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=23 egg sperm n=23 2n=46 zygote 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 Homologous chromosomes separate Sister chromatids separate Results of meiosis: 4 haploid cells are produced from one diploid cell Haploid Diploid Meiosis I Meiosis II Haploid Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II
Meiosis I : homologous chromosomes separate Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Nucleus Spindle fibers Nuclear envelope
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