Download Meiosis

11.4 – Meiosis &
Sexual Reproduction
Regents Biology
2007-2008
Objectives
§  Summarize the events of meiosis.
§  Contrast meiosis and mitosis.
Regents Biology
Cell division/Asexual reproduction
§  Mitosis
-  produce identical cells/
clones
Ex:
-  single-celled
eukaryotes (yeast,
protists)
-  simple multicellular
eukaryotes (hydra)
§  What might be some
advantages of asexual
reproduction?
Disadvantages?
Regents Biology
http://www.youtube.com/watch?v=ofxDIS7fbCE
How about the rest of us?
§  What if a complex multicellular organism (like us) wants
to reproduce?
§  Do we make egg and sperm by mitosis?
Regents Biology
Regents Biology
Regents Biology
Meiosis – production of gametes
§  chromosome number must
be reduced (2n → n)
§  fertilization restores
chromosome number (n
→ 2n)
Regents Biology
Regents Biology
Double division
of meiosis
DNA replication
1st division of
meiosis separates
homologous pairs
Meiosis 1
2nd division of
meiosis separates
sister chromatids
Regents Biology
Meiosis 2
Preparing for meiosis
§  1st step of meiosis: DNA
replication
§  DNA replicated in S phase of
interphase of meiosis (just like
in mitosis)
Regents Biology
M1 prophase
2n = 4
2n = 8
Meiosis 1
2n = 4
§  1st division of meiosis separates
homologous pairs
prophase 1
2n = 8
metaphase 1
2n = 8
reduction
telophase 1
Regents Biology
1n = 4
Meiosis 2
§  2nd division of
1n = 4
meiosis separates
sister chromatids
prophase 2
1n = 4
metaphase 2
1n = 2
Regents Biology
telophase 2
Regents Biology
Trading pieces of DNA
§  Crossing over
-  during prophase 1, sister
chromatids intertwine
-  homologous pairs swap
pieces of chromosome
-  DNA breaks and re-attaches
§  Advantages?
synapsis
tetrad
Regents Biology
prophase 1
Mitosis vs. Meiosis
Regents Biology
Variation from crossing over
§  Crossing over creates completely new combinations of
traits on each chromosome – genetic recombination!
§  creates an infinite variety in gametes
Regents Biology
Variation from random fertilization
§  Sperm + Egg = ?
§  any 2 parents will produce a zygote with over 70 trillion
(223 x 223) possible diploid combinations
Regents Biology
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both genetic
similarity and differences
Regents Biology
Sperm production
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
-  a continuous Cross-section
and prolific
process
of
seminiferous tubule
-  each ejaculation
contains ~
100-600
million
Regents
Biology
sperm
MEIOSIS II
Egg production
-  eggs in ovaries halted
before anaphase 1
-  meiosis 1 completed during
maturation
-  Meiosis 2 completed after
fertilization
-  1 egg + 2 polar bodies unequal divisions
Meiosis 1 completed
during egg maturation
Meiosis 2 completed
triggered
Regents
Biology
by fertilization
ovulation
Egg Production
germinal cell
(diploid)
primary follicles
fallopian tube
fertilization
primary
oocyte
(diploid)
MEIOSIS I
secondary
oocyte
(haploid)
first polar body
MEIOSIS II
after fertilization
second
polar body
Regents Biology
ovum
(haploid)
developing
follicle
mature follicle with
secondary oocyte
ruptured follicle
(ovulation)
corpus luteum