WHY ARE THEY NECESSARY?

SPECIALIZED SEX CELLS

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 ReproductionGametes have only one of each chromosomeRequires special cell division: Meiosis

2n = 46

+n (23)

n (23)

Diploid cells (2n) Gametes (n) • Takes place in gonads (testis, ovary)

Mitosis Cells divide 1x

MeiosisCells divide 2x

How is Meiosis different from Mitosis?

2n

2n 2n

2n

n n

n n n n

Diploid Cells

Haploid Cells

Mitosis Meiosis

Identical cells Different cells

Mitosis Meiosis

Cell division is over

Cells divide again; sister chromatids line up

Done

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

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.)

CQ#3: The different copies of chromosome 21 in John and Jane are shown to the right.

Parent #1 Parent #2

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.

1. Katherine of Aragon2. Anne Boleyn3. Jane Seymour4. Anne of Cleves5. Katherine Howard6. 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?

YESNO

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 oogenesisTakes place in the ovary of the

femaleThe 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.