Meiosis
- Created by: rosieevie
- Created on: 20-05-17 18:57
Significance of Meiosis
Meiosis important = produces haploid gametic cells
Fertilisation causes mixing of genomes from two parents - increases ploidy and variability
All organisms have alteration between haploid and diploid geneations
Fungi - haploid generation dominant and transient diploid stage
Humans - diploid generation dominant and transient haploid stage
Process of Meiosis
Somatic (diploid body) cells - pair of homologous chromosomes in each cell (1 maternal, 1 paternal)
Each haploid gamete has only 1 of each homologous choromosome
Fertilisation - 1 haploid homologous chromosome from each parent gamete = new diploid pair = restoration of somatic diploid level
Prophase 1
- Leptotene - attachment plaque
- Zygotene - synaptonemal complex
- Pachytene - tetrad/bilavent recombination nodules chiasmata
- Diplotene
- Diakinesis
Metaphase 1, Anaphase 1, Telophase 1
Prophase 2, Metaphase 2, Anaphase 2, Telophase 2
Leptotene
- Chromosomes condense
- Chromosome ends attached to nuclear membrane by attachment plaque
- Movement and recognition of homologous chromosomes = allows them to be brought together
- e.g. C1 attatchment plaque meets with other C1 plaque, recognise and form a pair
Zygotene
- Initiation of synapsis formation
- Chromosomes zipped together = synaptonemal complex
- Genetic recombination requires close association
- Attachment plaques move homologues around and brings ends together
- Two chromatids zip together w/ ladder-like protein on either sde of homologues
Pachytene
Chiasmata - point which paired chromosomes remain in cotact in first prophase of meiosis, crossing over and exchange of genetic material occurs between strands
- When synapsis complete - bivalent (homologous chromosomes in pairs) tetrad formed
- Tretrad - chromatids that make up 2 homologous chromosomes
- Large recombination nodules appear - mediate chromosomal chaismata - 90nm diameter
- Nodules bring non-sister chromatids 100nm together
Evidence for recombination nodule function:
- Recombination nodule number same as ciasmata number in prophase 1
- Recombination nodule distribution similar to chiasmata distribution
- Drosophil mutations w/ reduced recombination frequency have fewer recombination nodules
- Genetic recombination involves DNA synthesis at chiasmata
- Radioactive precursors were preferentially incorporated in pachytene DNA at or near chiasmata
Cells may remain at pachytene for multiple days
Diplotene
- Synaptonemal complex dissolces
- Homologues still attatched at chiasmata
- Some relaxing of DNA and active RNA synthesis
Can last for years as oocytes
Diakinesis
- RNA synthesis stops
- DNA becomes highly condensed
- Chromosomes detach from nuclear envelope
- Chiasmata sites can be clearly observed
Chiasmata
Chiasmata important in:
- Chromosome orientation
- Random assortment
- Act as centromeres
Mutants that do not form chiasmata often fail to undergo normal segregation
Meiosis 1
Chiasmata formation occurs - lasts until Anaphase 1 where chromosomes move like blocks around nucleus
Homologous chromosme pairs seperated in Anaphase 1 using 1 kinetochore attachment per centromere
1 homologous chromsome moves into one cell
Meiosis 2
Results in 4 haploid cells
Each contain single chromatid from 1 or other homologue - may have exchanged material
Differences between Mitosis and Meiosis
- Meiosis 1 - kinetochore fibres attach only to 1 side of centromere - important for orientation, independent segregation and correct distribution
- Mitosis - homologous chromosomes line up independently at metaphase plate
- Meiosis 2 - independent line up but unlike mitosis homologoues are not present as seperated in meiosis 1
- Meiosis 1 - chiasmata divide but in Meiosis 2 and mitosis centromeres divide
Generation of Variation during Mitosis
For each pair of homologous chromsomes - 1 maternal, 1 paternal
Causes of variation:
- Independent segregation of 23 non-homolgous chromosome pairs at Meiosis 1 (male and female do not line up on seperate sides)
- Fertilisation - varied chance of genetics
- Chiasmata
Comments
No comments have yet been made