Tuesday Apr 8, 2025 Meiosis

Review important chromosome words

• As human we have 23 different (i.e., types of) chromosomes (different species have different numbers of chromosomes)

• Each type of chromosome (#1-23) has a unique set of genes (which together, constitute our entire genome) 

• As diploids, we have 2 sets of each of the 23 chromosomes (1 set from mom, 1 set from dad - so 46 total)

• In other words, we have a pair of each type of chromosome (#1-23)

• The two chromosomes in a pair contain the same set of genes, we call them homologous chromosomes

Different Biological Situations Mitotic vs. Meiotic Cell Division Would be Used

• Both somatic and germ cells undergo cell division 

  • But M Phase is different in somatic vs. germ cells

• Somatic Cells Divide DNA By Mitosis 

  • Biological situation: 

    • Embryonic development 

    • Wound healing 

    • Maintenance of tissue from regular wear and tear 

    • Regeneration 

• Germ Cells Divide DNA By Meiosis 

  • Biological situation:

    • Production of haploid gametes (sex cells)

Processes & Outputs of Mitotic vs. Meiotic Cell Division

• If you’re a somatic cell

  • Do mitosis - divide replicated genome into 2 genetically identical daughter cells 

    • Includes 1 round of nuclear division 

    • Produces genetically identical, diploid daughter cells 

• If you’re a germ cell

  • Do meiosis - produce haploid gametes 

    • Includes 2 rounds of nuclear division 

    • Produces NOT genetically identical haploid gametes (sperm & eggs) 

Useful compare/contrast table for mitosis vs. meiosis 

Overall flow: 

• Leading up to meiosis (i.e., during interphase DNA replicates) 

• (meiosis I) homologous chromosomes pair up, then separate into two daughter cells 

• So by the end of 1st meiotic division, each daughter will have a full set of chromosomes (big & small) but only 1 homolog of each (either mom’s or dad’s) 

  • But each chromosome is still replicated 

• Each daughter thus undergoes second round of meiotic division to separate sister chromatids evenly between two new daughter cells (similar to mitosis) 

Meiosis I

• Prophase I: Homologous Chromosomes Pair & Crossover 

  • (DNA replication has already occurred and each chromosome is now made up of 2 sister chromatids attached at centromere) 

1. Replicated chromosomes condense, homologous chromosomes find each other & pair up (synapsis). This allows crossing over between non-sister chromatids to occur. 

   1. At the end of prophase I, homologous chromatids are crossed over, the nuclear envelope starts to disappear, and the meiotic spindle starts forming

• Random crossing over may not look like much but it is the KEY to why sex is amazing evolutionarily speaking 

  • It increases genetic diversity 

  • Genetic diversity gives rise to evolutionary innovation (it’s how new traits arise)

  • Each “recombined” chromatid that will end up in a gamete is a uniquely random mix of paternal and maternal genes 

• Prometaphase I & Metaphase I: 

2. Prometaphase I: Nuclear membrane breaks down. Meiotic spindles attach to the centromeres on each chromosome 

3. Metaphase I: Homologous pairs line up in center of cell, with bivalents oriented randomly with respect to each other (meaning paternal & maternal homologs of each bivalent can face either pole of cell)

   1. In addition to crossing over, the random orientation of homologous pairs at the metaphase plate is yet ANOTHER contributor to genetic diversity 

• Anaphase I & Telophase I: Homologs separate & remain condensed for prophase II

4. Anaphase I: Homologous chromosomes (homologs) separate, but sister chromatids do not separate) 

5. Telophase I & Cytokinesis: Nuclear membrane reforms, daughter cells split apart and are ready to move into prophase II. Chromosomes remain mostly condensed.

• 2 daughter cells result from meiosis I, each containing a unique combination of homologs

Meiosis II 

• Sister chromatids separate, as in mitosis 

• Mitosis and meiosis II are similar (prophase, metaphase, etc. & chromatid separation)

• *For Metaphase II: Totally random whether recombinant or non-recombinant chromatid will face one pole of the dividing cell vs. the other

• End result is 2 daughter cells from each cell produced during meiosis I (4 total), each containing one full set of (unreplicated) chromosomes

• Important*

  • Cells at the beginning of meiosis have the same number of unique chromosomes as cells at the end, however there’s only one version (homolog) of each (so ½ as many total chromosomes)

Steps in Meiosis That Result in Genetic Variation 

• Prophase I: Crossing over

• Metaphase I: Random orientation of homologous pairs at metaphase plate 

• Metaphase II: Random orientation of chromatids from each chromosome at metaphase plate