Covers the genetic code and how codons are read during translation. It explains the experiments that decoded the 64 codons, the start and stop codons, and reading frames, including open and closed frames. Point mutations are classified as silent, missense, nonsense and frameshift. The note then walks through the participants in translation, such as tRNA, aminoacyl-tRNA synthetase and the ribosome, followed by initiation, elongation and termination.
Inside: 1 section · 3 labelled pictures · 6 min
Topic 8 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 1 section, about 831 words (6 min of reading).
In this note
Introduction
Introduction
Genetic Code:
Definition: The genetic code is the mechanism by which genetic information is translated from nucleic acids to proteins.
Codons:
Definition: Codons are nucleotide triplets that encode specific amino acids.
Decoding the Genetic Code:
Experimental Approach:
Scientists utilized the bacterial enzyme polynucleotide phosphorylase.
This enzyme binds nucleotides together without requiring a template.
Short artificial "mRNAs" were synthesized using this enzyme.
These synthetic mRNAs were then added to ribosomes for in vitro translation.
The peptides synthesized during this process were sequenced.
Outcome:
Through these experiments, scientists elucidated the meaning of each of the 64 codons.
Stop Codons:
Definition:
Three out of the 64 codons (UAA, UAG, and UGA) do not encode for an amino acid.
Instead, they serve as signals to stop protein synthesis.
These codons are referred to as stop, terminal, or nonsense codons.
START Codon:
Dual Function:
AUG codon has a dual role:
Encodes for the amino acid methionine.
Acts as the start signal for translation.
Reading Frame:
Definition:
A reading frame refers to the specific way in which RNA or DNA is divided into codons during translation or transcription.
It begins with a start codon (usually AUG) and ends with a stop codon (UAA, UAG, or UGA).
RNA has three possible reading frames, while DNA has six frames (three for each strand).
Open Reading Frame (ORF):
Definition:
In a gene with its true start codon, there are no stop codons for a significant length.
This absence of stop codons is due to natural selection.
Such reading frames are termed as open reading frames (ORFs).
Closed Reading Frame:
Definition:
Reading frames that contain randomly formed start and stop codons are termed as closed or blocked.
These reading frames typically have stop codons occurring frequently after each start codon.
Point Mutations and Genetic Code:
Definition:
Point mutations involve changes in a single nucleotide within the DNA sequence.
These mutations are relevant in understanding the genetic code's impact on protein synthesis.
Types of Point Mutations:
Substitution:
Involves the replacement of a single nucleotide.
Deletion:
Removal of a nucleotide.
Insertion:
Addition of a nucleotide.
Effects of Point Mutations:
Silent Mutations:
Many substitutions lead to another codon for the same amino acid due to the code's degeneracy.
These mutations have no effect on the resulting protein.
Missense Mutations:
Result from substitutions that produce a codon for a different amino acid.
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