n Homologous chromosomes 2 chromosomes with the same

n Homologous chromosomes: 2 chromosomes with the same genetic information q q The same genes, but different forms (alleles) of those genes 1 chromosome comes from the father, the other from the mother Tetrad: a pair of homologous chromosomes n

Meiosis n Where does meiosis take place? q q n In the reproductive organs (testes, ovaries) Only carried out by the sex cells to produce gametes Gametes: ovum (egg) and sperm cells q q Each contains 22 autosomes and 1 sex chromosome (X or Y) In humans: gametes have a haploid number n=23, somatic cells have a diploid number 2 n=46

n Cells which are produced by meiosis are haploid – they only have half of the genetic material that is found in a somatic cell q q q They have 1 copy of each chromosome – the number of chromosomes is reduced during the first stage of meiosis n = # of chromosomes that are genetically different The gametes do not have homologous chromosomes

n There are 2 stages of meiosis q q n Meiosis II Over the course of these 2 stages, the cell will divide 2 times

Genetic Diversity n Produced by 2 different mechanisms: q q n n Crossing over between homologous chromosomes Independent assortment Reorganization of genetic information, AKA genetic recombination Diversity is important for a species because certain combinations of genes can better ensure the survival of individual organisms

Prophase I n n Homologous chromosomes synapse (pair up) to form tetrads (2 chromosomes, 4 chromatids) Crossing over may occur q q An exchange of genes between the pairs Chromatids overlap, break & exchange genetic information

Crossing Over

Metaphase I n The tetrads line up on the equatorial plate q n Spindle fibre attached to the centromere of each chromosome Independent assortment happens here q q q No specific order in which the tetrads line up DNA from the father could go to either pole, same for the DNA from the mother Results in a random distribution of genetic information from the parents

Independent Assortment Mother XX Father x xx Mother Father X XX

Anaphase I n n The tetrads are separated & each chromosome is pulled towards a different pole Sister chromatids stay attached – there is no separation at the centromere

Telophase I n n Cytoplasmic division – like in mitosis Some cells: cytokinesis does not fully occur & the cells pass straight to the 2 nd divsion Each new cell now has only 1 copy of each chromosome – haploid The 2 daughter cells are not identical to each other or to the parent cell

Meiosis II n n n The stages of meiosis II are identical to the stages of mitosis The cells at the end of the 2 nd division are haploid New cells are now called “gametes” or “spores” (plant cells)

Gametogenesis n n Formation of sex cells (gametes) during meiosis Male & female gametes both undergo meiosis, but there are some differences Spermatogenesis – formation of sperm cells Oogenesis – formation of egg cells

Spermatogenesis n n n Occurs in the testes Males produce 1 billion sperm cells each day At the end of meiosis II: q q Cytoplasm is divided equally between the 4 daughter cells Cells become specialized – streamlined for movement, develop flagella

Oogenesis n n Occurs in the ovaries Females begin with 2 million oocytes (potential to develop in to egg cells) q q q ~300 000 remaining by time of sexual maturity ~400 – 500 actually go through meiosis (from ~ 12 years to ~ 50 years) Meiosis occurs once each month to produce an egg cell n if the cell is not fertilized, the egg and other tissues that would be used for pregnancy are released from the body during menstruation

n Each oocyte goes through meiosis to produce 4 daughter cells q q q Due to an unequal division of cytoplasm, only 1 becomes an ootid Produces 3 polar bodies which are reabsorbed by the body Egg cells need cytoplasm, organelles & nutrients for cell division in case of fertilization

Sex Chromosomes n n Females have 22 pairs of homologous autosomes & 1 pair of double X sex chromosomes One X chromosome came from her mother, one came from her father

n n n Males have 22 pairs of homologous autosomes, 1 X sex chromosome and 1 Y sex chromosome Although not the same shape, the XY pair acts as homologous during meiosis The X chromosome came from his mother, the Y chromosome came from his father

Karyotype n n n A karyotype is a picture of an organism’s chromosomes, laid out so that the homologous pairs are side by side Ordered by size Sex chromosomes are always last

Errors n n During meiosis – errors are possible Mutation – error in the DNA q n Can be transmitted from one generation to the next when 2 gametes come together to form a zygote Cells with too much or too little genetic information will not function correctly

Nondisjunction n n Nondisjunction – incorrect separation of chromosomes during Anaphase I or Anaphase II If it happens during. . . q q Anaphase I: results in 4 abnormal cells Anaphase II: results in 2 normal cells, 2 abnormal cells

n n If a gamete with an extra chromosome is fertilized by a normal gamete. . . Trisomy – one chromosome too many

Trisomy – the number refers to which homologous pair has an extra chromosome n Trisomy 21 – Down Syndrome q n Trisomy 18 – Edwards Syndrome q n Small stature, large, shortened head, round face, mental deficiency Numerous physical and mental anomalies Trisomy 13 – Patau Syndrom q Numerous physical anomalies

n More commonly: nondisjunction involves the sex chromosomes

The union of abnormal eggs with normal sperm The union of abnormal sperm with normal eggs

Sex Chromosome Trisomy n n XXY – Klinefelter Syndrome q Male with the presence of feminine secondary sex characteristics XXX – Triple X Syndrome q Normal female, not often diagnosed XO – Turner Syndrome q Female, short, thick body, short neck, non-functioning ovaries Petite stature, corps massif, court, ovaires non fonctionnels XYY Syndrome q Male, taller than average height

n If the sister chromatids do not separate during Anaphase II, the gamete will be diploid q Diploid + haploid = triploid (3 n) n q 3 sets of chromosomes More than 2 sets of chromosomes - polyploid