meiosis Flashcards
whats meiosis I overview
-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).
-Two stages: Meiosis I and Meiosis II
how does crossing-over and segregation occur and in which meiosis
-meiosis I- chromosomes line up side by side
-Centrioles + chromosomes are replicated (just like mitosis)
-Maternal + paternal homologs pair up
-Genetic diversity is generated by recombination between homologous chromosomes
-One complete chromosome (2 chromatids) pulled to sep. poles
-Crossing-over takes place when homologues pair up.
what happens in meiosis II
-resembles mitosis
-The main difference is that cells in meiosis II are haploid instead of diploid
what happens in Meiotic prophase I
-homologues pair up
-Pairing in facilitated by the synaptonemal complex (proteins) as well as DNA base pairing between homologues.
-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
-the formation of the synaptonemal complex
-Paired homologs are brought to 400nm apart- very close proximity
-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 Synaptonemal complex do
-This tight bringing together of sister chromatids by the synaptonemal complex
-aligns the two chromosomes
-and helps in homologous recombination
what does Homolog Segregation Depend on
-Several Unique Features of MeiosisI
-Fundamental difference:
=Mitosis: Sister chromatids separate
=Meiosis: Homologous chromosomes separate in anaphase
-What allows the difference?
(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
what are the abnormalities in chromosomes
-abnormalities in chromosome number
-chromosome structural rearrangements- small rearrangements can have profound effects- not good
whats Aneuploidy
-abnormal umber of chromosomes
-There are two types
=Monosomy – 1 copy of a chromosome
=Trisomy – 3 copies of chromosome
how does Aneuploidy happen
-Disorders of chromosome number are caused by chromosome non-disjunction
-Either:
=homologous chromosomes
=sister chromatids
-fail to separate in
=Meiosis I
=Meiosis II
=Mitosis
what happens to the gametes with non-disjunction in meiosis 1
- 2 are n+1
- 2 are n-1
what happens to the gametes with non-disjunction in meiosis 2
-2 are n+1
-2 are just n
What are the consequences of numerical chromosomal abnormalities
-polyploidy- Triploid (eg 69,XXX, XXY, XYY) - embryonic lethal
-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))
-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 some Syndromes arising from nondisjunction
-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 foetuses
-Autosomal Trisomy: trisomy 18
=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
Why do we see lethality with non-disjunction
-Haploinsuficiency – pseudoautosomal genes are expressed from both alleles and dose matters
-Imprinted genes on X – ie monoallelic expression which is lost. ie null
meiosis
-location: testes and ovaries
-products: haploid gametes
-DNA replication: 1 round of replication but 2 cell divisions
-extent of prophase: meiosis I is long (can take years to completes)
-pairing of homologs: yes in meiosis I
-recombination: relatively common
-relationship between daughter cells: different due to recombination and assortment of homologs
