BVMS1

Meiosis to produce haploid gametes (n)

• a) zygote
• b) mitosis

2 homologous chromosomes form 2 homologous sister chromatids prior to mitosis/meiosis

I I ------> II II

• used for growth and repair
• four stages (prophase, metaphase, anaphase, telophase)
• cytokenesis (5th stage) allows division into two daughter cells- actual cell division

• Interphase:
• preparation for mitosis
• centrioles visible
• chromosomes are not visible but have been replicated

• chromatin begin to condense
• centrioles move to opposite ends of the cell
• microtubules form and attach to centromeres
• chromosomes begin to move

• chromosomes align along the middle of the nucleus
• ensure that in the next phase, when the chromosomes are separated each new nucleus will receive one copy of each chromome

the paired chromosomes separate at the kinetochores (centromeres) and move to opposite sides of the cell

• chromatids arrive at opposite poles of the cell
• new membranes form around the daughter nuclei
• chromosomes disperse

fiber ring composed of actin around the center of the cell contracts. pinching the cell into 2 daughter cells- each with 1 nucleus

• form of cell division that underlies sexual reproduction- only occurs in germ cells
• involves one cycle of DNA replication but two cell divisions (meiosis and meiosis II)
• allows chromosome number to reduce from diploid (2n) to haploid (n)

• Leptotene- chromosomes condense
• zygotene- homologous chromosomes pair up- synapsis
• pachytene- crossing over occurs
• diplotene- chromosomes start to separate
• diakenesis- homologous chromosomes separate to form sister chromatids

pairing of homologous chromosomes, undergo crossing over- location of crossovers are termed chiasma (key-as-mas)

• metaphase I: nuclear membrane disappears chromosomes move to equator and bind to spindle
• anaphase I: disjunction- sister chromatids move to opposite poles (normal movement), each pole is haploid
• telophase I: new nuclear membrane forms

• Prophase II: nuclear membrane disappears, chromosomes condense
• Metaphase II: centromere divides and chromosomes move to equator
• Anaphase II: sister chromatids move to opposite poles
• Telophase II: new nuclear membrane forms

• occurs at prophase I
• homologous chromosomes forms a sunaptonemal complex
• crossing over happens in pachytene
• exchange of chromosome material (recombination)
• Important for causing genetic diversity!!!

• Endonuclease: makes nicks in DNA
• U Protein: unwinds regions of DNA
• R protein: facilitates reassociation
• Preferentially occur in non coding regions

• pachytene checkpoint
• prevents exit from pachytene and inhibits cell cycle progression
• (only triggered when synapsis or recombination is ongoing, or when there is a defect in synapsis or recombination)