BMB/MICRB 252 Spring ‘05
Notes: Prof. P. Mitchell
LECTURE 25
Meiosis, part 1
CHAPTER 20: GERM CELLS AND FERTILIZATION
Figures covered: Fig. 20-1 through 20-13, 20-16-18
Asexual vs Sexual Reproduction
Most higher organisms reproduce sexually and have only a very brief haploid
phase. Sexual reproduction increases rate of evolution by increasing genetic
variation in the population. Life cycles of higher and some lower eukaryotes
involve alternations between diploid and haploid generations of cells. Haploid
cells for sexual reproduction are generated from diploid precursor cells by
meiosis.
Meiosis
• The formation of eggs and sperm begins in a similar way, with meiosis.
• Meiosis requires one round of replication followed by two successive cell
divisions (I and II).
• Prophase of division I is the longest part of meiosis; it resembles G2 of
mitotic cell cycle (replicated DNA maintained for prolonged time)
o replicated homologous chromosomes pair up at the metaphase
plate as bivalents (2 pairs of tightly-associated sister chromatids)
o complementary DNA base pairing between homologous
chromosomes allows crossing over to occur between non-sister
chromatids
o Crossing over has important role in genetic reassortment in
gametogenesis & has been useful to geneticists mapping relative
gene locations
o results in chiasmata that hold the pair of homologs together during
metaphase I
o synaptonemal complex distributes recombination nodules so cross-
over events are spread out
• At anaphase I, homologues (not chromatids) separate (additional means
of reassorting maternal and paternal genes)
• At anaphase of division II, sister chromatids separate, haploid daughter
cells result
Primordial germ cells and sex determination in mammals
• Small number of cells in gastrulating mammalian embryo are signaled by
neighbors to become PGCs
• These cells migrate into genital ridges (the primordial gonads)
• A developing gonad will become an ovary unless its somatic cells have Y
chromosome
•
Sry
gene on the Y is a testes-determining gene
• Sry is a transcription factor expressed in subset of somatic cells in the
developing gonad; it induces these cells to become Sertoli cells
• Sertoli cells produce signals that promote male development and suppress
female development, and induce male PGCs to commit to sperm
development
Terms to know:
sexual vs asexual reproduction
haploid vs diploid
