Meiosis: Key Overview

• Meiosis: specialized cell division that yields 4 nonidentical, haploid gametes in sexually reproducing diploid organisms
  • Only occurs specialized (diploid) cells like testes & ovaries in humans to make sperm and egg
  • Involves two rounds of division (meiosis 1 & 2)
• Purpose is to produce genetic variation in gametes; process that ensures each gamete receives both maternal and paternal chromosomes

Meiosis 1 and Meiosis 2: Process Overview

• Meiosis 1: focuses on the separation of homologous chromosomes
• Meiosis 2: separate sister chromatids (analogous to mitosis)

Meiosis 1: Detailed Process

Prophase 1	Metaphase 1
Chromatin begin to condense into chromosomes Homologs pair up; there are 2 of these, called Tetrads ●        Crossing over occurs	Homologous chromosomes/tetrads line up along the metaphase plate –          Alignment determines independent assortment –          The same apparatus moves the pairs: microtubules, spindle fibers, asters –          Diff to mitosis bcuz they line up in pairs –          One pole attaches to one of homologs kinetochore
Anaphase 1	Telophase/Cytokinesis 1
Homologous pairs separate and move to opposite sides of the cell, guided by the spindle apparatus Cohesins are not cleaved so sister chromatids remain attached at the centromere and move as one unit toward the pole	Each half of the cell has a haploid set of chromosomes Cytokinesis: two haploid daughter cells

Meiosis 2: Detailed Processes

Prophase 2	Metaphase 2
No synapsis, chiasmata or crossing over of homologs A spindle apparatus forms. In late prophase 2, chromosomes (made up of two chromatids) move towards metaphase plate	Sister chromatids line up in single file line at middle of the cell
Anaphase 2	Telophase/Cytokinesis 2
Sister chromatids are separated and move to opposite sides of the cell, now as two newly individual chromosomes. Because of crossing over in meiosis 1, the two sister chromatids of each chromosome are no longer identical.	Chromosomes arrive at opposite poles. Nuclei form, and the chromosomes begin decondensing. Result: FOUR haploid cells that are not identical to each other or parent

Meiosis and Genetic Diversity

Crossing Over Specifically Occurs in Prophase 1

• Synapsis: Homologous chromosome pair with each other
  • Allows for crossing over                                                                                                 
• Crossing Over: Chromosome from each parent align in a way so DNA sequences cross over and exchange genetic material → combines maternal and paternal alleles into single chromo (recombinant chromosome) → increased genetic diversity among gametes
• When homologs cross over, specific proteins break/unwinds pieces of genetic material on a chromosome & attaches it to non-sister chromatids on homolog (recombines) → exchanges DNA segments
• Physical constraints: probability of crossing over between genes is proportional to distance between two genes (increases as distance becomes larger)
• Chiasmata (sing. Chiasma): Point of contact between two non-sister chromatids where crossing over will occur  
  • Holds homologous pairs together due to sister chromatid cohesion
  • Absence leads to aneuploidy                                                                               
• Chromosomes that look like its parent (1 and 4) parental
• Chromosomes that are crossed over: ( 2 and 3): recombinant

 

 

Mitosis vs Meiosis

• Both have separation of sister chromatids

Genetic Variation Contributes to Evolution

• When genes mutate, they take multiple forms with each gene slightly differing in sequence of base DNA (alleles)

 

Three mechanisms to contribute to genetic variation

• Independent assortment of chromosomes
  • Gametes get random chromosome from parent → many combinations
    • The number of combinations possible when chromosomes assort independently into gametes is 2^(n), where n is the haploid number.
  • Crossing over
    • Creates chromosomes w/ new combination of alleles + mixes up pre-existing genes and mutations

                                         Random fertilization: random chance of any sperm and egg fusing is astronomical

Fertilization

• Occurs when sperm penetrates the membrane of egg → combine maternal and paternal genes in a fertilized egg (zygote) [offspring with both maternal & paternal traits]→ series of cleavage divisions (rapid cell division w/o cell growth) forms fetus
  • Process increases genetic variation in populations by creating new combinations of alleles in zygotes

Fertilization restores diploid number here & allows process of meiosis to be repeated