Meiosis and Sexual Reproduction

SEXUAL REPRODUCTION Sex cells formed by meiosis Requires 2 parents Offspring not identical Meiosis The process that results in the formation of gametes Gametes Sex cells, egg or sperm, which have haploid chromosomes Haploid Haploid means HALF, as in the number of chromosomes, written as n

The union of an egg and a sperm Fertilization n 2n The union of an egg and a sperm Zygote The cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes. Somatic Cells All body cells except for sex cells. Have diploid chromosomes Diploid Diploid means they have a full set of chromosomes, written as 2n

Homologous Chromosomes Similar chromosomes that are found in pairs. The paired chromosomes come from the mother and father. * Human body cells have 46 chromosomes each * Human body cells have 23 homologous pairs Meiosis and Fertilization Important for survival of many species, because these processes result in genetic variation of offspring. Meiosis A kind of cell division that results in gametes (sex cells) with half the number of chromosomes.

Gametes do not have homologous pairs,(a full set of chromosomes) but instead they have only half the chromosomes from each pair. Gametes are referred to as haploid (n). B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n). C. In meiosis, specialized cells divide 2 times producing 4 daughter cells.

Meiosis (2 Cell divisions – Meiosis I & Meiosis II) During Meiosis, crossing over occurs between homologous chromosomes and genes are exchanged causing genetic variation

End of Meiosis I **** Cells have a full set of “mixed” up chromosomes, Diploid, and prepare to divide again.

End of Meiosis II Cells divide, resulting in 4 haploid cells – gametes (sex cells) also known as reduction division.

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

Boy or Girl? The Y Chromosome “Decides”

Mitosis vs. Meiosis

Mitosis vs. Meiosis