20.4: Epigenetic control of gene expression

Question 1

What is epigenetics

Answer 1

Heritable changes in gene function without a change in the DNA base sequence

Question 2

What is the epigenome

Answer 2

The epigenome of a cell is the lifetime accumulation of chemical signals an organism has received.

Question 3

What does the epigenome mean

Answer 3

The epigenome determines the shape of the DNA-Histone complex. For example, it keeps genes that are inactive in tightly packed arrangement therefore transcriptional factors cannot bind.

Question 4

What can change chromatin

Answer 4

Acetylation

Methylation (ch3)

Question 5

Different forms of Chromatin

Answer 5

Chromatin can be LOOSE or TIGHT

tight = gene not expressed
loose = gene expressed

Question 6

Acetylation of histones

Answer 6

Acetylation occurs with histones

Increased acetylation of histones decreases the positive charge on the histones causing the attraction to phosphate groups on DNA to lessen. This loosens the chromatin

Decreased acetylation of histones increases the positive charge on histones and therefore increases their attraction to the phosphate groups of DNA. This tightens the chromatin and transcriptional factors cannot attach

Question 7

Methylation of DNA

Answer 7

Methylation occurs on DNA

Increased methylation of DNA (methyl groups adding to cytosine bases of DNA) inhibits transcription in 2 ways.

1. Physically stops transcriptional factors binding
2. Attracts proteins which condense the DNA histone complex

Decreased methylation of DNA, converse of above ^

Question 8

Epigenetics and inheritance

Answer 8

It is thought in sperm and eggs during the earliest stages of development a specialised cellular mechanism searches the genome and erases its epigenetic tags.

However, some escape this process.

Mother with diabetes = child with heightened chance of diabetes

Question 9

The effect of small interfering rna

Answer 9

1. A double stranded RNA molecule is broken down by an enzyme to form siRNA
2. One of the two siRNA strands combines with an enzyme
3. The siRNA molecule guides the enzyme to a messenger RNA molecule by pairing up its bases with the complementary ones on a section of the mRNA molecule
4. Once in position, the enzyme cuts the mRNA into smaller sections
5. These smaller sections are incapable of being translated into a polypeptide. Therefore their gene isn’t expressed.