cellular control

• mutation - random mistake in the structure of DNA

  • can happen when dna is replicating during interphase
  • rate of mutation is increased by mutagens
• types of mutation

  • substitution - swapping bases for another base, change the amino acid
  • deletion - loss of one or more bases causes a frame shift, all following bases are disrupted
  • insertion - addition of a new base causes a frame shift
• silent mutations - effect

  mutations often occur in non coding dna called introns, does not code for protein therefore will have no effect

  a mutation may code for a different amino acid but it may be similar to the previous amino acid and therefore not change active site of enzyme
• harmful mutations - missense mutations

  a mutation may change the amino acid in a protein causing it to be less effective or not function at all

  a single base substation may change the primary structure and therefore tertiary structure of a protein
• harm mutations - nonsense mutations

  mutations may cause a stop codon to be introduced partway along a gene, lead to short non-functional polypeptides

  in frame shifts all amino acids after deletion or insertion are affected
• beneficial mutations

  occasionally a change in an amino acid in a protein can make the protein more effective
• somatic mutations - not inherited but can cause ageing and cancer, result of mutations in normal diploid cells
• germline mutations - due to mutation during gamete formation. can cause genetic diseases and are passed
• gene expression - some proteins are only needed at certain times, therefore they are only made when they are needed
• transcriptional level regulation of gene expression - genes may be switched on/off

  amount of mRNA produced through transcription can be regulated
• histone modification when dna wrapped around proteins cannot be used for protein synthesis
• dna methylation - methyl groups may attach to dna altering gene expression, causes dna to bind more tightly to histone proteins.
• transcription factors - proteins that bind to dna to regulate rna polymerase

  structural genes that code for proteins work together and sometimes have a single promoter gene - this is called an operon
• the lac operon is a collection of genes responsible for lactose digestion and their regulation
• lactose permease - transports lactose into the cell acting as a carrier protein
• beta galactosidase - hydrolyses lactose into glucose and galactose
• IPOZY

  I = regulator gene

  p = promoter region

  o= operator region

  z= gene coding for beta galactosidase

  y - gene coding for lactose permease
• if no lactose present - lac operon

  regulator gene transcripted and transcribed, binds to operator region and blocks transcription of structural genes by rna polymerase
• if lactose present - lac operon

  1. regulatory gene produces repressor proteins, lactose binds altering the shape of
  2. rna polymerase joins to promoter region, if repressor protein bound rna polymerase cannot bind and no transcription/translation
  3. if lactose is present, rna polymerase can transcribe structural genes, enzyme for lactose digestion produced
• transcription factors in eukaryotes

  • proteins bind to promoter allowing rna to bind (activators)
  • proteins bind to promoter preventing rna to bind (repressors)
• post transcriptional level regulation of gene expression

  • eukaryotic genes contain introns (non coding) and externs (coding)
  • splicing is used to remove the introns to create mature rna
  • primary mRNA must be edited to remove introns
  • endonuclease/restriction enzymes can do this
• post translational level regulation of gene expression

  • proteins that have been transcribed and translated may need to be activated
  • proteins modified in Golgi apparatus
  • eg cyclic AMP
• homeobox genes - control development of a zygote to a complete organism

  switch on whole set of genes affecting body plan

  180bp long coding for 60 amino acids - homeodomain

  found in - plants, animals and fungi
• hox genes - animals only, responsible for correct positioning of body parts
• post transltion control by cAMP and phosphorylation

  1. signalling molecule binds to receptor
  2. activates a G protein
  3. G protein activates adenylyl cyclase
  4. adenylyl cyclase catalyses formation of CAMP
  5. cAMP activates protein kinases
  6. phosphorylates other protein
  7. acts as transcription factor to regulate transcription
• apoptosis - programmed cell death

  happens during foetal development to shape limbs

  in humans, kills tissue linking fingers to prevent being webbed

  ineffective t lymphocytes removed

  some homeboy genes may code for transcription factors which switch on genes that lead to apoptosis
• apoptosis procedure

  1. cell receives signals including cytokines and hormones
  2. cytoskeleton broken down by enzymes, cell shrinks and membrane blebs
  3. nucleus breaks down and chromatin condenses
  4. cell fragments made with intact plasma membranes around it, encases the hydrolytic enzymes
  5. cell fragments are ingested and digested by phagocytic cells
• not enough apoptosis leads to formation of tumours

  too much apoptosis leads to degeneration of tissues

  cell signalling maintains balance