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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
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
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
• 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
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