B5 OCR 21st Century (Does not contain the plants bits!)

Mitosis and Meiosis:

Mitosis and Meiosis are two ways in which cells reproduce.

Mitosis: a cell splits to create two identical copies of the original cell.

Each new cell is identical to the parent cell, same genes, and same number of chromosomes. To enable Mitosis to take place, cells go through a cycle of growth and division. The cell cycle repeats itself until the cell can no longer divide.

Growth Phase of the Cycle:

·       The number of organelles increase

·       chromosomes are copied (The DNA molecule is made up of 2 strands, as they separate or unzip, new strands or nucleotides are made alongside each of them, making two new copies.)

Division Phase of the Cycle:

·       The copies of the chromosomes separate.

·       The cells divide.

Meiosis:cells split to form new cells with half the number of chromosomes, to produce gametes (sex cells eg egg and sperm) for sexual reproduction. This takes place in the testes and ovaries.

           The Process:

·       When there is a cell with 2 pair’s of chromosomes...

·       Each chromosome replicates itself

·       Chromosomes part pairs and move to opposite sides with their copies

·       Cell divides for the first time, making 2 cells.

·       Then the copies separate and move to opposite sides

·       The second cell division takes place

·       4 gametes are produced, each with half the number of chromosomes as the parent cell.

Fertilisation:

·         A sperm and egg (gametes) fuse to produce a single body cell called a Zygote.

·         Gametes have half the number of chromosomes as the parent cell, so the Zygote has one whole set of chromosomes.

·         The Zygote then divides by mitosis to produce a cluster of cells called an Embryo.

·         The Embryo continues to divide by mitosis (from one cell, to two, to four, to eight etc)

·         The cells…