Genetic Code and Translation

Introduction to the Genetic Code

The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells. It is nearly universal among organisms and is fundamental to the process of gene expression.

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal during translation.

• Start Codon: AUG (codes for methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal termination of translation.

• Degeneracy: Most amino acids are encoded by more than one codon.

• Unambiguous: Each codon specifies only one amino acid.

• Nonoverlapping: Each nucleotide is part of only one codon.

• Commaless: Codons are read sequentially without breaks.

Example: The codon sequence AUG-GUG-UUG-AGC translates to the amino acids Met-Val-Leu-Ser.

Codon Table and Wobble Hypothesis

The codon table shows which triplet codons correspond to which amino acids. The wobble hypothesis explains how some tRNAs can recognize more than one codon due to flexible base pairing at the third position.

• Wobble Position: The third base of the codon allows for non-standard pairing, enabling one tRNA to pair with multiple codons.

• Base-Pairing Rules: Standard Watson-Crick pairing applies at the first two positions, but the third position can tolerate mismatches (e.g., G-U pairing).

Example: tRNA with anticodon 3'-UAI-5' can pair with codons 5'-AUA-3', 5'-AUU-3', and 5'-AUC-3'.

Exceptions to the Universal Genetic Code

While the genetic code is nearly universal, some exceptions exist, especially in mitochondria and certain protozoa.

Additional info: These exceptions are important in evolutionary genetics and biotechnology applications.

Gene Mutations and Their Effects

Types of Mutations Affecting the Genetic Code

Mutations can alter the genetic code and impact protein synthesis.

• Missense Mutation: A single nucleotide change results in a codon that codes for a different amino acid.

• Nonsense Mutation: A change that converts a codon for an amino acid into a stop codon, leading to premature termination of translation.

• Frameshift Mutation: Insertions or deletions that shift the reading frame, altering all downstream codons.

Example: DNA sequence change from 5'-TGG-3' (Trp) to 5'-TGA-3' (Stop) results in a truncated protein.

Epigenetic Memory and Imprinting

Epigenetic Effects: Dutch Hunger Winter

Epigenetic memory refers to heritable changes in gene expression that do not involve changes to the underlying DNA sequence. The Dutch Hunger Winter is a classic example of how environmental factors can affect epigenetic marks.

• Pregnant women exposed to malnutrition during the first 10 weeks after conception had children who, as adults, showed increased rates of obesity, diabetes, and cardiovascular disease.

• These effects are linked to reduced methylation of the IGF2 gene (a growth factor gene), indicating an imprinting error.

Additional info: DNA methylation is a key epigenetic modification that regulates gene expression and can be influenced by environmental factors.

Hybrid Genetics: Tigons

Genetics of Hybrid Animals

Hybrids such as tigons (offspring of a female lion and a male tiger) demonstrate unique genetic phenomena related to growth and imprinting.

• Tigon: Offspring of a female lion (Panthera leo) and a male tiger (Panthera tigris).

• Tigons are typically the same size as their parents or smaller, in contrast to ligers (offspring of a female tiger and a male lion), which are often larger than both parents.

• This size difference is due to genomic imprinting—certain growth genes are expressed differently depending on whether they are inherited from the mother or father.

Example: The absence of paternal growth gene expression in tigons leads to normal or reduced size.

Transcription and Translation Overview

Transcription

Transcription is the process by which mRNA is synthesized from a DNA template.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Template Strand: The DNA strand used to synthesize mRNA.

• Directionality: Transcription proceeds in the 5' to 3' direction.

Example: The TATA box is a common promoter element in eukaryotes.

Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Initiation: Begins at the start codon (AUG).

• Elongation: tRNAs bring amino acids corresponding to each codon.

• Termination: Occurs when a stop codon is encountered.

Equation:

Reading Frames and Overlapping Genes

Reading Frames

A reading frame is a way of dividing the sequence of nucleotides in mRNA into consecutive, non-overlapping triplets (codons).

• There are three possible reading frames in any mRNA sequence.

• Mutations that shift the reading frame (frameshift mutations) can drastically alter the resulting protein.

Example: The mRNA sequence 5'-AUGCCGACUGCUAACCG-3' can be read in three different frames, each producing a different amino acid sequence.

Overlapping Genes

Some viruses have overlapping genes, where the same DNA sequence encodes more than one protein depending on the reading frame.

• Overlapping genes allow for compact genomes but can complicate gene regulation.

• Disadvantage: Mutations in overlapping regions can affect multiple proteins.

Additional info: Overlapping genes are rare in eukaryotes but common in viruses and some prokaryotes.