Gene Expression: From Gene to Protein

Introduction to Gene Expression

Gene expression is the process by which information encoded in DNA directs 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 and phenotype, and the flow of genetic information is often summarized as the central dogma: DNA → RNA → Protein.

Evidence for the One Gene–One Polypeptide Hypothesis

Historical Experiments

• Archibald Garrod (1902): Proposed that genes dictate phenotypes through enzymes that catalyze specific reactions.

• Beadle and Tatum: Demonstrated that each gene encodes a specific enzyme by studying nutritional mutants in Neurospora (bread mold).

• The hypothesis evolved from "one gene–one enzyme" to "one gene–one protein," and finally to "one gene–one polypeptide," as many proteins are composed of multiple polypeptides.

Basic Principles of Transcription and Translation

Overview of the Processes

Transcription is the synthesis of RNA from a DNA template, producing messenger RNA (mRNA). Translation is the synthesis of a polypeptide using the information in mRNA, occurring at ribosomes. In prokaryotes, translation can begin before transcription is complete, while in eukaryotes, the nuclear envelope separates these processes and RNA undergoes processing before translation.

The Central Dogma

The central dogma of molecular biology describes the directional flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

The Genetic Code

Codons and the Triplet Code

The genetic code is based on codons, which are sequences of three nucleotides in mRNA that specify amino acids. There are 64 possible codons, 61 of which code for amino acids and 3 serve as stop signals. The code is redundant (more than one codon can specify the same amino acid) but not ambiguous (each codon specifies only one amino acid).

Template and Coding Strands

• The template strand of DNA is used to synthesize a complementary RNA transcript.

• The coding strand has the same sequence as the mRNA (except T is replaced by U in RNA).

Universality of the Genetic Code

The genetic code is nearly universal among all organisms, allowing genes from one species to be expressed in another. This universality supports the idea of a common evolutionary origin.

Transcription: DNA-Directed Synthesis of RNA

Molecular Components and Steps

Transcription is catalyzed by RNA polymerase, which binds to the promoter region of DNA and synthesizes RNA in the 5′ to 3′ direction. The process involves three main stages: initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter with the help of transcription factors (in eukaryotes, the TATA box is important).

• Elongation: RNA polymerase unwinds DNA and adds RNA nucleotides complementary to the template strand.

• Termination: In bacteria, transcription ends at a terminator sequence; in eukaryotes, it ends after the polyadenylation signal sequence is transcribed.

RNA Processing in Eukaryotes

Modification of mRNA Ends

Before mRNA leaves the nucleus, it undergoes processing:

• The 5′ end receives a modified nucleotide cap.

• The 3′ end receives a poly-A tail.

• These modifications facilitate export, protect mRNA, and help ribosomes recognize the mRNA.

RNA Splicing

Most eukaryotic genes contain introns (noncoding regions) and exons (coding regions). Introns are removed and exons are joined together by spliceosomes, which are complexes of proteins and small RNAs. Some RNA molecules (ribozymes) can catalyze their own splicing.

Alternative RNA Splicing

Alternative splicing allows a single gene to code for multiple proteins by varying which exons are included in the final mRNA. This increases protein diversity without increasing the number of genes.

Protein Domains and Exon Shuffling

Proteins often have modular regions called domains, which are frequently encoded by separate exons. Exon shuffling can lead to new proteins with novel functions.

Translation: RNA-Directed Synthesis of a Polypeptide

Molecular Components

Translation converts the genetic information in mRNA into a specific sequence of amino acids in a polypeptide. This process requires:

• tRNA (transfer RNA): Brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon.

• Ribosomes: Facilitate the coupling of tRNA anticodons with mRNA codons and catalyze peptide bond formation.

Structure and Function of tRNA

• tRNA molecules have a cloverleaf structure with an amino acid attachment site at the 3′ end and an anticodon loop that pairs with mRNA codons.

• tRNA is charged with the correct amino acid by aminoacyl-tRNA synthetase.

• Wobble pairing at the third codon position allows some tRNAs to recognize multiple codons.

Ribosome Structure and Function

• Ribosomes have three binding sites for tRNA: the A site (aminoacyl), P site (peptidyl), and E site (exit).

• Translation occurs in three stages: initiation, elongation, and termination.

Stages of Translation

• Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (carrying methionine), then the large subunit joins to form the initiation complex.

• Elongation: Amino acids are added one by one to the growing chain through codon recognition, peptide bond formation, and translocation.

• Termination: When a stop codon is reached, a release factor binds, causing the polypeptide to be released.

Protein Folding and Post-Translational Modifications

Newly synthesized polypeptides fold into their functional three-dimensional shapes. Some undergo further modifications, such as cleavage or the addition of chemical groups, to become fully functional proteins.

Targeting Proteins to Specific Locations

Proteins may be targeted to specific cellular locations by signal peptides. Free ribosomes synthesize cytosolic proteins, while bound ribosomes (on the ER) synthesize proteins for secretion or for the endomembrane system.

Mutations: Changes in Genetic Information

Types of Mutations

• Point mutations: Changes in a single nucleotide pair, including substitutions, insertions, and deletions.

• Substitutions: Can be silent (no effect), missense (change one amino acid), or nonsense (introduce a stop codon).

• Insertions and deletions: Can cause frameshift mutations, altering the reading frame and usually resulting in nonfunctional proteins.

Mutagens and Gene Editing

Mutations can occur spontaneously or be induced by mutagens (physical or chemical agents). Modern gene editing techniques, such as CRISPR-Cas9, allow scientists to introduce targeted mutations or correct genetic defects.

What Is a Gene?

Modern Definition

A gene is a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule (such as mRNA, tRNA, or rRNA). The concept of the gene has evolved from a simple unit of inheritance to a complex functional element in the genome.