How cells create gametes Meiosis How cells create gametes

Fertilization When a sperm and an egg meet, fertilization occurs inside a sex cell. Cell division begins after fertilization. Sex cells are involved in Meiosis.

Gametes/Zygotes/Embryos Gametes are sex cells. When a sex cell is fertilized, it is called a zygote. The zygote will eventually develop into an embryo.

Number of Chromosomes Body Cells (Mitosis) Sex Cells (Meiosis) Humans have 46 chromosomes, 23 pairs Sex Cells (Meiosis) Humans have 23 chromosomes, not in pairs

Diploid Di means 2 (or pairs) Diploid cells have pairs of chromosomes Body cells are diploid because they contain 23 pairs of chromosomes Mitosis produces diploid cells Zygotes (2n) are diploid because they contain pairs of chromosomes from both the sperm and egg

Diploid Cells Continued… Mitosis produces diploid body cells If 2 diploid cells combine, offspring would have TWO times as many chromosomes as the parent and the animal would not survive Body cells are produced by asexual reproduction So how do we get to the correct number of chromosomes?

Haploid Haploid means half the number of chromosomes as body cells Gametes or sex cells are haploid That means that sperm are haploid (n) and eggs are haploid (n) They have 23 chromosomes, which are half the number of (46) chromosomes in body cells

Haploid Cells Continued… Meiosis produces haploid sex cells through sexual reproduction Meiosis ensures offspring will have the same diploid number as its parent

Meiosis I See pg 128-129 in your textbook for diagrams. In Meiosis I, the nucleus divides and produces 2 new cells with 1 nucleus each. This is the same as the process of mitosis, but with sex cells. An exact copy is made of the original cell.

Meiosis II The nuclei divide and the chromatids separate producing 4 cells with half the number of chromosomes of the original nucleus. From a human cell with 46 chromosomes, meiosis produces 4 sex cells, each with 23 unpaired (haploid) chromosomes.

Did you know?? There are more than 8 million combinations of genes in humans, per parent. With 2 parents’ genes, there is a possibility of 70 trillion combinations. Meiosis takes an average of 74 hours in humans. The fertilized egg, with too few or too many chromosomes may die, or the growing embryo may have birth defects. Down Syndrome occurs in humans when the offspring receives an extra 21st chromosome.

Take a closer look… http://www.pbs.org/wgbh/nova/miracle/divi_flash.html