Control Of Gene Expression Flashcards
1
Q
What is chromatin remodelling? What type of control?
A
- Chromatin = DNA/histone complex.
- Transcription of genes not possible when tightly wound (heterochromatin) as RNA polymerase cannot access the genes.
- In euchromatin genes can be freely transcribed.
- DNA synthesis occurs during interphase, in time for cell division to occur.
- Transcriptional control.
2
Q
What is histone modification? What type of control?
A
- Negatively charged coils around positively charged histones.
- Histones can be modified to increase or decrease the degree of packing/condensation.
- The addition of acetyl groups or phosphate groups reduces the positive charge of the histones, so DNA coils less tightly (weaker attraction). This allows certain genes to be transcribed.
- Addition of methyl groups makes the histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly and preventing transcription of genes.
- Transcriptional control.
3
Q
What is an operon?
A
- An operon is a group of genes that are under the control of the same regulatory mechanism and are expressed at the same time.
4
Q
Why does E. coli need the Lac operon?
A
- When glucose is in short supply, lactose can be used as a respiratory substrate.
- Different enzymes are required for the metabolism of lactose.
5
Q
What are the different parts of the lac operon?
A
- 3 structural genes: lacZ (B-galactosidase), lacY (lactose permease), lacA (transacetylase). These 3 enzymes are transcribed onto a single long molecule of mRNA.
- A regulatory gene: lacI. Located near to the operon, codes for a repressor protein.
- Promoter: where RNA polymerase binds.
- Operator: where repressor protein binds if lactose isn’t present.
6
Q
Describe how the presence of lactose influences the lac operon.
A
- LacI codes for a repressor protein that prevents the genes being transcribed in the absence of lactose.
- The repressor protein is constantly produced and binds to an area called the operator, which is also close to the strucutral genes. The binding of this protein prevents RNA polymerase binding to the DNA and beginning transcription. Down regulation
- The section of DNA that is RNA poly’s binding site is called the promoter.
- When lactose is present, it binds to the repressor protein causing it to change shape so it can no longer bind to the operator.
- As a result RNA poly can bind to the promoter, genes transcribed, enzymes synthesised, lactose metabolised.
7
Q
What is the role of cyclic (c) AMP?
A
- Binding of RNA polymerase results in a relatively slow rate of transcription. Must be unregulated to produce the required quantity of enzymes to metabolise lactose efficiently.
- This is achieved by the binding of another protein, cAMP receptor protein (CRP), to DNA. CRP can only bind to DNA when already bound to cAMP.
- CAP helps RNA polymerase bind to the promoter, resulting in high levels of transcription when glucose levels are low.
- When glucose levels are high, less cAMP will be produced, so that CAP can’t bind, therefore transcription of the lac operon is slower.
8
Q
What is capping and tailing? What level of control?
A
- RNA processing.
- A cap = a modified nucleotide. 5’.
- A tail = a long chain of adenine molecules. 3’.
- Both help to stabilise mRNA and delay degradation in the cytoplasm.
- Cap also aids binding to ribosome.
9
Q
What is capping and tailing? What level of control?
A
- RNA processing.
- A cap = a modified nucleotide. 5’.
- A tail = a long chain of adenine molecules. 3’.
- Both help to stabilise mRNA and delay degradation in the cytoplasm.
- Cap aids binding to ribosome.
- Post-transcriptional.
10
Q
- What is splicing?
A
- Where the RNA is cut at specific points, introns are removed and the exons are joined together. Occurs within the nucleus.
- Certain parts can be removed to produce multiple different mRNA molecules from the same gene and therefore a different polypeptide.
- Post-transcriptional.
11
Q
What is RNA editing? What level of control?
A
- The nucleotide sequence of some mRNA molecules can be edited by adding, removing or substituting bases. Same effect as point mutations. Multiple proteins can be made from the same gene.
- Post-transcriptional.
12
Q
What mechanisms regulate the process of protein synthesis in translational control?
A
- Degradation of mRNA. The more resistant the molecule, the longer it will last in the cytoplasm. Therefore a greater quantity of protein will be synthesised if it does not degrade.
- Binding of inhibitory proteins to mRNA prevents it binding to ribosomes and the synthesis of proteins.
- Activation of initiation factors which aid the binding of mRNA to ribosomes (the eggs of many organisms produce large quantities of mRNA which is not required until after fertilisation, at which point initiation factors are activated.
13
Q
What are protein kinases? What level of control are they?
A
- A group of enzymes that catalyse the addition of phosphate groups to proteins.
- The addition of a phosphate group changes the 3 structure and so the structure of the protein.
- Many enzymes are activated by phosphorylation, so kinases are therefore important regulators of cell activity.
- Kinases are regularly activated by cAMP.
- Translational control.
14
Q
What is translational control?
A
- Switching translation on and off.
15
Q
What is post-translational control?
A
- Modifying the polypeptides to make proteins of specific functions.
