Review of Mitosis Four Phases Asexual reproduction because daughter cells reproduced are identical to parent cell Purpose is to replenish cells, assist in growth and development of an organism

Meiosis By the end of the lesson: I will be able to define meiosis I will be able to define diploid & haploid I will be able to define gamete I will be able to identify the stages of meiosis I will be able to compare and contrast mitosis and meiosis

copyright cmassengale Meiosis Animation copyright cmassengale 3

What is Meiosis? A type of cell division that reduces the number of chromosomes Reduces the chromosome number by half by splitting up pairs of chromosomes A cell with 2n chromosomes will have n number of chromosomes after meiosis

Why go through the Trouble? Each parent produces new combinations of genes through crossing over in gametes. A single set of parents creates a unique combination of genes in a baby when they combine gametes. This leads to new types of organisms with new adaptations.

Meiosis Overview 1. Involves 2 phases: Meiosis 1 and Meiosis 2 2. Results in 4 cells with 1/2 the normal genetic information. 3. Meiosis results in the formation of haploid sex cells called gametes.

Gametes These are the Ova (egg) and sperm. Ova are produced in the ovaries in females (oogenesis) Sperm are produced in the testes of males (spermatogenesis).

Spermatogenesis & Oogenesis

New Vocabulary Diploid (2N) - Normal amount of genetic material Haploid (N) - 1/2 the genetic material. Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote

Fertilization – “Putting it all together” 1n + 1n copyright cmassengale 10

MEIOSIS I

Prophase 1 Prior to division, amount of DNA has doubled Homologous Chromosomes are paired to form a tetrad. Homologous chromosomes carry the same genes. One pair comes from the father and one from the mother Homologous chromosomes Tetrad

copyright cmassengale Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring copyright cmassengale 13

So why even go through the extra work? Results in new genetic combinations or variation of offspring. This is the main advantage of sexual reproduction

Why is it advantageous to have genetic variation in a population?

Homologous pairs of chromosomes align along the equator of the cell Metaphase I Homologous pairs of chromosomes align along the equator of the cell copyright cmassengale 16

copyright cmassengale *Anaphase I Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres. copyright cmassengale 17

copyright cmassengale Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. copyright cmassengale 18

MEIOSIS II

Meiosis II DNA does not double  IT IS REDUCED from diploid to haploid The rest of meiosis II is similar to mitosis

Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Telophase II Anaphase II 4 Genetically Different haploid cells copyright cmassengale 21

Meiosis The most important thing to remember in distinguishing Mitosis from Meiosis is: Mitosis Meiosis body cells (somatic cells) sex cells (Gametes) results in 2 genetically identical diploid cells. produces 4 genetically different haploid cells. 1 round 2 rounds

Humans Every organism must inherit a single copy of every gene from both its parents. With us, we get 23 from our mom and 23 from our dad. 46 total. So when you produce your own gametes (sex cells) these 2 sets of genes have to be separated from each other so that each gamete contains just one set of genes.

Meiosis 1 diploid parent cell divides into 4 haploid daughter cells (for reproduction). Meiosis forms sperm or eggs called gametes that are haploid. Each parent donates 1 set of 23 chromosomes to the child (46 total for the baby).