Gene Expression: From Gene 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: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide (protein) from an mRNA template.

• Central Dogma: The flow of genetic information follows the sequence: DNA → RNA → Protein.

Transcription and Translation: Basic Principles

Transcription

Transcription is the process by which a segment of DNA is copied into RNA by the enzyme RNA polymerase. The resulting RNA is complementary to the DNA template strand.

• RNA Polymerase: Catalyzes RNA synthesis.

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

• Terminator: Sequence signaling the end of transcription (in prokaryotes).

• Transcription Unit: The stretch of DNA that is transcribed.

Translation

Translation is the process by which the sequence of an mRNA is decoded to produce a polypeptide. Ribosomes facilitate the coupling of tRNA anticodons with mRNA codons during protein synthesis.

• Ribosomes: Sites of translation.

• tRNA: Transfers specific amino acids to the growing polypeptide chain.

• Codons: Three-nucleotide sequences in mRNA that specify amino acids.

Genetic Code and Codons

Triplet Code and Codons

The genetic code is based on a triplet code, where each codon consists of three nucleotides. There are 64 codons, 61 of which code for amino acids and 3 are stop signals.

• Redundancy: Multiple codons can code for the same amino acid.

• Reading Frame: Codons must be read in the correct grouping for proper translation.

• Universal Code: The genetic code is nearly universal across all organisms.

Transcription and Translation Process

One DNA strand (the template strand) is used to synthesize a complementary RNA strand. The nontemplate strand (coding strand) matches the RNA sequence except for thymine (T) replaced by uracil (U).

• Template Strand: Used for RNA synthesis.

• Coding Strand: Matches mRNA sequence (except T/U).

Transcription: A Closer Look

Initiation, Elongation, and Termination

Transcription occurs in three stages: initiation, elongation, and termination. In eukaryotes, transcription factors and a TATA box promoter are crucial for initiation.

• Initiation: RNA polymerase binds to promoter, DNA unwinds.

• Elongation: RNA polymerase adds nucleotides to the growing RNA strand.

• Termination: Transcription ends at terminator (prokaryotes) or after polyadenylation signal (eukaryotes).

RNA Processing in Eukaryotes

Modification of mRNA Ends

Before mRNA leaves the nucleus, it undergoes processing. The 5′ end receives a cap, and the 3′ end receives a poly-A tail. These modifications facilitate export, protect mRNA, and help ribosome binding.

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

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

RNA Splicing

Most eukaryotic genes contain introns (noncoding regions) and exons (coding regions). Introns are removed, and exons are spliced together to form mature mRNA.

• Spliceosomes: Complexes that remove introns and join exons.

• Alternative Splicing: Allows production of different polypeptides from the same gene.

Translation: A Closer Look

Molecular Components of Translation

Translation requires mRNA, tRNA, ribosomes, and various protein factors. tRNA molecules carry amino acids and have anticodons that pair with mRNA codons.

• tRNA Structure: Cloverleaf shape, with amino acid attachment site and anticodon.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches amino acids to tRNA.

• Wobble: Flexible pairing at the third base of a codon.

Ribosome Structure and Function

Ribosomes have three binding sites for tRNA: P site (polypeptide), A site (amino acid), and E site (exit). They facilitate the assembly of amino acids into polypeptides.

• P Site: Holds tRNA with growing polypeptide.

• A Site: Holds tRNA with next amino acid.

• E Site: Exit site for discharged tRNA.

Stages of Translation

Translation occurs in three stages: initiation, elongation, and termination. Each stage requires specific protein factors and energy.

• Initiation: Small ribosomal subunit binds mRNA and initiator tRNA (methionine).

• Elongation: Amino acids added to chain via codon recognition, peptide bond formation, and translocation.

• Termination: Stop codon reached, release factor binds, polypeptide released.

Post-Translational Modifications and Protein Targeting

Protein Folding and Modifications

After translation, polypeptides fold into specific shapes and may undergo modifications such as glycosylation or methylation, which are necessary for proper function.

• Folding: Spontaneous formation of secondary and tertiary structure.

• Modifications: Addition of functional groups (e.g., sugars, methyl groups).

Targeting Proteins to Specific Locations

Proteins are directed to specific cellular locations by signal peptides. Ribosomes can be free (cytosol) or bound (ER), depending on the protein’s destination.

• Signal Peptide: Directs polypeptide to ER or other organelles.

• SRP: Signal-recognition particle escorts ribosome to ER.

Polyribosomes and Coupling of Transcription and Translation

Polyribosomes

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome. This allows rapid production of proteins.

• Polyribosome: Several ribosomes translating one mRNA.

• Coupling: In prokaryotes, transcription and translation occur simultaneously.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the genetic information. Point mutations affect a single nucleotide pair and can have various effects on protein structure and function.

• Substitutions: Replace one nucleotide pair with another.

• Silent Mutation: No effect on amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Changes amino acid codon to stop codon.

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

Mutagens

Mutagens are physical or chemical agents that cause mutations. Most carcinogens are mutagens.

• Spontaneous Mutations: Occur during DNA replication or recombination.

• Chemical Mutagens: Various categories, often carcinogenic.

Summary Table: Types of Mutations

Key Vocabulary

• Transcription

• Translation

• mRNA

• tRNA

• Ribosome

• Codon

• Anticodon

• Mutation

• Introns

• Exons

• Spliceosome

• Polyribosome

Important Equations

• Central Dogma:

• Triplet Code: