Chapter 14: Gene Expression – From Gene to Protein

14.1 Genes Specify Proteins via Transcription & Translation

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

• Genes provide the instructions for making proteins, which perform most life functions.

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

  • RNA contains ribose sugar (instead of deoxyribose in DNA).

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

  • RNA is usually single-stranded.

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

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

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

• Eukaryotes: Transcription and translation are separated by the nuclear envelope; RNA transcripts undergo processing before translation.

The Genetic Code

• Specifies which amino acids are used to build a protein.

• Redundant: More than one codon can specify the same amino acid.

• Not ambiguous: Each codon specifies only one amino acid.

Codons: Triplets of Nucleotides

• Codon: Sequence of three nucleotides on mRNA that codes for a specific amino acid.

• Each codon is recognized by a tRNA with a complementary anticodon.

• There are more codons (64) than amino acids (20).

• Start codon: AUG (codes for methionine; signals initiation of translation).

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

14.2 Transcription is the DNA-Directed Synthesis of RNA: A Closer Look

Transcription is the process by which a DNA sequence is copied into RNA by the enzyme RNA polymerase.

• Initiation: RNA polymerase binds to the promoter region (a specific DNA sequence signaling the start of a gene).

• RNA polymerase: Enzyme that synthesizes RNA from the DNA template in the 5’ to 3’ direction; does not require a primer.

• Stages of Transcription:

  1. Initiation: RNA polymerase binds to promoter and unwinds DNA.

  2. Elongation: RNA nucleotides are added to the 3’ end of the growing RNA strand; polymerase untwists DNA as it moves.

  3. Termination: Transcription ends when RNA polymerase reaches a terminator sequence (in bacteria) or a polyadenylation signal (in eukaryotes).

• In eukaryotes, the pre-mRNA is released and undergoes further processing.

14.3 Eukaryotic Cells Modify RNA After Transcription

In eukaryotes, the primary RNA transcript (pre-mRNA) is processed before it is translated into protein.

• RNA Processing: Modifies pre-mRNA to mature mRNA.

  • Both ends of the RNA are modified.

  • Non-coding regions (introns) are removed; coding regions (exons) are spliced together.

• Alteration of mRNA Ends:

  • 5’ end receives a modified guanine cap.

  • 3’ end receives a poly-A tail (adenine bases).

  • These modifications facilitate export from the nucleus, protect mRNA from degradation, and help ribosomes attach for translation.

• RNA Splicing:

  • Introns: Non-coding sequences removed from pre-mRNA.

  • Exons: Coding sequences joined together to form the final mRNA.

14.4 Translation is the RNA-Directed Synthesis of a Polypeptide: A Closer Look

Translation is the process by which the sequence of an mRNA molecule directs the incorporation of amino acids into a polypeptide chain.

Molecular Components of Translation

• Transfer RNA (tRNA):

  • Single RNA strand (~80 nucleotides) that folds into an L-shape.

  • One end has an anticodon that base-pairs with mRNA codon; the other end carries a specific amino acid.

  • Charged tRNA: tRNA bound to its amino acid.

• Ribosomes:

  • Composed of protein and ribosomal RNA (rRNA); consist of large and small subunits.

  • Facilitate the matching of tRNA anticodons with mRNA codons.

  • Three binding sites:

    • A site: Holds tRNA carrying the next amino acid.

    • P site: Holds tRNA with the growing polypeptide chain.

    • E site: Exit site for tRNA after amino acid is added.

Stages of Translation

• Initiation:

  • mRNA binds to small ribosomal subunit.

  • Small subunit scans for start codon (AUG).

  • Charged tRNA enters P site; large subunit joins to form the complete ribosome.

• Elongation:

  • Charged tRNA enters A site; peptide bond forms between amino acids in P and A sites.

  • tRNA in P site moves to E site and exits; tRNA in A site moves to P site.

  • Process repeats as new tRNAs enter A site.

• Termination:

  • Occurs when a stop codon (UAA, UAG, UGA) is reached in the A site.

  • A release factor binds, causing the addition of a water molecule instead of an amino acid, releasing the polypeptide.

• Energy for translation is provided by hydrolysis of GTP.

Protein Folding and Post-Translational Modifications

• Polypeptide chains fold spontaneously into their functional 3-D shapes.

• Some proteins require post-translational modifications (e.g., addition of sugars, lipids, phosphate groups; cleavage; assembly of multiple polypeptides).

Polyribosomes

• Multiple ribosomes can translate a single mRNA simultaneously, forming polyribosomes (polysomes).

• This increases the efficiency of protein synthesis.

14.5 Mutations of One or a Few Nucleotides Can Affect 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 of a gene.

• Can result in abnormal proteins (e.g., sickle-cell disease).

• If occurring in gametes, mutations can be passed to offspring.

Types of Small-Scale Mutations

• Single nucleotide-pair substitutions: One nucleotide and its partner are replaced by another pair.

• Nucleotide-pair insertions or deletions: Addition or loss of nucleotide pairs.

Substitution Mutations

• Silent mutations: No effect on amino acid sequence due to redundancy in the genetic code.

• Missense mutations: Change one amino acid to another; may affect protein function.

• Nonsense mutations: Change an amino acid codon to a stop codon, leading to a truncated, usually nonfunctional protein.

Insertions and Deletions

• Often have more severe effects than substitutions.

• May cause frameshift mutations, altering the reading frame and potentially changing every amino acid downstream.

Table: Types of Point Mutations and Their Effects

Example: Sickle-Cell Disease

• Caused by a single nucleotide substitution in the gene encoding the β-globin subunit of hemoglobin.

• Results in abnormal hemoglobin and sickle-shaped red blood cells.

Additional info: The central dogma of molecular biology summarizes the flow of genetic information: DNA → RNA → Protein. The accuracy of transcription and translation is critical for proper cell function, and mutations can have a range of effects from benign to severe, depending on their nature and location.