19.2 Control Of Gene Expression

Question 1

What is gene expression?

Answer 1

All cells in an organism carry the same genes (DNA) but carry out different functions and have different structures.
This because not all genes in a cell are expressed (transcribed and used to make a functional protein), they are selectively switched on and off.
Because cells show different gene expression, different proteins are made and these proteins modify the cell.

Question 2

Different levels of gene expression

Answer 2

Gene expression (and therefore protein synthesis) can be controlled at the transcriptional, post-transcriptional and post-translational level.

Question 3

Describe and explain transcription level control

Answer 3

• gene expression can be controlled at the transcriptional level by altering the rate of transcription of genes (increased transcription of mRNA)
• this is controlled by transcription factors - proteins that bind to DNA and switch genes on or off by increasing or decreasing the rate of transcription
  Factors that start transcription are called activators
  Factors that stop transcription are called repressors

Question 4

Describe how transcription factors work

Answer 4

The shape of a transcription factors determines whether it can bind to DNA or not, and can sometimes be altered by the binding of some molecule e.g. hormones and sugars

• in eukaryotes, transcription factors bind to specific DNA sites near the start of their target genes
• in prokaryotes control of gene expression often involve transcription factors binding to operons

Question 5

Describe and explain operons

Answer 5

An operon is a section of DNA that contains a cluster of structural genes that are all transcribed together, as well as control elects. And sometimes a regulatory gene.

The structural gene code for useful proteins, such as enzymes.

The control elements include a promoter (a DNA sequence located before the structural genes that RNA polymerase binds to) and an operator (a DNA sequence that transcription factors bind to)

The regulatory gene codes for an activator or a repressor.

Question 6

Describe and explain how Lac operons work

Answer 6

E.coli is a bacterium that respires glucose, but it can use lactose if glucose isn’t available.

The genes that produce enzymes needed to respire lactose are found on an operon called the lac operon.
The lac operon has three structural genes — lacZ, lacY, lacA, which produce proteins/ enzymes that help bacteria digest lactose (enzymes include B-galactosidase and lactose permease)

Lactose not present:
The regulatory gene (lacI) produces the lac repressor, which is a transcription factor that binds to the operator site when there;s no lactose present. This blocks trancription because RNA polymerase can’t bind to the promoter.

Lactose present:
When lactose is present, it binds to the lac repressor, changing the repressor’s shape so i can no longer bind to the operator site.
RNA polymerase can now begin transcription of the structural genes (lacZ, lacY, lacA)

Question 7

Describe and explain post-transcriptional level control

Answer 7

• after transcription, mRNA in eukaryotic cells is edited — thats because genes in eukaryotic DNA contain sections that don’t code for amino acids. These sections are introns. All sections of DNA that do code for amino acids are called exons.

During transcription the introns and exons are copied into mRNA.
- mRNA strands that contain introns and exons are called primary mRNA (or pre mRNA)

• introns are removed from pre-mRNA stands by a process called splicing - introns are removed and exons are joined together to form mature mRNA strands

Thus rakes place in the nucleus, the mature mRNA then leaves the nucleus for translation

Question 8

Describe and explain post-translation control

Answer 8

Some proteins aren’t functional straight after they have been synthesised, they need to be activated to work (become a functional protein). Protein activation controlled by molecules e.g. hormones and sugars.

Question 9

Describe and explain the role of cAMP in post-translation level control

Answer 9

• some molecules that control protein activation work by binding to cell membrane and triggering the production of cAMP inside the cell.
• cAMP then activates proteins inside the cell by altering their 3D structure e.g. altering the 3D structure can change the active site of an enzyme, making it more or less active

E.g. PKA (protein kinase A) is an enzyme made of four subunits
When cAMP binds it causes a change in the enzymes 3D structure, releasing he active subunits -PKA is now active