gene regulation

• transcription -

  1. The DNA molecule unwinds and hydrogen bonds between the bases are broken causing it to unzip.
  2. Free nucleotides bind (hydrogen bonds) to the exposed bases via complimentary base pairing
  3. RNA polymerase catalyses the formation of phosphodiester bonds between the sugars and phosphates
  4. A messenger RNA strand forms on the 3’ antisense strand
  5. The mRNA molecule breaks off from the template strand and travels out of the nucleus to a ribosome
• what are gene probes?

  • They are used to identify a particular strand of DNA or RNA
  • A gene probe is a small section of fluorescent RNA that binds to DNA by DNA-RNA hybridisation (complimentary base pairing)
• Bone marrow cells, neurones etc. have the gene for haemoglobin as part of their DNA in their nuclei. But only in red blood cells will you also find it being expressed on RNA during transcription.
• transcription factors -

  • control gene expression
  • First way to do this is by choosing which genes are expressed or “switched on.” E.g. we have about 20,000 genes but only half of those might be expressed in a neuron or maybe 8000 in a skin cell.
  • To turn DNA (the instructions) into proteins you need transcription of DNA to make some mRNA and then translation to use that mRNA to make a protein.
  • Anything that influences any of these processes is called a “transcription factor” because it influences gene expression.
• • Transcription factors are proteins that bind to DNA in the nucleus and affect transcription
  • Most transcription factors bind to regions called promotor sequences on the 5’ end of DNA
  • Other transcription factors can bind to regions called enhancer sequences
  • These change the structure of chromatin making it either more easy or more difficult for transcription to occur effectively switching genes on or off.
  • The other crucial feature is that this can happen at different stages of development to produce differentiation. E.g. foetal development
• • DNA has coding and non-coding regions (Exons and Introns)
  • Pre-mRNA is a copy of this base sequence
  • mRNA is transcribed missing out the introns
  • It is then altered by transcription factors before it is translated on the ribosome.
• • Spliceosomes are enzymes that join exons together once introns are removed
  • They can bind exons together in different ways meaning the same bit of DNA can create multiple, slightly different, but still functional mRNA strands.
• In alternative splicing, particular exons found within a gene may be included within or excluded from the final mature mRNA.  As a result, the polypeptide chains translated from alternatively spliced mRNAs will contain differences in their amino acid sequence (primary structure) and therefore in their resulting tertiary structure.  Ultimately, this means that the range of proteins synthesised from one gene will differ in their biological function.
• • Different mRNA strands = different amino acid sequence
  • Different amino acid sequence = different polypeptide chain
  • Different polypeptide chain = different protein
  • Different protein = variation in phenotype
• DNA methylation -

  • Addition of a methyl group (CH3) group
  • Always occurs on cytosine
  • Silences/represses gene expression
• histone modification -

  • Histones are positively charged proteins.
  • The double helix winds around them to form chromatin
  • Chromatin forms chromosomes
  • During mitosis the chromatin condenses to form chromosomes.
  • It is not available to be copied and to make any new proteins; it is known as heterochromatin.
  • When it becomes loosely wound and available during interphase it is called euchromatin.
• Acetylation – addition of an acetyl group (COCH3). Usually opens up the structure allowing transcription of genes
• Methylation – addition of a methyl group. This can inactivate a gene or even a whole chromosome 
• Gene regulation can occur at different stages. E.g.

  • During transcription
  • After transcription
  • During translation
  • After translation
• during transcription -

  • DNA acetylation
  • Histone acetylation
  • Histone methylation
  • DNA methylation
• post transcription -

  • mRNA splicing
• An operon is a group of genes that are all expressed together.

  This is useful because you can switch off all of them at once/ switch all of them on at once when needed.
• The Lac Operon

  • When glucose is in short supply lactose can be used as a respiratory substrate.
  • Different enzymes catalyse these reactions.
  • The switch between the two needs to be rapid.
  • The Lac Operon controls the metabolism of lactose
• LacZ, LacY, LacA are genes that code for enzymes.

  LacI is a regulatory gene.
• cAMP is a secondary messenger molecule in cells.

  • When Glucose is low cAMP levels increase.
  • cAMP bind to CRP (cAMP receptor protein).
  • This complex binds to DNA.
  • This allows RNA polymerase to bind to promoter region.
  • Lac operon up-regulated.
• Gene regulation during translation -

  • Degradation of mRNA – the more resistant an mRNA molecule is the longer it stays in the cytoplasm the more protein is produced.
  • Inhibitory proteins bind to mRNA preventing them from binding to ribosomes – no proteins synthesised.
  • Initiation factors – promotes mRNA binding to ribosomes – more proteins synthesised.
  • Protein kinases – enzymes that catalyse the addition of phosphate groups to proteins. This changes their tertiary structure and therefore function.
• Gene regulation after translation -

  • Addition of non-protein groups e.g. carbohydrates, lipids.
  • Modification of amino acids and formation of bonds such as disulphide bridges.
  • Folding/ shortening proteins.
  • Modification by cAMP e.g. in the lac operon.