Gene Expression: From DNA to Protein

Overview of Gene Expression

Gene expression is the process by which information encoded in DNA is used to direct the synthesis of proteins, which are responsible for an organism's traits. This process involves two main stages: transcription and translation. Proteins serve as the link between genotype (genetic makeup) and phenotype (observable traits).

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: Ribosomes read the mRNA and synthesize a polypeptide (protein).

• Mutations: Changes in DNA can alter protein function, leading to changes in phenotype.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Transcription: Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• RNA Processing: Eukaryotic cells modify RNA before translation.

• Translation: Occurs in the cytoplasm at ribosomes.

Transcription: DNA-Directed Synthesis of RNA

Basic Principles

• RNA Polymerase: Enzyme that synthesizes RNA from a DNA template.

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

• Stages: Initiation, Elongation, Termination.

• Base Pairing: RNA uses uracil (U) instead of thymine (T).

Initiation of Transcription

• TATA Box: A promoter element crucial for initiation in eukaryotes.

• Transcription Factors: Proteins that help RNA polymerase bind and initiate transcription.

Elongation and Termination

• RNA polymerase moves along the DNA, synthesizing RNA in the 5' → 3' direction.

• Termination differs between prokaryotes and eukaryotes.

RNA Processing in Eukaryotes

Modification of mRNA Ends

Eukaryotic pre-mRNA undergoes modifications:

• 5' Cap: Modified guanine nucleotide added to the 5' end.

• Poly-A Tail: 50–250 adenine nucleotides added to the 3' end.

• These modifications protect mRNA and aid in export and translation.

RNA Splicing

• Introns: Noncoding regions removed from pre-mRNA.

• Exons: Coding regions joined together to form mature mRNA.

• Spliceosome: Complex responsible for removing introns.

Alternative Splicing and Protein Domains

• Alternative splicing allows a single gene to code for multiple proteins.

• Proteins often have modular domains encoded by different exons.

Translation: RNA-Directed Synthesis of Polypeptides

Genetic Code and Codons

• The genetic code consists of triplets of nucleotides (codons) in mRNA.

• Each codon specifies an amino acid or a stop signal.

• The code is redundant but not ambiguous.

Reading DNA and mRNA

• DNA template strand is used to synthesize complementary mRNA.

• mRNA is read in the 5' → 3' direction during translation.

Transfer RNA (tRNA) Structure and Function

• tRNA carries specific amino acids and matches them to codons in mRNA via its anticodon.

• tRNA structure allows it to form hydrogen bonds and attach amino acids.

Aminoacyl-tRNA Synthetase

• Enzyme that attaches the correct amino acid to its tRNA.

• Requires ATP for the attachment process.

Ribosome Structure and Function

• Ribosomes have large and small subunits, each with tRNA binding sites: A, P, and E.

• Facilitate the coupling of tRNA anticodons with mRNA codons.

Stages of Translation

• Initiation: Small ribosomal subunit binds mRNA and initiator tRNA; large subunit joins.

• Elongation: Amino acids are added one by one; involves codon recognition, peptide bond formation, and translocation.

• Termination: Stop codon is reached; release factor causes polypeptide release.

Post-Translational Modifications and Protein Targeting

• Proteins may undergo modifications (glycosylation, methylation) after translation.

• Signal peptides direct proteins to specific cellular locations.

• SRP (signal-recognition particle) helps target proteins to the ER.

Polyribosomes

• Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome.

• Allows rapid production of many polypeptides.

Mutations and Their Effects on Protein Structure

Types of Mutations

• Point Mutations: Change in a single nucleotide pair.

• Substitutions: Replace one nucleotide pair with another.

• Insertions/Deletions: Add or remove nucleotide pairs, often causing frameshift mutations.

Effects of Mutations

• Silent Mutation: No effect on protein due to redundancy in the genetic code.

• Missense Mutation: Changes one amino acid to another.

• Nonsense Mutation: Changes an amino acid codon to a stop codon, leading to a truncated protein.

• Frameshift Mutation: Alters the reading frame, often resulting in a nonfunctional protein.

Mutagens

• Mutagens are physical or chemical agents that cause mutations.

• Most carcinogens are mutagens.

Summary Table: Types of Mutations

Key Vocabulary

• Gene Expression

• Transcription

• Translation

• mRNA

• tRNA

• Ribosome

• Codon

• Mutation

• Introns/Exons

• Spliceosome

• Polyribosome

Important Equations and Concepts

• Base Pairing in Transcription:

• Central Dogma:

Example: Using the Genetic Code

• Given mRNA sequence: AUG-GCU-UAA

• Using the genetic code table, AUG = Methionine (start), GCU = Alanine, UAA = Stop

• Resulting polypeptide: Methionine-Alanine

Summary

Understanding gene expression, transcription, translation, and mutations is fundamental to molecular biology. These processes explain how genetic information is converted into functional proteins and how changes in DNA can impact traits and health. Mastery of these concepts is essential for success in General Biology.