L27 Meiosis Flashcards
What are replicated in meiosis?
Centrioles and chromosomes are replicated.
How is genetic diversity generated in meiosis?
It is generated by recombination between homologous chromosomes.
What are the two stages of meiosis?
Meiosis 1 and 2.
Give an overview of meiosis.
- Diploid organisms have two versions of each chromosome (homologues).
- Homologues are either paternal or maternal.
- Only one homologue for each chromosome is packaged into a gamete.
- Meiosis resembles mitosis except that there are extra steps that segregate homologous chromosomes.
- Pairing of homologues before segregation allows for crossing-over (homologous recombination).
When does crossing over in meiosis 1 take place?
Crossing-over takes place
when homologues pair up.
What do sex chromosomes behave like in meiosis 1?
the sex chromosomes (X and Y) behave like homologues during sperm formation due to small regions of homology.
What do meiosis 2 resembles and what is the main difference?
Meiosis II: resembles mitosis, The main difference is that cells in meiosis II are haploid instead of diploid.
When does meiotic prophase 1 occur?
Meiotic prophase I is when homologues pair up.
Explain meiotic prophase 1?
Pairing is facilitated by the synaptonemal complex (proteins) as well as DNA base pairing between homologues.
What are the purposes of meiotic prophase 1?
Serves two purposes:
1) It aligns the chromosomes up ready for anaphase (along with the formation of the synaptonemal complex)
2) It allows for genetic recombination between paternal and maternal DNA on the same chromosome.
What does chromosome homologs pairing up lead to?
Chromosome Homologs pairing up leads to the formation of the synaptonemal complex
Explain the formation of the synaptonemal complex.
Paired homologs are brought to 400nm apart
Possibly a recombination complex (which recognises ds breaks) helps bind the homologs together
The axial core (proteins that bind the chromatin via cohesion) are cross-linked by transverse filaments to form the Synaptonemal complex
What does the tight bringing together of sister chromatids by the synaptonemal complex do?
aligns the two chromosomes
and helps in homologous recombination
What factors does homolog segregation depend upon in meiosis 1?
Fundamental difference:
Mitosis: Sister chromatids separate
Meiosis: Homologous chromosomes separate
in anaphase
What allows the difference for homolog segregation in meiosis 1?
(i) BOTH kinetochores (on one chromosome) attach to the same spindle pole
(in mitosis you to avoid this)
This is done by a protein complex that is removed after meiosis I
(ii) Crossing over = physical linkage between homologs
(iii) Cohesin is only removed from the arms
How many crossovers occur per bivalent during meiosis?
There is typically at least one crossover per bivalent (homologous chromosome pair) during meiosis, but usually no more than four.
Why aren’t there more crossovers per bivalent?
- Breakage Formation: Double-strand breaks (DSBs), which initiate crossovers, tend to occur in regions of open chromatin.
- Regulation:
Mechanisms exist to ensure at least one crossover per bivalent.
Crossover interference limits the number of crossovers in close proximity to each other.
These factors contribute to the observed range of 1-4 crossovers per bivalent.
How does meiosis go wrong?
Generally two categories of chromosome abnormalities:
* abnormalities in chromosome number
* chromosome structural rearrangements.
chromosome carries hundreds of genes small rearrangement= severe effects
What is aneuploidy?
Either through mitosis or meiosis followed by fertilisation you have to maintain a diploid chromosome number
There are two types
Monosomy – 1 copy of a chromosome
Trisomy – 3 copies of chromosome
You can get whole extra sets of chromosome, called polyploidy
How does aneuploidy occur?
- Disorders of chromosome number are caused by chromosome non-disjunction.
- Either homologous chromosomes or sister chromatids fail to separate in Meiosis I, Meiosis II or Mitosis.
Look at chromosome structure in slide 17.
Slide 18 and 19 diagram of normal and disjunction in meiosis.
What are the consequences of numerical chromosomal abnormalities?
- Polyploidy
- Aneuploidy(autosomes)
- Aneuploidy( sex chromosomes)
What are the problems cause by each abnormality? - Polyploidy
Triploid (eg 69,XXX, XXY, XYY) - Embryonic lethality.
What are the problems cause by each abnormality? - Aneuploidy (autosomes)
Nullsomy (missing a pair of chromosomes) - Pre-implantation lethal
Monosomy (missing one chromosome) - Embryonic lethal
Trisomy (one extra chromosome) - Usually lethal (but some exceptions
What are the problems cause by each abnormality? - Aneuploidy (sex chromosomes)
Additional sex chromosome - 47,XXX; 47, XXY, 47, XYY – minor problems relatively normal lifespan
Lacking a sex chromosome - 45,X; Turners, 99% abort, rest normal but infertile
45, Y; not viable
What are the syndromes arising from nondisjunction? (Trisomy 22)
Autosomal Trisomy
Trisomy 22 -
* Undeveloped midface (midface hypoplasis)
* Malformed ears
* Wide-spaced eyes (hypertelorism)
* Microcephaly
* Congenital heart disease
Rare in live born (usually die shortly after birth) is found in miscarried foetus
What are the syndromes arising from nondisjunction? (Trisomy 18)
Trisomy 18 (Edwards syndrome):
Symptoms include:
* Severe intellectual disability
* Low birth weight
* A small, abnormally shaped head
* A small jaw and mouth
* Clenched fists with overlapping fingers
* Congenital heart defects
* Various abnormalities of other organs
Death before birth or within 1st month of life
Rare survival to teenage years
What are the syndromes arising from nondisjunction? (Monosomy)
45, XO Turner’s syndrome
Complete or partial absence of a second sex chromosome in phenotypic females
Common diagnostic factors:
* Poor growth
* Short stature
* Delayed/absent pubertal development
* Congenital heart defects
* Skeletal abnormalities
UK: 1 in 2000 females at birth
Only 1% survive to term with 50% mosaic
Why do we see lethality in syndromes arising from nondijunction?
Two explanations
Haploinsuficiency – pseudoautosomal genes are expressed from both alleles and dose matters
Imprinted genes on X – ie monoallelic expression which is lost. ie null
Explain abnormalities caused by structural rearrangements?
Not surprisingly – tend to occur when homologous recombination is occurring, ie meiosis,(sperm and egg production).
Struc. rearrangements represent approx. 4.7% of 1st trimester abnormalities.
You could also get structural rearrangements caused by DNA-damage induced homologous recombination (S & G2).
These could occur at any time when the cells are dividing].
