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 / cw/content/index.htmlhttp://wps.aw.com/bc_martini_eap_4/40/1 0469/ 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 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 / cw/content/index.htmlhttp://wps.aw.com/bc_martini_eap_4/40/1 0469/ 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 21 st 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 using amniotic fluid that contains embryo cells Chromosome abnormalities are legal reasons for abortion in most western countries