Meiosis Flashcards
What is meiosis
- nuclear division
- forms 4 daughter nuclei that are genetically different from parent
- reduction division (half the number of chromosomes in daughter nuclei compared to parent)
- occurs during formation of gametes
What are homologous chromosomes
- same size
- same centromere position
- same genes (diff alleles)
- identical sequence of gene loci
- pair tgt during prophase I of meiosis
What happens prior to meiosis in meiosis cell cycle?
Interphase
- DNA replicates during S phase of interphase
- DNA in form of chromatin
- centrioles replicate
First step of meiosis I?
Prophase I
- chromatin condenses into chromosomes
- nucleolus and nuclear envelope disintegrate
- centrioles move to opposite poles
- spindle fibres form, attached to centromeres of chromosomes via kinetochore
- homologous chromosomes pair up through synapsis, chiasmata formed between non-sister chromatids (enables crossing over)
- pair of homologous chromosomes = bivalent/ tetrad
Crossing over : breakage and rejoining of corresponding sections of chromosome in non-sister chromatids
- exchange of genetic material = new combinations of alleles
Second step of meiosis I?
Metaphase I
- Bivalents are arranged along equator/metaphase plate
- INDEPENDENT ASSORTMENT: In each bivalent, the direction the chromosomes are facing is independent of that of other bivalents
Third step of meiosis I
Anaphase I
- homologous chromosomes separate and move to opposite poles due to shortening of spindle fibres
- sister chromatids remain attached at centromere and move as a single unit towards the same pole
Last step of meiosis I
Telophase I
- homologous chromosomes reach opposite poles
- spindle fibres disintegrate
- nuclear envelope and nucleolus reform
- 2 daughter nuclei formed (each has 1 set of chromosomes)
First step of meiosis II
Prophase II
- nucleolus and nuclear envelope disintegrate
- centrioles move to opposite poles
- spindle fibres form at right angles to the spindle axis of meiosis I, attached to centromeres of chromosomes via kinetochore
Second step of meiosis II
Metaphase II
- chromosomes arranged along equator/ metaphase plate (perpendicular to that of metaphase I)
- if crossing over occurred in prophase I , 2 sister chromatids are not genetically identical (independent assortment)
Third step of meiosis II
Anaphase II
- centromeres of each chromosome divide
- sister chromatids separate and move towards opposite poles due to shortening of spindle fibres
- separated chromatids = individual chromosomes
Last step of meiosis II
Telophase II
- chromosomes reach opposite poles
- chromosomes decondense into chromatin
- disintegration of spindle fibres
- nucleolus and nuclear envelope reform
- after cytokinesis, 4 haploid daughter cells (gametes) formed, each cell is genetically different from parent and each other
Why is meiosis significant?
- Prevents doubling of chromosomes in sexual reproduction
- 2 haploid gametes fuse = diploid zygote (restores diploid number of chromosomes)
- separation of homologous chromosomes during anaphase I = formation of haploid gametes - Generates genetic variation
- crossing over (prophase I) = new allele combinations
- independent assortment (metaphase I) = random assortment of maternal and paternal gametes after anaphase I
How does fertilisation result in genetic variation?
- end of meiosis results in genetically variable gametes
- random fusion of male and female gametes = genetically variable zygote
- 2^23 possible combinations of chromosomes in gametes
- unlikely that 2 gametes from same parent are genetically identical
How to calculate amount chromosomes/DNA
Number of chromosomes = number of centromeres
Number of DNA molecules = number of chromatids
What are abnormal events that can occur during meiosis?
- Structural aberration
- prophase I: homologous chromosomes become intertwined - Numerical aberration
- non-disjunction (failure to separate) during anaphase I/II
- can occur in 1/more of chromosomes = change in chromosome number (aneuploidy/polyploidy)
