Gamete Formation

Genetics

Human Gamete Formation

• Gametes are the sperm and egg

• Both haploid (n), meaning they have only one of each type of chromosome

• Produced through a special cell division called meiosis

• Somatic cells are diploid (2n) with 46 chromosomes; gametes have 23

Flowering Plant Gamete Production

• Ovule: a compartment inside the ovary where female gametes are produced

• Anther: where male gametes (pollen grains) are produced

• Gametes are always haploid; the zygote is always diploid

Gamete Formation

Meiosis

• Two divisions: goes through prophase, metaphase, anaphase, and telophase twice

• In Meiosis I the pairs of chromosomes (and their copies) separate in anaphase

• In Meiosis II, the individual chromatids separate

Mitosis vs Meiosis

Gamete Formation

• Spermatogenesis begins with a germ cell called a spermatogonium

• Two divisions follow (meiosis I and II)

• End result is four haploid sperm

Spermatogenesis

• Sperm formation passes through these cell stages:

• Spermatogonium• Primary spermatocyte• Secondary

spermatocyte• Spermatid• Mature sperm

Oogenesis

• Oogenesis begins with a diploid cell called a oogonium

• Two divisions follow (meiosis I and II)

• Result is 3 polar bodies and 1 mature ovum (egg)

Oogenesis

Oogenesis

• Ovum formation passes through the following cells stages:

• Oogonium• Primary oocyte• Secondary oocyte• Ovum

Oogenesis

• At birth, a female has all the primary oocytes already formed

• Frozen in prophase I• After puberty, meiosis

continues with 1 or 2 oocytes each month

• Only complete meiosis if fertilized

4 Sperm; 1 Ovum

Variety: arrangement and crossing over

Fertilization

Twinning

• Dizygotic: two eggs and two sperm

• No more closely related than any siblings

• “Fraternal”

Twinning

• Monozygotic: one egg and sperm

• The embryo separates at an early stage and each continues normal development

• Genetically identical

Twinning

• Conjoined twins: a monozygotic pair that does not separate all the way

• May or may not share vital organs

• Most often, if separated, one lives and the other dies

Aging

• Genes control cell division and apoptosis, so aging is somewhat genetically regulated

• By about age 30, the body begins to decline

• Genes control aging both passively (structures break down, cells are not replaced) and actively (causing new activities)

Aging

• The disease progeria can give clues as to what causes aging

• In one type, the gene for helicase is altered. Helicase unzips the DNA for repair and replication. This enzyme is missing in progeria

Aging

Aging

Aging

• Environment influences aging as well as genes

• America’s oldest person is 113, and is the second oldest person in the world