Meiosis I Stages and Prophase I

• Meiosis is cell division.
• Zanda Daneberga is an associate professor.
• Meiosis is from the Greek for "reduction."
• Diploid cells produce haploid gametes through meiosis.
• Two rounds of division with one round of DNA replication accomplish this.
• Germ cells uniquely undergo meiosis.
• Within gonads, germ cells use mitosis until they're signaled to use meiosis.
• Meiosis involves two cell divisions: Meiosis I and Meiosis II.

Meiosis I Stages

• Meiosis I has four stages.
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

Prophase I

• Prophase of meiosis I is complicated with multiple defined stages.
• Leptotene means "thin threads" in Greek.
• During Leptotene, chromosomes begin to condense.
• Zygotene means "paired threads" in Greek.
• During Zygotene, chromosomes become closely paired.
• Homologous chromosomes align along their lengths, form synapsis, and are held together by a synaptonemal complex.
• Pachytene means "thick threads" in Greek.
• During Pachytene synapsis completes and each homologue pair appears as a bivalent.
• Crossing over occurs.
• Diplotene means "two threads" in Greek.
• During Diplotene, the synaptonemal complex breaks down following recombination.
• The homologous chromosomes begin to separate but remain attached at the chiasmata.
• Diakinesis means "moving through" in Greek.
• During Diakinesis, chromosomes condense and separate.
• Terminal chiasmata only connect the two chromosomes.

Crossing Over

• Crossing over swaps genetic material between non-sister chromatids of homologous chromosomes during meiosis.
• Crossing over creates new allelic combinations in daughter cells.
• Crossing over (genetic recombination) generates genetic variation in chromosomes.
• Homologous recombination ensures crossing over.
• Homologous recombination is the process where DNA molecules break and fragments rejoin in new combinations.
• Double-strand breaks (DSB) begins homologous recombination followed by homologous reparation (re-joining the strands based on sequence of homologous chromosome).
• The recombination complex involves many proteins.
• Formation of DSBs is catalyzed by conserved proteins that have topoisomerase activity.
• Without recombination, chromosomes may not align properly for the first meiotic division leading to nondisjunction which increases chromosomal loss.
• Poor fertility can result from a failure in homologous recombination.

Metaphase I

• The nuclear membrane disappears.
• A spindle forms.
• Paired chromosomes align on the equatorial plane with their centromeres toward different poles.

Anaphase I

• Each bivalent's two members move apart.
• Respective centromeres with attached sister chromatids is drawn toward opposite poles of the cell.
• Disjunction is the process termed.
• Maternal and paternal chromosome in homologous pairs segregate randomly into daughter cells in meiosis I.
• Original paternal and maternal chromosome sets are sorted into random combinations.
• 2^23 (more than 8 million) is the possible number of combinations from 23 chromosome pairs that can be present in the gametes.

Telophase I and Cytokinesis

• Telophase I sees the two haploid sets of chromosomes grouped at opposite poles of the cell.
• Cytokinesis divides the cell into two haploid daughter cells which enter meiotic interphase.
• The interphase is brief, called interkinesis, and Meiosis II begins.

Meiosis II

• The second meiotic division is like mitosis, but cells entering Meiosis II are haploid (n).
• There is no DNA replication before the next division.