Meiosis Flashcards
What is meiosis?
It produces haploid cells with genetic material for sexual reproduction.
What does diploid mean?
There are two copies of every chromosome, maternal and paternal.
What does haploid mean?
One copy of each chromosome.
What is a gamete?
A sperm or egg cell.
What is a zygote?
When two gametes fuse.
What happens in meiosis 1?
The homologous chromosomes segregate.
What happens in meiosis 2?
The sister chromatids segregate.
What is homologous recombination?
When the maternal and paternal DNA mixes up.
When does homologous recombination occur?
In the first round of meiosis.
What are the phases in meiosis 1?
Interphase, prophase, metaphase 1, anaphase 1 and telophase 1.
What happens in meiosis 2?
Interphase 2, metaphase 2, anaphase 2, telophase 2 and cytokinesis.
What are the stages in prophase 1?
Leptotene, zygotene, pachytene, diplotene and diakinesis.
What happens in leptotene?
There is condensation of sister chromatids - they look like thin threads.
What happens in zygotene?
When homologous chromosomes pair up.
What happens in pachytene?
When pairing is complete and homologous chromosomes begin to exchange genetic material.
What happens in diplotene?
When pairing becomes less tight and sister chromatid pairs are visible and there is evidence of crossing over.
What happens in diakinesis?
The chromatid pairs begin to separate and the nuclear envelope breaks down and spindle forms.
How do chromosomes pair?
Via complementary DNA sequences.
What is synapsis?
When two pairs of sister chromatids form a four-chromosome bivalent.
What is a synaptonemal complex?
When two pairs of sister chromatids form a four-chromosome bivalent.
What are the three elements of synaptonemal complexes?
Cohesins, axial core and transverse fibers.
What are heteroduplexes?
DNA double helixes composed of strands that originate from the two different duplexes. There s rapid zippering when c
How do homologous strands find eachother?
It is a rare event, unless double stranded break occurs.
Why is there a programmed double strand break in meiosis i?
For recombination.
What is an advantage of having two sets of each chromosome during meiosis?
Broken DNA can be repaired.
What does the meiosis-specific double strand break repair process favour?
Maternal-paternal chromosome heteroduplexes.
What is the first step in double strand break repair?
Broken end processing: Nucleases chew broken ends to leave single stranded DNA overhangs.
What is the second step in double strand break repair?
Strand exchange: protein complexes bind the ssDNA and a double helix which it pulls apart. If the sequences match, pairing of different strands can occur.
What is the third step in double strand break repair?
DNA synthesis to fill in the gaps.
What is the fourth step in DNA strand break repair?
Cut and ligate the repaired strands.
What is holiday junction?
A cross-shaped structure that forms during recombination.
What are the two ways in which holiday junctions can be re-sealed?
Cut and re-seal along the V axis or cut and re-seal along the H axis. (horizontal or vertical).
What is it called when the holiday junction is cut and resealed along the vertical axis?
Crossover.
What is it called when the holiday junction is cut and resealed along the horizontal axis?
Conversion.
What are chiasmata?
Where the strands of DNA cross and intersect during crossing over.
How often does crossing over occur?
10% of the time.
How many crossing overs are there per chromosome?
At least one.
What is gene conversion?
When part of a maternal strand is copied into the paternal (or vice versa). There may be some mis-matched bases due to differences in the DNA sequences (cut along the horizontal axis).
When are sister chromatid kinetochores fused?
Meiosis 1.
When is the synaptonemal complex degraded?
Prophase 1.
What are cohesins near kinetochores protected by in meiosis 1?
Separase.
When do sister chromatid pairs move to poles?
Anaphase 1.
What does the reorientation of kinetochores involved in metaphase 1 and 2?
Cohesin depletion and the pulling force from microtubules in metaphase 2.
What is Sgo1?
A target of the APC in meiosis 2. It is also involved in sensing tension at kinetochores.
What factors play into genetic diversification?
Independent assortment of maternal and paternal chromosomes, crossing over, gene conversion and non-disjunction.
How many possible combinations of chromosomes are there for independent assortment?
2^23.
What is crossing over?
Swapping large segments of homologous chromosomes.
What is gene conversion?
Non-crossing over homologous recombination. Mis-match repair may copy maternal sequence into the paternal chromosome or vice versa.
What is non-disconjunction?
When chromosome segregation goes wrong.
What are some conditions in which non-disconjunction occurs?
Down syndrome, trisomy 28 or 13, turner syndrome, klinefelter syndrome or where there are XYY chromosomes present.
How many haploid sperm are produced per meiosis?
4.
How many egg cells are produced per meiosis?
1.
