Gene Expression: From DNA to Protein (Transcription, Translation, and Mutations)

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. This process involves two main stages: transcription and translation. In eukaryotes, gene expression also includes RNA processing and post-translational modifications.

Transcription

Definition and Steps of Transcription

Transcription is the synthesis of messenger RNA (mRNA) from a DNA template. It occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotes.

• Initiation: RNA polymerase binds to the promoter region of the gene, unwinding the DNA strands.

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

• Termination: Transcription ends when RNA polymerase reaches a terminator sequence, releasing the newly formed mRNA.

Key Parts of Transcription:

• Promoter: DNA sequence where RNA polymerase attaches.

• Terminator: Sequence signaling the end of transcription.

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

RNA Processing (Eukaryotes Only)

Modifications of Pre-mRNA

In eukaryotes, the primary transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA:

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

• Polyadenylation: Addition of a poly-A tail to the 3' end.

• Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons).

Example: The mature mRNA is exported from the nucleus to the cytoplasm for translation.

Translation

Definition and Steps of Translation

Translation is the process by which ribosomes synthesize a polypeptide using the sequence of codons in mRNA.

• Initiation: The small ribosomal subunit binds to the mRNA and the initiator tRNA. The large subunit then joins to form the complete ribosome.

• Elongation: tRNAs bring amino acids to the ribosome, matching their anticodons to the codons on the mRNA. Peptide bonds form between amino acids.

• Termination: When a stop codon is reached, the ribosome releases the completed polypeptide.

Key Parts of Translation:

• mRNA: Carries the genetic code from DNA.

• tRNA: Brings amino acids to the ribosome and matches them to the mRNA codons.

• Ribosome: Site of protein synthesis, composed of rRNA and proteins.

Using the Triplet Code to Create a Polypeptide

The genetic code is read in sets of three nucleotides (codons), each specifying an amino acid.

• Example: DNA: TAC → mRNA: AUG → Amino Acid: Methionine (start codon)

Formula:

$\text{DNA (template)} \rightarrow \text{mRNA (codons)} \rightarrow \text{Amino Acids (polypeptide)}$

Prokaryotic vs. Eukaryotic Transcription and Translation

Key Differences

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus (transcription), Cytoplasm (translation)
RNA Processing	None	5' cap, poly-A tail, splicing
Timing	Coupled (can occur simultaneously)	Separated by nuclear envelope

How Ribosomes Read mRNA

Ribosomes move along the mRNA in the 5' to 3' direction, reading each codon and facilitating the addition of the corresponding amino acid to the growing polypeptide chain.

• Start Codon: AUG (methionine)

• Stop Codons: UAA, UAG, UGA

Post-Translational Modifications

Types and Functions

After translation, polypeptides may undergo modifications that affect their function:

• Phosphorylation: Addition of phosphate groups

• Glycosylation: Addition of carbohydrate groups

• Cleavage: Removal of signal peptides or segments

• Folding: Assisted by chaperone proteins

Example: Insulin is synthesized as a precursor and then cleaved to its active form.

Types of Mutations

Classification and Effects

• Point Mutation: Change in a single nucleotide

• Silent Mutation: No change in amino acid sequence

• Missense Mutation: Changes one amino acid

• Nonsense Mutation: Introduces a premature stop codon

• Frameshift Mutation: Insertion or deletion that alters the reading frame

Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

Additional info: These notes expand on the original outline by providing definitions, examples, and a comparison table for prokaryotic and eukaryotic gene expression. The content is structured to cover all major aspects of transcription, translation, RNA processing, and mutations, as relevant to a college-level biology course.