3 The Chromosomal Basis of Heredity

2

3 Chromosome Structure Eukaryotic chromosome contains a single DNA molecule of enormous length in a highly coiled stable complexes of DNA and protein called chromatin The basic structural unit of chromatin is the nucleosome, a core particle of histone proteins that the DNA wraps around in ~200bp segments Each nucleosome particle consists of an octamere of pairs each of four histone proteins H2A, H2B, H3, and H4; a fifth histone protein, H1, binds the core particle to the linker DNA

4 (gene rich regions) (poorly staining regions) (gene poor regions) (heavily staining regions) Chromosome Structure

5 centromeres The centromere is a specific region of the eukaryotic chromosome where the kinetochore (the complex of DNA and proteins to which the spindle fibers) attach and pull the chromosomes during both mitosis and meiosis

6 telomeres G-rich repetitive sequences at the ends of the chromosomes Require special mechanisms to replicate Telomere length shortens with age Telomerase function is associated with cancers and aging

7 Chromosomes The chromosome complement = the complete set of chromosomes of plants and animals The nucleus of each somatic cell contains a fixed number of chromosomes typical of the particular species The number of chromosomes vary tremendously among species and have little relationship to the complexity of the organism

8 Ploidy The chromosomes in the nuclei of somatic cells are usually present in pairs, Diploid. Humans have 23 pairs of chromosomes The germ cells, or gametes, are Haploid and contain only one set of chromosomes The haploid gametes unite in fertilization to produce the diploid state of somatic cell. Thus each pair has one chromosome derived from the maternal parent and the other from the paternal parent

9 Cell Cycle

10 Mitosis Prophase chromosomes condense. Each chromosome is already doubled (each is called a chromatid for some reason) and held together at a specific region of the chromosome called the centromere. Metaphase chromosomes line up at the center of the cell. mitotic spindle (a bunch of microtubules) microtubules attaches the kinetochores to the centrosomes Anaphase the two sister chromatids move toward opposite poles (each sister chromatid is called a chromosome again). Telophase A nuclear envelope re-forms around each compact group of chromosomes, the chromosomes undergo decondense, and the cell divides in two

11 In animals, meiosis takes place in specific cells called meiocytes The oocytes form egg cells and the spermatocytes form sperm cells In the females of both animals and plants, only one of the four products develops into a functional cell (the other three disintegrate) Meiosis

12 Fig. 3.5

13 Meiosis Prophase I chromosomes condense (leptotene), homologous pairs of chromosomes synapse (zygotene) and crossovers or exchanges occur between nonsister chromatids (pachytene). The crossovers, called chiasmata, become visible as the chromosomes separate a bit (diplotene), and condense a bit more (diakinesis). Metaphase I chromosome pairs line up at the center of the cell. Oriented randomly. Anaphase I the two bivalent chromosome pairs move toward opposite poles Telophase I the chromosomes usually only partially decondense, and the second division begins Prophase II chromosomes condense Metaphase II chromosomes line up at the center of the cell Anaphase II the the two sister chromatids move to opposite poles Telophase II A nuclear envelope re-forms the chromosomes decondense and cell division occurs

14 Meiosis

15 2 Ways of Generating Variation (Recombinant Progeny) Crossing over (molecular recombination) Random assortment of different molecules

16