Download Meiosis Review

Meiosis vs. Mitosis
Animation
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▫ 2 divisions
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daughter cells
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genetically
▫ division
different
from parent ▫ Begins
with
produce
interphase
4 cells
▫ PMAT
2n  1n
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produces
gametes
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▫ crossing
over!!
1 division
daughter cells
genetically
identical to
parent
▫ produces 2 cells
▫ 2n  2n
produce cells for
growth & repair
NO crossing
over
Meiosis
Stages of Meiosis Review
BioFlix Animation
Unique Features of Meiosis
Crossing Over
• Homologous Chromosomes
• Synapsis
• Chiasma form
• Cross over at matching
regions of nonsister
chromatids
• genetic recombination increases
variation!!!
• Process itself varies
• Bacteria – asexual reproduction
Putting it all together…
meiosis  fertilization  mitosis + development
gametes
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meiosis
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egg
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zygote
fertilization
sperm
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mitosis
development
An Intro to Animal Development
• Fundamental biological question: How does a single
cell—the fertilized egg—develop into a multicellular
individual?
• Development proceeds in ordered phases through
organism’s life cycle:
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Gametogenesis
Fertilization
Cleavage
Gastrulation
Organogenesis
Gametogenesis –
sperm & egg production
Oogenesis
primary follicles
germinal cell
(diploid)
fallopian tube
fertilization
primary
oocyte
(diploid)
MEIOSIS I
secondary
oocyte
(haploid)
first polar body
MEIOSIS II
developing
follicle
mature follicle with
secondary oocyte
ruptured follicle
(ovulation)
after fertilization
second
polar body
ovum
(haploid)
corpus luteum
Egg Structure and Function
• Relatively large & nonmotile
• Size largely due to nutrient storage, required for early
embryonic development
• Quantity of nutrients varies across species
▫ Relatively small mammalian egg supplies nutrients for
early development ONLY  embryos start to obtain
nutrition through placenta shortly after fertilization
▫ Egg-laying species form larger eggs  yolk of the egg is
embryo’s SOLE nutritional source prior to hatching
Spermatogenesis
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
▫ continuous & prolific
process
▫ 100-600 million sperm
can be produced daily
MEIOSIS II
• Fertilization - haploid sperm and egg cells fuse to
form diploid zygote (a fertilized egg).
• Certain conditions MUST be met before a zygote can
form:
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Location: gametes in same place at same time.
Recognition: recognize & bind to each other.
Fusion!
Trigger onset of development.
Successful fertilization leads to
development – how do we
become a person from 2 single
cells? Do you think the process
is the same for all living things?
Explain and describe your vision
of this process.
Why Does Only One Sperm Enter the Egg?
• Animals use different mechanisms to avoid polyspermy fertilization by more than one sperm
• Ex: sea urchins - fertilization stimulates creation of a
physical barrier
▫ Ca2+-based signal is rapidly induced & propagated
throughout the egg, form a fertilization envelope keeps
away additional sperm
• Ex: mammals - cortical granules release enzymes 
modify egg cell receptors  prevent binding by
additional sperm.
Cleavage
• Cleavage - set of rapid cell divisions in animal zygotes
immediately after fertilization.
▫ 1st step is embryogenesis - process that makes single-celled
zygote into multicellular embryo.
▫ partitions egg cytoplasm without additional growth of the
zygote.
▫ Cells created by cleavage divisions called blastomeres.
▫ Completed cleavage results in embryo consisting of mass of
blastomere cells called a blastula.
Gastrulation
• Gastrulation - extensive & highly organized cell
movements radically rearrange embryonic cells into
structure called the gastrula.
▫ results in formation of embryonic tissue layers
(**tissue is integrated set of cells functioning as a unit)
▫ most embryos have 3 primary tissue layers: ectoderm,
mesoderm, and endoderm.
 these tissues are called germ layers because they give rise
to adult tissues and organs.
• Ectoderm - forms outer covering of adult body
& nervous system
• Mesoderm - gives rise to muscle, most internal
organs, & connective tissues like bone &
cartilage
• Endoderm - produces lining of digestive tract
or gut, along with some associated organs
What if something goes
wrong?
What happens then???
• nondisjunction - chromosomes do not
segregate correctly during meiosis
• Incorrect chromosome #
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Deletion
Duplication
Inversion
Translocation
** Extra chromosomes
often means survival**
**Missing chromosomes
often means DEATH**
• Monosomy - One less chromosome
due to missing chromosome in
gamete
Ex: Turner Syndrome
K
A
R
Y
O
T
Y
P
E
• Trisomy - Gamete has an extra
chromosome
• Ex: Trisomy 21 (Down Syndrome)
• Polyploidy - Complete
EXTRA sets of chromosomes
– almost ALWAYS lethal to animals
– plants can be healthier & larger
Genetic testing
• Amniocentesis in 2nd trimester
▫ sample of embryo cells
▫ stain & photograph chromosomes
• Analysis of karyotype
Sex chromosomes abnormalities
• Human development more tolerant of wrong
numbers in sex chromosomes
Results in variety of distinct syndromes
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XXY = Klinefelter’s syndrome male
XXX = Trisomy X female
XYY = Jacob’s syndrome male
XO = Turner syndrome female