epigenetic control of gene expression

Question 1

define epigenetics

Answer 1

environmental factors such as diet, stress can cause heritable changes in gene function without changing the base sequence of DNA

Question 2

epigenetics can subtly alter what?

Answer 2

the genetic inheritance of an organism’s offspring

Question 3

what does the epigenome refer to?

Answer 3

• both the DNA and histones are covered in chemicals, sometimes called tags
• these chemical tags form a second layer known as the epigenome

Question 4

how does the epigenome cause epigenetic silencing?

Answer 4

• epigenome determines the shape of the DNA-histone complex
• e.g. it keeps genes that are inactive in a tightly packed arrangement and therefore ensures they cant be read (keeps them switched off)
• this is known as epigenetic silencing

Question 5

how does epigenome cause genes to be switched on (opposite to epigenetic silencing?

Answer 5

• unwraps active genes so DNA can be easily transcribed

- switches them on

Question 6

the genetic code whereas the epigenome is

Answer 6

flexible

Question 7

wby is the epigenome referred to as flexible?

Answer 7

its chemical tags respond to environmental factors like diet and stress can cause the chemical tags to adjust the wrapping and unwrapping of the DNA so switches the genes on and off

Question 8

what is the epigenome of a cell?

Answer 8

• the accumulation of the signals it has received in its lifetime
• in early development the signals come from within the cells of the fetus and the nutrition from the mother is important in shaping the epigenome at this point
• after birth and throughout life, environmental factors affect the epigenome, environmental factors affect the epigenome although signals from within the body such as hormones also influence it
• these factors cause the epigenome to activate or inhibit a set of genes

Question 9

the environmental signal stimulates proteins to carry its message inside the cell to the nucleus; here the message passes to a specific protein which can be attached to a specific sequence of bases on DNA. Once attached the protein has two possible effects:

Answer 9

1- acetylation of histones leading to the activation or inhibition of a gene
2- methylation of DNA by attracting enzymes that can add or remove methyl groups

Question 10

what does acetylation (opposite to deacetylation) refer to?

Answer 10

process whereby an acetly group is transferred to a molecule; in this case by acetylcholine

Question 11

explain how deacetylation of associated histones switches off genes

Answer 11

• decreased acetylation increases the positive charges on histones and therefore increases their attraction to the phosphate groups of DNA
• the association between histones and DNA is stronger and the DNA is not accessible to transcription factors
• these transcription factors cannot initiate the production of mRNA from DNA
• so gene is switched off

Question 12

what does methylation refer to?

Answer 12

the addition of a methyl group to a molecule; in this case the methyl group is added to the cytosine bases of DNA

Question 13

methylation normally inhibits the transcription of genes in 2 ways:

Answer 13

1- prevents binding of transcription factors to DNA
2- attracting proteins that condense the DNA-histone complex (by inducing deacetylation of histones) making DNA accessible to transcription factors

Question 14

give an example of the effects of epigenetics on inheritance

Answer 14

• in humans when a mother has a condition known as gestational diabetes, the fetus is exposed to high concentrations of glucose
• these high glucose concentrations cause epigenetic changes in the daughter’s DNA
• increasing the likelihood she will develop gestational diabetes herself

Question 15

explain the link between methylation and cancer

Answer 15

• in some cases, the activation of normally inactive genes can cause cancer
• in other cases, it is the inactivation of a normally active gene that can cause cancer
• has been discovered that diseased tissues taken from patients with colon cancer had less DNA methylation that normal tissues
• as increased methylation normally inhibits transcription, decreased methylation leads to higher than normal gene activity- genes switched on
• there are specific sections of DNA, ones near promoter regions that have no methylation in normal cells
• in cancer cells these regions become highly methylated and causing genes that should be active to switch off; this abnormality happens in the early development of cancer

Question 16

explain how epigenetics can increase the incidence of mutations

Answer 16

• some active genes normally help repair DNA and so prevent cancers
• in people with various types of inhibited cancer it is found that increased methylation leading to protective genes being switched off
• so damaged base sequences in DNA are not repaired and so can lead to cancer

Question 17

what are the 2 main uses of epigenetics in disease treatment?

Answer 17

• cancer treatments

- diagnostic tests

Question 18

how can epigenetics treat cancer?

Answer 18

• these treatments use drugs to inhibit certain enzymes involved in either histone acetylation or DNA methylation
• e.g. drugs that inhibit enzymes that cause DNA methylation can reactivate genes that have been silenced

Question 19

why must epigenetic therapy be specifically targeted on cancer cells?

Answer 19

if the drugs were to affect normal cells, they could activate gene transcription and make them cancerous, so causing the very disorder they were designed to cure

Question 20

explain the use of epigenetics in diagnostic testing

Answer 20

• diagnostic tests that help to detect the early stages of diseases such as cancer, brain disorders and arthritis
• these tests can identify levels of DNA methylation and histone acetylation at an early stage of disease
• this allows those with these diseases to seek early treatment and so have a better chance of cure

Question 21

regarding the effect of RNA interference on gene expression, in eukaryotes and some prokaryotes,the translation of mRNA produced by a gene can be inhibited by breaking mRNA down before its coded information can be translated into a polypeptide- what type of RNA is involved?

Answer 21

small interfering RNA (siRNA)

Question 22

explain how siRNA inhibits the translation of mRNA? (effect of siRNA on gene expression)

Answer 22

1- double stranded RNA is broken up by an enzyme into siRNA
2- one of the two strands of siRNA combines with an enzyme
3- the siRNA strand pairs with complementary bases on an mRNA strand
4- the enzyme cuts mRNA into smaller sections
5- mRNA no longer capable of being translated into a polypeptide
6- this means the gene hasn’t been expressed

Question 23

suggest how siRNA could be used to identify the role of genes in a biological pathway

Answer 23

• some siRNA that blocks a particular gene could be added to the cells
• by observing the effects, or lack of them, we could determine what the role of the blocked gene is

Question 24

suggest how siRNA could be used to treat certain diseases

Answer 24

siRNA could be used to prevent disease by blocking the gene that causes it