Meiosis

Need For Reproduction Reproduction is the life process in which living things produce other living things of the same species. Reproduction is not necessary for the life of one individual organism. It is necessary for the continued survival of a particular group of organisms.

Cell Division All cells arise from other cells by cell division. When cells grow to a certain size they must either divide or die. If a cell continued to grow without dividing, the surface area of the cell would become too small to hold the cell’s contents.

Cell Division Cell division is a complex series of changes in the nucleus of a cell that leads to the production of two new cells. The new cells are called daughter cells. The nuclei of the daughter cells are usually identical to each other and to that of the parent cell. The daughter cells grow and increase in size until they divide and produce two more daughter cells. This process continues, resulting in organism growth and reproduction.

Mitosis All cells in the body (except sex cells) are produced by the process of mitotic cell division. Mitosis involves a complex series of changes in the nuclei of body cells that produce identical (same) daughter cells. They have the same number and type of chromosomes as the parent cells.

The Cell Cycle – Stages of Mitosis Although the events of mitosis are an ongoing process, they are generally described in terms of separate phases, or stages. Interphase Prophase Metaphase Anaphase Telophase

Meiosis The process of meiosis involves two cell divisions and produces cells that are different from the parent cell. Meiosis produces cells that have one-half the number of chromosomes as the parent cells. If meiosis did not take place, the fertilized egg would have double the amount of chromosomes it needs. Meiosis takes place during sexual reproduction when sex cells, called the egg and sperm, are produced in sex organs.

The Cell Cycle- Stages of Meiosis During meiosis, two divisions of the nucleus occur. These divisions are called meiosis I and meiosis II. Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

Meiosis One-half the number of chromosomes is called the monoploid, (or n) number. In humans, the monoploid number (n) is 23. When the egg and sperm unite during fertilization, the species normal chromosome number called the diploid or (2n) number is restored (brought back). The diploid number in humans is 46. n (sperm) + n (egg) = 2n

Summary of Meiosis Two cells form during meiosis I. In meiosis II, both of these cells form two cells. The result is four sex cells. Each sex cell has one-half the number of chromosomes in its nucleus that was in the original nucleus. From a human cell with 46 paired chromosomes, meiosis produces four sex cells each with 23 unpaired chromosomes.

Comparison of Mitosis and Meiosis Characteristic Mitosis Meiosis Number of daughter cells 2 4 Number of cell division 1 Daughter cells are diploid or haploid Diploid Haploid Daughter cells are identical or different Identical Different Parent cell is diploid or haploid