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.

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