Review
• Spermatogenesis, Oogenesis, Crossing
Over, Imprinting---A Review
Primary germ cells migrate from
the yolk sac to the ovaries or
testes where they undergo mitosis
and become oogonium or
spermatogonium or remain germ
cells.
Spermatogenesis
• Spermatogenesis, or sperm production,
begins around puberty and continues for
the remainder of a man's life
• A young healthy man produces several
hundred million sperm per day.
Spermatogenesis
Oogenesis
• During embryonic development, diploid cells in
the ovaries called oogonia divide by mitosis to
produce primary oocytes. The primary oocytes
are diploid.
• Primary oocytes start meiosis. They complete
interphase and prophase I. They are “frozen” at
the end of prophase I and remain this way until
the female reaches puberty.
• A female is born with about 2 million primary
oocytes. By the time she reaches puberty, about
400,000 are left.
Oogenesis cont….
• After puberty, each month one of the primary
oocytes is selected for ovulation (release from
the ovary). Just prior to ovulation, the primary
oocyte is “unfrozen” and completes meiosis I,
forms the first polar body, and is then frozen in
meiosis II at metaphase II.
• After ovulation, if a sperm penetrates the
secondary oocyte (egg) after it is released from
the ovary, the secondary oocyte will be
stimulated to complete meiosis II forming one
more polar body and a mature egg. The first
polar body formed from meiosis one may also
complete meiosis II to form another polar body.
Prophase I
Fertilization takes place in the
fallopian tubes
Crossing Over –Genetic Variation
Crossing over
Actual photograph of crossing over
No crossing over during
gametogenesis
Crossing Over
A
B
a
b
1. When does this
happen in the formation
of the egg and sperm?
2. Show the gametes
formed using the letters
“A” and “B” and “a” and
“b”.
Imprinting
• Imprinting
Evidence
o Uniparental embryos
o Uniparental disomy
o Differences in maternal and paternal gene
function
Epigenetics and Disease:
Genomic imprinting
Parent specific expression or repression of genes
or chromosomes in offspring.
So… even though two copies of a given gene are inherited,
one from each parent, only the maternal or paternal allele
is expressed.
The non-expressed allele is said to be “imprinted.”
Life Cycle of an Imprint
Wilms Tumor
-Childhood Tumor of the kidney (nephroblastoma)
Accounts of 7% of all childhood cancers
-Caused by a defect in imprinting of the Insulin-like
Growth Factor 2 (IGF2) gene
-IGF2 is usually only expressed from the paternal locus, ie
maternally imprinted
-Defects in imprinting that cause expression of the maternal
locus lead to cancer
Genomic imprinting and disease:
Several diseases are associated with genomic imprinting
e.g.
Beckwith–Wiedemann syndrome
Prader–Willi syndrome
Angelman syndrome
Wilms Tumor
Fragile X syndrome
Myotonic dystrophy (congenital)