Cellular Control

Cellular Control 

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Transcription

Transcription

Transcription - creation of a single stranded mRNA copy of the DNA coding strand

  • Gene to be transcribed unwinds and unzips - DNA helicase breaks hydrogen bonds between bases - exposed bases known as cistron
  • RNA polymerease reads the exposed DNA bases and allocates free floating complementry RNA nuclotides to form mRNA
  • condensation reaction takes place between adjancent RNA nuleotides
  • mRNA produced is complemetry to the DNA molecule and is therfore a copy of the base sequence and is released out of the nuleaus via a nucleur pore to a ribosome
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Translation

Translation

Translation - the assembly of polypeptides at ribosomes via mRNA and amino acids 

Codon - group of 3 bases which corresponds to a particular amino acid

  • mRNA molecule binds to ribosome on the rough endoplasmic reticulum so that 6 bases are exposed
  • tRNA molecules have anticodons which are associated with specific amino acids
  • tRNA molecule with anticodon and amino acid that is complementry to first mRNA codon binds to it
  • the next tRNA molecule binds to the next exposed mRNA codon bringing another amino acid
  • condensation reaction between amino acids forms peptide bonds 
  • mRNA moves along ribosome to expose new bases 
  • first tRNA molecule breaks away and is reloaded with another amino acid to repeat the process 
  • gradually an entire polypeptide is formed until the 'stop' codon is reached, which stops the process 
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Mutations

Mutations 

Mutation - a change in the seqeunce or amount of bases on a DNA polynucleotide chain 

Mutations can have beneficial, neutral of harmful effects

Substitution/ Point Mutations - where one base replaces another - tends to have no effect because DNA code is degenerate, meaning each amino acid is coded for by more than one triplet code

Insertion/Deletion - whereby one or more nuclotide pairs are added or deleted from a length of DNA, causing a frame shift - causes the whole DNA sequence to be read differently - produces a different primary structure which could affect the secondary and teritary structure, causing a disruption in the function of the protein 

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Mutations - Sickle Cell Anaemia

Mutations - Sickle Cell Anaemia

  • caused by a substituion mutation in gene that codes for one of the polypeptide chains in haemoglobin
  • haemoglobin made of Alpha and Beta chain polypeptides - sick cell anaemia causes change in beta chains
  • base sequence for first glutamate amino acid changes from CTT to CTA, which codes for valine
  • Glutamate has hydrophillic side chain which lies on outside to make haemoglobin soluble
  • Valine in the place of glutamate causes a hydrophobic side chain on the outside 
  • When blood oxygen levels fall valine side chains stick together, forming long chains of stuck-together heamoglobin
  • this pulls the shape of the red blood cell inwards, becoming sickle shaped
  • this prevents the heamoglobin carrying oxygen but can also stops the cells moving through capillaries, causing blockages, pain, and damage to tissue
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The Lac Operon

The Lac Operon 

Operon - a length of DNA containing a base sequence that codes for the proteins ( structural genes), and other base sequences that determine wether or not the gene will be switched on (Promotor and Operator)
Promotor - a gene which initiates transcription. RNA polymerase binds to the promotor to begin transcription
Operator - a gene which regualtes the promotor, either 'activating it' or 'deactivating it'
Funtional/Structural Genes - genes that code of polypeptides
Reguator - gene that codes for production of repressor protein. Repressor protein binds to operator gene, stopping structural genes from being transcribed

E.coli bacteria have genes that code for Lactose permease (enables cell to take up lactose) and Beta galactosidase (hydrolyses lactose to glucose). If bacteria is grown on medium with just glucose then enzymes are not needed and genes are turned off. However if bacteria is grown on medium containing lactose then the enzymes are needed and the genes are switched on. The Lac Operon enables this process to function

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The Lac Operon - 2

The Lac Operon - 2

When Lactose is Absent from Growth Medium

  • Regulator gene is expressed forming repressor protein 
  • repressor protein binds to operator, covering part of the promotor - RNA polymerase cannot bind to promotor and so cannot transcribe structural genes
  • structural proteins (Beta Galactosidase and lactose permease) are not synthesised - 

When Lactose is Added

  • Lactose (inducer) molecule binds to repressor protein changing its shape, preventing it from binding to operator 
  • this leaves promotor region free - RNA polymerase can now bind and initiate transcription of mRNA for the structural genes
  • structural genes are expressed and proteins (Beta Galactosidase and lactose permease) can be synthesised 
  • E.coli can use lactose permase to take up the lactose and use bata galactosidase to convert the lactose to glucose and galactose, which can then be used for respiration
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Apoptosis

Apoptosis

Apoptosis - programmed cell death that occurs in multi cellular organisms 
Necrosis - untidy and damaging cell death

  • Enzymes break down cell cytoskeleton
  • cytoplasm becomes dense with organelles becoming tightly packed
  • cell surface membrane changes to form 'blebs'
  • chromatin condeses and nuclear envelope breaks - DNA breaks into fragments
  • cell breaks up in vesicles which are engulfed by phagocytes (phagocytosis) 
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