Meiosis Objectives To learn the phases of Meiosis To understand its role in sexual reproduction To compare and contrast with mitosis

What is meiosis? It is a major part of sexual reproduction Meiosis is a process to convert a diploid cell to a haploid gamete, and It causes a change in the genetic information to increase diversity in the offspring because the gametes produced have a mixture of parent chromosomes It includes recombination in which pairs of chromosomes swap genes.

Gametes Gametes for fertilization usually come from separate parents Female- produces an egg Male produces sperm Both gametes are haploid, with a single set of chromosomes

What is sexual reproduction? 1 Formation of new individual by a combination of two haploid sex cells (gametes). 2 Fertilization- combination of genetic information from two separate cells that have one half the original genetic information 3 The new individual is called a zygote, with two sets of chromosomes (diploid).

Chromosomes All our cells except our sex cells contain 23 pairs (46) chromosomes We get one of each pair from each of our parents. 22 of these pairs are always identical in size The 23rd pair are the sex chromosomes (X or Y) In women they are identical XX In men they are different XY

Gametes & Chromosomes each gamete (egg or sperm) has: 23 chromosomes which includes one sex chromosome (X or Y) meiosis gives us genetic variability in these haploid cells so each of our sperm or eggs contains a different combination of our parental pairs of chromosomes In theory, a gamete could contain all the chromosomes you inherited from your mother or from your father – but normally contains a random mix of both.

Fertilisation after fertilisation (sperm enters egg) the zygote has 23 pairs or 46 chromosomes The nuclei of the sperm and egg fuse. Two random sets of chromosomes, one from each parent are combined to make a future child. The randomness is the cause of variation This is why we are all unique. Even though siblings have the same parents – they have different combinations of those parents’ chromosomes

Vocabulary meiosis: a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of pairs of chromosomes in a diploid cell homologous: each of the chromosomes that came from the male parent has a corresponding pair from the female parent diploid (“two-sets”): a cell that contains pairs of chromosomes haploid (“one-set”): contain only a single set of chromosomes tetrad: a structure made when two corresponding homologous pairs join – 4 chromatids in a tetrad crossing-over: exchange of alleles between homologous chromosomes and produces new combination of alleles – an additional cause of variation.

In meiosis, how are the 4 daughter cells produced? prophase metaphase anaphase telephase cytokinesis

Key features At Telophase 1 pairs of chromosomes pulled apart instead of copies of same chromosome Number of chromosomes halved at the end of telophase 1 Daughters of first division not identical – contain mix of maternal and paternal chromosomes Recombination may occur in metaphase 1 exchanging material between chromosomes: adding more variation.

What does meiosis create? Four new daughter cells, which gives the species genetic variability

For clarification http://www.youtube.com/watch?v=7_gHHeo6FDE

Abnormalities

Mendel’s Peas Characteristics

Mendel’s peas: a cross

Try this test http://www.bbc.co.uk/bitesize/higher/biology/genetics_adaptation/meiosis/quiz/q45456255/ You would benefit from reading the associated notes first.

Plenary 1. How many chromosomes has a human cell 2. How many chromosomes in an egg or sperm 3. Why the difference? 4. Why does meiosis intrinsically produce variation in gametes? 5. What else adds to variation? 6. Why do two children with the same parents not look identical? 7. Meiosis Activities: tutorial