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