Meiosis Flashcards
What is the outcome of mitosis ? Meiosis ?
Mitosis: 2 diploid cells
Meiosis: 4 haploid cells (gametes)
How many rounds of replication and division take place in mitosis ? In meiosis ?
Mitosis: 1 round of replication 1 round of division
Meiosis: 1 round of replication 2 rounds of division
What are the main differences between meiosis and mitosis ?
- 2 diploid daughter cells vs 4 halpoid daughter cells
- 1 round of replication 1 round of division vs 1 round of replication 2 rounds of division
- Duplicated chromosomes pair early on in meiosis, unlike in mitosis
- Recombination in meiosis, not mitosis
Describe the main steps of meiosis.
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents (by process of synapsis, come towards each other), crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
How do homologous chromosomes line up in mitosis ? in meiosis ?
Mitosis: Homologous chromosomes lined up at the metaphase plate independently
Meiosis 1: Homologous chromosomes are paired at the metaphase plate
What is a bivalent ?
A pair of synapsed (aligned) homologous chromosomes
What is a chiasma ? How many chiasma may there be per chromosome ?
“The points at which members of a chromosome pair are in contact during prophase 1 of meiosis and because of which recombination, or crossing over, occurs on separation”
2 or 3 chiasma along chromosome pair
What are the main features of recombination ?
what is it? how does it occur?
- Process that creates new combinations of genes by shuffling the linear order of the DNA.
- If genes close, more likely to be inherited together
- Occurs through Holliday Junction
What is a Holliday Junction ?
The cross-strand structure formed when two DNA duplexes cross in a recombination event.
Holliday junction is a specific four-way DNA structure that forms during genetic recombination, a process where DNA molecules exchange genetic material. It looks like a cross-shaped structure, resulting from the exchange of DNA strands between two homologous chromosomes. The holiday junction plays a crucial role in the genetic diversity and stability of organisms by facilitating genetic recombination and repair processes.
May the outcomes of a recombination event be predicted ?
No
How does pairing and recombination of sex chromosomes in males occur ?
- X and Y cannot pair perfectly (not the same size) but still pair up
- Pseudo-autosomal regions (on tip of X and on tip of Y) allow X and Y to pair up and undergo recombination.
- Rest of Y does not undergo recombination (or very little)
How may the Y chromosome be used to trace paternal ancestry ?
Because most of Y chromosome (except for pseudo-autosomal region) does not undergo much recombination so stable region of genome
What are the events in meiosis which help create genetic diversity ?
- Independent assortment of maternal and paternal homologs during meiosis 1: “Pairs of homologous chromosomes (tetrads) are lined up independently of each other (maternal and paternal chromosomes are also lined up on the metaphase plate at random)”
- Recombination: in prophase 1
What is the pause between meiosis 1 and 2 called ?
Interkinesis
What is MPF ?
Maturation Promoting Factor (a CDK-cyclin complex)
Describe the levels of MPF throughout meiosis.
During stages of meiosis (both 1 and 2) high activity of MPF, but dips at resting stage between meiosis 1 and meiosis 2.
Which protein is key is both meiotic divisions ?
How so ?
Cohesins
-In meiosis 1, degradation of cohesin complexes holding the homologous chromosomes together
whilst leaving those holding the sister chromatids together alone (cohesins lost at the arms but not at the center)
-In meiosis 2, degradation of cohesin complexes holding sister chromatids together
What are possible meiotic errors ?
meiois 1 and 2
Non-disjunction in meiosis 1: one of potential gametes inherits both maternal and paternal copies of a chromosome. When that precursor undergoes meiosis 2, 2 copies of said chromosome in 2 gametes (aneuploid gametes), and 2 gametes lacking a copy of chromosome. May lead to trisomy (3 copies of one chromosome).
Non-disjunction in meiosis 2: sister chromatids both inherited in the same gamete, leading to a aneuploid gamete with 2 IDENTICAL copies of a chromosome (from same parent, unlike non-disjunction in meiosis 1, wherein aneuploid gametes have one paternal and one maternal copy of a chromosome), and a gamete with no copies of that chromosome.
What are the consequences of errors in meiosis 1, and 2 ? (following fertilisation of the gametes)
Outcomes of error in meiosis 1 = Uniparental heterodisomy
-2 non-identical chromosomes
-If fertilised, may end up with trisomic zygote
with three different chromosomes
(eg one paternal, two from maternal grandparents)
Outcomes of error in meiosis 2 = Uniparental isodisomy
-2 identical chromosomes
-If fertilised, trisomic zygote with 2 identical chromosomes
but the third will not be different
Define uniparental disomy. Give the two types of it.
Uniparental disomy (UPD) is a genetic condition where both copies of a chromosome pair are inherited from a single parent, instead of one from each parent as typically occurs. Both members of a chromosome pair are inherited from one parent, and the other parent’s chromosome
for that pair is missing. Happens because sometimes, extra copy of chromosome can be removed very early in development (i.e. third copy of chromosome lost). Depending which chromosome lost, may end up with individuals inheriting two chromosomes from same parent
or from different parents.
Unidisomy may lead to Angelman Syndrome, Prada-Willi Syndrome, Beckwith-Wiedemann Syndrome.
Uniparental Heterodisomy = an individual inherits two different copies of a chromosome from one parent. This typically occurs when nondisjunction (the failure of homologous chromosomes or sister chromatids to separate properly during cell division) happens in one meiosis, followed by a normal disjunction event in the other meiosis. As a result, one chromosome pair is homologous while the other pair is non-homologous.
Uniparental Isodisomy = an individual inherits two identical copies of a chromosome from one parent.As a result, both copies of the chromosome are identical, which can lead to genetic disorders caused by homozygosity for a recessive allele or due to the effects of imprinted genes.
