Meiosis Flashcards
Describe, generally, what happens during meiosis I
- Centrioles + chromosomes replicated (like mitosis)
- Homologous chromosomes pair up
- Homologous recombination occurs
- 2 Chromatids pulled to separate poles (Anaphase I)
- Cytoplasm splits
Describe, generally, what happens during meiosis II
- Sister chromatids separate in Anaphase II
- Produces haploid daughter cells
What is the main difference between meiosis and mitosis?
Meiosis largely resembles mitosis except there are extra steps that segregate homologous chromosomes
What happens during meiotic prophase I?
Homologous chromosomes pair up
- facilitated by synaptonemal complex & DNA base pairing between homologues
What facilitates the pairing up of homologous chromosomes?
the synaptonemal complex (proteins)
DNA base pairing between homologues
What is the purpose of homologous chromosomes pairing in meiotic prophase I?
2 purposes:
1. Aligns chromosomes up ready for anaphase (along with the formation of synaptonemal complex)
2. Allows for genetic recombination between paternal and maternal DNA = genetic variation
What is a chiasma?
Point of contact between homologous chromosomes in crossing over = chromosomes swap regions of genetic info
Describe how the homologous chromosomes are actually brought together.
- Paired homologues brought to 100nm apart
- Thought that a recombination complex (which detects double stranded breaks) helps bind the homologues together
- The axial core (proteins that bind chromatin via cohesion) are cross-linked by transverse filaments to from the synaptonemal complex
What is the fundamental difference between mitosis and meiosis?
Meiosis: homologous chromosomes separate = haploid daughter cells
Mitosis: sister chromatids separate = diploid daughter cells
Describe how is crossing-over is regulated
There’s at least one crossing over site per bivalent (arm) but no more that 4
But, if these sites from at DS breaks - why aren’t there more?
- DS breaks tend to form in open chromatin (not all chromatin)
- Coupled with, Crossover interference - once one forms it inhibits others close by
= No. of crossing over sites limited
What are the 2 general categories for meiosis going wrong?
Abnormalities in chromosome number (nondisjunction)
Chromosome structural rearrangements (caused by homologous recombination)
Why can even small structural arrangements on chromosomes have severe effects?
Homologous recombination can run risk of DNA being swapped over incorrectly
= due to huge no. of genes even small rearrangements can have severe effects
What is aneuploidy?
the condition of having an abnormal number of chromosomes in a haploid set
What are the 2 types of aneuploidy?
Monosomy → 1 copy of a chromosome
Trisomy → 3 copies of chromosome
What is Spectral karyotyping (SKY) and why is it useful?
→ uses DNA probes to show artificial colours on chromosomes during metaphase
Useful bc it is much easier to identify rearrangements due to bright colours
What causes aneuploidy?
Homologous chromosomes / sister chromatids fail to separate in: Meiosis I, Meiosis II or mitosis
Describe the effects of nondisjunction in meiosis I
serious effect: all gametes will either be trisomic / monosomic
Describe the effects of nondisjunction in meiosis II
less serious than meiosis I: ½ gametes ‘normal’, ¼ trisomic, ¼ monosomic
What is nullisomy?
missing pair of chromosomes
What are the consequences of aneuploidy in sex chromosomes?
Additional sex chromosome = minor probs but normal lifespan
Lacking a sex chromosome = 45,X can be highly lethal, some normal but infertile. 45, Y not viable
Give an example of syndromes involving autosomal trisomy
Trisomy 22:
Microcephaly
Congenital heart disease
Malformed ears
Rare in live born, usually found in miscarriages
Give an example of a syndrome involving monosomy
45, XO Turner’s syndrome:
Complete or partial absence of 2nd sex chromosome in phenotypic females
Common symptoms: Poor growth, short, delayed/ absent puberty, congenital heart defects
Only 1% survive to term with 50% mosaic (nondisjunction in only some of the cells)
Why do we see lethality from lacking an X chromosome e.g Turner’s syndrome?
Thought that X chromosome inactivation in ‘females’ would mean these conditions would have no effect, but researchers found:
Haploinsufficiency → pseudoautosomal genes (on X chromosome) expressed from both alleles, so if one X is lost the dose matters
Where does meiosis occur?
Only in testes / ovaries
What are the products of meiosis?
Haploid gametes
How many cell divisions occur in meiosis?
2
What is the relationship like between daughter cells like in meiosis?
Genetically different due to:
- Recombination
- indepndent assortment of homologues