20- Regulation of Transcription and Translation

What is oestrogen an example of?

A steroid hormone

Explain how transcription of a gene is stimulated by transcription factors.

• Transcription factor moves from the cytoplasm into the nucleus

• Binding site of transcription factor binds to promoter region of DNA

• RNA polymerase is stimulated to bind and process of transcription begins

• mRNA is produced

Explain how transcription of a gene is inhibited by transcription factors.

• Transcription factor binding site binds to promoter region of DNA

• RNA polymerase is prevented from binding and transcription does not occur

• mRNA is not produced

Explain how oestrogen stimulates the transcription of a gene. (5 marks)

• Oestrogen is lipid-soluble, so diffuses through the phospholipid bilayer and into the cell

• Oestrogen binds with complementary oestrogen receptor site on transcription factor

• Oestrogen causes the transcription factor to change shape, so that it can now bind to promoter region on DNA

• Transcription factor (still bound to oestrogen) enters the nucleus through a nuclear pore and binds to promoter region on DNA

• RNA polymerase is stimulated to bind to promoter region of DNA and transcription begins

Why does oestrogen only affect target cells?

Only target cells have the oestrogen receptor

What is epigenetics?

The field of science involving heritable changes in gene function, without changes to the base sequence of DNA, caused by environmental factors

What is DNA wrapped around?

Proteins called histones

What is the epigenome?

• Chemical tags found on DNA and histones

• Which determines the shape of the DNA-histone complex

What is epigenetic silencing?

When the epigenome keeps inactive genes in a tightly packed arrangement so that they cannot be read (and are kept switched off)

What are examples of environmental factors that may affect the epigenome?

• Diet

• Stress

How is transcription affected when the association of histones with DNA is weak?

• DNA-histone complex is less condensed (loosely packed)

• DNA is more accessible to transcription factors which can initiate the production of mRNA

• Transcription is stimulated

How is transcription affected when the association of histones with DNA is strong?

• DNA-histone complex is more condensed (tightly packed)

• DNA is less accessible to transcription factors so they cannot initiate the production of mRNA

• Transcription is inhibited

What are the two ways in which transcription can be inhibited?

• Decreased acetylation of histone proteins

• Increased methylation of DNA

What is acetylation?

The process in which an acetyl group is transferred to a molecule by acetylcoenzyme A

What is deacetylation?

The process in which an acetyl group is removed from a molecule

How does decreasing acetylation affect transcription?

• Transcription is inhibited

• DNA-histone complex is more condensed (tightly packed)

• DNA is not accessible to transcription factors so they cannot initiate the production of mRNA

(AI- acetylation increases transcription)

What is methylation?

The process in which a methyl group (-CH3) is added to a molecule

Which DNA base is the methyl group added to in the methylation of DNA?

Cytosine

What are the two ways in which DNA methylation inhibits transcription?

• It prevents the binding of transcription factors to DNA

• It attracts proteins that condense the DNA-histone complex, making the DNA inaccessible to transcription factors

What is the name of the type of chromatin produced when the DNA-histone complex is more condensed?

Heterochromatin

What is the name of the type of chromatin produced when the DNA-histone complex is less condensed?

Euchromatin

How could epigenetics cause cancer?

• Increased methylation of genes which would normally prevent cancer

• This would cause the protective genes to be switched off

How could you treat cancers triggered by epigenetic changes?

• Using drugs to inhibit certain enzymes involved in DNA methylation or histone acetylation

• Targeted at cancer cells

How does siRNA (small interfering RNA) inhibit gene expression? (5 marks)

• Enzyme breaks down double-stranded RNA into siRNA

• One of the two siRNA strands combines with an enzyme

• siRNA pairs with complementary bases on target mRNA

• Enzyme cuts mRNA into smaller fragments

• mRNA cannot be translated into a polypeptide and the gene is not expressed