CHAPTER 10—PAGES 268-286

Meiosis and Human Reproduction

I. What do we know already?

A. Each human cell contains 46 chromosomes.B. Each human cell contains about 25,000

genes, which are located on the chromosomes.

1. Check out the chart:

How Does the Human Genome Stack Up?

A genome is an organisms’ complete set of DNA

About how many genes do human cells contain in their genome?

About how many genes does the HIV virus contain?

How about a lab rat?

C. Think about it—we know chromosomes are different sizes, and each contains a different number of genes; but each chromosome probably has at least 1,000 different genes!

D. Remember: genes code for proteins.

E. Remember: the proteins that you have in your body determine your traits, such as:

Eye color Hair color Height Tongue rolling Hitchhiker thumb

II. Karyotypes

A. A karyotype is a display of chromosomes from a cell.

B. In a karyotype, the chromosomes are lined up by size, from largest to smallest by pairs.

C. A karyotype from a normal human cell has 46 chromosomes, or 23 pairs.

D. Is this karyotype from a male or female?

One from mom

One from dad

XY = male

XX = female

E. Remember: one chromosome #1 is from the mom, and one chromosome #1 is from the dad, etc. How about the sex chromosomes? The X can come from mom or dad, and the Y chromosome can only come from the dad—more about this later!

1. Before we move on, let’s briefly review conception as it occurs in

humans.

This is a diagram of the female reproductive system:

Each month, 1 egg cell matures. A woman is born with about 5 million immature egg cells already in her ovaries—this is all she’ll ever have.

This egg cell is released from the ovary into the fallopian tube in a process called ovulation.

If, while in the fallopian tube, the egg cell meets up with a sperm cell, the two cells will fuse and make one cell called a zygote. This process is called fertilization, and it is the start of a fetus, meaning that the woman is pregnant. The zygote (which is a cell) starts to undergo mitosis, and in 9 months, if all goes well, the baby is ready to be born.

E. This is a karyotype from a human that has a disease. What

is abnormal?

This person has ___Trisomy 21___

= _Down’s Syndrome_

XY = male

F. This is a karyotype from a human that has a disease. What

is abnormal?

This person has _Trisomy 13__

G. How does this happen to a person? The answer lies in understanding the process of meiosis.

III. There are 2 types of cell division:

1. MitosisA. Most body cells undergo mitosisB. The goal is to produce 2 daughter cells that are

identical to the parent cell with respect to the number and type of chromosomes

C. Diagram

2. MeiosisA. Egg and sperm cells are produced by the process of

meiosis. Therefore the only cells that undergo meiosis are primary egg cells and primary sperm cells.

B. The goal of meiosis is to produce egg and sperm cells, which are necessary for the process of sexual reproduction. These sperm and egg cells should only contain half the amount of chromosomes (n) as the parent cell. For humans, this means that sperm and egg cells only contain 23 chromosomes: 1 #1 chromosome, 1 #2 chromosome, etc. and either an X or a Y sex chromosome.

1. Picture:

C. There are 2 types of meiosis

1. SpermatogenesisA. Occurs as a primary sperm cell undergoes meiosis

to produce 4 regular sperm cells. This process takes about 2 months.

B. Primary sperm cells are found in the testes of males, more specifically in the seminiferous tubules.

C. Once a man reaches puberty, many primary sperm cells undergo spermatogenesis on a daily basis, producing over 5,000,000 sperm cells a day, which are then stored in the epididymis.

D. Picture:

E. The goal of spermatogenesis is for each primary sperm cell to produce 4 sperm cells. Keep in mind that human sperm cells are not identical to each other, but they each have 23 chromosomes. One of these chromosomes is either an X or a Y. A sperm cell is about .00024” long—much smaller than the egg.

2. OogenesisA. Occurs once a month as an immature egg cell

undergoes meiosis to produce 1 regular egg cell and 3 polar bodies.

B. Immature egg cells are found in the ovaries of females.

C. A female is born with all the egg cells she will ever have (about 5 million)—only about 500 of these egg cells will ever mature and be released into the fallopian tube during a woman’s life.

D. The goal of oogenesis is to produce one egg cell that could be fertilized in the fallopian tube. Keep in mind that human egg cells from the same person are not identical to each other, but they each have 23 chromosomes. One of these is always an X and never a Y.

D. The numbers that we use to represent chromosomes in the our diagrams do not mean that the same thing in the mitosis drawing—check out the Oogenesis/Spermatogenesis lab!

III. Genetic Variation

A. You have completed the meiosis lab. At this point, you have either produced some egg cells or some sperm cells. If these were human egg and sperm cells, they would each have 23 chromosomes. Although we are pretending that these egg and sperm cells are human cells, they each only have 4 chromosomes.

B. You should have noticed that all the egg cells are not identical to each other.

C. You should have noticed that all the sperm cells are not identical to each other.

D. You may have some cells that are identical to each other with respect to the chromosomes—this is a limitation to the lab—it does not show the process of crossing over.

E. Crossing Over

1. Occurs during Prophase I of meiosis.2. Phases of meiosis:

3. Terminology

A. Homologous chromosomes a pair of chromosomes in the same individual that carry the same type of information (e.g. eye color) but not necessarily the same alleles (e.g. blue or brown)—I from mom, one from dad.

B. Homologous chromosomes after they replicate themselves in the S phase of interphase (see drawing). Each chromosome attaches to its duplicate with a centromere.

Drawing:

C. Next, the cell will enter into Prophase I. During this time, these four chromosomes attach tightly and closely to each other. It is during this time that crossing over can occur, producing many, many different types of chromosome/gene combinations in egg and sperm cells.

It is estimated that each tetrad undergoes about 3 or 4 crossovers.

We could not do this with our lab, so our babies will look a bit more like each other than normal siblings.

bbb

Time for the Meiosis Lab—all right!