3 The Chromosomal Basis of Heredity

1
3The Chromosomal Basis of Heredity
2
(No Transcript)
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
Chromosome Structure
(gene poor regions) (heavily staining regions)
(gene rich regions) (poorly staining regions)
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
Meiosis

• 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)

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)
Random assortment of different molecules
Crossing over (molecular recombination)
16
(No Transcript)