Meiosis

Need For ReproductionReproduction 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 DivisionAll 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 DivisionCell 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.

MitosisAll 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.InterphaseProphaseMetaphaseAnaphaseTelophase

Mitosis

MeiosisThe 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

MeiosisOne-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 MeiosisNumber of daughter cells 2 4

Number of cell division 1 2

Daughter cells are diploid or haploid

Diploid Haploid

Daughter cells are identical or different

Identical Different

Parent cell is diploid or haploid

Diploid Diploid