chapter 19

• housekeeping genes

  genes coding for enzymes and essential metabolic pathways such as respiration
• protein based hormones that are needed for growth and development are only required by some cells for short lived responses and are coded for by tissue specific genes
• genes can be turned on or off and the rate of produce synthesis increased or decreased depending on the demand
• expressing genes only when the products are needed also prevents vital resources being wasted
• 4 ways that genes can be regulated

  1. transcriptional = genes can be turned on or off
  2. post transcriptional = mRNA can be modified which regulates translation and type of protein made
  3. translational = translation can be stopped or started
  4. post translational = proteins can be modified after synthesis which changes their function
• exon

  coding region of DNA, expressed region
• intron

  non coding region of DNA, not expressed region
• operon

  group of genes that function as a single transcription unit
• transcription factor

  protein or short non coding RNA that can combine with a specific site on a length of DNA and inhibit or activate transcription of the gene
• acetylation

  addition of acetyl or phosphate group
• methylation

  addition of a methyl group
• chromatin

  uncondensed DNA in a complex with histones
• heterochromatin

  tightly wound DNA causing chromosomes to be visible during cell division
• euchromatin

  loosely wound DNA present during interphase, transcription of genes is not possible when DNA is tightly wound because RNA polymerase cannot access the genes so genes in euchromatin can be freely transcribed
• protein synthesis does not occur during cell division but during interphase of cell divisions, simple form of regulation that ensures the proteins necessary for cell division are synthesised in time, also prevents complex and energy consuming process of protein synthesis from occuring when cells are dividing
• DNA coils around histones because they are positively charged and DNA is negatively charged, histones can be modified to increase or decrease the degree of condensation
• acetylation or phosphorylation reduces the positive charge on the histones to make them more negative causing DNA to coil less tightly allowing more genes to be freely transcribed
• methylation increases the positive charge and makes the histones more hydrophobic causing the DNA to coil more tightly and preventing genes to be transcribed
• epigenetics

  control of gene expression by the modification of DNA and regulation
• gene

  a length of DNA coding for a polypeptide
• allele

  an alternative version of a gene
• point mutation

  a mutation in which one base replaces another
• operons are more common in prokaryotes than eukaryotes because of smaller and simpler structure of their genomes, also efficient way of saving resources because if certain gene products aren't needed, then all genes involved in their production can be switched off
• glucose is easier to metabolise and is the preferred respiratory substrate of E coli and other bacteria, if glucose is in short supply, lactose can be used instead, different enzymes are needed to metabolise lactose
• lac operon components

  group of 3 genes:

  • lacZ
  • lacY
  • lacA

  involved in metabolism of lactose, they are structural genes

  and contain lacI which is a regulatory gene, a promoter region and an operator region
• lac operon

  section of DNA in the bacterial genome containing a number of genes