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

Transcription and the Genetic Code

Overview of Transcription and Translation

Transcription and translation are fundamental processes in molecular genetics, enabling the conversion of genetic information from DNA to functional proteins. Transcription is the synthesis of RNA from a DNA template, while translation is the process by which ribosomes synthesize proteins using the information encoded in messenger RNA (mRNA).

• Transcription: DNA is used as a template to produce RNA, specifically mRNA, in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble amino acids into a polypeptide chain, forming proteins.

• Key molecules: tRNA (transfer RNA) brings amino acids to the ribosome, matching mRNA codons via its anticodon.

Example:

DNA sequence: GAAGGTTTATTTAA Transcription produces mRNA: CUUCCAAAUAAAUU Translation yields polypeptide: Met–Lys–Phe

The Genetic Code

Properties of 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 composed of codons, which are sequences of three nucleotides.

• Linear form: The code is read in a linear sequence, three nucleotides at a time.

• Codon: Each triplet of nucleotides (codon) specifies a single amino acid.

• Unambiguous: Each codon specifies only one amino acid.

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

• Start and stop signals: Specific codons signal the initiation (start codon, usually AUG) and termination (stop codons: UAA, UAG, UGA) of translation.

• No internal punctuation: Codons are read sequentially without pauses.

• Non-overlapping: Each nucleotide is part of only one codon.

• Nearly universal: The genetic code is conserved across most organisms.

Example:

The codon AUG codes for methionine and serves as the start codon for translation.

The Triplet Nature of the Genetic Code

Frameshift Mutations and Codon Structure

The triplet nature of the genetic code was determined through experiments involving frameshift mutations. Insertion or deletion of nucleotides shifts the reading frame, altering the resulting protein sequence.

• Frameshift mutation: Addition or removal of one or two nucleotides disrupts the reading frame, while addition or removal of three nucleotides preserves the frame but may alter one amino acid.

• Triplet code: The reading frame is maintained by reading three nucleotides at a time.

Example:

Original mRNA: AUG ACC CAU GAU UAC GGA UUC Frameshift by insertion: AUG ACC CAA UGA UUA CGG AUU C Result: Different amino acid sequence due to altered reading frame.

Table: Effects of Frameshift Mutations

Mutation Type	Insertion/Deletion	Translational Reading Frame
Wild-type sequence	None	THE BIG BOY SAW THE NEW CAT EAT THE HOT DOG
Single insertion	(+)	TH EBI GBO YSA WTH ENE WCA TEA TTH EHO TDO G
Single deletion	(-)	TEH IGB OYS AWT HEN EWC ATE ATT HEH OTD OG
Double insertion	(+)(+)	THE BIG OYS AWT HEN EWC ATE ATT HEH OTD OG
Double deletion	(-)(-)	TH EBI GBO YSA WTH ENE WCA TEA TTH EHO TDO G
Triple insertion	(+)(+)(+)	THE BIG BOY SAW THE NEW CAT EAT THE HOT DOG
Triple deletion	(-)(-)(-)	THE BIG BOY SAW THE NEW CAT EAT THE HOT DOG

Nonoverlapping Nature of the Genetic Code

Evidence for Nonoverlapping Codons

The genetic code is nonoverlapping, meaning each nucleotide is part of only one codon. This was determined by analyzing protein sequences and mutation effects.

• Nonoverlapping code: Each base is read only once in a single codon.

• Mutation effects: A single base mutation affects only one amino acid, not adjacent ones.

Example:

If the code were overlapping, a mutation would change two adjacent amino acids. In a nonoverlapping code, only one amino acid is affected.

Table: Overlapping vs. Nonoverlapping Codons

Code Type	Possible Codons	Mutation Effect
Overlapping	GTAC, TAC, GTA, ACA	Single mutation affects two amino acids
Nonoverlapping	202 possible combinations	Single mutation affects one amino acid

Translation: tRNA and Ribosomes

Mechanism of Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. tRNA molecules bring specific amino acids to the ribosome, matching mRNA codons via their anticodon regions.

• Initiation: The ribosome assembles at the start codon (AUG) on the mRNA.

• Elongation: tRNA molecules enter the ribosome, matching their anticodon to the mRNA codon, and the ribosome catalyzes peptide bond formation.

• Translocation: The ribosome moves along the mRNA, reading each codon and adding the corresponding amino acid.

• Termination: When a stop codon is reached, translation ends and the polypeptide is released.

Example:

tRNA carrying methionine recognizes the start codon AUG and initiates protein synthesis.

Summary Table: Key Features of the Genetic Code

Feature	Description
Codon Length	Three nucleotides (triplet)
Unambiguous	Each codon specifies one amino acid
Degenerate	Multiple codons can specify the same amino acid
Start/Stop Signals	AUG (start), UAA/UAG/UGA (stop)
Nonoverlapping	Each base is part of only one codon
Universal	Code is nearly universal across species

Additional info: The notes and images provided are consistent with foundational topics in college-level genetics, including the structure and function of the genetic code, transcription, translation, and the molecular basis of gene expression.