MeiosisThe Production of Sex Cells

How many chromosomes does a human body cell have?

46

How many chromosomes does a human sex cell have?

23 each

The sex cell of females is called?

• Egg

The sex cell of males is called?

• Sperm

• Cell size: http://learn.genetics.utah.edu/content/begin/cells/scale

Why is it important that sex cells have half the usual number of

chromosomes?

• So that the fertilized egg receives a normal number of chromosomes

During what process are sex cells made?

• Meiosis– Produces sex cells with half the number of

chromosomes.

1

Prophase 2

7)How many steps? (Not counting cytokinesis)

9

8) How many chromosomes are in a human cell at the start of

meiosis?• 46

9) How many times do the chromosomes

make copies of themselves during

meiosis?

• once

1

Prophase 2

10) How many chromosomes are in the human cell after

interphase?

• 92

11) How many divisions does meiosis have?

1

Prophase 2

2

12) How many cells do you start with in meiosis?

How many cells do you end with?

1

Prophase 2

1

4

13) How many chromosomes are

present in each human cell at the end of meiosis?

1

Prophase 2

23

14) Is each cell produced at the end

of meiosis the same?Why?

1

Prophase 2

No

To allow for genetic variations

Steps of Meiosis1. Interphase

- each chromosome copies itself

Steps of Meiosis2. Prophase I

- Homologous chromosomes (chromosomes that are the same) line up together

Steps of Meiosis

- Homologous chromosomes line up in the middle and the spindle attaches to the centromere

3. Metaphase I

Steps of Meiosis

- The homologous chromosomes move to opposite ends of the cell

- The centromere does NOT split

4. Anaphase I

Steps of Meiosis5. Telophase I

- Each cell now has a copy of each homologous chromosome

- Cytokinesis occurs making 2 new cells

Steps of Meiosis• Between Telophase 1 and Prophase 2:

The cell now rests, no duplication of chromosomes occurs

Steps of Meiosis

6. Prophase II

Steps of Meiosis

7) Metaphase II

Steps of Meiosis8. Anaphase II

The centromere splits and sister chromatids separate

Steps of Meiosis9. Telophase II

Result

- 4 new daughter cells that are all genetically different

- each new cell has half the number of chromosomes as the original cell

Spermatogenesis• Males produce 4 sperm each round of meiosis

Oogenesis• Females produce 1 egg (ovum) and 3 polar

bodies each round of meiosis

Fertilization• Fertilization will

restore the original # of chromosomes

• 23 from egg + 23 from sperm 46 total

chromosomes to make a human