Chapter 4 Life

Picture of mitosis above.

Chapter 4 section 3 Notes, Talon
Sexual Reproduction and Meiosis

Vocabulary:
Sexual Reproduction: two cells sometimes called an egg and a sperm come together (2 organisms)
Sperm: Formed in the male reproductive organs (testes)
Egg: Formed in the female reproductive organs (ovaries)
Fertilization: the joining of an egg and a sperm
Zygote: The cell that forms as a result of fertilization
Diploid: Containing 2 sets of chromosomes
Haploid: Containing 1 set of chromosomes
Meiosis: The process that produces haploid sex cells

Your body forms 2 types of cells- body cells and sex cells
Body cells are diploid… they have two sets of chromosomes.
In the human body diploid cells have 23 pairs of chromosomes, you get one pair from your mother and one pair from your father.

Sex cells are haploid… They have only one set of chromosomes. Instead of having 23 pairs, they have only 23 in humans.

Sometimes we say that haploid cells have 1n and diploid cells have 2n. (n would represent a set of chromosomes, 2*n would mean 2 sets)
How many chromosomes would be in an egg? How many would be in a sperm?

How do an egg and sperm make a zygote?





Look at the Pictures in your book on pages 106 and 107.

Meiosis produces haploid (1n) sex cells. This process is similar to mitosis but the resulting cells have only one set of chromosomes not 2.
During meiosis, two divisions of the nucleus occur. These are called meiosis I and meiosis II. Each step has names like those in mitosis.
Meiosis I- ( follow along on your diagram)
Prophase I- chromosomes visible, centrioles are at each end, spindle fibers are beginning to form.
Metaphase I- chromosomes line up in the middle of the cell and attach to a spindle fiber
Anaphase I- Two pairs of chromatids move away from each other toward opposite ends of the cell. The centromere (center piece of chromosome) is attached to the spindle so the chromatids do not separate like in mitosis.
Telophase I- cytoplasm divides and two new cells form. Each new cell has one duplicated chromosome from each similar pair.

What is different about the above steps from mitosis?
Meiosis II- The chromatids of each of the duplicated chromosomes will be separated in meiosis II. Meiosis II deals with the duplicated chromosomes. They will be separated and put into the sex cells.
Prophase II- duplicated chromosomes and spindle fibers reappear in each new cell with centrioles on each end of the cell and spindle fibers stretching along the cell.
Metaphase II- the duplicated chromosomes move to the center of the cell and line up. Each centromere attaches to 2 spindle fibers.
Anaphase II- The centromere divides and each chromatid moves to opposite ends of the cell. The chromatids are no longer in pairs so they are individual chromosomes.
Telophase II- spindle fibers disappear and a nuclear membrane forms around the chromosomes at each end of the cell. The cytoplasm then divides.

Summary- Meiosis I produces 2 cells. Both of these produce 2 cells. Two divisions of the nucleus result in 4 sex cells. Each of these sex cells has one-half the number of chromosomes. This process can make eggs or sperm.

Mistakes in Meiosis- Because this process happens so much in organisms mistakes can be made. Sex cells with too many or too little chromosomes can be produced. Sometimes these sex cells die. If they live the resulting organism will have more than the necessary number of chromosomes. Sometimes they will be unfertile (unable to have children). They may not grow normally. Downs syndrome.