Polytene Chromosomes

1
Chapter 11

• Part 2

2
Polytene Chromosomes

• Found in salivary glands and other tissues of
  flies and species of protozoans
• Seen in the nucleus during interphase
• See linear series of alternating bands and
  interbands, distinctive banding for each
  chromosome in a given species

3
Polytene continued

• Chromomeres individual bands seen in chromosome
  are lateral condensations of material along the
  chromosome
• Polytene are believed to be
• paired homologs which are usually dispersed as
  chromatin in somatic cell and not as chromosomes
• large size and distinctiveness result from many
  strands of DNA that replicate but do not separate
  or do cells undergo cytoplasmic division
• 1000-5000 DNA strands in parallel alignment that
  gives to banding pattern

4
Puffs

• Bands are believed to be individual genes, uncoil
  to undergo transcription and creates a puff
• know it is transcription is because it
  incorporates a lot of radioactive nucleotides in
  this area
• Study of the bands reveal differential gene
  activity characteristic of pattern of
  activation during development
• not sure of the number of genes in each band
  based on size of band could be 50-100 average
  sized genes

5
Lampbrush Chromosomes

• Looks like a brush
• See in most vertebrate oocytes and spermatocytes
  of some insects meiotic chromosomes
• Isolated in diplotene stage of 1st prophase
  direct metabolic activities in developing cells
• they do not condense like the rest of chromosomes
  initially, later they coil into a normal
  chromosome

6
Lampbrush Structure

• Condensed areas in loops along a central axis
  chromomeres pair in lateral loops adjacent
  loops present along 1 of 2 axes of chromosome
• More DNA than to code a single gene
• Chromosomal loop 1 strand double helix while
  central axis is 2 double helices loops are
  active in RNA transcriptions

7
Eukaryotic DNA Organization

• Chromosome only visible during cell division
• During interphase it is referred to as chromatin,
  becomes replicated and cells undergo mitosis
  where chromatin condenses into chromosomes
• Human DNA is 2 m in length when lined up end to
  end needs to fit into a 5-10 ?m cell nucleus

8
Chromatin Structure

• DNA is associated with proteins in all phases of
  cell cycle
• Protein is a positively charged histones less
  positive-charged non-histones
• histones are rich in Lys and Arg residues
  creates and electostatic interaction wit negative
  DNA
• 5 main histone proteins used to coil up DNA

9
Observations

• Digest chromatin with micrococcal nuclease
  yielded DNA fragments 200 bp long, not random as
  then the fragments would be varying lengths
• protection from endonuclease attack
• repeating units
• EM shows chromatin to be linear arrays of
  spherical particles beads on a string
  nucleosomes

10
Observations continued

• Histone proteins occur in 2 types of tetramers -
  1) (H2A)2(H2B)2 and 2) (H3)2(H4)
• one of each tetramer making up an octamer with
  200 bp of DNA in association with it
• Longer nuclease digestion can remove DNA from
  nucleosome to yield a nucleosome core particle of
  147 bp
• DNA lost is linker DNA between the nucleosomes
  that interacts with the 5th histone protein H1

11
Model

• 147 bp coil around octomer in a left-handed super
  helix
• Coil into nucleosome reduce 2 m to 1/3 original
  length
• 11 nm chromatin further packed in to thicker 30
  nm fiber called a solenoid
• Loop into 300 nm chromatin fiber and then into
  arms of the chromosome