The number of chromosomes per cell depends on the type of organism Human 46, reproductive cell ______ Fruit Fly 8, reproductive cell ______ http://morgan.rutgers.edu/morganwebframes/level1/p age2/ChromNum.html
Diploid Cells: 2 of each chromosome * Chromosomes vary in size and shape * Human reproductive cells = 23 chromosomes (sperm, egg) Homologues: 2 members of each pair of chromosomes
Fertilization: joining of sperm and egg Zygote: single cell of fertilized egg = 46 chromosomes
Haploid Cells: one chromosome of each homologous pair * Two haploids come together to form one diploid cell * Haploids are produced by Meiosis: process by which haploid nuclei are formed from diploid nuclei. Correct meiosis ensures gametes will have the right number and type of chromosomes.
I. Meiosis in Male Animals Meiosis I A.Chromosomes shorten and thicken B.Chromosomes replicate – two chromatids joined by centromere C.Homologous chromosome pairs attract and come together D.Tetrads move to the equator E.One chromosome pair to each pole F.Cleavage furrow forms 2 new cells with 2 chromosomes each pair
Meiosis II (males) A.Homologous chromosomes move to equator B.Chromatids pull apart C.Chromosomes move to poles D.Cells divide = 4 new cells with ½ the original chromosomes *Formation of 4 haploid cells (immature sperm) *a flagella forms from 1 centriole of each cell as it matures to propel the sperm cells
Diagram omits phases of prophase, metaphase, anaphase, telophase but those phases are understood to be present in order for cells to divide as they do 46 92 46 23 chromosomes each new immature cell
II. Meiosis in Female Animals Follows the same basic format as males except… Meiosis I in females begins before birth as early as 12 th week of development Meiosis then stops until sexual maturity, then several cells continue meiosis on cyclic basis – only 1 cell survives per cycle The surviving cell divides and the larger of the 2 enters meiosis II Meiosis II: the large cell divides again – Small cell dies and the larger develops into egg
Oogenesis = making of eggsSpermatogenesis = making of sperm Germ cell Meiosis I P M A T Meiosis II P M A T Ova Ovum Oocyte (Die off) Meiosis I P M A T Meiosis I P M A T sperm Spermatocyte spermatozoa Germ cell 2N – diploid 4N – replicated N – haploid Human #’s ???? DRAW THIS!!!!