Genetic Control of Proteins

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  • Genetic Control of Proteins
    • RNA
      • mRNA
        • Single helix molecule consisting of thousands of mononucleotides
        • Manufactured in the nucleus during transcription
        • Is a mirror copy of one of the DNA template strands
        • Exits through the nuclear pore and binds to a ribosome in the cytoplasm. Here it acts as a template for protein synthesis (translation).
      • tRNA
        • Small molecule - 80 nucleotides
        • Single stranded polypeptide chain, folded into clover-leaf shape
        • Has an anticodon that pairs with a complementary triplet codon on an mRNA molecule
        • One end of chain extends beyond the other, so that amino acids can easily attach
    • Transcription
      • Splicing of pre-mRNA
        • Happened in eukaryotic cells
        • Non-coding introns are removed from the DNA
        • The remaining exons (coding sections) can be rejoined in a variety of different combinations. This means that a single section of DNA can code for up to a dozen proteins depending on the order in which the exons are rejoined.
      • Is the process of making pre-mRNA using a section of DNA as a template
      • The process:
        • 1. DNA helicase breaks the hydrogen bonds between bases. The two strands separate and the nucleotide bases in that region are exposed
        • 3. Complementary bases pair up. (Cytosine and Guanine, Adenine and Uracil).
        • 2. RNA polymerase moves along the template strand, causing the nucleotides to join with complementary individual nucleotides present in the nucleus
        • 4. As RNA polymerase builds the strand of pre-mRNA the two original DNA strands rejoin behind it
        • 5. When RNA polymerase recognises a stop codon it detaches. The production of pre-mRNA is then complete
    • Translation
      • pre mRNA moves out of the nucleus via a nuclear pore and moves into the cytoplasm
      • The process:
        • 6. As the third tRNA molecule is brought to the mRNA strand and another peptide bond is formed, the first tRNA molecule is released from its amino acid. It is then free to bind to another amino acid in the cell
        • 5. The amino acids are joined by a peptide bond using ATP and an enzyme
        • 7. The process continues, building up a polypeptide chain
        • 4. The ribosome moves along the mRNA bringing two tRNA molecules at a time to pair with the complementary codons on the mRNA strand
        • 8. Up to 15 amino acids can be linked per second, and up to 50 ribosomes can pass immediately behind the first so many identical polypeptides can be assembled simultaneously
        • 3. Another tRNA molecule, carrying another amino acid, pairs up with the next codon on the mRNA because it has the complementary anticodon
        • 2. A tRNA molecule with the complementary anticodon moves to the ribosome and pairs up with the codon on the mRNA. The tRNA molecule is also carrying an amino acid.
        • 8. This polypeptide synthesis continues until a stop codon is reached by the ribosome. The ribosome, mRNA and tRNA molecule seperates and the polypeptide chain is complete
        • 1.A ribosome attaches to the starting codon (AUG)at one end of the mRNA molecule
    • The Genetic Code
      • Degenerate - most amino acids have more than one codon
      • Non - overlapping - each base in the sequence/ each triplet is read only once
      • Universal - the same codon codes for the same amino acid in all organisms
    • Mutations
      • Substitution of Bases
        • Nonsense mutation - base change results in the formation of a stop codon
        • Mis-sense mutation - base change results in a different amino acid being coded for
        • Silent mutation - although different, the substituted base still codes for the same amino acid
      • Deletion of Bases
        • A nucleotide is lost from the normal DNA sequence. Consequences can mean the sequence of the entire polypeptide is altered.
        • Frame shift can occur due to the genetic code being read in triplets. The deletion of one nucleotide means the reading frame has been shifted to the left by one letter. Effects are more considerable when a base is deleted near the start of the sequence.
      • Causes
        • Chance - arising spontaneously during DNA replication
        • Mutagenic Agents
          • High energy radiation - disrupts the DNA molecule
          • Chemicals - alter the DNA structure or interfere with transcription
      • Control of Cell Division
        • Proto-oncogenes - stimulate cell division
          • Growth factors attach to a receptor protein on cell surface membrane. Relay proteins (in the cytoplasm)  carry the message to the nucleus where the genes necessary for DNA replication are 'switched on'.
          • Proto-oncogenes can be mutated into Oncogenes. Oncogenes can affect cell division in 2 ways:
            • The receptor protein on the cell surface membrane may be permanently activated. This means cell division is switched on even in the absence of growth factors
            • The oncogene can code for a growth factor that is then produced in excessive amounts. This stimulates excessive cell division
        • Tumour suppressor genes
          • Inhibit cell division, therefore maintaining normal rates of cell division and preventing the formation of tumours
          • Mutation of these genes can cause them to be inactivated. Cell division would therefore increase. Mutated cells usually die as they are structurally and functionally different from other cells, however survivors are capable of cloning themselves.


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