Gene Expression: From Gene to Protein

Overview of Gene Expression

Gene expression is the process by which information encoded in DNA directs the synthesis of proteins, which are essential for cellular structure and function. This process involves two main stages: transcription and translation. Proteins serve as the link between genotype and phenotype, determining the traits of an organism.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide (protein) using the information in mRNA.

• Central Dogma: The flow of genetic information is typically DNA → RNA → Protein.

DNA vs. RNA

DNA and RNA are nucleic acids that differ in structure and function. DNA stores genetic information, while RNA acts as a messenger and functional molecule in protein synthesis.

• DNA: Double-stranded, contains deoxyribose sugar, bases are A, T, C, G.

• RNA: Single-stranded, contains ribose sugar, bases are A, U, C, G.

Transcription: From DNA to RNA

Basic Principles of Transcription

Transcription is the process by which a segment of DNA is used as a template to synthesize a complementary RNA strand. In eukaryotes, this occurs in the nucleus; in prokaryotes, it occurs in the cytoplasm.

• RNA polymerase catalyzes RNA synthesis, binding to the DNA at the promoter region.

• The template strand of DNA is used to assemble a complementary RNA molecule.

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

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter, aided by transcription factors in eukaryotes (e.g., TATA box).

• Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' to 3' direction.

• Termination: In prokaryotes, transcription stops at a terminator sequence; in eukaryotes, the transcript is released after a polyadenylation signal.

RNA Processing in Eukaryotes

In eukaryotic cells, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

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

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

• RNA Splicing: Removal of noncoding regions (introns) and joining of coding regions (exons).

Alternative RNA Splicing

Alternative splicing allows a single gene to code for multiple proteins by varying which exons are included in the final mRNA.

Translation: From RNA to Protein

The Genetic Code

The genetic code is a set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. Each three-nucleotide sequence (codon) specifies a particular amino acid.

• There are 64 codons: 61 code for amino acids, 3 are stop signals.

• The code is redundant but not ambiguous.

• Codons must be read in the correct reading frame.

Molecular Components of Translation

Translation occurs in the cytoplasm and involves mRNA, transfer RNA (tRNA), and ribosomes.

• tRNA: Brings amino acids to the ribosome; has an anticodon that pairs with mRNA codons.

• Ribosome: Site of protein synthesis; has three binding sites (A, P, E) for tRNA.

Stages of Translation

• Initiation: Small ribosomal subunit binds mRNA and initiator tRNA (carrying methionine) at the start codon (AUG).

• Elongation: Amino acids are added one by one to the growing polypeptide chain.

• Termination: Occurs when a stop codon is reached; a release factor binds, releasing the polypeptide.

Protein Folding and Targeting

After translation, polypeptides fold into their functional three-dimensional shapes and may be targeted to specific cellular locations.

• Signal peptides direct proteins to the endoplasmic reticulum or other organelles.

• Polypeptides may be modified after translation (post-translational modifications).

Mutations and Gene Editing

Types of Mutations

Mutations are changes in the genetic material that can affect protein structure and function.

• Point mutations: Change a single nucleotide pair (substitutions, insertions, deletions).

• Silent mutations: Do not change the amino acid sequence.

• Missense mutations: Change one amino acid to another.

• Nonsense mutations: Change an amino acid codon to a stop codon.

• Frameshift mutations: Insertions or deletions that alter the reading frame.

CRISPR and Gene Editing

CRISPR-Cas9 is a revolutionary gene-editing technology that allows scientists to make precise changes to DNA, including correcting disease-causing mutations.

• Cas9 protein, guided by RNA, cuts the target DNA sequence.

• DNA repair mechanisms can introduce or correct mutations.

Summary Table: Key Steps in Gene Expression