Genetic Code
The genetic code is the set of rules by which information encoded in
mRNA sequences is converted into a polypeptide sequence
Codons: Triplets of bases which correspond to a particular amino acid
The order of the codons determines the amino acid sequence for a protein
• A coding sequence always begins with a start codon (AUG)
• A coding sequence is terminated with a stop codon
The genetic code has two key features:
• Universality – All organisms use the same genetic code
• Degeneracy – Multiple codons may code for the same amino acid
Topic 2.7: TRAnSCRIPTIOn & TRAnSLATIOn!
Transcription
Transcription is the synthesis of an RNA sequence from a DNA template
• This process occurs within the nucleus of a cell
Transcription is mediated by the enzyme RNA polymerase, which:
• Separates the DNA strands (breaks H bonds between base pairs)
• Covalently joins free complementary RNA nucleotides together
After transcription, the RNA is released to the cytoplasm (for translation)
Types of RNA
Three main types of RNA may be produced:
• mRNA – Transcript used to make protein
• tRNA – Transfers amino acid to ribosome
• rRNA – Catalytic component of ribosome
mRNA rRNA tRNA
UUU"
Phe"
UCU"
Ser"
UAU"
Tyr"
UGU"
Cys"
UUC" UCC" UAC" UGC"
UUA"
Leu"
UCA" UAA" STOP" UGA" STOP"
UUG" UCG" UAG" STOP" UGG" Trp"
CUU"
Leu"
CCU"
Pro"
CAU"
His"
CGU"
Arg"
CUC" CCC" CAC" CGC"
CUA" CCA" CAA"
Gln"
CGA"
CUG" CCG" CAG" CGG"
AUU"
Ile"
ACU"
Thr"
AAU"
Asn"
AGU"
Ser"
AUC" ACC" AAC" AGC"
AUA" ACA" AAA"
Lys"
AGA"
Arg"
AUG" Met" ACG" AAG" AGG"
GUU"
Val"
GCU"
Ala"
GAU"
Asp"
GGU"
Gly"
GUC" GCC" GAC" GGC"
GUA" GCA" GAA"
Glu"
GGA"
GUG" GCG" GAG" GGG"
Translation
Translation is the process of polypeptide synthesis by the ribosome
• Messenger RNA (mRNA) is transported to the ribosome
• A ribosome reads an mRNA sequence in base triplets called codons
• Each codon codes for a specific amino acid (as per the genetic code)
• Amino acids are transported to ribosomes by transfer RNA (tRNA)
• Each tRNA aligns opposite a codon via a complementary anticodon
• The ribosome moves along the mRNA sequence (5’ → 3’) and joins
amino acids together with peptide bonds (condensation reaction)
• The synthesis of a polypeptide is initiated at a start codon (AUG)
and is completed when the ribosome reaches a STOP codon
tRNA
polypeptide amino acid
codon
anticodon
mRNA
ribosome
Gene
→
Protein
A gene is a sequence of DNA which encodes a polypeptide sequence
• One gene = one polypeptide (proteins may have multiple polypeptides)
There are exceptions to this fundamental relationship:
• Genes may be alternatively spliced (one gene = many polypeptides)
• Genes encoding tRNA or rRNA are transcribed but not translated
• Genes may be mutated to alter the original polypeptide product
DNA (gene)
mRNA
polypeptide
transcription
translation
