SPECIALIZED SEX CELLS WHY ARE THEY NECESSARY?

Why do diploid organisms need to have specialized sex cells? Sex cells (gametes: sperm or egg) allow traits to be combined from two organisms 2n (46) 2n (46) + 4n = 92 too many

Sexual Reproduction + Gametes have only one of each chromosome Requires special cell division: Meiosis Diploid cells (2n) Gametes (n) Takes place in gonads (testis, ovary) n (23) n (23) + 2n = 46

How is Meiosis different from Mitosis? Cells divide 1x Meiosis Cells divide 2x 2n 2n 2n 2n n n Diploid Cells n n n n Haploid Cells

Mitosis Meiosis Different cells Identical cells

sister chromatids line up Mitosis Meiosis Cells divide again; sister chromatids line up Cell division is over Done

How many unique gametes can a cell with two pairs of chromosomes make?

(2)23 > 8 million different possible gametes Why bother? For humans with 23 pairs of chromosomes? (2)23 > 8 million different possible gametes For a couple possible unique offspring (8 million) x (8 million) = (64,000,000,000,000) But wait! There’s more!

Crossing over enables even greater variety Exchange of equivalent sections between homologous chromosomes. Occurs at random locations along chromosome. Creates new versions of chromosomes.

CQ#1: The cell to the right shows a diploid organism with two chromosomes (2n=2). The pictures below show some of the steps this cell may go through during mitosis or meiosis. A. B. C. D. E. F. G. H. Place the appropriate steps in order for a cell going through mitosis. (Note: you may use just some or all of the steps.)

CQ#2: The cell to the right shows a diploid organism with two chromosomes (2n=2). The pictures below show some of the steps this cell may go through during mitosis or meiosis. A. B. C. D. E. F. G. H. Place the appropriate steps in order for a cell going through meiosis. (Note: you may use just some or all of the steps.) 11

Parent #2 Parent #1 CQ#3: The different copies of chromosome 21 in John and Jane are shown to the right. Which of the following is a normal gamete that might be produced by either John or Jane? A B C D E

KING HENRY VIII King Henry married six times in hopes of producing a strong line with male heirs. Katherine of Aragon Anne Boleyn Jane Seymour Anne of Cleves Katherine Howard Katherine Parr

CHILDREN Katherine of Aragon had six pregnancies: 3 sons, 3 daughters, only one daughter survived Anne Boleyn had three pregnancies: 1 son, 1 daughter, and 1 unknown, only 1 daughter survived. Jane Seymour had one pregnancy: 1 son, who survived None of Henry’s remaining wives had pregnancies

WHO WAS AT FAULT? Henry divorced Katherine, and beheaded Anne due to his belief that his wives were denying him male heirs. CQ#4: Was it in fact the fault of his wives? YES NO

WHY? Females can provide only X’s to their children Males can provide either an X or a Y

Gamete Formation – Sperm This process is called spermatogenesis Takes place in the testes of the male The process begins with a diploid cell called the spermatogonium

The spermatogonium enlarges and undergoes meiosis I and II. The final product is 4 haploid sperm cells Each sperm cell has the same number of chromosomes and the same amount of cytoplasm Each sperm cell also has a long, tail-like flagellum used for movement

Gamete Formation - Eggs This process is called oogenesis Takes place in the ovary of the female The process begins with a diploid cell called the oogonium

The oogonium enlarges and undergoes meiosis I and II At the end of meiosis one, however, the cytoplasm is not equally divided between the two daughter cells. The cell that receives most of the cytoplasm is called the primary oocyte, the other cell is called the polar body

The polar body is not a viable gamete As the primary oocyte undergoes meiosis II, the cytoplasm again divides unevenly. The cell that receives most of the cytoplasm is called the Ovum or egg.

Why the unequal division? The unequal division of the cytoplasm during oogenesis is to provide the ovum with enough nutrients to support the developing zygote for a few days after fertilization

What is the zygote? The zygote is the new cell formed through the process of fertilization.