Genetic Code

Replication, Transcprition, Translation and DNA Fingerprinting

  • Created by: ava.scott
  • Created on: 16-09-14 20:40

Semi-Conservative Theory and Proof

Why? Chromosones must divide so that each daughter cells have an exact copy of the gentic code after mitosis.

Who? Watson and Crick suggested the semi-conservative methods after disocvering DNA's structure.

Proving it:

  • Bacteria grown with heavy N15 isotope incooperated into their DNA. The bacterial DNA was extracted and centifruged; it settled at a low point in the test tube.
  • Heav-isoptope bacteria were then washed and allowed to grow in a lighter isotope, dividing once more. This generation's DNA showed up at a mid-point after centifruging.
  • Bacteria divided once again, and this second generation's DNA was half light and half at midpoint; this proves semi-conservative theory.
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Semi Conservative Replication


  • Weak hydrogen bonds between bond pairs break by DNA helicase enzyme.
  • Each chain acts as a template for the new chromosone.
  • DNA polymerase enzyme attaches new nucleotides to the exposed base pairs.
  • Two new strands are made from one original DNA helix.
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Characteristics of the code

DNA is a store of genetic information. Chromosones are split into thousands of genes. A gene is a length of chromosone that codes for a particular polypeptide.

DNA is degenerate (many codons for a single amino acid). Codons are non-overlapping and universal.

Triplet Codes

  • Each amino acid has its own code of bases. T, A, C and G code for over 20 amino acids.
  • If each amino acid only needed one base, only 4 amino acids could be coded for.
  • Three bases for a codon gives 64 codes, surplus for amino acids.
  • There are three codons that 'STOP' a polypeptide.
  • Codons are universal in all organisms.
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Protein Synthesis: Transcription

1) DNA is a template made from nucleotides.

2) A mRNA molecule is made from complentary nucleotides via transcription.

3) mRNA acts as a template for complementary tRNA molecules to attach, along with teh amino acids they carry. This is called translation.


  • DNA helicase acts on the chosen cistron of the DNA, breaking H-bonds and exposing bases.
  • RNA polymerase links to template strand at the beginning, and the DNA unravels.
  • Free RNA nucleotides line up next to complementary bases (C to G, A to U)
  • RNA polymerase runs down the strand, catalysing the phospho diester bonds to form between free nucleotides, to create a strand of mRNA.
  • RNA polymerase separates on reaching a STOP code.
  • mRNA travels out through a nuclear pore and into the cytoplasm/RER.
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Protein Synthesis: Transcription

The Ribosome: This organelle has two sub units, of which the smaller has two attacjment points for tRNA.They switch between holding the next tRNA molecule and the growing polypeptide chain. The ribosome holds the codon-anticodon complex.


  • mRNA attaches to the ribosome.
  • The first codon attaches to the complementary anticodon of a tRNA, and the same happens on the second attavhemnet point. 
  • Each tRNA's amino acid are held close enough to join by peptide bonds.
  • This continues until the STOP cod (end of mRNA molecules.)
  • Multiple ribosomes are found on one mRNA strand, creating many of the same polypeptide.


The anticodon on the tRNA molecule is specific to its amino acid, so mRNA still codes for precise amino acids. Once released from the ribosome, tRNA is free to go and collect another amino acid. Attachment is caled activation and requires ATP.

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Polypetptides are convereted into proteins and are sjed for varied functions.

Most polypetides have to be converted to atleast secondary proteins, but some primary proteins have functions too.

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Applications of Genetics

The Human Genome project aims:

  • identify all genes in human DNA
  • determine sequences of 3 billion chemical base pairs
  • improve data anlysis tools
  • transfer technologies to private sector
  • address ethical, social and legal issues.


  • Carrier testing for faulty genes: this can allow people to decide not to have children/ a child if they carry a faulty gene.
  • Pre-symptomatic estimating and prediction: Thus allows those at risk of diseases to have prevention treatments, or drugs developed to fit the structure of the faulty protein.
  • New-born baby screening.
  • Confirmation of diseases
  • Identity/forensic tests
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Drawbacks of genetic testing


  • Commercialised gene tests for onset of diseases are controversial. They are targeted at pre-symptomatic who could be at risk, but interpretation of results can be unreliable, as mutations don't always lead to the disease.
  • Laboratory errors could make teets unreliable and invalid (but isn't this the same for all tests?)
  • Telling some one they are predicted to a have a disease can actually encourage symptoms to appear.


  • who shoudl have access to this genetic information?
  • who owns and controls information?
  • Do parents ahve the right to test their children for mutations?
  • Cloning???


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Genetic Fingerprinting

The same as someones DNA Profile. It is dertermined by soemones DNA introns, calkled hyper variable regions (HVR.) These are passed onto offspring.


  • DNA extracted and cut up using endonucleases.
  • Fragments are separted by electrophoresis. Larger DNA travels less than smaller DNA, and tehy are separtaed into bands.
  • The plate is covered by a nylon mebranem and the fragmebts are transferred onto the mebrane by siuthern blotting.
  • Radioactive probes are attached to specific fragments, and the rest is washed away.
  • Membrane with fragments are x-rayed abd the radioactive probes expose the film.
  • This autoradiograph is unique to all individuals.
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Uses of Genetic Fingerprinting

  • PATERNITY TESTS: White blood cells takn from parents. Mothers radiograph bands are subtracted from the the childs; the father must have all remaining bands.
  • FORENSIC EVIDENCE: uses polymerase chain reaction

PCR: Semi conservative replicationin a test tube. DNA is dissolved in a buffer and mmixed with enzymes, nucleotides and primers (these act as signals for DNA polymerase to begin replicating a specific part of DNA)

  • Denaturation- Heat DNA with buffer at 95 centigrade. This separates the strands of the helix
  • Annealing- add primers and cool tp 55 centigrade.
  • Extension- heat to 70 centigarde and enzyme catalyses reaction between nucleotides to create new DNA helices.
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Issues with PCR and privacy

The vast quatities of data rases issues of storage and personelle- whos hould be able to see the data?

Could insurance companies penalise the ill using the data?

Wider issues of biodiversity- human variaty is preserved as effort have been made to store genetic material firm many tribes and races.

Ethical standards must be maintained.

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Gene Therapy

= using DNA as a drug to treat diseases by delivering therapeutic DNA into patients cells.

SOMATIC- ex vivio in vivo

  • (normal body cells)
  • This only targets the patients and ot future generations. It is seen as a sfaer, and more ocnservative methods, but is a therapy and not a cure- somatic cells die and are replaced.


  • (gametes)
  • This causes permanent change in the individual and their offpsrings DNA. Its gives the possibility of liberating a family from a genetic disease. However, it is very ntroversial as people beleive it is like 'playing God.'
  • Unforeseen consequences could occur- we don't know enough about the genome yet.
  • Ethical factors over eugenics are important to consider.
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