Genetic code

Genetic code

Objectives

At the end of this
lecture, students will be able to:

• Describe genetic code

• Explain the concept of degeneracy

• Describe the effect of mutations on genetic code

Content

• Genetic code

• Concept of degeneracy

• Effect of mutation on genetic code

Genetic code

• A set of rules by which information encoded within genetic
material is translated into proteins

• Decoding is accomplished by ribosomes

• Ribosome links amino acids in an order specified by mRNA
using tRNA

• Code defines the sequence of these nucleotide triplets – codons

• Every triplet of nucleotides in a nucleic acid sequence
specifies a single amino acid

• Majority of genes are encoded with exactly same code – canonical or standard genetic code

• Some variant codes have evolved

• Protein synthesis in human mitochondria relies on a
genetic code that differs from standard genetic code

• 64 different codon combinations are possible with a
triplet codon of 3 nucleotide

• 64 codons are assigned to amino acid or stop signal

• RNA sequence – UUUAAACCC – reading frame (5’to 3’)

• Consists of 3 codons – UUU, AAA, CCC – specifies one amino
acid

Degeneracy
of genetic code

• Codons GAA and GAG – specify glutamic acid

• Exhibit redundancy, neither specify any other amino acid

• Codons encoding amino acids may differ in any of the three
positions in 2nd or 3rd position

• Glutamic acid is specified by GAA and GAG codon (diff in 3^rd
position)

• Leucine – UUA, UUG, CUU, CUC, CUA, CUG (diff. in 1^st
& 3^rd position)

Salient
features of genetic code

Sequence reading
frame

• Way of dividing the sequence of nucleotides

• Set of consecutive, non-overlapping triplets

• The actual frame work in which a protein sequence is
translated – start codon

• Open reading frame – region of nucleotide sequence from
start codon (ATG) to stop codon

Start/stop codons

• Translation starts with a chain initiation codon (start
codon)

• Start codon+ nearby sequences+ initiator factors – to
start translation

• Most common start codon – AUG

• 3 stop codons – UAG, UGA and UAA

• Stop codons – termination or non-sense codons

Effect of
Mutation

• Most common is transition

• One purine changed to other purine

• One pyrimidine to other pyrimidine

• Transversion – pyrimidine change to purine & vice versa

• Can have an impact on the phenotype of organism if occurs
in coding sequence of gene

• Very low error rates – 1 in every 10-100 million bases –
due to proof reading ability of DNA polymerase

• Missense and nonsense mutations – Point mutations

• Genetic disease – Sickle cell anemia & Thalassemia

• Mutations disrupting reading frame sequence – Frameshift mutations

Transfer of
information via genetic code

• Genes coding for proteins consists of tri –nucleotide unit
– Codon

• Nucleotide – Phosphate+ deoxy sugars + nitrogen bases

• Each protein coding gene is transcribed into RNA polymer
in prokaryotes and to mRNA in eukaryotes

• mRNA is inturn translated on a ribosome into a chain of
amino acids – polypeptide

Summary

• Genetic code is a set of rules by which information
encoded within genetic material is translated into proteins

• Code defines the sequence of these nucleotide triplets
known as codons

• 64 different codon combinations are possible with a
triplet codon of 3 nucleotide

• The actual frame work in which a protein sequence is
translated is known as start codon

• Most common start codon – AUG and 3 stop codons – UAG, UGA
and UAA

• Transitions and transversion are the mutational changes
that may occur in genetic code