DNA and Protein Synthesis

• Methionine is the start amino acid.
• Every three bases in the sequence of a gene is called a triplet.

  Each triplet codes for a specific amino acid.
• There are 4 different nucleotides, which means there are 4^3= 64 possible combinations of triplets that code for 20 amino acids.
• The genetic code is degenerate. This means most amino acids are coded for by more than one triplet.
• There are three triplets that don't code for amino acids- these are stop triplets.

  A stop triplet is found at the end of each gene.
• The genetic code is universal- it is the same in all organisms.

  The same codon/ triplet always codes for the same amino acid no matter the organism.
• The genetic code is non-overlapping. This means that each base is read only once and is part of only one triplet.
• To synthesise proteins, two types of RNA (mRNA and tRNA) are needed in addition to the DNA.
• RNA is a single stranded polynucleotide.

  The nucleotides compromise a phosphate, a ribose sugar and one of four nitrogenous bases ( adenine, uracil, guanine and cytosine).
• Messenger RNA (mRNA) molecules are made up of a few thousand nucleotides long. The bases do not bond to each other.
• In mRNA, every three bases is called a codon.
• mRNA is linear.
• Transfer RNA (tRNA) molecules are about 75 nucleotides long.

  The strand folds back on itself and hydrogen bonds form between complementary base pairs.

  The overall shape is called a clover leaf.
• At one end of a tRNA molecule is a binding site.

  Three bases are exposed. These are called the anticodon.
• There are 61 types of tRNA molecules (64 - 3 stop codons= 61).
• Each of the 61 tRNA molecules has a different sequence of bases in the anticodon. Each type of tRNA carries one specific type of amino acid.
• The anticodon on a tRNA molecule binds to a complementary codon on a mRNA molecule during the process of translation.
• To make a protein, a specific gene is transcribed to make a molecule of pre-mRNA. The whole gene is transcribed including the introns and exons.
• In eukaryotes, the pre-mRNA is spliced to form mRNA, which is a copy of the exon sections of the gene only.

  This mRNA is then translated to produce polypeptides.
• Transcription (1):

  • One gene unwinds due to hydrogen bonds between complementary bases being broken.
  • Complementary free RNA nucleotides bind to the exposed bases on the template strand (adenine with uracil, thymine with adenine and guanine with cytosine).
  • The sugar phosphate backbone between free RNA nucleotides is then joined using RNA polymerase.
• Transcription (2):

  • Both the intron and exon sections of the gene are then transcribed to make pre-mRNA.
  • Transcription stops at a stop triplet such as ATC. This marks the end of the gene and causes disengagement of RNA polymerase.
• Transcription (3):

  • A gene is transcribed repeatedly to make many pre-mRNA at one time.
  • Most genes are transcribed to make pre-mRNA, however a few genes are transcribed to make ribosomal RNA and transfer RNA.
• Transcription summary:

  • One gene unwinds as hydrogen bonds between complementary base pairs are broken.
  • Complementary free RNA nucleotides bind to exposed bases on the template strand according to the complementary base pairing rule.
  • The sugar phosphate backbone between free RNA nucleotides is joined by RNA polymerase.
  • Both intron and exon sections make up pre-mRNA.
  • Transcription stops at the stop triplet, and RNA polymerase disengages
• RNA polymerase joins nucleotides together to form mRNA, pre-mRNA and RNA. It forms phosphodiester bonds between the ribose sugar and phosphates of RNA nucleotides.
• In eukaryotic nuclei, after transcription the pre-mRNA is then spliced. The non-coding sections (introns) are cut out and the coding sections (exons) are edited together to form mRNA.
• Prokaryotic DNA and mitochondrial and chloroplast DNA does not contain introns, so the mRNA does not need splicing.
• After transcription/ splicing, the mRNA leaves the nucleus through a nuclear pore and binds to a ribosome. Ribosomes contain ribosomal RNA and proteins.
• Translation (1):

  1. The ribosome binds to the first two codons on the mRNA.
  2. A tRNA carrying a methionine amino acid comes to the ribosome.
  3. The anticodon on the tRNA binds to the complementary codon on the mRNA.
• Translation (2):

  1. A second tRNA binds to the next codon, carrying a specific amino acid.
  2. The two amino acids undergo a condensation reaction and a peptide bond forms between them.
  3. The first tRNA then leaves the ribosome and picks up another amino acid.
• Translation (3):

  1. The ribosome moves along the mRNA to cover the next codon.
  2. The anticodon of another tRNA, also carrying a specific amino acid, binds to the next codon.
  3. The amino acid binds to the growing polypeptide chain.
  4. When the ribosome reaches a stop codon it disengages from the mRNA (there are no tRNA with anticodons complementary to stop codons).
• Translation (summary):

  • The ribosome binds to the first two codons of the mRNA.
  • The anticodon of a tRNA carrying a methionine amino acid binds to the complementary first codon of the mRNA.
  • A second tRNA carrying a specific amino acid binds to the next codon.
  • The two amino acids undergo a condensation reaction to form a peptide bond between them.
  • The first tRNA leaves the ribosome to pick up another specific amino acid. The anticodon of another tRNA binds to the complementary next codon.
  • This process continues until the ribosome disengages at the stop codon.
• ATP is required to provide the energy for the bond formation between the amino acid and the tRNA molecule, allowing the tRNA to carry an amino acid to the ribosome during translation.