Regulation of transcription and translation

• controlling transcription:

  • transcription is when a gene is copied from DNA into mRNA
  • the enzyme responsible for synthesising mRNA from DNA is called RNA polymerase
• • all cells in an organism carry the same genes (DNA) but the structure and function of different cells varies
  • this is because not all the genes in a cell are expressed (transcribed and used to make a protein)
  • bc different genes are expressed, different proteins are made and these proteins modify the cell - determine the cell structure and control cell processes (including the expression of more genes, which produce more proteins)
• the transcription of genes is controlled by protein molecules called transcription factors
• The role of transcription factors:

  • in eukaryotes, transcription factors move from the cytoplasm to the nucleus
  • in the nucleus they bind to specific DNA sites called promoters, which are found near the start of their target genes - the genes they control the expression of
  • transcription factors control expression by controlling the rate of transcription
• • some transcription factors called activators, stimulate or increase the rate of transcription
  • e.g. they help RNA polymerase bind to the start of the target gene and activate transcription
• • other transcription factors called repressors inhibit or decrease the rate of transcription
  • e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription
• Oestrogen:

  • the expression of genes can also be affected by other molecules in the cell e.g. oestrogen
  • a steroid hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor
  • forming an oestrogen-oestrogen receptor complex
• not all cell types have oestrogen receptors - so not all cells are affected by oestrogen
• The oestrogen-oestrogen receptor complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene. The complex an act as an activator of transcription e.g. helping RNA polymerase bind to the start of the target gene
• in some cells, the oestrogen-oestrogen recpetor complex can act as a repressor of transcription rather than an activator - depends on the type of cell and the target gene
• oestrogen is a steroid hormone found in mammals

  • steroid hormones are small, hydrophobic, lipid-based hormones that can diffuse through the cell membrane and can pass directly into the nucleus through nuclear pores
• oestrogen is involved in controlling the female fertility cycle and is also responsible for stimulating sperm production in males

  • up to 100 different genes are controlled by oestrogen
• the oestrogen stimulation pathway:

  • oestrogen diffuses through the cell surface membrane into the cytoplasm
  • oestrogen diffuses through a nulear pore into the nucleus
  • within the nucleus, oestrogen attached to an ERa oestrogen receptor to undergo a conformational change
  • the new shape of the ERa oestrogen receptor allows it to detatch from the protein complex and diffuse towards the gene to be expressed
  • The ERa oestrogen receptor binds to a cofactor which enables it to bind to the promoter region of the gene, this stimulates RNA polymerase binding and gene transmission
• RNAi:

  • in eukaryotes, gene expression is also affected by RNA interference (RNAi)
  • RNAi is where small, double-stranded RNA molecules stop mRNA from target genes being translated into proteins
  • a similar process to RNAi can also occur in prokaryotes
  • the molecules involved in RNAi are called siRNA (small interfering RNA) and miRNA (microRNA)
• How RNAi works:

  siRNA (and miRNA in plants):

  • once mRNA has been transcribed, it leaves the nucleus for the cytoplasm
  • in the cytoplasm, double-stranded siRNA associated with several proteins and unwinds
  • one of the resulting single strands of siRNA is selected and the other strand is degraded (broken down)
  • the single strand of siRNA then binds the the target mRNA
  • the base sequence of the siRNA is complementary to the base sequence in sections of the target mRNA
• • the proteins associated with the siRNA cut the mRNA into fragments - so it can no longer be translated
  • the fragments then move into a processing body, which contains 'tools' to degrade them
• a similar process happens with miRNA in plants:

  • like siRNA, the base sequence of plant miRNA is complementary to its target mRNA sequence
  • and so binding results in the cutting up and degradation of the mRNA
  • however, its production in the cell is similar to that of mammalian miRNA
• miRNA in mammals:

  • in mammals, the miRNA isn't usually fully complementary to the target MRNA
  • this makes it less specific than siRNA and so it may target more than one mRNA molecule
• • when miRNA is first transcribed, it exists as a long, folded strand
  • it is processed into a double strand, and then into 2 single strands by enzymes in the cytoplasm
  • like siRNA one strand associates with proteins and binds to target mRNA in the cytoplasm
  • instead of the proteins associated with miRNA cutting RNA into fragments, the miRNA-protein complex physically blocks the translation of the target mRNA
  • the mRNA is then moved into a processing body, where it can either be stored or degraded
  • when stored it can by returned and translated at another time
• • mRNA leaves the nucleus
  • miRNA (not fully complementary) and associated proteins bind to the target mRNA, blocking translation
  • in the processing body mRNA is degraded or stored
• • siRNA and associated proteins bind to the target mRNA
  • mRNA is cut up unto small fragments
  • in the processing body - mRNA fragments are degraded
• RNAi molecules are small lengths of non-coding RNA (don't code for proteins)
• siRNA is double stranded unlike mRNA or tRNA
• double stranded siRNA is unwound into 2 single-stranded siRNA molecules by an enzyme
• siRNA is about 20-25 nucleotides long
• siRNA has a potential use in treating genetic disorders, for example stopping a known harmful gene from being expressed.

  siRNA molecules with a base sequence complementary to the mRNA from that gene could be inserted into the affected cells - they will bind to the mRNA and so block translation of that protein
• the unused single strand of miRNA is degraded in the cytoplasm (like the second strand of siRNA)
• example of a gene expression system in bacteria = the lac repressor
• The lac repressor:

  • E. coli is a bacterium that respires glucose, but it can use lactose if glucose isn't available
  • if lactose is present E. coli makes an enzyme (beta-galactosidase) to digest it
  • but if there is no lactose, it doesn't waster energy making an enzyme it doesn't need
  • the enzymes gene is only expressed when lactose is present
  • the production of the enzyme is controlled by a transcription factor - the lac repressor
• • when there is no lactose, the lac repressor binds to the DNA at the start of the gene stopping transcription
  • when lactose is present it binds to the lac repressor, stopping it binding to the DNA, so the gene is transcribed
• The experiment:

  • different E. coli mutants were isolated and grown in different media e.g. with lactose or glucose
  • the mutants have mutations (changes in their DNA bases) that mean they act differently from normal E.coli e.g. they produce beta-galactosidase when grown with glucose
  • to detect whether active (working) beta-galactosidase was produced, a chemical that turns yellow in the presence of active beta-galactosidase was added to the medium
  • the production of mRNA that codes for beta-galactosidase was also measured
• Mutant 1


  mRNA and active beta-galactosidase were produced even when grown with only glucose - the gene is always being expressed
• Mutant 1


  Has a faulty lac repressor e.g. in the absence of lactose the repressor isn't able to bind to DNA, so transcription can occur and mRNA and active beta-galactosidase are produced
• Mutant 2


  mRNA is produced but active beta-galactosidase isn't when lactose is present - the gene is being transcribed but it isn't producing active beta-galactosidase
• Mutant 2


  Is producing faulty beta-galactosidase e.g. because a mutation has affected its active site