Eukaryotic Chromosome Structure

Nucleosome Structure of Chromosomes

Nucleosome - fundamental packaging structure of DNA that is found in all eukaryotic chromosomes; DNA is wrapped around an octamer of small basic proteins called histones; 146 bp is wrapped around the core and the remaining bases link to the next nucleosome; this structure causes negative supercoiling

Core histones  in Nucleosomes

• H2A, H2B, H3 and H4
• form histone octamer (two of each protein)
• these basic proteins have an affinity for DNA
• the most abundant proteins associated with DNA
• the amino acid sequence of these four histones is conserved suggesting a similar function for all

Core DNA in Nucleosomes

• DNA that is associated with the core histones
• this length is invariant and is always 146 base pairs
• the core DNA forms two loops around the octamer
• this permits two regions that are 80 bp apart to be brought into close proximity; this may allow coordination between two distal sequences for gene regulation.

Linker DNA in Nucleosomes

• Linker DNA is the DNA that is between each histone octamer
• Can vary in length from 8 to 114 base pairs
• Variation is species specific
• Variation in linker DNA length has also been associated with the developmental stage of the organism or specific regions of the genome.