Meiosis & Sexual Reproduction

Chapter 7

Gametes are haploid cells – contain one set of chromosomes2 gametes join to form a zygote (first cell in a new organism)

Meiosis – Used to form sex cells (sperm and egg); Begins with one normal cell that has all the chromosomes of the human (46) and ends with four cells that have half the chromosomes (23)

Steps of Meiosis: 2 divisions of the nucleus

• Before Meiosis begins, the DNA in the original cell is replicated to start with homologous chromosomes (chromosomes that are similar in size, shape, and genetic content)

• Meiosis I– Prophase I– Metaphase I– Anaphase I– Telophase and Cytokinesis I

• Meiosis II– Prophase II– Metaphase II– Anaphase II– Telophase and Cytokinesis II

crossing-over = occurs when portions of a chromatid on one homologous chromosome are broken down & exchanged with corresponding chromatid portions of other homologous chromosomes

Prophase I• Chromosom

es become visible and homologues pair along their length

• Nuclear Envelope breaks down

• Crossing-over occurs

Metaphase I

• Pairs of homologous chromosomes move to the equator of the cell

Anaphase I

• Homologous chromosomes separate (move to the opposite poles of the cell)

Telophase and Cytokinesis I

• Chromosomes gather at the poles of the cell.

• The cytoplasm divides.

Prophase II

• A new spindle forms around the chromosomes

Metaphase II

• Chromosomes line up at the equator

Anaphase II

• Centromeres divide.

• Chromatids, now called chromosomes, move to opposite poles of the cell

Telophase and Cytokinesis II• A nuclear envelope

forms around each set of chromosomes.

• The cytoplasm divides.

• 4 haploid offspring cells result

• known as gametes

Where are these gametes formed in humans??

• Males: in the testes– In SPERMATOGENESIS: a diploid reproductive

cell divides meiotically to form FOUR haploid spermatids, each of which develop into a mature sperm cell

• Females: in the ovaries– In OOGENESIS: a diploid reproductive cell divides

meiotically to produce ONE mature egg cell (ovum)• Why not four?

– During cytokinesis I and II, the cytoplasm of the cell is divided unequally between new cells….one cell receives most of the cytoplasm and therefore it can become the egg. The other three products that will eventually degenerate are called polar bodies.

Where are these gametes formed in humans??

Reproduction- Process of producing offspring, sexual or asexual

asexual – single parent passes copies of all its genes to each of its offspring; creates clones (an organism is genetically identical to parent)

– allows organisms to produce many offspring in a short period of time without having to find a mate; little genetic variation which is a disadvantage in a changing environment

Sexual – 2 parents each form reproductive cells that have ½ the number of chromosomes (23); offspring has traits from both parents but not exact copies

– allows organism to make different combos of genes; genetic diversity leads to evolution