Cell Division (2) Meiosis: two-stage cell division in which the chromosome number of the parent cell is reduced by half. Meiosis is the process by which gametes are formed. Gametes: sex cells that have a haploid (n) chromosome number

Meiosis Terminology Haploid: refers to the number of chromosomes in a gamete (n). A gamete is a sex cell. Diploid: refers to twice the number of chromosomes in a gamete (2n). Every cell of the body, with the exception of sex cells, contains a diploid chromosome number. In humans: Haploid Chromosome # = 23 Diploid Chromosome # = 46

Offspring carry genetic information from each of the parents Each of the 23 chromosomes you receive from your father is matched by 23 chromosomes from your mother. Meiosis Homologous chromosomes: paired chromosomes similar in shape, size, gene arrangement, and gene information

Meiosis Terminology During fertilization, a haploid (n = 23) sperm cell unites with a haploid (n = 23) egg cell to produce a diploid (2n = 46) zygote. Zygote: a cell resulting from the union of a male and female sex cell, until it divides and then is called an embryo

Mitosis vs Meiosis

Meiosis

Interphase  DNA replication occurs before meiosis

Stages of Meiosis Overall: 2 nuclear divisions that produce 4 haploid cells Meiosis I  Reduction Division – diploid reduced to haploid Prophase I Metaphase I Anaphase I Telophase I Meiosis II  Separation of two chromatids Prophase II Metaphase II Anaphase II Telophase II

Meiosis I

Meiosis II

Meiosis I: Prophase I Homologous chromosomes come together Each pair is made up of 4 chromatids Tetrad: a pair of homologous chromosomes, each with two chromatids Synapsis: the pairing of homologous chromosomes

Meiosis I: Prophase I Crossing over: the exchange of genetic material between two homologous chromosomes  recombination shuffles the allele content between homologous chromosomes (increasing genetic diversity)

Meiosis I: Metaphase I Attached spindle fibers line up the homologous pairs at the equatorial plate

Meiosis I: Anaphase I & Telophase I Homologous chromosomes are moved to opposite ends One member of each homologous pair will be found in each daughter cell

Meiosis II Pairs of chromatids separate and move to opposite poles  There is NO replication of chromosomes before meiosis II  each cell has half the # of chromosomes as the parent cell

Meiosis II: Prophase II The nuclear membrane dissolves, the centrioles move to opposite poles, and the spindle fibres begin to form

Meiosis II: Metaphase II The chromosomes attach themselves to the spindle fibres and line up at the equatorial plate

Meiosis II: Anaphase II The sister chromatids are separated and move to opposite poles The nuclear membrane begins to re-form around the single chromosomes

Meiosis II: Telophase II The cytoplasm separates. The chromosome number has been reduced by half.

Cytokinesis Four haploid daughter cells are formed.

Worksheet 1 Worksheet 2

Formation of Sex Cells in Humans The formation of gametes in animals is called gametogenesis In humans, gametes are produced in the testes (males) and in the ovaries (female)

Spermatogenesis (Males) A primary spermatocyte (diploid sex cell) undergoes meiosis I and II and forms 4 spermatids (haploid), which will develop into sperm Males can produce one billion sperm cells every day!

Oogenesis (Females) A primary oocyte (diploid sex cell) undergoes meiosis I, but the division of the cytoplasm is unequal, resulting in the formation of a polar body The larger cell (ootid) and the polar body both go through meiosis II However, the polar bodies die and the nutrients are absorbed by the body Only one egg (ovum) is produced by meiosis Women have about oocytes, of which only 400 – 500 will be released during the reproductive years

Worksheet 3-Homework Worksheet 4

Sex Chromosomes sex chromosomes: the pair of chromosomes that have a role in the sex of an individual autosomes: the chromosomes not involved in sex determination Although they are physically different, the X and the Y chromosomes are able to synapse for part of their length and function as homologous chromosomes during meiosis.

Sex Determination in Humans In humans, xx is female, xy is male. Sex determination depends on the sex chromosome inherited from the father, who is xy.

Sex Determination in Non-Humans In birds, the mother determines the sex of the child. – WZ is female, ZZ is male In ants, bees, and wasps, sex is determined by the number of chromosome sets an individual has. – males are haploid and females are diploid In turtles, sex is determined by the environment, particularly the temperature at which the eggs are incubated. – HOT=female, COLD=male

Amniocentesis In Amniocentesis, fetal cells are obtained from 2 tablespoons of amniotic fluid A 3 to 4 inch needle is pushed through the abdomen and into the amniotic fluid. Do 3 to 4 months after pregnancy. This procedure is done without an anaesthetic. The cells are then cultured and a karyotype or other testing can be done. This test can be used to detect genetic disorders such as downs syndrome (trisomy 21).

Chorionic Villus Sampling (CVS) CVS- cells are obtained from the chorion of the placenta (tissue from the placenta) using a syringe. A needle is inserted through the abdomen or through the vagina and cervix, using the ultrasound for guidance. Do after 10 or 12 weeks after pregnancy. The cells are then cultured and a karyotype or other testing can be done. This test can be used to detect genetic disorders such as downs syndrome (trisomy 21).

Karyotype Chart karyotype chart: a picture of chromosomes arranged in homologous pairs monosomy: the condition where there is a single chromosome in place of a homologous pair trisomy: the condition where there are three homologous chromosomes in place of a homologous pair

Abnormal Meiosis: Nondisjunction Nondisjunction occurs when two homologous chromosomes move to the same pole during meiosis. The result is that one of the daughter cells will be missing one chromosome while the other will retain an extra chromosome. -can occur in any cell during mitosis BUT effects are most devastating during the formation of sex cells in meiosis. Can cause Down Syndrome. Nondisjunction means “doesn’t break apart”.

Down Syndrome Down syndrome: a trisomic disorder in which a zygote receives three homologous chromosomes for chromosome pair number 21 -regardless of race: a round, full face; enlarged and creased tongue; short height; and a large forehead. -1 in 1000 babies is born with Down syndrome

Down Syndrome increased risk as mother gets older

Turner Syndrome (X) Turner syndrome: a monosomic disorder in which a female has a single X chromosome -the individual has 45 chromosomes -The only condition where humans can survive without one of a pair of chromosomes. -1 in every 5000 female babies -In 98% of fertilized eggs, the egg is spontaneously aborted.

Turner Syndrome short (average is 4 feet and 8 inches) web of skin between neck and shoulders sterile breasts and other secondary sex characteristics develop incompletely normal intelligence but individuals do have learning difficulties

Klinefelter Syndrome (XXY) Klinefelter syndrome: a trisomic disorder in which a male carries an XXY condition -child appears to be a male at birth -however, as he enters sexual maturity, he begins producing high levels of female sex hormones. Males with Klinefelter syndrome are sterile. -1 in 1000 males have genotype XXY

Klinefelter Syndrome (XXY) Small testes Female features including breast development Long limbs. Slightly taller than average. (about 6 feet) Language impairments, speak later. Might have more X chromosomes: XXXY or XXXXXY. These individuals have mental retardation.

Super Males (XYY) occurs in 1 in 1000 males very tall (about 6 feet 2 inches) moderate to severe acne average to slightly lower than average intelligence most men who are XYY don’t even know it!

Metafemales (XXX) occurs in 1 in 1000 females sterile no other physical or mental problems

Review

Worksheet 5

Karyotype Lab Part 1 Hand in your lab at the end of class.