• Chromatin consists of DNA and proteins
• Chromatin is the material of which chromosomes are composed
• Chromatin is only visible by light microscopy during mitosis when it condenses
G1 – everything multiples except for chromosomes
S – chromosomes duplicates
G2 – checks if everything is duplicated
M – mitosis
The amount of compaction of DNA differs from one from another
Some parts are replicated earlier than others.
For ex. Some bubbles are larger than others in the other diagram
Even during interphase, you can see different chromosomes associated with different
Chromosome appear to be anchored by their centromeres.Adjacent interchromosomal
domains that are DNA scarce separate chromosomal territories.Active genes appear to be
near these boundaries perhaps because transcription components have easier access or
RNA diffusion is more rapid.
Chromatin – DNA + protein
Chromosome is made up of chromatin.
The proteins are diff. types. The main types are histone proteins. Other proteins are
nonhistone proteins. Just focus on histone proteins.
• Each chromosome is ~ half DNA and half protein
• About half of the proteins are histone proteins, small baasic proteins that tightly
• The remaining proteins, the nonhistone proteins, are very diverse and perform a
variety of tasks in the nucleus
• Histones are the major class of proteins associated with DNA
• There are 5 major types of histones in all eukaryotes H1, H2A, H2B, H3 and H4
• 20-30% of the amino acids in histones are lysine and arginine—DNA has a
• The remaining nonhistone proteins, are very diverse and perform a variety of
tasks in the nucleus
• Histones form circular structures called nucleosomes and DNA is wound around
them. 8 protein structure. 2 H2A, H2B, H3, and H4
• 2 H3/H4 dimers assemble to form a tetramer. Once this is formed, 2 H2A/H2B
dimers associate with the tetramer to form the octamer.
• H1 is not part of the core nucleosome H1 helps pack the histone octomers together.
Looks like beads on a string
Chromatin loops of 20 to 100 kb are anchored to the chromosome scaffold by nonhistone
proteins at sites called MARs (matrix attachment regions)
The radial loop-scaffold model suggests that the loops gather into “rosettes” and are
further compressed by nonhistone proteins
Metaphase chromatin is compacted 250-fold compared to the 300-nm fiber
The regions of DNA that has to be copied into RNA has to be more relaxed. The part of
has to decondensed so RNA poly can access that region. It is the region being actively
Chromosome compaction allows for efficient separation of chromosomes at anaphase.
The chromatin loops formed during cond