10.1 Meiosis Flashcards
Where do chromosomes replicate before meiosis?
Interphase
What are the 3 cell cycle phases in interphase?
- G1
- S
- G2
What happens in the S phase of interphase?
DNA is repilicated so that each chromosome consits of two chromatids
At the start of meiosis, what do the chromosomes do?
They condense and are visible as two chromatids, called sister chromatids
Where do pairing or synapsis occur in meiosis?
Occurs where homologous chromosomes come to align beside each other
What a tetrad?
The combination of chromatids as it is composed of four chromatids
Also a homologous pair
What is a tetrad also referred to as and why?
Bivalent - as it is composed of a homologous pair
What is the protein-based structure that forms between the homologous chromosomes called?
Synaptonemal complex
In what type of cells do synaptonemal complexes form?
Eukaryotic cells
What is crossing over?
The exchange of DNA material between non-sister homologou chromatids
When do breaks in the DNA occur in meiosis?
Prophase I
What happens during crossing over?
Non-sister chromatids “invade” a homologous sequence on a non-sister chromatid and bind in the region of the break
What are the connection points where crossing over occured called?
Chiasmata (P) or Chiasma (S)
What does the chiasmata formation between non-sister chromatids in a bivalent result in?
An exchange of alleles
What are the two consequences of chiasmata formation?
- Increased stability of bivalents at chiasmata
- Increase genetic variability if crossing over occurs
What are the two consequences of chiasmata formation?
- Increased stability of bivalents at chiasmata
- Increase genetic variability if crossing over occurs
What chromosomes are exchanged in crossing over?
The process of crossing over results in the exchange of DNA between the maternal and paternal chromosomes
What leads to independent assortment?
Crossing over can decouple linked combinations of alleles and therefore lead to independent assortment
How many times can crossing over occur?
Multiple times and between different chromatids within the same homologous pair.
When does crossing over and independent assortment occur?
Crossing over - Prophase I
Independent assortment - Metaphase I
What are the 3 methods by which meiosis creates genetic variation?
- Crossing over - Prophase I
- Independent assortment - Metaphase I
- Random Fusing of gametes
What are the 3 methods by which meiosis creates genetic variation?
- Crossing over - Prophase I
- Independent assortment - Metaphase I
- Random Fusing of gametes
How are recombinant chromatids fromed?
When exchange of alleles occurs due to crossing over
Compared to before, what do recombinant chromatid still contain? Whats different?
Recombinant chromatids still contain the same genes but a different combination of alleles
After crossing over, what are chromatids of each chromosome no longer?
They are no longer genetically identical
What are the 3 features shared between homologous chromosomes?
- Same length
- Same centromere position
- Same gene content
What is a difference between homologous chromosomes?
They will have different combinations of alleles
What is a chiasma?
An X-shaped knot-like strcuture that forms where crossing over has occurred
What do chiasmata do to homologous chromosomes?
They hold homologous chromosomes together for a while
What do chiasmata do to homologous chromosomes?
They hold homologous chromosomes together for a while
Which round of meiosis resembles mitosis?
The first meiotic division is unique while the second round resembles mitosis
How does meiosis 1 differ from mitosis and meiosis 2?
- Sister chromatids remain associated with each other
- The homologous chromosomes behave in a coordinated fashion in prophase
- Homologous chromosomes exchange DNA leading to genetic recombination
- Meiosis I is a reduction division in that it reduces the chromosome number by half
Which stage of meiosis initiates the processes that result in the creation of genetic variety of gametes?
Meiosis 1
When does the segregation of homologous chromosomes occur?
During anaphase I, resulting in two haploid cells, each with only one copy of each homologous pair
How does independent assortment occur?
independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I
What is independent orientation?
The direction in which one pair is facing does not affect the direction in which any of the other pairs are facing. This is called independent orientation
If an organism is heterozygous for a gene, how would you expect their chromosomes to be like in a cell?
In its cells, one chromosome in a pair will carry one allele of the gene and the other chromosome will carry the other allele
How is it determined which allele moves to which pole in meiosis?
The orientation of the pair of chromosomes will determine which allele moves to which pole
What is the chance of each allele moving to a particular pole?
50%
By is there a 50% chance of an allele moving to a particular pole?
Because there is random orientation of chromosome pairs
What are separated in meiosis II?
Sister chromatids
What do daughter cells of meiosis I not do before entering meiosis II?
After meiosis I, the daughter cells enter meiosis II without passing through interphase
How is meiosis II similar to mitosis?
The replicated chromosome is separated into chromatids
Why are the sister chromatids separated in meiosis II likely to be non-identical?
Due to the occurrence of crossing over
