Post on 27-Dec-2015
Meiosis
Stages of Meiosis ReviewComparison of Meiosis and
Mitosishttps://www.youtube.com/watch?v=PICF-_yiAQ8
Meiosis vs. Mitosis▫1 division▫daughter cells
genetically identical to parent
▫ produces 2 cells▫ 2n 2n
▫produce cells for growth & repair
▫NO crossing over
▫2 divisions▫daughter cells
genetically different from parent
▫produce 4 cells
▫2n 1n▫produces
gametes▫crossing
over!!
▫division▫Begins
with interphase
▫PMAT
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookmeiosis.html
Point where chromatid connect
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
SpermatogenesisEpididymis Testis
Coiledseminiferoustubules
Vas deferens
spermatozoa
spermatids(haploid)
secondaryspermatocytes(haploid)
primaryspermatocyte(diploid)
germ cell (diploid)
MEIOSIS II
MEIOSIS I
▫ continuous & prolific process
Oogenesis
MEIOSIS I
MEIOSIS II
first polar body
secondpolar body
ovum(haploid)
secondaryoocyte(haploid)
primaryoocyte(diploid)
germinal cell(diploid)
primary follicles
mature follicle withsecondary oocyte
ruptured follicle(ovulation)
corpus luteum
developingfollicle
fertilization
fallopian tube
after fertilization
mitosis
zygote
Putting it all together…
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2346
egg
sperm
46
meiosis 4623
23
fertilization
development
meiosis fertilization mitosis + development
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46
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4646 46
gametes
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.▫ 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:▫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.
• nondisjunction - chromosomes do not segregate correctly during meiosis
• Incorrect chromosome #
** Extra chromosomes often means survival**
**Missing chromosomes often means DEATH**
• 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)