Meiosis Reproductive Cell Division Meiosis cell division during

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 (1 n) ** n = # of chromosomes • Female gametes = eggs (1 n) • Fertilization = uniting of male and female gametes; resulting fertilized egg = zygote (2 n) 1 n (egg) + 1 n (sperm) = 2 n (offspring)

Fertilization n=23 egg sperm n=23 2 n=46 zygote

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 Sister chromatids separate Homologous chromosomes separate Meiosis II Diploid Prophase I Metaphase I Anaphase I Telophase I Haploid Prophase II Metaphase II Anaphase II Telophase II Haploid Results of meiosis: 4 haploid cells are produced from one diploid cell

Meiosis I : homologous chromosomes separate Spindle fibers Nucleus Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Nuclear envelope Anaphase I (diploid) Telophase I (diploid)

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

Tetrad Genetic variation

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