do not form in telophase I.
All of the stages of meiosis II have the same events as the stages of mitosis, with the possible exception of prophase II. In some species, the chromosomes are still condensed and there is no nuclear envelope. Other than this, all processes are the same.
Why would an individual with a mutation that prevented the formation of recombination nodules be considered less fit than other members of its species?
The chromosomes of the individual cannot cross over during meiosis if the individual cannot make recombination nodules. An individual who cannot produce diverse offspring is considered less fit than individuals who do produce diverse offspring. Does crossing over occur during prophase II? From an evolutionary perspective, why is this advantageous? Crossing over does not occur during prophase II; it only occurs during prophase I.
In prophase II, there are still two copies of each gene, but they are on sister chromatids within a single chromosome rather than homologous chromosomes as in prophase I. Therefore, any crossover event would still produce two identical chromatids. Because it is advantageous to avoid wasting energy on events that will not increase genetic diversity, crossing over does not occur.
Skip to content Meiosis and Sexual Reproduction. Learning Objectives By the end of this section, you will be able to do the following: Describe the behavior of chromosomes during meiosis, and the differences between the first and second meiotic divisions Describe the cellular events that take place during meiosis Explain the differences between meiosis and mitosis Explain the mechanisms within the meiotic process that produce genetic variation among the haploid gametes.
Meiosis I Meiosis is preceded by an interphase consisting of G 1 , S, and G 2 phases, which are nearly identical to the phases preceding mitosis. Prophase I Early in prophase I, before the chromosomes can be seen clearly with a microscope, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins.
Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.
Crossover occurs between nonsister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes. Prometaphase I The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Metaphase I During metaphase I, the homologous chromosomes are arranged at the metaphase plate —roughly in the midline of the cell, with the kinetochores facing opposite poles.
In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2 n , where n equals the number of chromosomes in a set.
In this example, there are four possible genetic combinations for the gametes. Anaphase I In anaphase I, the microtubules pull the linked chromosomes apart. Telophase I and Cytokinesis In telophase, the separated chromosomes arrive at opposite poles. Link to Learning. Meiosis II In some species, cells enter a brief interphase, or interkinesis , before entering meiosis II.
Prophase II If the chromosomes decondensed in telophase I, they condense again. Prometaphase II The nuclear envelopes are completely broken down, and the spindle is fully formed. Metaphase II The sister chromatids are maximally condensed and aligned at the equator of the cell.
Anaphase II The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midline of the cell the metaphase plate in metaphase I.
In anaphase I, the homologous chromosomes separate. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids separate. Telophase II and Cytokinesis The chromosomes arrive at opposite poles and begin to decondense. Comparing Meiosis and Mitosis Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells.
Meiosis and mitosis are both preceded by one cycle of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell. Evolution Connection.
None of this happens in mitosis. In metaphase I meiosis, the tetrads correspond to the metaphasic plaque. In mitosis there is only one division and it produces two daughter cells. Meiosis, on the other hand, is used in the human body for only one purpose: the production of gametes - gametes or sperm and egg cells.
The goal is to produce daughter cells with exactly half the number of chromosomes of the original cell. Synapse also called synthesis is the connection of two homologous chromosomes that occurs during meiosis.
Mitosis also has prophase but does not usually link two homologous chromosomes. Cytokinesis is part of the M phase, but it is not part of mitosis. Mphase consists of nuclear division mitosis and cytoplasmic division cytokinesis.
And yes, telophase is part of mitosis, including the M phase. The other name for mitosis is division of equations. Therefore, the number of chromosomes in the resulting progeny is similar to that of the parent cell. All eukaryotic cells reproduce by mitosis, except the germ cells, which pass through meiosis see below to produce gametes ova and sperm.
Mitosis occurs in all cells of the body except the sex cells, which are formed through meiotic cell division. Chromosomal transition or crossover is the exchange of genetic material between two homologous chromosomes, non-sister chromatids, which result in recombinant chromosomes during sexual reproduction.
Mitosis has five different phases: interphase, prophase, metaphase, anaphase and telophase.
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