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Lecture 9

BIMS 201 Lecture Notes - Lecture 9: Synaptonemal Complex, Diplo, Meiosis

Biomedical Science
Course Code
BIMS 201
Elizabeth A.Crouch

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Chapter 3a and 3b-Notes
Meiosis: only occurs in germ/sex cells; oocytes and spermatocytes; sperm and egg
o Purpose: genetic diversity, reduction division in meiosis I
Medel’s st law: alleles separate during meiosis; meiosis I
Prophase I: L)PDD; **know proteins-cohesins, separins, securins; these 5 steps only
happen in meiosis I
o Leptonema
lepto- thead—chromosomes very threadlike/stringy
very beginning of prophase=things start condensing, nuclear membrane
starts dissolving
o zygonema
zygo- oig togethe – beginning of pairing of homologous
bivalent: 2 homologous chromosomes together or tetrad (chromatids)
forming synaptonemal complexes b/w homologs
o pachynema: chromosomes molecularly zip together; exchange/recombo. of
genetic material
synaptonemal complexes: have protein structure around them
woven together
build b/w zygonema and pachynema
terminalization: crossing-oe ouig; uzippig
tetrads=4 chromatids
pah- thik – chromosomes look very thick; almost @ max.
crossing-over event: opening DNA on both homologs and cross strands
soe ase pais o’t ath eatl, ut esoled i et ell
move in concert slowly to metaphase plate
o diplonema
diplo- to – crossing-over still occurring, but homologous
hoosoes stat to pull apat figue ’s; lattie ok; loops;
sometimes circles)
starts repelling except where connected
o diakinesis: crossing-over complete; homologous chromosomes still paired
synaptonemal complex unzips itself and resolves; stay attached
w/cohesins but not criss-crossed @ DNA level
terminalization: unzipping stops
intimately associated but NOT wrapped around each other
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Metaphase I: pulling homologous chromosomes to opp. poles
o All mixed up and random=point of variation (223 combos)
o Ind. assortment=lining up
Medel’s 2nd law: alleles segregate from each other ind.; meiosis I
No 2 alleles have to segregate together
The do segegate togethe he the’e losel spaed togethe
bc not enough room for crossing-over to occur
Genes can recombine freely
When 2 genes=fa eough o hoosoe, the’e id. assotig 
enough room for them to cross-over
o Closer 2 genes are together = < likely to cross-over
Recombo. freq.
Anaphase I: each chromosome still has 2 sis chromatids = need 1 > division
Pophase II: e uik; do’t eed nuclear membrane yet; not much to do; cytoplasm
divides and immediately start lining up metaphase II
Look @ ed up hapte ’s!
Meiosis II is > like mitosis than meiosis I
Sperm-very specialized cell
o Mito. in mid-piece; drive whipping of tail
o Tail: made of microtubules
o Very little cytoplasm
o Enzymes in head (very specialized enzyme pockets in lysosomal body that
degrades covering/outside of egg)=acrosomal body/acrosomes
o DNA wrapped around protamine (proteins) NOT histones (normal)
Non-histone proteins: everything but 5 histone proteins
Proteins that make up kinetochore
Proteins that make up synaptonemal complex
Transcription factor
Transcriptional activators/inhibitors
Scaffolding proteins
H1 safety pin, holds wrap together
H2A, 2B, 3, 4 form ball that DNA is wrapped around
o 23 chromosomes
o 2ndary oocyte is actually what gets fertilized, not technically egg
o pauses in prophase I in diplonema 1st (sitting in follicle in ovary)
o pauses again in metaphase II (waiting to be fertilized)
o 2ndary oocyte is shed during menstruation
o egg: when sperm DNA is already inside
nondisjxn: improper pairing; spermatogenesis and oogenesis ( > likely to occur here bc
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