Lecture 14 – 10/5 – Dr. Wormington
Centrosome
& DNA Replication
Same as in Mitosis
Cell is Tetraploid 4n
Homologous
Chromosomes
Align Along Lengths
Synapsis
Recombination
Between Homologous
Chromosomes Occurs
4n
2n
2n
Phenotypes without Recombination
Statall
Ptydemocrat
Stashort
Ptyrepublican
tall & short are alleles of the Stature gene (Sta)
democrat & republican are alleles of the
Party gene (Pty) located on the same chromosome
Statall
Ptydemocrat
Stashort
Ptyrepublican
Proof that Recombination Generates
Genetic Diversity
Sta and Pty genes illustrate linkage &
Recombination provides genetic maps
Stay tuned to Chapter 10 for more details
4n
2n
Note: Only 1 Kinetochore
Forms per Pair of
Sister Chromatids
Recall 1/chromatid
in mitosis!
Homologous
Chromosomes
Segregate to Opposite
Poles – Sister Chromatids
Remain Attached
2n
2n Cells Undergo
Cytokinesis
Sister Chromatids
Condense
No DNA Replication
Interkinesis
2n
1 Kinetochore Formed on
Each Chromatid as in Mitosis
Sisters Separate& Migrate
to Opposite Poles
As in Mitosis
2n
n
n
Spermatogenesis
Generates 4 sperm
Oogenesis
Generates 1 egg +
3 polar bodies
Nuclear Envelope Reforms
Chromosomes Decondense
Cytokinesis Occurs
n
n
Each cell
Contains single set of chromosomes
Fertilization Generates 2n Zygote
Interesting Connections Between Meiosis & Human Reproduction
• Humans have 23 chromosome pairs
Even w/o recombination, can generate 223 (~107) different haploid gametes
•A human female fetus in the7th month of gestation
already has 7x106 oocytes arrested in Meiotic Prophase I
•At birth, this has already declined to ~106 oocytes
•Most oocytes undergo apoptosis
•Only 1-2 oocytes typically resume meiosis and proceed to Metaphase II
during each monthly ovulation between the ages of 12–50
•12–50 years may elapse between when an oocyte was 1st formed
and when it completes meiosis & is ovulated
•Only 400/106 oocytes ever complete meiosis
•75-80% of fertilized eggs never develop into a viable embryo
Primarily due to nondisjunction events generating trisomies & monosomies
•In contrast – Spermatogenesis requires only 60 days
•A single ejaculation releases 2x108sperm = 24 hrs production
Note: Only 1 of
These 4 cells would
be an egg
The other 3 are
Polar bodies & would
Not be fertilized!
Still 50:50 if egg
Lacks chromosome
Or is disomic
Why remaining arrested in
Meiotic Prophase I
is risky business!
Nondisjunction
Both homologues
Segregate to same pole
These gametes
have an extra
chromosome
These gametes lack
a chromosome
Zygotes are monosomic
for 1 chromosome
Typically an embryonic
lethal event owing to
Recessive lethal mutations
Present on 1 chromosome
Not "covered" by wild-type
Gene on missing chromosome
Zygotes are
Trisomic
e.g., Trisomic Chr 21
Down Syndrome
Monty Python and the Holy Grail 1974
Scene 2
DEAD PERSON: I'm not dead!
CART MASTER: 'Ere. He says he's not dead!
CUSTOMER: Yes, he is.
DEAD PERSON: I'm not!
CART MASTER: He isn't?
CUSTOMER: Well, he will be soon. He's very ill.
DEAD PERSON: I'm getting better!
CUSTOMER: No, you're not. You'll be stone dead in a moment.
murder
suicide
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Cells Arrested by Cell Cycle Checkpoints Initiate Apoptosis
Within Hours Unless Damage/Defect is Repaired
Check out articles by Vogelstein & Kinzler and Evan & Vousden
on BIO 201 website for relevance to cancer
p53 Tumor Suppressor
activates DNA repair
enzymes in DNA damage
checkpoint arrested cells
Go is typically the only phase
where cells can remain
arrested indefinitely without
undergoing apoptosis
Mammalian oocytes which
remain arrested in G2 of
meiotic prophase I for years
are an obvious exception.