Gene Expression: From Gene to Protein (Chapter 14 Study Notes)

Gene Expression: From Gene to Protein

Basic Principles of Transcription and Translation

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein. This process involves two main stages: transcription and translation.

• Genes provide the instructions for making proteins, which are essential for cellular structure and function.

• RNA acts as the bridge between DNA and protein synthesis.

  • RNA is chemically similar to DNA but contains ribose sugar instead of deoxyribose.

  • RNA uses the base uracil (U) instead of thymine (T).

  • RNA is usually single-stranded.

• Gene expression involves:

  • Transcription: Synthesis of RNA from a DNA template, producing messenger RNA (mRNA).

  • Translation: Synthesis of a polypeptide from the mRNA transcript, occurring at ribosomes.

• Bacteria: Translation can begin before transcription is finished due to the absence of a nucleus.

• Eukaryotes: The nuclear envelope separates transcription from translation, and RNA transcripts undergo processing before becoming mature mRNA.

The Genetic Code

The genetic code specifies which amino acids will be used to build a protein. It is universal, redundant, and unambiguous.

• Redundant: More than one codon may specify a particular amino acid.

• Not ambiguous: No codon specifies more than one amino acid.

Codons: Triplets of Nucleotides

Codons are sequences of three nitrogenous bases on mRNA that specify particular amino acids.

• Each codon is a three-letter "word" that corresponds to an amino acid.

• tRNA molecules carry amino acids and have anticodon sequences complementary to mRNA codons.

• There are more codons than amino acids.

• Start codon: AUG (signals initiation of translation).

• Stop codons: UAA, UAG, UGA (signal termination of translation).

Transcription: DNA-Directed Synthesis of RNA

Transcription is the process by which RNA is synthesized from a DNA template. It involves initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter, a DNA sequence signaling the start point.

• RNA polymerase: Enzyme that catalyzes RNA synthesis in the 5’ to 3’ direction; does not require a primer.

• Elongation: RNA polymerase moves along DNA, untwisting the helix and adding nucleotides to the 3’ end of the growing RNA strand.

• Transcription progresses at about 40 nucleotides per second in eukaryotes.

• Multiple RNA polymerases can transcribe a gene simultaneously.

• Termination:

  • In bacteria: RNA polymerase stops at the terminator; mRNA can be translated immediately.

  • In eukaryotes: RNA polymerase transcribes a polyadenylation signal (AAUAAA); pre-mRNA is released for further processing.

Eukaryotic RNA Processing

Before translation, eukaryotic RNA transcripts are modified to become mature mRNA.

• Both ends of the RNA transcript are altered:

  • 5’ cap: Modified guanine added to the 5’ end.

  • Poly-A tail: Addition of adenine bases to the 3’ end.

• These modifications facilitate mRNA export from the nucleus, prevent degradation, and help ribosomes attach to the mRNA.

• RNA splicing: Removal of introns (non-coding regions) and joining of exons (coding regions).

Translation: RNA-Directed Synthesis of a Polypeptide

Translation is the process by which a polypeptide is synthesized from an mRNA template. It involves tRNA, ribosomes, and several steps.

• Transfer RNA (tRNA):

  • Recognizes and binds specific amino acids based on anticodon sequences.

  • Charged tRNA is bound to an amino acid.

  • tRNA is about 80 nucleotides long and folds into an L-shaped 3D structure.

• Ribosomes:

  • Facilitate coupling of tRNA anticodons with mRNA codons.

  • Composed of protein and ribosomal RNA (rRNA), forming small and large subunits.

  • Three sites:

    • A site: Holds incoming charged tRNA.

    • P site: Holds tRNA with growing polypeptide chain.

    • E site: Holds uncharged tRNA before exit.

• Stages of Translation:

  • Initiation: mRNA binds to small ribosomal subunit; start codon recognized; large subunit joins.

  • Elongation: Charged tRNA enters A site; peptide bond forms; tRNA moves through P and E sites.

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

• Protein factors and GTP hydrolysis provide energy and aid translation.

Protein Folding and Post-Translational Modifications

After translation, polypeptides fold into their functional 3D shapes and may undergo further modifications.

• Polypeptide chains spontaneously coil and fold during synthesis.

• Post-translational modifications include addition of sugars, lipids, phosphate groups, cleavage, or assembly of multiple polypeptides.

• Proteins may be targeted to specific cellular locations.

Polyribosomes (Polysomes)

Multiple ribosomes can translate a single mRNA simultaneously, forming polyribosomes.

• Allows rapid and efficient protein synthesis.

• Observed in both bacteria and eukaryotes.

Mutations: Effects on Protein Structure and Function

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

• Point mutations: Chemical changes in a single nucleotide pair.

  • Can lead to abnormal proteins (e.g., sickle-cell disease).

  • If in gametes, may be inherited by offspring.

• Types of Small-Scale Mutations:

  • Substitutions: Replace one nucleotide pair with another.

    • Silent mutations: No effect due to redundancy.

    • Missense mutations: Change one amino acid to another.

    • Nonsense mutations: Change an amino acid codon to a stop codon, often resulting in a nonfunctional protein.

  • Insertions and Deletions: Addition or loss of nucleotide pairs.

    • Often have more severe effects than substitutions.

    • Can cause frameshift mutations, altering the reading frame and resulting in abnormal proteins.

Summary Table: Types of Point Mutations

Mutation Type	Description	Effect on Protein
Silent	Substitution that does not change amino acid	No effect
Missense	Substitution that changes one amino acid	May alter protein function
Nonsense	Substitution that changes codon to stop codon	Usually results in nonfunctional protein
Frameshift	Insertion or deletion that shifts reading frame	Usually results in nonfunctional protein

Key Equations and Concepts

• Central Dogma of Molecular Biology:

• $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Transcription Direction:

• $\text{RNA is synthesized in the 5' \rightarrow 3' direction}$

Example: Sickle-Cell Disease

Sickle-cell disease is caused by a missense mutation in the gene encoding the β-globin subunit of hemoglobin. This single nucleotide change results in the substitution of valine for glutamic acid, altering the protein's structure and function.