10.1 Meiosis Flashcards
What happens in the S phase?
This is in the interphase of meiosis. The DNA is replicated, so that each chromosome consists of two chromatids.
When does crossing over occur?
Prophase 1.
How does crossing over work?
Crossing over is the exchange of DNA material between non-sister homologous chromatids.
During Prophase 1 of meiosis breaks in the DNA occur. Following these chromosome breaks, non-sister chromatids “invade” a homologous sequence on a non-sister chromatid and bind in the region of the break. They swap sections of DNA between the chromatids. Once crossing over is complete, the non-sister chromatids continue to adhere at the sight where crossing over occurred. These connection points are called chiasmata or chiasma. Evidence suggests that connections via chiasmata are essential for the successful completion of meiosis.
What is the consequence of crossing over?
Increased stability of bivalents at chiasmata.
Increased genetic variability if crossing over occurs.
Crossing over produces new combinations of alleles on the chromosomes of haploid cells.
What is a chiasma?
A chiasma is an X-shaped knot-like structure that forms where crossing over has occurred. Chiasmata hold homologous chromosomes together for a while, but then slide to the end of the bivalent, allowing the chromosomes to move to opposite poles of the cell.
What is synapsis?
The pairing up of doubled homologous chromosomes to make a bivalent.
How does meiosis 1 differ from mitosis 1 and meiosis 2?
1) Sister chromatids remain associated with each other (exactly the same genes and alleles)
2) The homologous chromosomes behave in a coordinated fashion in prophase
3) Homologous chromosomes exchange DNA leading to genetic recombination.
4) Meiosis 1 is a reduction division in that it reduces the chromosome number by half.
Why is the first meiosis a reduction?
Because the homologous chromosomes are separated in anaphase. These are the chromosomes with the same genes but different alleles of those genes. They are both doubled so there is 4 and then separated, so that they are in their IDENTICAL pairs of 2 and separated so different alleles go to different poles.
What is independent assortment?
The inheritance of one trait does not influence the inheritance of another. Linked genes are the exception because they are on the same chromosomes.
If someone is heterozygous for a trait (they have 2 different alleles) then the random orientation of the bivalents will decide which homologous chromosome is pulled to which pole and therefore which gamete gets that allele. There is each time a 50% chance it will move to that pole. The position of one does not effect the position of any others and this is independent assortment.
What is meiosis produces variety?
1) Crossing over
2) Random orientation of bivalents - If someone is heterozygous for a trait (they have 2 different alleles) then the random orientation of the bivalents will decide which homologous chromosome is pulled to which pole and therefore which gamete gets that allele. There is each time a 50% chance it will move to that pole. The position of one does not effect the position of any others and this is independent assortment.
3) Homologous chromosomes separated
What happens in meiosis 2?
Meiosis 2 follows mitosis 1 without the cells going through interphase. The sister chromatids are separated. These were identical but may no longer be because of the crossing over that occurred in prophase 1.
