MEIOSIS SxSx Cell Reproduction

Meiosis The form of cell division by which gametes (Sex cells), with half the number of chromosomes, are produced. Diploid (2n)  haploid (n)Diploid (2n)  haploid (n) Meiosissexual reproduction.Meiosis is used for sexual reproduction. Two divisionsmeiosis I meiosis IITwo divisions (meiosis I and meiosis II) 4 cells are produced at the end of meiosis. 100 % of original DNA50 % of original DNA

2n=46 human sex cell diploid (2n) n=23 meiosis I n=23 sperm haploid (n) meiosis II

Fertilization spermeggzygoteThe fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Homologous Chromosomes chromosomesmother’sfather’sPair of chromosomes (mother’s and father’s) that are similar in shape and size. Homologous pairs carry genes controlling the same inherited traits (eye color, hair color, nose shape, etc.) homologous chromosomes (46 total chromosomes)Humans have 23 pairs of homologous chromosomes (46 total chromosomes) autosomes (body structures) a.22 pairs of autosomes (body structures) sex chromosomes (Determine gender) b.1 pair of sex chromosomes (Determine gender)

Homologous Chromosomes PaternalMaternal eye color hair color hair color

Genes- sections of a chromosome responsible for one trait (eye color, height, etc)

Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue

Sex Chromosomes XX chromosome - femaleXY chromosome - male In Humans the “Sex Chromosomes” are the 23 rd set

Crossing Over Crossing over (variation)Crossing over (variation) may occur between chromosomes. Crossing overCrossing over: pieces of nonsister chromatids break and reattach to the other chromatid.

Crossing Over = variation nonsister chromatids chiasmata: site of crossing over variation Tetrad

Crossing-Over in PROPHASE I Chromosomes exchange genetic material between homologous pairs!

WHYYYYYY is Genetic Variation Important?

Because of Crossing over…… 1.More than 8 million different kinds of eggs and sperm can be produced trillion different zygotes possible

Fertilized egg with diploid set of chromosomes is called a zygote. After fertilization, the zygote divides by mitosis and becomes an embryo. Later, the embryo becomes a fetus Zygote>Embryo>Fetus