3.5 DNA and Protein Synthesis

    Cards (17)

    • Transcription:
      1. Just the gene that is going to be transcribed unwinds.
      2. Only one strand acts as a template strand.
      3. Free RNA nucleotides in the nucleus complementary base pair with exposed bases on DNA template strand. Cytosine -> Guanine, Adenine -> Uracil (Thymine on DNA).
      4. RNA polymerase forms phosphodiester bonds between adjacent RNA nucleotides.
      5. Strand of mRNA is formed until it reaches a stop triplet and mRNA is complete and detached.
    • In protein synthesis, 2 processes called transcription and translation takes place.
    • Transcription is when the DNA genetic code is copied into mRNA.
    • Translation is when mRNA is read by ribosomes to form a polypeptide chain.
    • Splicing is the removal of introns from pre-mRNA by enzymes.
    • The process of splicing involves cutting out intron sequences using endonucleases and joining exon sequences together again using ligases.
    • During transcription, only one strand of DNA serves as a template for mRNA formation. The other strand remains coiled around histone proteins forming chromatin fibres.
    • DNA helicase breaks the hydrogen bonds between the DNA strands.
    • The bonds formed between the DNA strand and the RNA nucleotide are hydrogen bonds.
    • Adjacent RNA nucleotides are joined by a phosphodiester bond.
    • RNA polymerase catalyses transcription.
    • mRNA leaves the nucleus by a nuclear pore.
    • RNA nucleotides bond together in a condensation reaction.
    • Translation:
      1. mRNA attaches/associates with a ribosome.
      2. Ribosome moves to a start codon on mRNA.
      3. tRNA molecules bring a specific amino acid to the ribosome - 2 tRNA molecules can fit into the ribosome, so mRNA codons are read 2 at a time.
      4. tRNA anti-codon binds to the complementary mRNA codon.
      5. Ribosome moves along to the next codon and previous tRNA molecule leaves the ribosome.
      6. Amino acids are joined together in a condensation reaction forming peptide bonds and requiring ATP.
      7. Process continues until a stop codon is reached and a polypeptide chain is released from the ribosome.
    • The polypeptide formed from translation would initially be a long strand. What protein structure is this?
      Primary structure.
    • Give the two types of molecule from which a ribosome is made.

      rRNA and proteins.
    • In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA. Explain why.
      Because pre-mRNA gets spliced (introns are removed by enzymes) to become mRNA.