From mRNA to Protein (College Board AP Biology)

• After transcription and the post-transcriptional modifications are complete, the mRNA moves out of the nucleus via the nuclear pore and diffuses into the cytoplasm towards a ribosome for translation
• This stage of protein synthesis occurs on ribosomes
  • Ribosomes are found in the cytoplasm of eukaryotic and prokaryotic cells
  • On the rough endoplasmic reticulum of eukaryotic cells
• Translation involves taking the genetic code from the mRNA and synthesizing a polypeptide
  • A polypeptide is a sequence of amino acids covalently bonded together
  • The order of the amino acids is based on the information stored in the genetic code of the mRNA

Protein Synthesis in Prokaryotes

• Conversely to eukaryotes, in prokaryotes, transcription and translation occur simultaneously
• mRNA is transcribed from the circular loop of genetic material and the transcript immediately binds to a ribosome for translation
• This is possible because
  • The genetic material in prokaryotes is found in the cytoplasm rather than being contained inside the nuclear membrane 
  • Prokaryotic DNA does not undergo post-transcriptional processing, e.g. splicing. as prokaryotes have far fewer introns than eukaryotic cells

Comparing Protein Synthesis in Prokaryotes and Eukaryotes

Eukaryotic cells carry out transcription independently of translation whereas prokaryotes carry out both processes simultaneously

The Process of Translation in Eukaryotes

• Translation occurs in the cytoplasm of the cell
• After leaving the nucleus via a nuclear pore, the mRNA molecule attaches to a ribosome
• Initiation of translation 
  • Near the beginning of the mRNA is a triplet of bases called the start codon (AUG)
  • This is a signal to start off translation 
  • AUG codes for an amino acid called methionine

• • In the cytoplasm there are free molecules of tRNA (transfer RNA)
    • tRNA is a single-stranded molecule of RNA that folds into a clover-like structure
    • tRNA molecules have a triplet of unpaired bases at one end, known as the anticodon, and a region at the other end where a specific amino acid can attach
    • There are about 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site
  • The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
  • The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet on the mRNA molecule called the codon
• The elongation phase
  • Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
  • A peptide bond is then formed, via a condensation reaction, between the two amino acids
  • This process continues until a ‘stop’ codon on the mRNA molecule is reached
• Termination of translation
  • The stop codon acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
  • The amino acid chain then forms the final polypeptide

Translation Diagram

The process of translation

• Codons of three bases on mRNA correspond to one amino acid in a polypeptide
• The four nucleotide bases in mRNA are not enough to code for 20 separate amino acids
• Pairs of nucleotides would only give 16 combinations (4² = 16), which is still not enough
• Triplets of nucleotides would yield 64 combinations (4³ = 64), which is more than enough
• Different triplets code for the same amino acid, giving some protection against mutation
  • A triplet is a sequence of three DNA bases that codes for a specific amino acid
  • A codon is a sequence of three mRNA bases that codes for a specific amino acid
  • A codon is transcribed from the triplet and is complementary to it
• An anticodon is a sequence of three transfer RNA (tRNA) bases that are complementary to a codon
  • The transfer RNA carries the appropriate amino acid to the ribosome
  • The amino acid can then be condensed onto the growing polypeptide chain
• Certain codons carry the command to stop translation when the polypeptide chain is complete ('Stop codons')

mRNA Codons and Amino Acids Table

Step 1: Work out the antisense sequence using A-T and C-G base pairing rules

AAG CTC GTA ATG CGG

Step 2: Work out the mRNA codons, complementary to the antisense strand

UUC GAG CAU UAC GCC

Step 3: Use the mRNA Codons and Amino Acids Table (above) to work out the first amino acid

First base in codon = U, second base = U, third base = C

So we're looking in the top-left box of the table; this amino acid is Phe

Step 4: Repeat for the remaining 4 codons

GAG = Glu

CAU = His

UAC = Tyr

GCC = Ala

Answer: The final sequence of amino acids is Phe-Glu-His-Tyr-Ala

Reverse Transcription in Viruses

• The role of reverse transcriptase in the transfer of a gene into an organism is to produce a single strand complementary DNA molecule (cDNA) that contains the code for the desired characteristic, this will then be inserted into a vector (after being converted into a double stranded DNA molecule)
• Reverse transcriptase enzymes are sourced from retroviruses and they catalyze the reaction that reverses transcription
• The mRNA (with the genetic code for the desired gene) is used as a template to synthesize a single strand of complementary DNA (cDNA)
• Reverse transcriptase enzymes are often used as it is easier for scientists to find mRNA with the specific characteristic because specialized cells make very specific types of mRNA (eg. β-cells of the pancreas produce many insulin mRNA) and mRNA does not contain introns
• Reverse transcription is the best known example where the directional flow of the steps of gene expression is reversed