Chromsome structure and DNA

How does DNA become supercoiled?

• When double stranded DNA is supercoiled one of two strands is cut/nicked

• This nick is then sealed with DNA ligase to lock in the supercoiling.

Eukaryotic DNA consists of

1. one large linear molecule of DNA

2. Large amounts of 5 histones (positive charged)

3. Large amounts of non-histone proteins

how does DNA and histones interact?

DNA is hihly negatively charged while histone proteins are highly positively charged.

1. DNA first level of packaging - Nucleosomes

• 1 and a bit turns around nucleosome

How many nucleotides does each nucleosome have looped around it once.

146

linker region

the double stranded DNA between each nucleosome (can be 8-114 nucleotide pairs long)

Nucleosome core components

• Core → consists of histones (8 proteins) 4 on top 4 on bottom.

• The only difference between the core and the complete nucleosome is the addition of the fifth histone (in green) its like glue that holds the Double strand DNA around the core

2 - DNA second level of packaging

squishing nuclosome toegyher 11nm→ 30nm

• faciliated by enzymes

3. DNA THIRD level of packaging

30 -→ 300 nm

• tether 30nm to green line → chrosmomsome scaffold made up of non histone proteins

4. DNA 4th level of packaging

300 → 250

250 coils into 1400 nm chromsome