CHROMOSOMES DNA deoxyribonucleic acid is a long thin

CHROMOSOMES • DNA (deoxyribonucleic acid) is a long, thin molecule that directs cellular functions and heredity. • DNA contains information that is encoded in segments called genes. • A single molecule of DNA contains thousands of genes.

Chromosome Structure • A chromosome forms when a single DNA molecule, and its associated proteins (histones), coil tightly prior to cell division. • DNA only coils in the form of chromosomes when the cell is preparing for cell division

By the time chromosomes coil and become visible, the DNA has been copied, and each chromosome consists of two copies ensuring that each new cell will have the same genetic information. • The identical strands of DNA are each called chromatids and are held together by a centromere.

• Body cells (somatic cells) contain two copies of each chromosome, referred to as homologous chromosomes. • Homologous chromosomes are similar in size, shape and have the same type of genetic information (code for the same characteristics). • One homologue originates from the female parent and the other from the male parent.

• Human cells have 23 different PAIRS of homologous chromosomes. – 1 pair is the sex chromosomes(determine sex and related characteristics) • In humans and many other species, females are designated XX and males XY. are designated – 22 pairs are autosomes (determine other traits)

• The number of chromosomes in cells is constant within a species. • Diploid cells (2 n)contain two homologues of each chromosome. –Somatic cells • Haploid cells (n)contain one homologue of each chromosome. –Sex cells –gametes(egg and sperm) • For humans 2 n = 46. (∴n = ? ) For fruit flies 2 n = 8. (n = 4)

The haploidg ametes are fused in fertilization which restores the diploid number in the zygote(the fertilized egg).

A karyotype is a pictoral spread of an individual’s chromosomes. • It can be used to detect some genetic abnormalities (Down’s Syndrome)

MEIOSIS • Meiosis is a form of cell division that halves the number of chromosomes in cells. • Meiosis takes place in order to make gametes • Four haploid gametes are produced from one diploid cell. • Meiosis consists of two successive nuclear divisions.

Interphase • Replication of DNA must occur before cell division

Prophase I • Chromosomes condense • Nuclear membrane disappears • Spindle fibers begin to form • Homologous chromosomes pair

Prophase I cont. • Homologous chromosomes pair to form a tetrad • Crossing over results in genetic variation

CROSSING OVER • Crossing over(exchange of genetic material between homologues) occurs during Prophase I. It produces genetic recombination.

Metaphase I • Homologous chromosomes line up along the equator of the cell

Anaphase I • Homologous chromosomes separate move to opposite poles

Telophase I • Two cells are formed • Each has ½ the genetic information of the parent cell

Overview of Meiosis I

Prophase II • Condensed chromosomes attach to spindle fibers • Nuclear membrane disappears

Metaphase II • Chromosomes line up along equator of cell

Anaphase II • Sister chromatids are pulled apart

Telophase II • Each cell forms 2 new (1 n) cells

Meiosis II • Meiosis II is much like mitosis with chromatids separating • End result of meiosis is 4 cells with half the genetic material as the parent cell

Overview of Meiosis II

Comparing Meiosis & Mitosis Meiosis • Homologous chromosomes independent • Homologous chromosomes pair, forming homologues until anaphase I • Identical daughter cells • Daughter cells haploid Daughter cells have new assortment of parental chromosomes • Chromatids not identical; crossing over

Formation of Gametes In animals, meiosis produces haploid reproductive cells called gametes Only occurs within their reproductive organs In humans, meiosis occurs in the testes and in the ovaries Testes – production of male gametes known as sperm cells. Diploid reproductive cell divides meiotically to form four haploid cells called spermatids. Production of sperm cells is called spermatogenesis This process produces 4 viable sperm cells

Meiosis – mouse testes Parent cell 1 st division 2 nd division 4 gametes http: //www. djusd. k 12. ca. us/davishigh/amoriarty/Meiosis. htm

Formation of Gametes Ovaries- Oogenesis is production of mature egg cells Diploid reproductive cell divides meiotically to produce one mature egg cell During cytokinesis I and II of oogenesis, cytoplasm of the original cell is divided unequally between new cells One egg cell is produced by meiosis. Other three products of meiosis called polar bodies, degenerate (break down)

Mistakes in Meiosis • Mistakes in meiosis cause genetic abnormalities • One such genetic abnormality is Down syndrome or Trisonomy 21

Meiosis – division error Chromosome pair

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilised then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?