Ch. 6 Sexual Reproduction (c) McGraw Hill Ryerson 2007

Review: Chromosomes = packages of DNA Body (somatic) cells have 2 versions of each chromosome One from mom & one from dad Humans have 23 pairs; 46 total Reproductive cells (gametes) have only 1 version

DNA from a human somatic cell

Egg Sperm

Cell Division Two types of division: Mitosis – makes new somatic (body) cells Meiosis – makes gametes (egg/sperm) used in sexual reproduction

6.1 Meiosis Meiosis = process that produces cells with ½ the original number of chromosomes Has 2 separate nuclear divisions Important aspect of sexual reproduction

(c) McGraw Hill Ryerson 2007 Sexual Reproduction Sexual reproduction = reproduction that requires the participation of two individuals each contributing a gamete Produces genetic diversity Variation produces individuals that may have advantages over one another. See pages 188 - 189 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Role of Gametes Body cells have a diploid chromosome number (chromosomes are in pairs) Gametes have half (haploid) the normal number of chromosomes Ex) In humans, sperm & egg each have 23 chromosomes so when fertilization occurs: 23 (egg) + 23 (sperm) = 46 (zygote) See page 190 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Zygote (fertilized egg) develops into an embryo, and on into a fetus (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Gamete Formation Before meiosis, DNA replication must occur (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Stages of Meiosis (c) McGraw Hill Ryerson 2007 See pages 191 - 192

Meiosis Events Meiosis I Matching chromosome pairs (homologous chromosomes) move to opposite ends (poles) of the cell - two daughter cells result. See pages 191 - 193

(c) McGraw Hill Ryerson 2007 Crossing Over In meiosis I, chromatids of chromosome pairs can cross over each other & exchange DNA segments increases genetic possibilities produces more variation (c) McGraw Hill Ryerson 2007

Meiosis II Chromatids of each chromosome are pulled apart Resulting in four haploid cells These final cells develop into gametes

Independent Assortment Pairs of chromosomes separate independently creates different combinations of chromosomes in daughter cells

(c) McGraw Hill Ryerson 2007 Review of Events: (c) McGraw Hill Ryerson 2007

Meiosis Details Gametes do not form equally in males & females See pages 194 - 195 Gametes do not form equally in males & females In males, all 4 cells develop into sperm. In females, 1 cell gets most of the cytoplasm & becomes the egg.

(c) McGraw Hill Ryerson 2007 Chromosome mutations sometimes occur spontaneously During meiosis, parts of chromosomes can be deleted, inverted, duplicated or moved to another spot (translocated). (c) McGraw Hill Ryerson 2007

Chromosome mutations can occur because of mutagens Chromosome changes, sometimes leading to genetic disease or death, can be cause by mutagens such as radiation or chemicals.

Down syndrome karyotype: Failed separation of chromosomes has serious consequences: a gamete may end up with no chromosome or too many Zygotes that result from these gametes rarely survive, and if they do, they will have serious genetic disorders. Ex. Turner syndrome (XO), Klinefelter syndrome (XXY), Down syndrome Karyotypes allow geneticists can view an individual’s chromosomes. Down syndrome karyotype: See pages 196 - 197 Take the Section 6.1 Quiz