Meiosis in Humans
Chapter 13
What you need to know!
• The role of meiosis and fertilization in
sexually reproducing organisms
• Meiotic abnormalities
Spermatogenesis
• Creation of sperm
• Diploid germ cells undergo meiosis I, II,
and maturation
• Onset/duration: puberty/death
• Meiosis I to maturity: 70 days
• Location: seminiferous tubules in testes
• Quantity: 100 million per day
Sperm Development
• During meiosis cells migrate from outside
the seminiferous tubule wall to the inside
• The seminiferous tubule leads to the
epididymis
• The epididymis is a 6m coiled duct where
sperm mature
Sperm Structure
• Acrosomes are digestive
enzymes that dissolve the
protective layer around the
egg
• The nucleus is haploid
• Mitochondria in the midpiece
power the sperm on it’s 2
day journey
Ejaculation
• Delivers 30ml of semen containing
approximately 100 million sperm
• Many sperm die in the acidic environment
of the vaginal canal
– Semen contains antacids to help some
survive
• Many sperm have defects
• Age, stress, and environment contribute to
sperm count
Spermatogenesis Animation
• http://wps.aw.com/bc_martini_eap_4/40/10
469/2680298.cw/content/index.html
Oogenesis
• Creation of eggs
• A Female embryo produces approximately 450
primary oocytes
– Primary oocytes are germ cells hibernating in
Prophase I
• Onset: puberty
– One primary oocyte per month completes M1
• Creation of a secondary oocyte and a polar body
– M2 begins upon the secondary oocyte’s release from
the ovary
– M2 is not complete until fertilization
Oogenesis
• Location: Ovaries and Fallopian Tubes
• Oocytes do an unequal division of the
cytoplasm during cytokinesis
– One ovum gets the maximum amount of
nutrition and organelles
• Yields 1 ovum and 2 polar bodies
Menopause
• Women run out of primary oocytes
between 40-50 years of age
• Coincides with a drop in sex hormones
• Leads to the stop of menstruation by the
mid 50’s (on average)
Oogenesis Animation
• http://wps.aw.com/bc_martini_eap_4/40/10
469/2680298.cw/content/index.html
Stages of Meiosis in Humans
Non-Disjunction
• Failure to separate homologous
chromosomes during anaphase 1 or sister
chromatids during anaphase 2
• Causes include: cleavage furrows early,
kinetochore spindle fibers shorten to
unequal lengths
• Leads to some sex cells with missing
chromosomes and other sex cells with too
many of a chromosome
Aneuplody
• Abnormal sex cells undergo fertilization and
there is an abnormal/unequal number of
chromosomes
• Most are lethal (typically miscarry)
• Survivors usually have aneuplodies of the sex
cells
– Survival only requires one X chromosome
Examples:
• Trisomy – organism has three copies of one
chromosome
• Monosomy – organism has one copy of a
chromosome
Syndromes
Down’s Syndrome = trisomy of the 21st pair
• Effect: physical and mental retardation, heart problems, and
sterility
• Exponentially increases in relation to age of the mother
Turner Syndrome = monosomy of the sex chromsomes (X only)
• Effect: always female, short stature, webbed neck,
underdeveloped female characteristics, low sex hormone
levels, sterile (can be treated with estrogen)
Klinefelter Syndrome = trisomy or more of sex chromosomes
(XXY, XXXY)
• Effect: male, underdeveloped male characteristics, some
female characteristics, usually normal intelligence (probability of
retardation increases with the number of chromosomes)
• XXX (no syndrome) = sterile female
• XYY (no syndrome) = sterile male
Diagnosis
• Photomicrography: analysis of a picture of
an organism’s chromosomes taken during
prophase of mitosis (karyotype)
• Amniocentesis: karyotyping embryo
between week 12-16 using amniotic fluid
that contains embryo cells
• Chromosome abnormalities are legal
reasons for abortion in most western
countries