topic 8

• Gene expression


  When the DNA or a gene is expressed or a protein is created
• Gene expression


  • Includes transcription and translation
• Epigenetics


  A heritable change in gene function where the DNA base sequence is unchanged
• Epigenetics


  Controls gene expression by initiating or preventing transcription
• Factors that can cause epigenetic changes

  • Diet
  • Stress
  • Toxins
• Epigenome


  The single layer of chemical tags around the DNA
• Heterochromatin


  Tightly coiled DNA where transcription is inhibited
• Chromatin


  Loosely coiled DNA where transcription can occur
• Methylation


  Adding a methyl group to the DNA, resulting in tightly coiled heterochromatin and inhibited transcription
• Acetylation


  Adding an acetyl group to the histones, resulting in loosely coiled chromatin and enabled transcription
• Increased methylation


  Inhibits transcription
• Increased acetylation


  Enables transcription
• Tumor suppressor genes


  Genes that code for proteins that slow down cell division and can cause cell death
• Mutation in tumor suppressor genes


  Linked to cancer development
• Hypermethylation of tumor suppressor genes

  Causes them to be switched off, leading to excess cell growth and tumor development
• Hypomethylation of oncogenes


  Causes them to be permanently switched on, leading to excess cell division and tumor development
• Transcriptional factors


  Molecules that can move from the cytoplasm into the nucleus and either initiate or inhibit transcription
• Transcriptional factors


  • They are proteins with a unique 3D shape
  • Part of the protein binds to DNA
  • Another part is a receptor for another molecule to attach to
• Transcriptional control


  1. Transcriptional factor binds to DNA

  2. This enables RNA polymerase to bind

  3. Transcription starts
• Transcriptional factor is bound to DNA


  Transcription can occur
• Transcriptional factor is not bound to DNA


  Gene is turned off/inactive
• Estrogen


  A steroid hormone that can activate a transcriptional factor
• Activation of transcriptional factor by estrogen

  1. Estrogen enters cell and binds to receptor part of transcriptional factor

  2. This changes the shape of the DNA binding site

  3. Activated transcriptional factor can now bind to DNA

  4. RNA polymerase can then bind and transcription can occur
• Transcriptional control is the control of DNA to mRNA
• Translational control will be covered in a later video
• RNA interference (RNAi)


  The process of regulating translation
• RNAi


  • Occurs in eukaryotic and some prokaryotic organisms
  • Involves targeting particular mRNA sequences to inhibit certain genes
• RNAi mechanism


  1. Transcribed mRNA is destroyed in the cytoplasm before it can be used to code for a polypeptide chain

  2. Done by small interfering RNA (siRNA)
• Transcription


  The process that results in the production of mRNA
• Translation


  The process where mRNA is used to code for a polypeptide chain
• Protein synthesis


  Transcription and translation in summary
• RNAi mechanism


  1. Double-stranded RNA is cut into small sections by enzymes

  2. Small sections are made single-stranded (siRNA)

  3. siRNA binds to complementary mRNA sequences

  4. Enzyme attached to siRNA cuts the bound mRNA
• siRNA binds to complementary mRNA


  Cuts the mRNA, preventing translation of that gene
• Genome


  The entire genetic material of an organism
• Genome in eukaryotes


  • Found in the nucleus of a cell
• Genome in prokaryotes


  • Loose within the cytosol, as they don't have a nucleus
• Genome sequencing


  Identifying the exact DNA-based sequence for the entire genome or for the entire DNA of a cell
• Every single cell in one organism contains the same DNA, except for red blood cells which don't have a nucleus, and gametes which have half the amount of DNA
• Organisms with sequenced genomes

  • Human
• Human Genome Project started

  1990