DNA replication, Transcription and Translation

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  • Created by: Megnag
  • Created on: 25-11-16 09:10
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  • DNA Replication, Transcription and Translation
    • DNA Replication
      • An essential feature of DNA is that it must be able to replicate itself accurately, so that when a cell divides, the genetic code it carries can be passed onto daughter cells
      • DNA Replication copies DNA precisely so that new molecules are produced with exactly the same sequence of bases as the original strands
      • DNA Replication occurs in the nucleus during interphase of the cell cycle when DNA is not tightly coiled
      • Process
        • 1. The first step in the process is the 'unzipping' of the two strands. DNA helicase moves along the double helix, unwinding the two strands, which separate from one another as the relatively weak hydrogen bonds between the bases are broken
        • 2. The unpaired nucleotides are exposed and each single strand now acts as a template for the formation of a new complementary strand. Free nucleotides move into place: C pairs with G and A pairs with T.
        • 3. The free nucleotider bases form complementary pairs with the bases on the single DNA strands. DNA polymerase is the enzyme involved in linking the new nucleotides into place. Finally, the two new DNA molecules are rewound, each one forming a new double helix
      • The two new DNA strands that are produced are absolutely identical to the original strands. Complementary base pairing between the template strand and the new strand ensures that an accurate copy of the original DNA is made every time replication occurs. DNA replication is Semi-conservative because no DNA molecule is completely new. Every double helix includes one 'original' strand and one 'new' strand
    • Transcription
      • Occurs in the nucleus
      • The first stage in the synthesis of a protein is the production of an intermediate molecule that carries the coded message of DNA from the nucleus into the cytoplasm where the protein can be produced
        • This intermediate molecule is called messenger RNA or mRNA
          • RNA (ribosenucleic acid) has similarities and differences to DNA
      • As we have seen, the building blocks for RNA are the RNA nucleotides that are found in the nucleus. Complementary base pairing of RNA to DNA occurs in the same way as in the replication process, but this time Uracil (U) pairs with Adenine (A) since the base Thymine (T) is not found in RNA.
      • Transcription results in the copying of one section of the DNA molecule, not its entire length
      • Process
        • 1. DNA is unzipped by the enzyme RNA polymerase and the two strands uncoil and seperate
        • 2. Free nucleotides move into place along one of the two strands
        • 3. The same enzyme, RNA polymerase, assembles the free nucleotides in the correct places using complementary base pairing. As the RNA nucleotides are linked together, a single strand of mRNA is formed. This molecule is much shorter than the DNA molecule because its a copy of just one section - a gene. The mRNA separates from the DNA and the DNA double helix is zipped up again by DNA polymerase
        • Once an mRNA molecule has been transcribed, it moves via pores in the nuclear envelope to the cytoplasm where the process of translation can take place.
    • Translation
      • Due to complementary base pairing, the sequence of bases along the mRNA molecule corresponds to the sequence on the original DNA molecule
      • Each sequence of three bases, called a triplet, corresponds to a specific amino acid, so the order of these triplets determines how amino acids will be assembled into polypeptide chains in the cytoplasm
      • Translation is the process by which the coded information in mRNA strands is used to construct polypeptide chains which in turn make functioning proteins
      • Each triplet of mRNA bases is called a codon and codes for one amino acid
      • Translation is carried out in the cytoplasm by structures called ribosomes and molecules of another type of RNA called transfer RNA or tRNA.
        • Ribosomes have binding sites for both the mRNA and tRNA molecules.
      • Process
        • 1. The ribosome binds to the mRNA and then draws in specific tRNA molecules with anticodons that match the mRNA codons
        • 2.Only two tRNA molecules bond to the ribosome at once. Each one carries with it the amino acid specified by its anticodon. The anticodon of tRNA binds to the complementary codon of the mRNA molecule with hydrogen bonds
          • These processes are repeated over and over again until the complete polypeptide is formed. The final codon that is reached is a 'stop' codon, which does not code for an amino acid but tells the ribosome to detach from the mRNA. AS it does so, the polypeptide floats free in the cytoplasm
            • 3. When two tRNA molecules are in place on the ribosome, a peptide bond forms between the two amino acids and the form a dipeptide. Once a dipeptide has been formed, the first tRNA molecule detaches from both the amino acid and the ribosome. The ribosome moves along the mRNA one triplet to the next codon.
            • 1. The ribosome binds to the mRNA and then draws in specific tRNA molecules with anticodons that match the mRNA codons
        • 3. When two tRNA molecules are in place on the ribosome, a peptide bond forms between the two amino acids and the form a dipeptide. Once a dipeptide has been formed, the first tRNA molecule detaches from both the amino acid and the ribosome. The ribosome moves along the mRNA one triplet to the next codon.

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