Meiosis Making Gametes

Review Asexual Reproduction is also called _______________ Mitosis starts with one parent cell and ends up with __________________________________________ Genetically, the daughter cells are ________________ to the parent cell Making an identical copy of a cell is also called _______________ What type of cells in YOUR BODY divide by Mitosis? ________________ The scientific term for Body cells is ______________ These cells are considered 2n (diploid). What does that mean? _______________________________________

Review Cont’d How many Chromosomes should a normal human have? _____ If a human skin cell must divide to heal a wound, how many chromosomes would each of the daughter cells have? _____ If a human bone cell must divide because a baby is growing, how many chromosomes would each of the daughter cells have? _________ What are two reasons for Mitosis? __________ & ___________ Which organelle is in control of cell division? ___________ What happens when the nucleus loses control? __________

Mitosis Asexual Cell Reproduction Necessary for growth & repair Chromosomes in Somatic Cells are in pairs 2N (diploid) Two new daughter cells are identical copies of original parent cell Clones

Cell Cycle Sequence of phases in the life cycle of the cell Notice! Mitosis is only a SMALL part of the cell cycle! In what phase do cells spend most of their time? _____________ Mitosis & Cell Cyle Sequence of phases in the life cycle of the cell

Sex Cells are Different Human beings have 46 chromosomes in every Somatic (Body) Cell This is a complete set But, if mom’s egg cell has 46… And, if dad’s sperm cell has 46… The Zygote (the fertilized egg cell) would have 92! Would THAT make a human?

Meiosis Cell division of reproductive cells (sex cells) Gametes or germ cells (sperm and egg) Produced in Gonads Each new cell is not exactly like the parent cell Allows for variation Important for the survival of our species!

Meiosis In each new daughter cell, the chromosomes are not in pairs NOT 2n (not diploid!) At the end of Meiosis, four daughter cells contain half of the original number of chromosomes (N) Each is unique! Not identical to the parent

Gametes (aka germ cell) Mature egg or sperm cell Produced in the Gonads Each gamete has half the original number of chromosomes 1N (haploid) HALF IS IMPORTANT!

Why is Half is Important? Meiosis makes unique sex cells! Half the number of chromosomes (haploid) is important for sexually reproducing organisms! N = The number of chromosomes in a gamete To create a new organism, two parents give half of their own chromosomes to make a baby with a complete set of chromosomes! Half from mom Half from dad N + N = 2N (Zygote!)

Sexual Reproduction Sexual Reproduction requires two parents Offspring have SOME traits inherited from each parent… but not all! YOUR DNA is not identical to either parent It is a mix of genes from both parents Meiosis

Sexual Reproduction Fusion of two (N) gametes to produce a single zygote (2N) Introduces greater genetic variation, allows genetic recombination A VERY GOOD THING!

Chromosome Pairing Homologous pair Homologous means similar Each chromosome in the pair are Homologous to the other (carry genes for same trait) Only one pair differs - sex chromosomes X or Y

Different! Metaphase I in Meiosis… Chromosomes form TETRADS (four together) Metaphase II in Meiosis… Chromosomes line up in the middle and then a SECOND DIVISION happens! The two daughter cells are divided into four and each has HALF the chromosome number!

Two equally probable arrangements of chromosomes at metaphase I Independent assortment of homologous chromosomes POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

Crossing over increases genetic variability The exchange of genes between two homologous chromosomes happens when in Tetrad formation

Tetrad Chaisma Centromere Figure 8.18A

SPERMATOGENESIS OOGENESIS spermatogonium oogonium Sperm and egg formation in humans. In sperm formation (spermatogenesis), diploid cells called spermatogonia produce primary spermatocytes. The primary spermatocytes are the diploid cells that go through meiosis, yielding haploid secondary spermatocytes. These spermatocytes then go through meiosis II, yielding four haploid spermatids that will develop into mature sperm cells. In egg formation (oogenesis), cells called oogonia, produced before the birth of the female, develop into primary oocytes. These diploid cells will remain in meiosis I until they mature in the female ovary, beginning at puberty. (Only one oocyte per month, on average, will complete this maturation process.) Oocytes that mature will enter meiosis II, but their development will remain arrested there until they are fertilized by sperm. An unequal meiotic division of cellular material leads to the production of three polar bodies from the original oocyte and one well-endowed egg. The egg can go on to be fertilized, but the polar bodies will be degraded. egg spermatids polar bodies (will disintegrate)

Figure: 10-07 Title: Big difference in size. Caption: A human egg surrounded by much smaller human sperm.

Review Mitosis & Meiosis Both are forms of cell division Both involve replication Both involve disappearance of the nucleus, and nucleolus, nuclear membrane Both involve formation of spindle fibers