GAMETE FORMATION AND ERRORS IN MEIOSIS GAMETE FORMATION

GAMETE FORMATION AND ERRORS IN MEIOSIS

GAMETE FORMATION • Gametogenesis – process of creating sperm and eggs • Spermatogenesis – male gamete formation • Occurs in the testes • Immature spermatogonia (germ cells) may continually duplicate through mitosis or undergo meiosis to eventually develop into sperm cells • In some species this can occur year round while in other species this is limited to certain times in the year

SPERMATOGENESIS • A diploid germ cell (spermatogonium) undergoes meiosis to create 4 haploid cells • Following meiosis II, most cytoplasm is lost and a tail develops to allow locomotion sperm • The entire process, from spermatogonium to functional sperm, takes approximately 76 days • At any one time, different cells may be in a different stage of the development process this allows sperm cell production to stay steady at about 200 million sperm per day

GAMETE FORMATION • Oogenesis – female gamete formation • Occurs in the ovaries • Diploid germ cell (oogonium) undergoes meiosis to create 1 haploid cell • After meiosis I, cytoplasm does not split equally and the majority goes to the secondary oocyte. The other cell is called a polar body and it is not a viable sex cell. • After meiosis II, the cytoplasm is again unequally divided and only one cell is viable as a sex cell (egg or ovum)

OOGENESIS • When a female is born, primary oocytes have already entered meiosis I but remain suspended in prophase I until puberty • Starting at the first menstrual cycle, meiosis will resume one oocyte at a time, once a month • Production of ova continues from puberty until menopause (between 40 and 55) • Oocytes left in ovaries nonfunction/unresponsive and no longer released

ABNORMAL MEIOSIS • Nondisjunction – failure of chromosomes to separate properly • 2 homologous chromosomes move to the same pole during meiosis • One of the daughter cells will be missing 1 chromosome, while the other retains an extra chromosome

NONDISJUNCTION • In humans, nondisjunction produces gametes with 22 and 24 chromosomes • The gamete with 24 chromosomes has both chromosomes from one of the homologous pairs • If joins with normal gamete, 24 + 23 = 47 chromosomes in zygote rather than 46 • Once zygote begins dividing, each cell of the body will contain 47 chromosomes • Trisomy: there are 3 homologous chromosomes instead of a homologous pair • Monosomy: there is a single chromosome instead of a homologous pair

DOWN SYNDROME • A trisomic disorder in which a zygote receives 3 homologous chromosomes for chromosome pair number 21 • Common traits: round, full face, enlarged and creased tongue, short height, large forehead, mental retardation • Woman in 40 s has 1 in 40 chance of having a child with Down syndrome • 1/600 babies

• Turner syndrome: monosomic disorder - a female has a single X chromosome • Sex chromosomes undergo nondisjunction • Appear female but do not usually develop sexually, short and have thick, widened necks 1/3000 female babies • Klinefelter syndrome: trisomic disorder - male carries an XXY condition • Nondisjunction in sperm or egg • Appears to be male at birth, but begins producing high levels of female sex hormones once entering sexual maturity Males are sterile • 1/500 male babies