Unit 4: Section 5- Genetics

A summary of the genetics topic from A2 level edexcel biology

  • Created by: R_Hall
  • Created on: 03-11-13 12:00

The Genetic Code and Protein Synthesis 1

  • DNA is a polynucleotide, made up of many nucleotides containing deoxyribose sugar and a base (adenine, thymine, guanine or cytosine)
  • It exists as a double helix- polynucleotide strands are joined by hydrogen bonds between bases. Bases join by complementary base pairing- A with T and C with G
  • Genes are sections of DNA found on chromosomes. They code for proteins, which are made from amino acids
  • The order of bases in a gene determines the order of amino acids in a protein. Each aa is coded for by a triplet of bases in the gene
  • Different sequences of bases code for different aa- the genetic code. The code is non-overlapping and degenerate (more combinations of triplets than aa, so some aa are coded for by more than one triplet)
  • Some triplets start and stop the production of the protein- start and stop codons
  • DNA molecules are found in the nucleus, but organelles for protein synthesis are in the cytoplasm. As DNA is too big to leave the nuclear envelope, it is copied into RNA by transcription
  • RNA- a single polynucleotide strand. Uracil (U) replaces thymine as a base in RNA
  • mRNA- carries genetic code nucleus -> cytoplasm. Used to make protein in translation. 3 adjacent bases are a codon
  • tRNA- carries aa which makes proteins to the ribosomes. Has aa binding site, sequence of 3 bases called anti-codon
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The Genetic Code and Protein Synthesis 2

  • Transcription starts when RNA polymerase attaches to the DNA at the beginning of a gene. The h bonds between bases break, and the DNA uncoils. 
  • One DNA strand becomes a template for the mRNA copy- called the antisense strand. RNA polymerase lines up free RNA nucleotides alongside the template strand. A reverse copy of the DNA is made in mRNA, by complementary base pairing
  • RNA polymerase moves along DNA, separating strands and assembling mRNA. When it reaches a stop codon, it detaches
  • The H bonds in the DNA reform once the RNA polymerase passes, and the mRNA moves through the a nuclear pore and attaches to a ribosome in the cytoplasm
  • Genes contain sections of DNA called introns that don't code for aa. The sections that do code are called exons
  • During transcription, introns and exons are both copied into mRNA, but the introns are then removed through splicing. The exons are joined in the nucleus to form mRNA strands
  • Exons can be joined together in different orders to form different mRNA strands- this means more than 1 protein can be produced from 1 gene
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The Genetic Code and Protein Synthesis 3

  • Translation occurs at the ribosomes in the cytoplasm. The mRNA from transcription attaches itself to a ribosome and tRNA molecules carry aa to the ribosome
  • A tRNA molecule with an anticodon complementary to the 1st codon on the mRNA attaches to the mRNA by complementary base pairing. A 2nd tRNA attaches to the next codon in the same way
  • The 2 aa attached to the mRNA are joined by a peptide bond, the 1st tRNA moves away. The process continues, producing a chain of amino acids until a stop codon is reached
  • The polypeptide chain moves away from the ribosome and translation is complete
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DNA profiling 1

  • Everyone's DNA is different, so everyone has a unique DNA profile. DNA profiling can be used to identify people and determine genetic relationships
  • 1. A DNA sample is obtained eg. blood, saliva
  • 2. Polymerase chain reaction (PCR) is used to amplify DNA. The reaction mixture contains DNA sample, free nucleotides, primers (short pieces of DNA complementary to start of wanted fragment) and DNA polymerase enzyme
  • DNA is heated to 95C to break H bonds between bases. It is then cooled to 50-65c to allow primers to anneal to the strands. The mixture is heated to 72C to allow DNA polymerase to line up free nucleotides alongside each template strand. Complementary base pairing means new complementary strands are formed
  • Two new copies of fragment DNA are formed and one cycle of PCR is completed. The cycle can start again, and each 'turn' doubles the amount of DNA present
  • 3. A fluorescent tag is added to all fragments so they can be viewed under uv light
  • 4. Gel electrophoresis is used to separate the DNA. The DNA is placed in a well in a slab of agarose gel, covered in buffer solution. An electric current is passed through the gel
  • DNA frag. are -vely charged, so they move to the +ve electrode at the far end of the gel. Short DNA frag. move faster and travel further, so fragments separate according to length
  • 5. The gel is viewed under uv light, and the frag. appear as bands
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DNA Profiling 2

  • DNA collected from crime scenes can be compared to samples from possible suspects, to link them to crime scenes
  • If the DNA profiles match (eg. the bands on the gel line up), the person can be linked to the crime scene
  • DNA profiling can be used to determine genetic relationships in humans. The more bands on two DNA profiles that match, the more closely related those two people are.
  • DNA profiling can be used on animals and plants to prevent inbreeding, which causes health, productivity and reproductive problems by decreasing the gene pool size.
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