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_4/40/10469/2680298.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_4/40/10469/2680298.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

SyndromesDown’s Syndrome = trisomy of the 21st pair• Effect: physical and mental retardation, heart problems, and

sterility• Exponentially increases in relation to age of the motherTurner 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