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KaryotypeKaryotypes
Meiosis Overview
Crossing Over During Meiosis
2n combinations!!!(n=haploid number)
n = 23
2n = ~8 million
The Genetic Power of Independent Assortment
Fertilization – The Players
Modified from: Human Physiology (2nd ed.), by Silverthorn
Caput epididymis
Corpus epididymis
Vas deferensCauda epididymis
Seminiferous
tubule
A sperm is born…
Fertilization – The Players
The Egg and I…
Model systems for studying fertilization
• Contains glycoproteins for species-specific sperm-egg recognition
Mammalian egg is surrounded by the zona pellucida (thick extracellular matrix material)
Sea urchin eggs:
sea urchins
frogs
zebrafish
mice
advantages and disadvantages for each?
Blocking Polyspermy
Fertilization Triggers the Blocks to Polyspermy and Egg Activation
Zona
Pellucida
Oocyte
1.
3.
4.
Acrosomal Exocytosis is a Calcium Dependent Step
2. [Ca2+]i
Sperm Penetration of Zona PellucidaYanagimachi and Phillips, 1984
(golden hamster sperm)
Recognition of Sperm and Egg
• The chemoattraction of the sperm to the egg by soluble molecules released by the egg
• The exocytosis of the acrosomal vesicle to release its enzymes
• The binding of sperm to the extracellular matrix surrounding the egg
vitelline layer – echinoderms
mammals – zona pellucida
(Invertebrates and Mammals)
• The passage of sperm through the extracellular matrix
• Fusion of sperm and egg plasma membranes
L. variegatus zygotes, viewed from the side. A. 1-cell zygote.
The fertilization envelope is visible as a large "halo" around the embryo.
The arrow points to the site of sperm penetration. B. 2-cell C. 8-cell D. 16-cell
E. 32-cell F. Hatched blastula. (from worms.zoology.wisc.edu/urchins/SUcleavage_stages.html)
https://youtu.be/dXpAbezdOho
https://youtu.be/qisrNX3QjUg
Telomeres Shorten during Normal DNA Replication
1. Helicase unwinds end of DNA helix (at end of chromosome).
5
5
3
3
CHROMOSOME SHORTENING DURING NORMAL DNA REPLICATION
RNA primer
33 5
SSBPs Helicase
Leading strand
Lagging strand
Sliding clampDNA polymerase
5
5
End of chromosome
2. DNA polymerase completes the leading strand. Primase synthesizes RNA primer at endof lagging strand.
Telomerase Prevents Shortening of Telomeres
5
DNA polymerase
TELOMERE REPLICATION
Missing DNA on
lagging strand
Sliding clamp
55
RNA primer
3
3
53
3Telomerase with itsown RNA template
5
5
5
3
3
1. When the RNA primer is removed from the5 end of the lagging strand (see Figure 14.14),a strand of parent DNA remains unreplicated.
4. Primase, DNA polymerase, and ligase thensynthesize the lagging strand in the 53direction, restoring the original length of thechromosome.
2. Telomerase binds to the “overhanging” sectionof single-stranded DNA. Telomerase addsdeoxyribonucleotides to the end of the parentDNA, extending it.
3. Telomerase moves down the DNA strand andadds additional repeats.
Proofreading and Repairing DNA
DNA polymerases proofread newly made DNA
• incorrectly placed nucleotides are replaced
error rate = 1 in 100,000 bp
Mismatch Repair:
repair enzymes correct errors of base pairing
Repairing DNA Damage:
Excision Repair
Xeroderma pigmentosum
