Meiosis

Homologous Chromosomes Homologous chromosomes contain the same genes, one from each parent Ex: Two copies of chromosome 3, one from mother and one from father, form a homologous pair. Homologous pair after DNA replication: L D This paternal chromosome 3 carries instructions for light hair This maternal chromosome 3 carries instructions for dark hair L D Both have the gene for hair color, but the color itself may be different!

Chromosome Number Diploid Haploid All eukaryotic cells can be categorized as either… Diploid Contain homologous pairs of chromosomes (di = two sets) All body cells except sperm and egg are diploid In humans, diploid chromosome number is 46 (23 pairs) Can be written as 2n Haploid Contain only one of each chromosome (hap = half as much) Only sperm and egg are haploid In humans, haploid chromosome number is 23 (no pairs, only singles) Can be written as n

How many homologous pairs? How many chromosomes? 3 6 How many homologous pairs? 3 Haploid or Diploid? Haploid Diploid 2n or n? n 2n

Meiosis Overview Meiosis is the creation of sex cells, or gametes. It occurs in the testes of males and the ovaries of females. It results in eggs (females) and sperm (males).

Mitosis vs. Meiosis Mitosis Meiosis Produces identical copies of somatic cells (body cells) Responsible for growth and healing Produces 2 diploid daughter cells Meiosis Produces unique gametes – no two are the same! Responsible for making egg and sperm Produces 4 haploid daughter cells

Phases of Meiosis Mitosis Meiosis Like mitosis, meiosis also follows the PMAT phases Unlike mitosis, meiosis goes through them twice (meiosis I and meiosis II) Mitosis Prophase Metaphase Anaphase Telophase Meiosis Prophase I Metaphase I Anaphase I Telophase I & cytokinesis Prophase II Metaphase II Anaphase II Telophase II & cytokinesis

Meiosis I In meiosis I, a reduction division occurs: chromosome number is halved from 2n to n (46 to 23) In metaphase I, homologous pairs line up at the equator In anaphase I, the pairs are separated Meiosis I

Animation Meiosis II Meiosis II looks identical to mitosis, except two cells are becoming four Meiosis II

Crossing Over When homologous chromosomes pair during meiosis I, crossing over may occur Chromosomes come into contact, crossed sections are exchanged Produces brand new combinations of genes

46 total 92 total 46 total in each 23 total in each Chromosomes replicate Homologous chromosomes pair up 1st division – homologous chromosomes separate 2nd division – sister chromatids separate

“Fun” Facts Females are born with as many eggs as they need- Around 1 million. About 40,000 will survive to puberty and develop one at a time each month. Males can produce sperm until they die, nearly 100 million per day, on average. Sperm live a maximum of five days. It takes only one sperm to fertilize one egg.

Critical Thinking Practice Diversity in Sexually Reproducing Species For an organism with four chromosomes, the diagrams of metaphase I below are equally likely. Using two different colors, draw the daughter cells each of these would produce at the end of meiosis I. How does this randomization of maternal and paternal chromosomes contribute to diversity in sperm and egg? Name one other source of variation during meiosis. OR ? ? ? ?