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6th Grade Science / Lesson 2 - Sexual Reproduction

Sexual Reproduction
What will we be learning today?

  • In this lesson, we are going to learn all about sexual reproduction.

Do Chromosome Numbers Change?

  • Humans, like most many-celled organisms, reproduce from two parents. Reproduction from two parents is called sexual reproduction. How does it work? Each parent produces a sex cell. The male sex cell is a sperm. The female sex cell is an egg. During sexual reproduction these two cells join into one cell.

Humans have 46 chromosomes in each body cell. If a sperm and an egg each had 46 chromosomes, what would happen when they joined? The resulting cell would have 46 x 2, or 92 chromosomes, twice as many chromosomes as it should

This does not happen, because sex cells are developed through a special kind of cell division. In a process called meiosis, the nucleus of a sex cell divides twice. As a result the final, or mature, sex cells have only half as many chromosomes as the other cells of an organism; in this case-23.

How Do the Numbers Change Back?

  • Just before they join, a human sperm and an egg each have only 23 chromosomes. When they join they become one cell with 46 chromosomes. The cell has received half the number of chromosomes from each parent cell. The joining of an egg and a sperm is called fertilization. The cell that forms is called a fertilized egg, or zygote.

  • The zygote begins to divide by mitosis. Each new cell continues to have 46 chromosomes. The zygote develops and grows into a new organism.

  • Humans have 46 chromosomes in each of their body cells. Other animals and plants have different chromosome numbers. But mitosis and meiosis work much the same way no matter how many chromosomes there are in a cell. Together these processes make sure that each cell of the new organism has the same number of chromosomes as in each parent. Half the number of chromosomes comes from each parent.

Let's Review What We Learned Today!

  • Now complete your worksheet!