20.3 - Regulation Of Transcription And Translation

Question 1

What are the stages of gene expression that can be regulated?

Answer 1

Gene expression can be regulated at several stages:

Control of transcription – Deciding which genes are transcribed.
Control of splicing – Modifying pre-mRNA to form mature mRNA.
Control of translation – Regulating protein synthesis in the cytoplasm (e.g., by siRNA).
Post-translational modification – Altering the polypeptide after synthesis.

Question 2

What must happen for a gene to be expressed?

Answer 2

The gene must be transcribed into mRNA in the nucleus and then translated into a polypeptide in the cytoplasm.

Question 3

What are transcription factors, and where are they found?

Answer 3

• Transcription factors are proteins that regulate gene expression by controlling transcription.
• They are present in the cytoplasm and have a specific tertiary structure, enabling them to bind to specific DNA sequences.

Question 4

How do transcription factors regulate transcription?

Answer 4

Promoter function: Some transcription factors promote transcription by aiding RNA polymerase binding to DNA.
Inhibitory function: Others inhibit transcription by preventing RNA polymerase from binding.
Their specificity comes from the specific shape of their binding site, which matches certain DNA base sequences.

Question 5

How does oestrogen regulate gene expression?

Answer 5

• Oestrogen, a lipid-soluble steroid hormone, enters the cell by simple diffusion through the phospholipid bilayer.
• It binds to its receptor protein ERα in the cytoplasm.
• Binding changes the shape of ERα, removing inhibitors and activating the transcription factor.
• The active ERα enters the nucleus and binds to the promoter region of DNA, enabling RNA polymerase to start transcription.

Question 6

What properties of oestrogen allow it to regulate transcription?

Answer 6

• Lipid-solubility: It diffuses through the cell membrane.
• Specific binding: It binds to ERα, whose shape is complementary to oestrogen.
• Activation of ERα: This removes inhibitors and promotes its binding to DNA.

Question 7

Describe how oestrogen activates a transcription factor.

Answer 7

• Oestrogen binds to its receptor, ERα, in the cytoplasm.
• This causes a conformational change in ERα, exposing its DNA binding site.
• The active transcription factor moves into the nucleus and binds to DNA to initiate transcription.

Question 8

What is siRNA, and how does it inhibit translation?

Answer 8

• siRNA is a short double-stranded RNA molecule.
• It binds to the RISC protein in the cytoplasm, which separates siRNA into single strands.
• The RISC-siRNA complex binds to complementary mRNA sequences, causing mRNA to be cleaved.
• Cleaved mRNA cannot be translated and is broken down by nuclease enzymes.

Question 9

How is siRNA formed, and what does it do?

Answer 9

• Double-stranded RNA is cleaved into small siRNA sections.
• In the cytoplasm, siRNA binds to RISC, which activates it by separating the strands.
• The RISC-siRNA complex binds complementary mRNA, cleaving it and preventing translation

Question 10

Where does regulation of transcription and translation occur?

Answer 10

• Transcription regulation occurs in the nucleus and involves transcription factors.
• Translation regulation occurs in the cytoplasm, often mediated by molecules like siRNA.

Question 11

How do transcription factors ensure specificity in gene expression?

Answer 11

Transcription factors bind to specific DNA sequences due to their specific tertiary structure and binding sites, ensuring they only control certain genes.

Question 12

How do transcription factors affect RNA polymerase?

Answer 12

• Promoting transcription: They aid RNA polymerase in binding to the DNA.
• Inhibiting transcription: They prevent RNA polymerase from binding to the DNA.

Question 13

What is post-translational modification, and why is it important?

Answer 13

It involves changes to the polypeptide after it is synthesized, such as folding, cleavage, or adding functional groups, which are critical for the protein’s functionality.

Question 14

What is the role of the promoter region in transcription?

Answer 14

The promoter region is a DNA sequence where transcription factors and RNA polymerase bind to initiate transcription.

Question 15

What is the role of a transcriptional factor?

Answer 15

Transcriptional factors stimulate transcription of a gene

Question 16

Describe how oestrogen stimulates the expression of a gene.

Answer 16

• Oestrogen diffuses through the phospholipid portion of a cell-surface membrane into the cytoplasm of a cell, where it binds with a site on a receptor portion of the transcriptional factor.
• Oestrogen changes the shape of the DNA binding site on the transcription factor so it can now bind with DNA.
• The transcriptional factor now enters the nucleus through a nuclear pore and binds with DNA, stimulating transcription of the gene that makes up that portion of DNA, i.e. it stimulates gene expression,