Meiosis Flashcards
Meiosis results in cells which are ?
Haploid
In meiosis, how many rounds of DNA replication and division are there ?
1 replication
2 divisions
Describe the chromosomes in Meiosis 1
Homologous chromosomes are paired at the metaphase plate.
Meiosis
The division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte.
Describe the chromosomes in Mitosis
Homologous chromosomes line up at the metaphase plate independently.
Describe Meiosis 1
Replicated chromosomes align to form a bivalent
Chiasma develop in prophase 1
Recombination occurs - important source of genetic diversity
Homologous chromosomes line up at spindle and are separated and move towards opposite poles.
Cytokinesis
Step 1 of Meiosis 1
The chromosomes replicated prior to Meiosis 1, each consist of genetically identical chromatids attached at the centromere.
Chromosomes condense and homologous chromosomes pair up.
Stage 2 of Meiosis 1
Chiasmata form at points of contact between the non-sister chromatids of a homologous chromosome and sections of DNA are exchanged.
Crossing over of DNA is random and produces genetically different recombinant chromosomes.
Stage 3 of Meiosis 1
Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle.
The orientation of the pairs of homologous chromosomes at the equator is random.
Stage 4 of Meiosis 1
The chromosome of each homologous pair are separated and move towards opposite poles.
Stage 5 of Meiosis 1
Cytokinesis occurs and 2 daughter cells form
When do chiasma develop ?
Prophase 1
Meiosis 2
Each of the 2 cells produced in Meiosis 1 undergoes further division during which the sister chromatids of each chromosome are separated.
Function of crossing over - recombination
Allows exchange of genetic information and is an important source of genetic diversity.
Useful tool for geneticists.
Holliday junction
Shows what happens at the crossing over point between different strands of DNA.
Recombination enzyme has the capacity to cleave DNA in different locations.
Describe the complex that forms at the Holliday junction
The key complex that does the nicking and ligating of the strands together.
Describe pairing and recombination of sex chromosomes in males
There is a small region of the X and Y chromosome called the pseudoautosomal region.
This allows the X and Y chromosome to pair up and undergo recombination within the short region.
Describe the Y chromosome
Shorter than the X chromosome
So, lack most of the corresponding alleles to the X chromosome.
Significance of pairing and recombination of sex chromosomes in males
Implications for tracing paternal ancestry, which can go back generations.
Feature of the pseudoautosomal region
Shox is a gene that lies in the pseudo-autosomal region
The pseudo-autosomal dominant region escapes X chromosome inactivation.
Describe X chromosome inactivation
IN homogametic females, one of the 2X chromosomes are inactivated randomly at an early stage of development.
This prevents a double dose of gene products which could be harmful to cells.
How is genetic diversity generated ?
Independent assortment of maternal and paternal homologs during Meiosis 1.
Crossing over during meitotic prophase 1.
Bivalent
Set of 4 paired chromatids
How many gametes are formed from one round of Meiosis ?
4
What is key to correct meiotic division ?
Degradation of cohesion complexes
What feature increases in Meiosis 1 and 2 ?
MPF - maturation promoting factor
CDK:cyclin complex
Key protein involved in degradation of cohesion complexes
Rec8 - containing mitotic divisions
In Meiosis 1 - loss of Rec8 from the arms of chromosomes NOT centromere
In Meiosis 2 - loss of Rec8 from the centromere
State a disease arising from an error from meiosis
Down syndrome
- error in meiosis 1
Describe the meiotic error in Down syndrome
Paternal and Maternal chromosomes
Normal duplication of chromosomes
Recombination can occur normally
Non-disjunction OCCURS
Gametes with 2 copies of chromosome 21
(occurs after DNA replication)
What is non-disjunction ?
Improper separation of bivalent into intermediates between Meiosis 1 and 2
Duplicated chromosomes in meiosis 2 can separate.
Describe an error in Meiosis 2
Bivalent structure is separated properly in Meiosis 1
Problem in Meiosis 2
Duplicated chromatids are not separated correctly.
Meiosis 1
Separation of homologs
Meiosis 2
Separation of sister chromatids
Describe fertilisation following meiotic errors
Trisomic zygote formed which can try to become an embryo.
- different chromosomes in the trisomic zygote, depending if the error was in Meiosis 1 or 2
Uniparental disomy after MI error
Uniparental heterodisomy
- Different chromosomes
- Same parent
Uniparental disomy after MII error
Uniparental isodisomy
- Same chromosome
- Same parent
What is a problem with uniparental disomy ?
Receiving 2 copies of a chromosome from one parent and none from another can have serious consequences.
State the syndromes caused by uniparental disomy
Angelman
Prada-Willi syndrome
Beckwith-Wedemann syndrome
What is strongly associated with meiotic errors ?
Maternal Age
Women >40 years old
Errors in Meiosis 1 are more common than Meiosis 2
Spermatogenesis
Occurs continuously from puberty onwards.
Meiosis - 24 days long
4 sperm are generated from 1 precursor
Oogenesis
Meiosis 1 begins during foetal development.
Undergoing Meiosis 1 during puberty.
- polar body generated
Secondary oocyte carries onto Meiosis 2
Meiosis 2 ends when the egg is fertilised.
Fertilisation - second polar body discarded
Differences between Meiosis in spermatogenesis and oogenesis
IN spermatogenesis, continuous meiosis, generating 4 gametes from 1 precursor
In females, discontinuous, very long periods of arrest and only generating 1 gamete from 1 precursor. Last stage only happens if the egg is fertilised.
What stage are the eggs arrested in Meiosis - in oogenesis ?
Long arrest at prophase 1
- if chromosomes are not held together properly, this may result in a meiotic error
Do chromosome cohesion defects underlie maternal age effect ?
Yes
Suggested that in older women chromosomes are not held together properly.
Based on a study with mice.
