Meiosis vs. MitosisAnimation

▫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

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

mitosiszygote

Putting it all together…

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egg

sperm

46

meiosis 4623

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fertilizationdevelopment

meiosis fertilization mitosis + development

4646

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4646 46

gametes

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:▫Gametogenesis▫Fertilization▫Cleavage▫Gastrulation▫Organogenesis

Gametogenesis – sperm & egg production

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

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

SpermatogenesisEpididymis Testis

Coiledseminiferoustubules

Vas deferens

spermatozoa

spermatids(haploid)

secondaryspermatocytes(haploid)

primaryspermatocyte(diploid)

germ cell (diploid)

MEIOSIS II

MEIOSIS I

▫ continuous & prolific process

▫ 100-600 million sperm can be produced daily

•Fertilization - haploid sperm and egg cells fuse to form diploid zygote (a fertilized egg).

•Certain conditions MUST be met before a zygote can form:▫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 - 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.

Gastrulation

•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 #• Deletion• Duplication• Inversion• Translocation ** Extra chromosomes often means survival**

**Missing chromosomes often means DEATH**

• Monosomy - One less chromosome due to missing chromosome in gameteEx: Turner Syndrome

KARYOTYPE

• 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 ▫XXX = Trisomy X female▫XYY = Jacob’s syndrome male▫XO = Turner syndrome female