Question

can someone answer 14-17

Genome structure (4) 14. (4 pts) Humans have about 20,000 protein genes encoded in a genome of about 3 billion bp. Rice has about 37,000 protein genes in 300 million bp. How might you explain this difference in gene density? Nuclear organization (14) 15. (6 pts) Describe the structure of a nucleosome. 16. (4 pts) How does chromatin structure affect gene expression? 17. (4 pts) Give two examples of specific chemical modifications which control chromatin structure.

Answer 1

14. Humans have 3 billion base pair long DNA whereas the rice genome is only 300 million base pair long but rice genome has 37000 protein genes compared to 20000 protein genes in humans. This means that the rice genome has far less non-coding regions composed of various repetitive sequences and transposable elements whereas 98.5% of the human genome is non-coding in nature made up of repetitive sequences and transposons.

15. Nucleosome structure: The nucleosome core particle contains 2 copies of each core histone protein H2A, H2B, H3 AND H4 known as the histone octamer around which 146 base pairs of super-helical DNA is wrapped 1.65 times in left handed orientation. The two nucleosomes are connected by the H1 linker histone. The nucleosome core along with around 165bp of DNA and linker histone is known as the chromatosome. The nucleosome causes first level of compaction of genome by organizing 200 bp of DNA and this can self assemble into higher order structures of chromatin which allows for genome compaction.

16. There are majorly two types of chromatic structure which affect gene expression: euchromatin and heterochromatin. The heterochromatin is tightly packed containing heavily methylated histones. They show very less transcriptional activity, late replicative and compactly coiled. The euchromatin is the actively transcribing region of the human genome. It is made up of loosely packed form of DNA found in the inner body of nucleus, loosely coiled and early replicative. They contain less methylated and highly acetylated histones.

17.  Example 1: H3K4me3: the trimethylation at the 4^th lysine residue of the core histone 3 is the hallmark of the transcriptionally active region of the human DNA.

Example 2: H3K9me3: the trimethylation at the 9^th lysine residue of core histone 3 is a hallmark of transcriptionally repressed region of the human DNA.