book 4 part 1 (organisation of genome)

Question 1

describe the organisation of DNA in eukaryotic genomes

Answer 1

1. DNA is wound around a histone octamer to form a nucleosome (1st level), whereby ionic bonds are formed between positively charged amino acids of histones and negatively charged DNA
2. 10nm chromatin fibre coils around itself and forms interactions between histone tails of neighbouring nucleosomes, linker DNA and the H1 histone to from 30nm chromatin fibre, solenoid.
3. there are 6 nucleosomes in one turn of the helix
4. solenoid then forms radial loops that are attached to a non-histone protein scaffold:
   - in interphase, the radial loops are attached to the inside of the nuclear lamina
   - in prophase, the radial loops are attached to the central protein scaffold to form an array of 300nm across
5. radial loops domains further coil and fold to compact and condense to form metaphase chromosome

Question 2

describe the structure and function of introns

Answer 2

intron is a non-coding DNA sequence found within genes.

• it allows the cell to make different mRNA and hence different proteins from the same gene via alternative splicing
• it also contains silencers or other regulatory sequences and hence play regulatory roles in gene expression

Question 3

describe the structure and function of centromeres

Answer 3

centromeres are non-coding sequences that are found at the constricted region in the metaphase chromosome where two sister chromatids are held together
- they are involved in adhesion of chromatids