15.2 The process of meiosis  (Page 2/17)

15.2a meiosis i

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.

Prophase i

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. In meiosis though, proteins will bind the homologous chromosomes together. 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 synapsis supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over ( [link] ).

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.

Figure 11.1

During synapsis, crossover     —or genetic recombination—occurs between the non-sister chromatids. Crossover is when sections of DNA from one chromosome of the pair are swapped for a corresponding section from the other chromosome. Because this occurs between homologous chromosomes, only different versions of genes are exchanged -- not totally different genes ( [link] ). As prophase I progresses, the proteins that contributed to synapsis break down and the chromosomes begin to condense. Once synapsis is over, the homologous chromosomes are still attached to each other at the centromere.

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.