Meiosis: Making Sex Cells with Half the Chromosomes

0:00Sex cells, or gametes, have half the usual number of chromosomes. How are they made?

0:07Chromosomes are long strands of DNA that carry genes. This body cell has four.

0:14They come in pairs: in each pair, one came from the mother and one from the father.

0:20Most human body cells have 46 chromosomes, in 23 pairs. Our cell has just 2 pairs.

0:28Meiosis is the cell division that makes sex cells. The cell divides twice.

0:34First, each chromosome copies itself. The two copies stay joined.

0:40Next, the two chromosomes of each pair line up side by side.

0:45They swap matching pieces of DNA. This is called crossing over.

0:50Then the pairs line up across the middle. Which way each pair faces is random.

0:57In the first division, the pairs separate into two new cells.

1:01In the second division, the two copies of each chromosome are pulled apart.

1:07That makes four new cells, each with one chromosome from every pair.

1:12Our cell had 4 chromosomes, and each new cell has 2: half the number.

1:18In humans, a body cell's 46 chromosomes become 23 in a sex cell.

1:25When a sperm and an egg join, 23 plus 23 makes 46 again.

1:31Crossing over and the random line-up of pairs make every sex cell different.

1:37Meiosis does not make identical cells. Mitosis does that, for growth and repair.

1:43Meiosis: one cell divides twice into four different cells, each with half the chromosomes.