An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over (Figure 3). In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. 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. Figure 2 Early in prophase I, homologous chromosomes come together to form a synapse. Recall that synapsis does NOT occur during mitosis. In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. The tight pairing of the homologous chromosomes is called synapsis (Figure 2). As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Prophase IĮarly in prophase I, the chromosomes can be seen clearly microscopically. This prepares the cell to enter prophase I, the first meiotic phase. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate.
Cohesin holds the chromatids together until anaphase II. 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.ĭuring 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. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. 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.
Homologous chromosomes can exchange parts in a process called "crossing over.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. In Metaphase I, homologous chromosome pairs line up. This shuffling process is known as recombination or "crossing over" and occurs while the chromome pairs are lined up in Metaphase I. Each sibling is 50% mom and 50% dad, but which 50% of each can vary in the siblings. But this happens independently for each trait, so just because you got your dad's brown eyes doesn't mean you'll get his blond hair too. Each sperm and egg will end up with either B or b from mom and either B or b from dad. This leads to four possibilities: You could get B from mom and B from dad, or B from mom and b from dad, or b from mom and B from dad, or b from mom and b from dad. Imagine, for example, that eye color was controlled by a single gene, and that mom could have B, the allele for brown eyes or b, the allele for blue eyes, and dad could also have B or b. But each non-identical-twin child of these parents ends up with a different combination. You ended up with half of mom's paired genes and half of dad's paired genes.
Your parents each have at least one pair of alleles (versions of a gene) for every trait (and many pairs of alleles for each polygenic trait).
0 kommentar(er)
