Transcriptional level

• (transcriptional level) All cells contain teh same DNA, but not the same proteins. Certain proteins like ATP Synthase, RNA Polymerase, Pyruvate dehydrogenase. These are expressed by house keeping genes. Other proetins are needed by certain tissues at certain times like haemoglobin, insulin, antibodies.These are expressed by tissue-specific genes.
• (transcriptional level) Some proteins are only required at certain time, so genes are switched on/off, such as thermogenin, lactase, foetal haemoglobin, hormones.
• (transcriptional level) Prokaryotic organisms are unicellular, therfore no tissue-specific genes, but housekeeping genes are on all the time.
• (transcriptional level) Some genes are expressed when a peotwin is required, which is part of an operon. An operon is a group of structural genes under the control of fa single promoter region.
• (transcriptional level) Structural genes code for proteins involved in metaboloism. regulatory genes code for proteins which control transcription (transcription factors).
• (Transcriptional level) An example is Lac Operon. A stretch og DNA in E.Cloi genome containing:

  • Structural genes coding for proteins required for lactose metabloism... followed by...
  • ... a regulatory section to switch gene expressions on/off.
• (transcriptional level) If Lactose is present and glucose is present, Lac Operon is Switched OFF. If lactose is present and glucose is absent, Lac operon is switched ON.
• (transcriptional level) For eukaryotic transcriptional factors are proteins (or short non-coding pieces of RNA) which is coded by regulatory genes. It regulated genes by

  1. directly by binding to the promoter region to repress or induce expression.
  2. Indirectly by acting as or through a cofactor which binds to the promoter region of the gene.
• (transcriptional level) Eukaryotic DNA is associated with histones. Histones are + charged proteins which package and order DNA. into structural units called nucleosomes. Chromatin is DNA wrapped around histones and is present in interphase.
• Euchromatin is DNA which is loosely wound around histones, so transcription occurs more easily.
• Heterochromatin is DNA which is tightly wound around histones, so it is harder for transcription to take place
• Acetylation of the histones makes histones less '+' charged, so DNA is loosened and less tightly wound aroiund histones (repelled). Transcriptional factors and RNA polymerase bind to DNA and genes are expressed by euchromatin. This converts heterochromatin to euchromatin.
• Methylation of histones makes histones and DNA more hydrophiphobic, so they pack more tightly together (cant form H+ bonds with Water). Transcription factors cant bind and genes arent expressed. This converts euchromatin to heterochromatin.