Meiosis
Stages of Meiosis Review
Comparison of Meiosis and Mitosis
https://www.youtube.com/watch?v=PICF-_yiAQ8
Meiosis vs. Mitosis
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▫ 2 divisions
▫
daughter cells
▫
genetically
▫ division
different
from parent ▫ Begins
with
produce
interphase
4 cells
▫ PMAT
2n  1n
▫
produces
gametes
▫
▫ crossing
over!!
1 division
daughter cells
genetically
identical to
parent
▫ produces 2 cells
▫ 2n  2n
produce cells for
growth & repair
NO crossing
over
Point where chromatid
connect
http://www.emc.maricopa.edu/faculty/farabee/biobk/
biobookmeiosis.html
Crossing Over
• Homologous Chromosomes
• Synapsis (pairing of homologous
chromosomes)
Chiasma form (Point where chromatid
connect)
• Cross over at matching
regions
• genetic recombination increases
variation!!!
• Process itself varies(only certain part of DNA, forms
bridge)
• Bacteria – asexual reproduction
Gametogenesis
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
MEIOSIS II
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
Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
23
meiosis
23
egg
23
46
23
zygote
fertilization
sperm
46
46 46
46 4646
46
46
46
mitosis
development
An Introduction to Animal Development
• How a single cell—the fertilized egg—develops into a
multicellular individual is one of the fundamental questions
in modern biology.
• Gametes are haploid reproductive cells. In animals, male
gametes are called sperm and female gametes are called eggs.
• Development proceeds in ordered phases through an
animal’s life cycle.
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Gametogenesis
Fertilization
Cleavage
Gastrulation
Organogenesis
Egg Structure and Function
• Egg cells are relatively large and nonmotile.
• Their size is largely due to nutrient storage, required for
early embryonic development.
• Quantity of nutrients varies across species.
▫ The relatively small mammalian egg only has to supply
nutrients for early development, as embryos start to obtain
nutrition through the placenta shortly following
fertilization.
▫ Egg-laying species produce larger eggs; the yolk of the egg
is the embryo’s sole source of nutrition prior to hatching.
• Fertilization occurs when a haploid sperm and egg
cells fuse, forming a diploid zygote (a fertilized egg).
• Many conditions must be met before a zygote can
form:
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Gametes must be in the same place at the same time.
Gametes must recognize and bind to each other.
Gametes must fuse together.
Fusion must trigger the onset of development.
***remember prezygotic – obstacle to mating or fertilization (behavioral or mechanical
postzygotic – prevent hybrid offspring from developing
Why Does Only One Sperm Enter the Egg?
• Animals employ different mechanisms to avoid
polyspermy, fertilization by more than one sperm.
• In sea urchins, fertilization stimulates the creation of a
physical barrier.
▫ After fertilization, a Ca2+-based signal is rapidly induced
and propagated throughout the egg, resulting in the
formation of a fertilization envelope, which keeps away
additional sperm.
• In mammals, the cortical granules release enzymes that
modify egg cell receptors, preventing binding by
additional sperm.
Cleavage
• Cleavage is the set of rapid cell divisions that take place
in animal zygotes immediately after fertilization.
• Cleavage is the first step in embryogenesis, the
process that makes a single-celled zygote into a
multicellular embryo.
• Cleavage partitions the egg cytoplasm without any
additional growth of the zygote.
• The cells created by cleavage divisions are called
blastomeres.
• When cleavage is complete the embryo consists of a
mass of blastomere cells called a blastula.
• During gastrulation, extensive and highly organized
cell movements radically rearrange the embryonic
cells into a structure called the gastrula.
• Gastrulation results in the formation of embryonic
tissue layers. A tissue is an integrated set of cells that
function as a unit.
• Most early embryos have three primary tissue layers:
ectoderm, mesoderm, and endoderm.
• These embryonic tissues are called germ layers
because they give rise to adult tissues and organs.
• Ectoderm forms the outer covering of the adult
body and the nervous system.
• Mesoderm gives rise to muscle, most internal
organs, and connective tissues such as bone and
cartilage.
• Endoderm produces the lining of the digestive
tract or gut, along with some of the associated
organs.
What if something goes
wrong?
What happens then???
• nondisjunction - chromosomes do not
segregate correctly during meiosis
• Incorrect chromosome #
** 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
▫ XXY = Klinefelter’s syndrome male (infertility, less
male hormone, possible learning disabilities)
▫ XXX = Trisomy X female(no physical difference)
▫ XYY = Jacob’s syndrome male(no physical diff)
▫ XO = Turner syndrome female
(infertility, puffiness in hands and feet, heart and kidney
Problems)