Meiosis and Sexual Reproduction

Sexual Reproduction

Diploid Cell A cell that has chromosomes located in similar pairs.

These cells are called body cells.– In humans, body cells have

23 pairs of chromosomes (46 total chromosomes).

Sexual Reproduction

Haploid Cell Chromosomes are not located in pairs.

Only have half of the number of chromosomes as a body cell.

These cells are called sex cells.– In humans, sex cells have

23 chromosomes.

Sexual Reproduction

Fertilization Occurs when a sperm cell joins with an egg cell to create a zygote.– The sperm and egg cells

are haploid cells.– The resulting zygote is

diploid.

Meiosis and Sex Cells

Meiosis Process that divides one diploid cell into four haploid cells.

Broken into Meiosis I and Meiosis II.

Meiosis and Sex Cells

Why is meiosis important?

Ensures that sex cells are only haploid so that when they are joined they form a diploid zygote.

Meiosis I

Prophase I Chromosomes copied before meiosis begins are visible as two chromatids.

Each chromatid pair links with its similar set of chromatids to form a tetrad.

Meiosis I

Prophase I

Meiosis I

Metaphase I Chromosome pairs line up in the center of the cell.

The centromere of each pair becomes attached to one spindle fiber.

Meiosis I

Metaphase I

Meiosis I

Anaphase I Each half of the tetrad moves to the opposite side of the cell.

Anaphase I

Meiosis I

Meiosis I

Telophase I Cytoplasm divides to form 2 new cells.

Meiosis I

Telophase I

Meiosis II

Prophase II Spindle fibers reappear in each of the new cells.

Meiosis II

Prophase II

Meiosis II

Metaphase II Duplicated chromosomes move to the center of the cell.

Each centromere is attached to two spindle fibers.

Meiosis II

Metaphase II

Meiosis II

Anaphase II Centromere divides and each chromatid moves to the opposite side of the cell.

Meiosis II

Anaphase II

Meiosis II

Telophase II Spindle fibers disappear.Nuclear membrane

reappears.Cytoplasm divides at the

end of meiosis.

Meiosis II

Telophase II

Summary of Meiosis

Meiosis - Briefly

Meiosis I – Starts with one diploid

cell– Forms two cells

Meiosis II– Each of the two cells

formed in meiosis I forms two cells

– End result is four haploid sex cells

Mistakes in Meiosis

Possible mistakes:

Haploid cells may end up with too few or too many chromosomes.– Zygotes formed from these

cells may die or have deformations.