cell division Flashcards
what happen in meiosis?
Stages in meiosis
To achieve the reduction in chromosome number, meiosis consists of:
one round of chromosome duplication
two rounds of nuclear division
The two rounds of division, are designated with a “I” or “II” where:
Meiosis I is the first round – it reduces the number of chromosome sets from two to one
Meiosis II is the second round, which takes place in a way that is like mitosis
what happpenes in prophase?
Crossing over/recombination
Meiosis is also crucial in creating variation in the population by generating new combinations of genetic material that differ from the combinations found in either parent.
Early in Meiosis I, the nuclear envelope begins to break down and the proteins associated with homologous chromosomes bring the pair close to each other. The tight pairing of the homologous chromosomes is called synapsis.
In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. Then, an exchange of chromosome segments between non-sister homologous chromatids occurs. This is called crossing over or recombination.
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Prophase I
Early in prophase I, the chromosomes can be seen clearly microscopically. 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 precisely aligned with each other. An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over. This process is revealed visually after the exchange as chiasmata (singular = chiasma).
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.
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As prophase I progresses, the close association between homologous chromosomes begins to break down, and the chromosomes continue to condense, although the homologous chromosomes remain attached to each other at chiasmata. The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the pairs are held together only at chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
The crossover events are the first source of genetic variation 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, it will carry some DNA from one parent of the individual and some DNA from the other parent. The recombinant sister chromatid has a combination of maternal and paternal genes that did not exist before the crossover.
WHAT IS The key event in prometaphase I ?
The attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres……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.
FF
ENGORGED WITH BLOOD
GLAND
FLACCID
BLADDER
compromised cell
abnormal cell
