Click to see full answer. Likewise, in what stage of meiosis do Tetrads line up? The tetrad occurs during the first phase of meiosis. It is the foursome of chromatids that forms when replicated homologous chromosomes align. It must be formed for crossing over to occur. It is broken apart when the homologous chromosomes separate in meiosis I.
Metaphase I: The tetrads of chromosomes are lined up on the equator of the cell with the spindle apparatus having already been completely formed.
It is during prophase I and metaphase I where genetic recombination occurs. What is a group of 4 chromatids called? The group of 4 chromatids that forms during synapsis is called. What is the product of meiosis 1? During meiosis one cell? These four daughter cells only have half the number of chromosomes? Meiosis produces our sex cells or gametes? How many chromosomes are at the end of meiosis 1? What events occur during meiosis I and meiosis II?
Both Meiosis I and II have the same number and arrangement of phases: prophase, metaphase, anaphase, and telophase. Both produce two daughter cells from each parent cell. Meiosis is preceded by an interphase consisting of the G 1 , S, and G 2 phases, which are nearly identical to the phases preceding mitosis.
The G 1 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 G 2 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. 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. 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. 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. 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.
Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. In anaphase I, the microtubules pull the attached 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.
In telophase I, the separated chromosomes arrive at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. Then 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. 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. Although there is only one chromosome set, each homolog still consists of two sister chromatids. Learning Objectives Describe the stages and results of meiosis I. During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over.
In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly.
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