3.5 DNA and Protein Synthesis

• 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.