Chapter 10.5 From Gametes to Offspring AP Biology Fall 2010

Objectives Describe gamete formation in plants and animals Understand and list the variation possibilities that exist in gamete formation and fertilization

Gamete Formation in Plants Life cycle of most plants alternates between sporophyte and gametophyte stages Sporophyte: multicelled spore-producing body that makes sexual spores by way of meiosis Spore: haploid reproductive cell that is not a gamete and that does not take part in fertilzation

Gamete Formation in Plants Spore undergoes mitotic cell divisions that give rise to gametophyte One or more gametes form inside this multicelled haploid body

Gamete Formation in Plants

Gamete Formation in Animals Diploid germ cells give rise to gametes

Gamete Formation in Animals In males, meiosis and gamete formation are called spermatogenesis – Germ cell (2n)  primary spermatocyte (2n)  Meiosis I  two secondary spermatocytes (n)  Meiosis II  four spermatids (n) – Spermatids change in form, each develops a tail to become mature sperm

Gamete Formation in Animals In females, meiosis and gamete formation are called oogenesis – Germ cell (2n)  primary oocyte (2n)  Meiosis I  secondary oocyte (n, and large in size) plus polar body (n, and small in size)  Meiosis II  one large ovum (n) plus 3 polar bodies (n, small) – The single ovum is the only cell capable of being fertilized by a sperm, has more cytoplasm and cytoplasmic machinery than polar bodies – Polar bodies wither and die

Gamete Formation in Animals

More Shuffling at Fertilization The diploid chromosome number is restored at fertilization when two very different gamete nuclei fuse to form the zygote

More Shuffling at Fertilization Variation present at fertilization is from 3 sources: 1.Crossing over occurs during prophase I 2.Random alignments at metaphase I lead to millions of combinations of maternal and paternal chromosomes in each gamete 3.Of all the genetically diverse gametes produced, chance will determine which two will meet in fertilization

More Shuffling at Fertilization

Review 1.T/F Animals form spores during gamete formation 2.T/F Plants first go through sporophyte formation and then gametophyte formation 3.T/F Gametes have a diploid number of chromosomes while the zygote has a haploid number of chromosomes

Answers 1.False- plants do 2.True 3.False- gametes are haploid, zygotes are diploid