Translation and the Genetic Code: Structure, Function, and Mutations

Translation: Protein Synthesis from mRNA

Overview of Translation

Translation is the process by which the genetic information encoded in messenger RNA (mRNA) is used to synthesize a polypeptide (protein). This process occurs in the ribosome and involves several key molecular components.

• mRNA: Serves as the template carrying the genetic code from DNA.

• Ribosomes: Complexes of rRNA and proteins that facilitate the assembly of amino acids into polypeptides.

• tRNA: Transfer RNA molecules that bring specific amino acids to the ribosome according to the mRNA codon sequence.

• Translation factors: Proteins that assist in initiation, elongation, and termination of translation.

• Enzymes: Catalyze reactions during protein synthesis.

Example: The synthesis of hemoglobin in red blood cells is a classic example of translation in action.

One Gene, One Polypeptide Hypothesis

Evolution of the Concept

Each gene (a specific DNA sequence) codes for a single polypeptide product. This concept has evolved over time:

• Originally: "One gene, one enzyme" (Beadle and Tatum)

• Later: "One gene, one protein"

• Current: "One gene, one polypeptide" (recognizing that some proteins are made of multiple polypeptides)

Example: The gene for insulin codes for a single polypeptide that is processed into the active hormone.

The Genetic Code

Structure and Properties

The genetic code is a set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells. It is read in triplets called codons.

• Codon: A sequence of three nucleotide bases in mRNA that specifies a particular amino acid.

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

• Start codon: AUG (codes for Methionine) signals the start of translation.

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

Example: The codon UUU codes for the amino acid Phenylalanine.

Codon	Amino Acid	Function
AUG	Methionine	Start codon
UAA, UAG, UGA	None	Stop codons
UUU, UUC	Phenylalanine	Amino acid coding
GGC	Glycine	Amino acid coding
AGC	Serine	Amino acid coding

Frameshift Mutations

Insertion and Deletion Effects

Frameshift mutations occur when nucleotides are inserted or deleted from the DNA sequence, altering the reading frame of the genetic code. This can drastically change the resulting protein.

• Normal sequence: Codons are read in groups of three bases.

• Insertion: Addition of a base shifts the reading frame, changing all downstream codons.

• Deletion: Removal of a base also shifts the reading frame.

• Effect: Often results in nonfunctional proteins due to incorrect amino acid sequence.

Example: Insertion of a T in the sequence ATG CAT TGC changes the reading frame and the resulting amino acids.

Nature of the Genetic Code

Transcription and Translation

The genetic code is transcribed from DNA to mRNA and then translated into a polypeptide chain. The process involves reading the mRNA in the 5' to 3' direction and assembling amino acids accordingly.

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

• mRNA: Contains codons that are translated into amino acids.

• Polypeptide: The chain of amino acids formed during translation.

Example: DNA sequence 5'-ATGGGCTCC-3' is transcribed to mRNA 5'-AUGGGCUCC-3', which is translated to Met-Gly-Ser.

Summary Table: Key Features of the Genetic Code

Feature	Description	Example
Triplet Code	Each codon consists of three bases	AUG, UUU, GGC
Start Codon	Signals initiation of translation	AUG (Methionine)
Stop Codons	Signal termination of translation	UAA, UAG, UGA
Degeneracy	Multiple codons code for the same amino acid	Leu: UUA, UUG, CUU, CUC, CUA, CUG
Frameshift Mutation	Insertion or deletion alters reading frame	ATG CAT TGC → ATT GCA TTG C

Key Equations and Concepts

• Number of possible codons: $4^3 = 64$

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Additional info: These notes expand on the provided slides by including definitions, examples, and tables for clarity and completeness, suitable for college-level Cell Biology study.