Meiosis Part 1 Flashcards
Importance of meiosis?
- Sexual reproduction involves the fusion of 2 nuclei to form a zygote during fertilisation
- Gametes must contain half the genetic information (they are haploid) to prevent the doubling of the chromosome number at fertilisation
- Gametes are formed by the process of gametogenesis.
Key points of meiosis?
4 genetically different daughter cells produced
Produces gametes with only one copy of each chromosome
Prevents doubling of chromosome number after fertilisation.
How does meiosis produce genetic variation
Random assortment of chromosomes Random Segregation of chromosomes Crossing Over Independent assortment of chromatids Independent segregation of chromatids
Stages of meiosis?
Meiosis I (reduction phase) 2n -> n Separates whole chromosomes Prophase 1 Metaphase 1 Anaphase 1 Telophase 1
Meiosis II (Mitotic Phase) - separates sister chromatids Prophase II Metaphase II Anaphase II Telophase II Cytokinesis
What happens in Prophase I
Chromosomes shorten and thicken
Centrioles move towards poles (IN AC only)
Microtubules form
Nucleolus and nuclear envelope breaks down
Synapsis occurs (pairing up of homologous chromosomes Crossing over occurs between no-sister chromatids
What happens in Metaphase I?
Bivalents line up on equator
Spindle fibres attach to centromere (made from microtubules)
What happens in Anaphase I?
Very rapid phase Spindle fibres contract Exerts pulling force on centromeres Centromere DOES NOT split Whole chromosome pulled to opposite poles
What happens in Telophase I?
Chromosomes reach poles
Chromosomes uncoil and lengthen
Spindle fibres disintegrate
Nuclear envelope reforms
Centrioles replicate
What happens in Prophase II specific to meiosis?
Poles are at 90 degrees to the position of the poles in meiosis I.
Other sources of variation (Not occurring in meiosis)?
Mutations
- Inversion
- Deletion
- Non-disjunction
- Translocation
Random Mating
Random fertilisation
What is non-disjunction?
The failure of homologous chromosomes to separate correctly in Anaphase I or sister chromatids in Anaphase II
What is translocation?
The movement of a gene fragment from one chromosome location to another, which often alters or stops expression
What is inversion?
A defect in the chromosome in which a segment of the chromosome breaks off and reinserted in the same place but in the reverse direction relative to the rest of the chromosome.
What is random mating?
The choice of mate by a female is a source of variation
Females select a male based on visible characteristics and behavioural traits
What is random fertilisation
One secondary oocyte is released each cycle.
Millions of spermatazoa are released per ejaculation
The process of fertilisation is random
So genetic variation occurs within the zygote.
