Meiosis - Reproductive Cell Division• Meiosis = cell division during which sex cells

(gametes) are produced • Remember: sex cells contain half the number of

chromosomes as the body cells (haploid)• Male gametes = sperms (1n)

** n = # of chromosomes• Female gametes = eggs (1n)• Fertilization = uniting of male and female

gametes; resulting fertilized egg = zygote (2n)

1n (egg) + 1n (sperm) = 2n (offspring)

n=23egg

sperm n=23

2n=46zygote

Fertilization

Meiosis Cont.

• Occurs only in reproductive organs called gonads (testes or ovaries)

• Meiosis in males = spermatogenesis (gives rise to sperms)

• Meiosis in females = oogenesis (gives rise to eggs)

Karyotype = collection of all organism’s chromosomes

Overview of Meiosis

Homologouschromosomesseparate

Sister chromatidsseparate

Results of meiosis: 4 haploid cells are produced from one diploid cell

Haploid

Diploid

Meiosis I Meiosis II

Haploid

Prophase IMetaphase IAnaphase ITelophase I

Prophase IIMetaphase IIAnaphase IITelophase II

Meiosis I : homologous chromosomes separate

Prophase I(early)

(diploid)

Prophase I(late)

(diploid)

Metaphase I(diploid)

Anaphase I(diploid)

Telophase I(diploid)

Nucleus Spindlefibers

Nuclearenvelope

Meiosis II : sister chromatids separate

Prophase II(haploid)

Metaphase II(haploid)

Anaphase II(haploid)

Telophase II(haploid)

Four nonidentical

haploid daughter cells

Why do we have meiosis?

I. To make sperm and eggs

II. To make new combinations of genes (due to crossing over and independent assortment of chromosomes) which leads to variation within a species

Homologous chromosomes

Tetrad

Genetic variation

Tetrad

Random (independent) assortment

CELL DEATH

2 types of cell death:

• Orderly, genetically programmed cell death = apoptosis

• Cell death due to injury = necrosis

CELL AGING

• Geriatrics = branch of medicine dealing with medical problems of elderly

As we age:• Many vital functions slow down• Telomeres (DNA on tips of chromosomes) shorten• Glucose is added to proteins and contributes to

stiffening• Free radicals damage proteins, lipids, and DNA