6.1 MEIOSIS BC Science 9: p. 188-203

But first lets test your mitosis knowledge

Mitosis produces cells that are clones, but there is a lot of genetic diversity in nature –why??

Sexual Reproduction Meiosis is an important aspect of sexual reproduction. Sexual reproduction, through the shuffling of DNA, produces genetic diversity. Variation within the offspring may produce individuals that have an advantage over other individuals.

The problem The chromosome # needs to be preserved from one generation to another, but during sexual reproduction the genetic contents of 2 cells are fused

Gametes A sex cell The male gamete is a sperm and the female gamete is an egg Fertilization is the process in which an egg cell is penetrated by a sperm cell, and the genetic information of both the male and female gametes combine A zygote is the result of this (a fertilized egg). It has two sets of chromosomes The zygote undergoes repeated mitosis to form an embryo that will develop until it becomes mature. As sexual maturity is reached, gametes (ie. either sperm or egg cells) will begin to be produced. Gametes are haploid (ie. half as many chromosomes).

Diploid vs Haploid Diploid: Haploid: A cell with a diploid # of chromosomes has 2 sets of chromosomes (one from Mom and one from Dad) Usually expressed as 2n  in humans, this # is 46 (23 pairs) All body cells have the diploid number Haploid: A cell with haploid # of chromosomes has only 1 set of chromosomes (only has 1 member from each pair (ex. Only 1 sex chromosome – 1X or 1Y) Usually expressed as n  in humans this # is 23 The only cells with haploid # are the gametes

Homologous Chromosomes A pair of chromosomes in which 1 was inherited from Mom, 1 from Dad They contain genes from the same traits at the same locations, although they may contain different variations of those genes (known as alleles) Example: Both will contain the gene that codes from eye colour, but one will code for blue eyes and the other for brown

Complete the fill in the blanks activities in your notes

Meiosis Meiosis is the process of producing gametes (sperm and egg) There is still only one round of DNA replication, but 2 rounds of cell division First cell division is called Meiosis I , the second called Meiosis II.

Meiosis I Matching chromosome pairs (ie. homologous chromosomes) move to opposite poles of the cell. (this is different from mitosis and Meiosis II, in which it is sister chromatids that get separated The end result is two diploid daughter cells.

Meiosis II Sister Chromatids of each chromosome are pulled apart (like mitosis) END RESULT: 4 daughter cells that one half from each homologous pair (ie. haploid with 23 chromosomes) Four cells are produced that are genetically different from one another.

How does meiosis produce genetic variation?? Two events during meiosis I occur: crossing over and independent assortment

Crossing Over Occurs between each homologous pair Pairs of non-sister chromatids “cross-over” each other and exchange segments of DNA Multiple crossovers can occur, leading to an infinite number of genetic possibilities  genetic diversity!

Independent Assortment During Metaphase I, the homologous chromosomes line up randomly at the equator, which means that the mixture of chromosomes that end up in the daughter cells varies For each of the 23 chromosome pairs, there are 223= 8 million possible combinations of separation in any egg or sperm cell With fertilization, 70 trillion different zygotes are possible from the combination of one sperm cell and one egg cell  More genetic diversity!!

Gamete Formation in Males and Females Although the process of meiosis is the same for males and females, gamete formation is not In males it occurs in the testes and produces 4 sperm cells In females it occurs in the ovaries and although it produces 4 cells, there is an unequal division of the cytoplasm and organeles  one cell gets most; the other 3 that don’t quickly disintegrate

Mutations During Meiosis Chromosome mutations can occur spontaneously. Parts of chromosomes may be inverted, deleted, duplicated or moved to another spot. Chromosome mutations can occur because of mutagens (eg. radiation). This can lead to genetic diseases or death.

Genetic Disorders Failed separation of chromosomes in meiosis has serious consequences. A gamete may end up with no chromosomes, or have too many of a chromosome. Zygotes that result from these gametes rarely survive. If the zygote does survive it will likely have a serious genetic disorder.

Karyotypes The chromosomes of an individual can be studied using a karyotype. Geneticists can diagnose genetic disorders or syndromes by viewing a karyotype. Eg. Down syndrome usually occurs when there is an extra twenty-first chromosome.

Plan for today Watch video on meiosis (ameoba sisters) Colour genetic variation worksheet Fill out venn diagram comparing mitosis and meiosis while going through online interactive (NOVA website- link on my website) Meiosis Taboo for practice