meiosis! Flashcards
What is the purpose of meiosis?
To reduce the chromosome number in the parent cell by half and produce 4 gamete cells (haploids). Meiosis occurs with germ cells as parents. This is a required process to produce egg and sperm, which would then undergo fertilization for sexual reproduction.
What is the difference between meiosis I and meiosis II?
Meiosis I is the separation of the homologous chromosomes, and meiosis II is the separation of the sister chromatids of a chromosome*. Meiosis II is very similar to mitosis.
- the sister chromatids make up one chromosome until they separate. Then they become individual chromosomes.
Prophase I
- chromosomes begin to condense, spindles form, nuclear envelope breaks down
- Synapsis: Homologous chromosomes pair up and become physically connected by a protein structure; synaptonemal complex
- Crossing over: Nonsister chromatids exchange genetic information at corresponding segments of DNA molecules. Begins during homologous pairing and is completed while chromosomes are in synapsis.
- Microtubules from both poles attach to the two kinetichores of a homologous pair, which then begin to move towards the metaphase plate.
What is a chiasma (plural chiasmata) and where does it exist? Why does it look the way it does?
A chiasma is an X-shaped region where crossing over has occured. It looks like a cross because sister chromatid cohesion still holds the sister chromatids together
Describe sister chromatid cohesion in depth.
Sister chromatids are closely associated along their lengths. During synapsis and crossover, the chiasmata appears as an X-like region because the sister chromatids of one chromosome are still stuck together due to sister chromatid cohesion.
When in the cell cycle does sister chromatid cohesion build and break during meiosis? Mitosis?
Sister chromatid cohesion builds during S phase.
MEIOSIS:
During anaphase I, sister chromatid cohesion breaks down along the sister chromatid arms to allow the homologous chromosome pair to separate.
In anaphase II, sister chromatid cohesion breaks at the centromeres, allowing the chromatids to separate.
*the cohesion is what allows the chromatid sisters to move as one unit towards the same pole when homologous pairs separate in anaphase.
MITOSIS: Cohesion breaks down at the start of anaphase to separate the sister chromatids.
What is the difference between synapsis and crossover?
Crossover occurs during synapsis.
Synapsis is the pairing of homologous chromosomes, and crossover is the exchange of genetic material during synapsis.
Metaphase I
Pairs of homologous chromosome pairs are lined up at the metaphase plate. Each chromosome of one pair face opposite poles.
Both chromatids of one homolog are attached at the kinetichore microtubules of one pole.
How does metaphase I promote genetic variation within offspring?
Chromosomes with one or more recombinant chromatids can be oriented in two alternative, inequivalent ways as sister chromatids are no longer identical. This different possibility in arrangement increases the number of genetic types of daughter cells that emerge.
