Control Of Gene Expression

When are enzymes which are necessary for reactions present in metabolic pathways like respiration reqired?

Constantly.

What are the genes that code for the enzymes necessary for reactions in metabolic pathways called?

Housekeeping genes.

When are protein-based hormones (required for the growth and development of an organism or enzymes) required?

They are only required by certain cells at certain times to carry out a short-lived response. They are coded for by tissue-specific genes.

Where is the entire genome of an organism present?

In every eukaryotic cell that contains a nucleus. This includes genes not required by that cell, so the expression of genes and the rate of synthesis of proteins like enzymes has to be regulated.

Depending on demand, what can happen to genes?

They can be turned on or off, and the rate of product regulated.

Why are bacteria able to respond to changes in the environment?

Because of gene regulation. Expressing genes only when the products are needed also prevents vital resources being wasted.

Is gene regulation the same in eukaryotes and prokaryotes?

It is fundamentally the same. However, the the stimuli that cause changes in gene expression and the responses produced are more complex in eukaryotes.

What do multicellular organisms have to respond to?

Not only changes in the external environment, but also changes in the internal environment.

What is gene regulation required for?

For cells to specialise and work in a coordinated way.

As there are a number of different ways genes are regulated, what are the different categories at which gene regulation operates?

• Transcriptional.

• Post-transcriptional.

• Translational.

• Post-translational.

Briefly, how are genes regulated at the transcriptional level?

Genes can be turned on and off.

Briefly, how are genes regulated at the post-transcriptional level?

mRNa can be modified, which regulates translation and the types of proteins produced.

Briefly, how are genes regulated at the translational level?

Translation can be stopped or started.

Briefly, how are genes regulated at the post-translational level?

Proteins can be modified after synthesis, which changes their functions.

What are the different mechanisms that can affect the transcription of genes?

• Chromatin remodeling.

• Histone modification.

• Lac operon.

• Role of cyclic AMP.

Since DNA is a very long molecule, and needs to be packed into the nucleus of a cell, what happens?

DNA is would around proteins called histones in eukaryotic cells, in order to be packed into the nucleus of a cell. The resulting DNA/protein complex is called a chromatin/

What is heterochromatin?

Tightly wound DNA, causing chromosomes to be visible during cell division.

What is euchromatin?

Loosely wound DNA present during interphase. These genes can be freely transcribed.

Why is the transcription of genes not possible when DNA is tightly wound?

Became RNA polymerase cannot access the genes.

Does protein synthesis occur during cell division or interphase between cell divisions?

Interphase.

Why are proteins synthesised during interphase and not during cell division?

This is a simple form of regulation ensures the proteins necessary for cell division are synthesised in time. It also prevents the complex and energy consuming process of protein synthesis from occurring when cells are actually divided.

Why does DNA coil around histones?

Because they are positively charged and DNA is negatively charged,

What can histones be modified to increase/decrease?

The degree of packing (or condensation) of DNA/

What does the addition of acetyl (acetylation) or phosphate groups (phosphorylation) do to histones?

It reduces the positive charge on the histones (making them more negative), causing DNA to coil less tightly, allowing certain genes to be transcribed.

What does the addition of methyl groups (methylation) do to histones?

It makes them more hydrophobic, so they bind more tightly to each other, causing DNA to coil more tightly, preventing the transcription of genes.

What is epigenetics?

A term that is increasingly used to describe this control of gene expression by the modification of DNA. It is sometimes used to include all of the different ways in which gene expression is regulated.

What is an operon?

A group of genes that are under the control of the same regulatory mechanism and are expressed at the same time.

Are operons more common in prokaryotes or eukaryotes?

Prokaryotes, due to the smaller and simpler structure of their genomes.

Why are operons an efficient way of saving resources?

Because if certain gene products are not needed, then all of the genes involved in their production can be switched off.

What substrate is easier for E.coli to metabolise?

Glucose, so it is the preferred respiratory substrate.

In E.coli, what happens if glucose is in short supply?

Lactose can be used as a respiratory substrate. Different enzymes are needed to metabolise lactose.

What is the lac operon?

A group of three genes. lacZ, lacY and lacA, involved in the metabolism of lactose.

Why are the genes in the lac operon structural genes?

They code for three enzymes (β-galactosidase, lactose permease and transacetylase) and they are transcribed onto a single long molecules of mRNA.

In lac operon, where is a regulatory gene (lacI) located?

Near to the operon, and codes for a repressor protein that prevents the transcription of the structural genes in the absence of lactose.

When is the repressor gene produced in the lac operon?

Constantly, and binds to an area called the operator, which is also close to the structural genes.

What does the binding of the repressor to the operator prevent?

RNA polymerase binding to DNA and beginning transcription. This is called down regulation.

What is the section of DNA that is the binding site for RNA polymerase called?

The promoter.

When lactose is present, what does it bind to?

The repressor protein, causing it to change shape so that it can no longer bind to the operator. As a result, RNA polymerase can bind to the promoter, the three structural genes are transcribed, and the enzymes are synthesised.

What does the binding of RNA polymerase still only result in?

A relatively slow rate of transcription that needs to be increased or up-regulated to produce the required quantity of enzymes to metabolise lactose efficiently.

How is the rate of transcription of the lac operon increased or up-regulated?

By the binding of another protein, cAMP receptor protein (CRP), that is only possible when CRP is bound to cAMP (a secondary messenger).

What does the transport of glucose into E.coli decrease?

The levels of cAMP, reducing the transcription of the genes responsible for the metabolism of lactose. If both glucose and lactose are present, then it will still be glucose, the preferred respiratory substrate, that is metabolised.

What is the product of transcription?

A precursor molecule, pre-mRNA. This is modified forming mature mRNA before it can bind to a ribosome and code for the synthesis of the required protein.

What happens to the pre-mRNA?

A cap (modified nucleotide) is added to the 5’ end and a tail (a long chain of adenine nucleotides) is added to the 3’ end.

What does adding a cap and tail to to the pre-mRNA?

They both help stabilise mRNA and delay degradation in the cytoplasm. The cap also aids binding of mRNA to ribosomes.

What else happens to the pre-mRNA, besides adding a cap?

Splicing.

What is splicing?

This occurs where the RNA is cut at specific points - the introns (non-coding DNA) are removed and the exons (coding DNA) are joined together.

Where does splicing and adding caps and tails happen?

Within the nucleus.

How can the nucleotide sequence of some mRNA molecules be changed?

Through base addition, deletion, or substitution. These have the same effect as point mutations and result in the synthesis of different proteins which may have different functions.

What does RNA editing increase?

The range of proteins that can be produced from a single mRNA molecule or gene.

What mechanisms regulate the process of protein synthesis?

• Degradation of mRNA.

• Bindings 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).

How does the degradation of mRNA regulate the process of protein synthesis?

The more resistant the molecule, the longer it will last in the cytoplasm, that is, a greater quantity of protein synthesised.

What are protein kinases?

Enzymes that catalyse the addition of phosphate groups to proteins. The addition of a phosphate group changes the tertiary structure and so, the function of the protein. Many enzymes are activated by phosphorylation,

Since many enzymes are activated by phosphorylation, what are protein kinases important for

They are important regulators of cell activities.

What are protein kinases themselves often activated by?

The secondary messenger cAMP.

What does post-translational control involve?

Modifications to the proteins that have been synthesised.

What does post-translational control include?

• Addition of non-protein groups such as carbohydrate chains, lipids or phosphates.

• Modifying amino acids and the formation of bonds such as disulphide bridges.

• Folding or shortening of proteins.

• Modification by cAMP - ie lac operon.