DLA 7 Packaging In Eukaryotes

Question 1

What are the functions of chromatin?

Answer 1

• Packaging of DNA to allow the genome to fit inside the eukaryote nucleus
• Maintain the structure of chromosomes during metaphase and protect it from damage
• Regulate gene expression by making regions of the genome accessible to transcription (uncondensed DNA) or repressing transcription by forming a condensed DNA structure and making that region of the genome inaccessible to the transcriptional machinery

Question 2

Contrast the two types of chromatin

Answer 2

Euchromatin- uncondensed and transcriptionally active

Heterochromatin- condensed and transcriptionally active

Question 3

What is Epigenetic regulation?

Answer 3

Different orders of chromatin packing of whole genome can give rise to the highly condensed metaphase chromosome

-In addition, different types of chromatin can form in specific regions of the genome during interphase and can affect gene expression

Question 4

Describe the nucleosome structure

Answer 4

Various histone proteins bind with each other along with DNA to forms a nucleosome. The core is composed of two molecules each of H2A, H2B, H3 and H4 Histone proteins

Question 5

Explain the beads on a string formation

Answer 5

Nucleosomes are connected together by linker DNA and H1 histone to produce the “beads on a string” extended form of chromatin

Question 6

How is heterochromatin formed?

Answer 6

A multi step process first involving DNA methylation. DNA methylation is the modification of the DNA base cytosine to 5-methyl-cytosine

Question 7

Explain DNA methylation and transcriptionally repression

Answer 7

DNA methylation tags a region of the genome for histone modification that causes the formation of heterochromatin.

1. Dnmt 3 methylates unmethylated DNA
2. MeCP2 recruiters HDAC1
3. HDAC1 deacetylates histones
4. Heterochromatin is formed

Question 8

How are histone acetyoated ?

Answer 8

Deeacetylated nucleosome - nucleosome is more tightly associated with the DNA reducing access to the DNA (heterochromatin = no transcription)- this is done with HDACs/HDs

Acetylated nucleosome- a more ‘open’ conformation is achieved, facilitating access to DNA (euchromatin = potential for transcription)

Question 9

What does acetylation do?

Answer 9

Reduces affinity between histone & DNA, leads to increased transcription