Epigenetics

Question 1

What is epigenetics?

Answer 1

Gene regulation system: for tissue-specific expression- turning genes on & off due to factors (e.g. environment, diet/nutrition)

Question 2

Types of modifications on histones?

Answer 2

• Methylation
• Acetylation
• Phosphorylation
• Ubiquitination

Question 3

Significance of epigenetics

Answer 3

Relevant to the study of:

• Somatic cell therapy
• cloning & transgenic therapy
• Cancer
• Viral taency
• Developmental abnormalities

Question 4

How variable expression can be useful in our knowledge in other fields eg cancer?

Answer 4

To understand what stimulates certain genes to be expressed and how it is silenced or enhanced.

Question 5

Where can the modifications take place that affect gene expression?

Answer 5

DNA (C) and Histone tails

Question 6

Histone methylation: including the amino acids are marked, # of methyl groups added, and the enzymes involved in methylation and demethylation.

Answer 6

• Lysine & Arganine on histone tail methylated (1-3x methyl group per lysine on tail)
• Added by: Histone Methyltransferase
• Removed by: Lysine-specific demethylase I

Question 7

What affect does methylation on histone have on chromatin structure, and why? How does this affect expression?

Answer 7

More methylation (hypermethylation) = more condensed chromatin = Heterochromatin = less active bc DNA binding molecules (DBP) can’t access DNA = genes not/less expressed

Question 8

Histone acetylation: including which amino acid are marked, # of acetyl group added, and the enzymes involved in acetylation and deacteylation.

Answer 8

• Lysines on histone tail acetylated (1x acyl per Lys group & not methylated)
• Added by: Histone acyl transferase
• Removed by: Histone de-acetylase

Question 9

What affect does acetylation have on chromatin structure? How does this affect expression?

Answer 9

More acetyl (Hyperacetylation) = less condensed chromatin = Euchromatin = more active bc DBP can access DNA = genes expressed

Question 10

DNA methylation: the CpG site, the enzymes involved in initiation, and in maintenance of methylation.

Answer 10

• CpG: The C (that has a G next to it on the same strand) is methylated
• Initiated by De Novo (make new) DNA methyl transferase > used for adding new methyl groups on DNA
• Maintained by Maintanance methyl transferase > used on a dividing cell that has replicated & match methylation pattern from parent- allele was inherited from

Question 11

What affect does DNA methylation have on gene expression (transcription)?

Answer 11

Methylation prevents BINDING of DBP on REGULATORY region (not coding region) ≠ transcribe

Question 12

Effects of DNA methylation on regulatory regions and on up-regulation & down-regulation genes.

Answer 12

Methylation inactivates REGULATORY region => incr. or decr. expression

• Switch off up-reg. gene = decr. expression
• Switch off down-reg. gene (e.g silencer) = incr. (/enhanced) expression

Question 13

How the marks are reprogrammed via DNA replication?

Answer 13

1. DNA replication => new strand from old strand
2. Maintenance transferase recognise hemi-methylated DNA (methyl on old strand but not on new) and adds methyl group on new strand on C (w/ CpG) => both strands methylated

Question 14

What happens in gametogensis? in terms of methyl groups

Answer 14

1. De-methylation in early development of primordial germ cells
2. Re-methylation occurs (males earlier)
3. Re-methylation completed prior to meiosis

Question 15

What happens in embryogenesis? in terms of methyl groups

Answer 15

1. De-methylation after fertilisation (paternal genome quicker rate bc has enzyme)
2. Re-methylated @ implantation

Question 16

a) What is imprinting? b) How does imprinting affect allele expression? c) How are the marks reset?

Answer 16

a) genes that are expressed depending on whether it was inherited from mum or dad (Mono-allelic expression)
b) Only one gene (from mum or dad) is active (the other is silenced)
c) Both alleles of offspring re-set to match methylation pattern from the allele which was inherited from the one parent (de/methylated)

Question 17

What is dosage compensation in mammals?

Answer 17

Inactivate 1X chromosome in females (compensate for the only X in males) by converting it into heterochromatin in early embryo development (1000 cell stage)

Question 18

Explain how dosage compensation in mammals is achieved.

Answer 18

1. 1X chromosome randomly selected to be inactivated = Barr body
2. A region on X chromo. Xic (X inactivation centre) = codes for RNA (Xist)
3. Xist coats chromo
4. => transcriptional gene silencing & chromatin left condensed = remain inactivated

Question 19

What happens in phosphorylation?

Answer 19

Serine AA phosphorylated

• Added by: Kinases
• Removed by: Phosphatases

Question 20

What happens in Ubiquitination?

Answer 20

Ubiquitin (protein) added to side chain of Lysine

• Added by: Ubiquitin conjugating enzyme
• can’t be removed bc end point command&raquo_space; recognised as trash/ useless

Question 21

What is bromodomain?

Answer 21

effector molecule that targets acetyl groups > regulates gene expression by winding or unwinding chromatin

Question 22

Give an example of X-inactivation

Answer 22

tortoiseshell coloured cat

• occurs in females (XX) & males (XXY)
• red allele on 1 X chromo. & black allele on another X chromo.
• random patches of red & black occur bc of random inactivation of X in early development

Question 23

What happens when Drosophila “white” gene is near heterochromatin region (instead of being more upstream)?

Answer 23

expression of gene stops = red w/ patches of white in eyes of fly (called variegation)

Question 24

How does methylation protect DNA?

Answer 24

heterochromatin/ tightly wound DNA can’t be accessed by endonucleases

Question 25

Epigenetic change results from…

Answer 25

activation of a gene that is normally repressed e.g. expression of telomerase on cancer cells