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Module: BIOM - 1006

Lecturer: Dr Sherwood

Date: 15/11/16

Chromosome Structure
o

DNA is packaged into chromosomes
 This is achieved by the use of proteins to wind up the DNA
 Each human chromosome has 22-48 million nucleotide pairs
 End to end, this is 1
...
Whereas, during interphase, they
decondense and become ~20x longer
 During interphase, the chromosomes are hard to visualise because they are long and thin

o

Chromatin is DNA that is package with proteins, of which, there are two types:
 Non-histone proteins
 These include DNA polymerase, scaffold proteins and heterochromatin protein 1, among
other proteins
 Histone proteins
 Breaking open a chromatin fibre during the cells
interphase allows us to see the nucleosome core
particles (see right)
 They are observed as beads on a string

o

The nucleosome core particles are defined as being DNA plus the
histone octamer
 The stretches of DNA between the core particles are known as strands of linker DNA
 One linker strand is ~4-80 nucleotides long
 Each nucleosome is technically classified as one core particle and one linker strand

o

Once the nucleosome core particle has been released
from the rest of the DNA strand (using the enzyme
nuclease) you can disassociate the DNA from the
protein using a high salt concentration
 This leaves the histone octamer and the
147bp DNA strand that was wrapped around
the octamer (right)
 The DNA wrapped around the protein will always be 147 nucleotides long
 You can further split the histone octamer into the 8 proteins that comprise it (see below):
 2x H2A
 2x H2B
 2x H3
 2x H4
 The octamer proteins all possess many
negatively charged amino acids (such
as arginine and lysine)
 These tightly bind to the
negatively charged DNA
phosphate

o

The histone protein H1 helps pull the nucleosomes
together to bundle them and condense the
chromatin fibre



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