MEIOSIS Flashcards
Diploid organisms have two versions of each chromosome. What are these called?
Homologues.
Homologues are either ……………………… or ………………………..
Maternal
Paternal
How is meiosis different from mitosis?
- Meiosis results in cells with half the genetic information required. Mitosis results in 23n cells.
- Meiosis undergoes PMAT twice where as this only occurs once in Mitosis.
- In prophase 1 of meiosis, homologous chromosomes pair with one another. In mitosis the chromosomes continue to condense.
- In prophase 1 of meiosis, homologous recombination occurs. This does not happen in mitosis.
- In anaphase 1 of meiosis, homologous chromosomes are pulled to opposite poles whereas in mitosis sister chromatids are separated.
What is the purpose of homologous recombination?
Serves two purposes:
- Aligns chromosomes ready for anaphase (along with formation of synaptonemal complex)
- It allows for genetic recombination between maternal and paternal DNA on the same chromosome.
What facilitates pairing and recombination of homologous chromosomes?
The synaptonemal complex.
What makes up the synaptonemal complex?
Chromatin loops bind to cohesin that are attached to the axial cores of the homologues. The axial cores of the homologues are then bound in a zip like fashion by transverse filaments to bring the homologous chromosomes to 400nm apart.
What are the stages of formation of the synaptonemal complex?
- Interphase
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
Homologous segregation (Anaphase 1) depends on sever unique features of Meiosis 1, what are they?
In mitosis it is sister chromatids that separate.
In meiosis 1 is is homologues that separate.
What allows homologues to separate and not for their sister chromatids to separate?
- Both kinetochores attach to the same spindle pole in meiosis 1.
- Due to crossing over, there is a physical link between homologues.
- In meosis 1, cohesin is only removed from the arms of the chromosomes.
How many times can crossing over occur in a single bivalent chromosome?
At least one but no more than four.
When crossing over occurs once, what happens close by?
Crossover interference means crossing over nearby is inhibited.
When meiosis goes wrong, what are the two categories of chromosome abnormalities?
- Abnormalities in chromosome number
2. Chromosome structural rearrangements.
Define aneuploidy.
Having an abnormal number of chromosomes.
What is monosomy?
Having only one copy of a chromosome.
What is trisomy?
Having 3 copies of a chromosome.
What is polyploidy?
Having extra sets of chromosomes.
How does aneuploidy occur?
Disorders of chromosome number are caused by chromosome non-dysjunction.
This means homologous chromosomes or sister chromatids have failed to separate in either Meosis 1 or 2, or Mitosis
Does chromosome non-dysjunction cause greater defects in Meiosis 1 or Meiosis 2?
Non-dysjunction occuring in Meiosis 1 is more severe as this will produce no normal gametes.
If this occurs in Meiosis 2, then only half of the gametes will be abnormal.
What is the expected quality of life in someone with additional sex chromosomes?
Most will have a normal life span with only minor issues.
What is the expected quality of life for someone lacking sex chromosomes?
45Y is not usually viable.
45X produces Turners Syndrome. In 99% if case of 45X, foetuses abort.
What is the expected quality of life for a person that is triploid ie 69 chromosomes
Embryonic lethal.
What is a nullsomy?
A missing pair of chromosomes, this is pre implantation lethal.
Most aneuploidies are lethal especially when it comes to what type of chromosome?
Autosomal chromosomes.
Some autosomal trisomies survive.
What autosomal trisomies can survive?
Trisomy 18 - (Edwards Syndrome)
Trisomy 22
What are some of the features of Edwards Syndrome?
Severe intellectual disability Low birth weight A small, abnormally shaped head A small jaw and mouth Clenched fists Overlapping fingers Congenital heart defects Abnormal organans
What are the features of Trisomy 22?
Undeveloped mid face
Malformed ears
Wide spaced eyes
Congenital heart disease
What are most cases of Trisomy 21 known as?
Down’s Syndrome
What are the two potential explanations as to why we see lethality in aneuploidy?
- Haploinsuffiency
- Imprinted genes on X
Usually, in females and X is inactivated. In Turners Syndrome, only one full X is present, if at random this X is inactivated then there is no transciption and therefore death.
If there is no inactivation of X, the single X is haploinssuficient because pseudoautosomal genes on sex chromosomes still require two copies for suffiecient expression and in this case not enough can be transcribed.
If a gene is imprinted and there is no other copy then monoallelic expression is lost and the gene is null.
Which model for lethality is actually correct?
Haploinsufficiency model.
Why is the haploinsuffieciency correct and how can we test for that?
- out of a group of hESC in culture, some will random losing a sex chromosome
- in screening both types, which are pseudoautosomal or escape x inactivation and what is expressed in XX that is not expressed in X0
- 37 genes meet this criteria
- out of these, only 3 are expressed and escape x inactivation
- test for imprinting - look for SNPs in the genome that are heterozygous but homozygous in the gene you are looking at - none showed this therefore it is the haploinsuffiency model that is correct.
