Gene Expression: From Gene to Protein (Chapter 17 Study Notes)

Gene Expression: From Gene to Protein

Introduction to Gene Expression

Gene expression is the process by which the information encoded in DNA directs the synthesis of proteins, ultimately determining an organism's traits. Proteins serve as the molecular link between genotype (genetic makeup) and phenotype (observable traits).

• Gene expression involves two main stages: transcription and translation.

• Transcription is the synthesis of RNA from a DNA template.

• Translation is the synthesis of a polypeptide using the information in mRNA.

Historical Evidence: Genes, Enzymes, and Metabolic Defects

Early studies established the connection between genes and enzymes, which catalyze specific biochemical reactions.

• Archibald Garrod (1902) proposed that genes dictate phenotypes via enzymes, based on his study of alkaptonuria, a hereditary disease caused by the absence of an enzyme that breaks down alkapton.

• Beadle and Tatum (1940s) used bread mold (Neurospora) mutants to show that each gene encodes a specific enzyme in a metabolic pathway.

• Further research by Srb and Horowitz identified mutants lacking different enzymes required for arginine synthesis.

Mutant Class	Enzyme Deficiency	Growth Requirement
Class I	Enzyme A	Requires ornithine, citrulline, or arginine
Class II	Enzyme B	Requires citrulline or arginine
Class III	Enzyme C	Requires arginine only

The Products of Gene Expression

Not all proteins are enzymes. The original "one gene-one enzyme" hypothesis was revised to "one gene-one protein," and later to "one gene-one polypeptide." However, many eukaryotic genes can produce multiple polypeptides via alternative splicing, and some genes code for functional RNA molecules.

• Polypeptide: A linear chain of amino acids; may combine to form a functional protein.

• Alternative splicing: Process by which different combinations of exons are joined to produce multiple protein variants from a single gene.

Basic Principles of Transcription & Translation

RNA acts as the bridge between DNA and protein synthesis.

• Transcription: DNA → RNA (specifically, messenger RNA or mRNA)

• Translation: mRNA → Polypeptide (protein)

• Ribosomes: Cellular structures where translation occurs.

In prokaryotes: Translation can begin before transcription is finished. In eukaryotes: The nuclear envelope separates transcription (in the nucleus) from translation (in the cytoplasm). Eukaryotic RNA transcripts undergo processing to become mature mRNA.

The Genetic Code

The genetic code is a set of rules by which information encoded in DNA is translated into proteins.

• Triplet code: Each amino acid is specified by a sequence of three nucleotides (codon).

• DNA template strand provides the sequence for complementary RNA nucleotides.

• mRNA codons are read in the 5' → 3' direction during translation.

• The non-template (coding) strand of DNA matches the mRNA sequence (except T is replaced by U).

Codon	Amino Acid
AUG	Methionine (Start)
UAA, UAG, UGA	Stop signals
UUU, UUC	Phenylalanine
GAA, GAG	Glutamic acid
... (total 64 codons)	20 amino acids + 3 stop signals

• The genetic code is redundant (multiple codons for one amino acid) but not ambiguous (each codon specifies only one amino acid).

• Codons must be read in the correct reading frame to produce the correct protein.

Evolution of the Genetic Code

The genetic code is nearly universal among all organisms, from bacteria to animals. Genes can be expressed in different species, indicating a common evolutionary origin.

• Transgenic organisms (e.g., tobacco plant expressing a firefly gene) demonstrate the universality of the genetic code.

Molecular Components of Transcription

Transcription is catalyzed by RNA polymerase, which synthesizes RNA using DNA as a template.

• RNA polymerase assembles RNA in the 5' → 3' direction.

• Unlike DNA polymerase, RNA polymerase does not require a primer.

• RNA synthesis follows base-pairing rules: A-U, C-G (uracil replaces thymine in RNA).

Synthesis of an RNA Transcript

Transcription occurs in three stages:

1. Initiation: RNA polymerase binds to the promoter region of DNA, unwinds the DNA, and begins RNA synthesis.

2. Elongation: RNA polymerase moves along the DNA, adding nucleotides to the 3' end of the growing RNA strand.

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

Initiation of Transcription

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• TATA box: A specific promoter sequence important in eukaryotes.

• Transcription factors: Proteins that help RNA polymerase bind to the promoter and initiate transcription (required in eukaryotes).

• Transcription initiation complex: The complete assembly of transcription factors and RNA polymerase at the promoter.

• Transcription unit: The stretch of DNA transcribed into RNA.

Elongation of the RNA Strand

• RNA polymerase untwists the DNA helix and adds nucleotides to the 3' end of the RNA.

• Multiple RNA polymerases can transcribe a gene simultaneously.

Termination of Transcription

• Bacteria: Transcription ends at a terminator sequence; mRNA is ready for translation.

• Eukaryotes: RNA polymerase II transcribes a polyadenylation signal (AAUAAA); the pre-mRNA is released and processed.

RNA Processing in Eukaryotes

Pre-mRNA undergoes several modifications before becoming mature mRNA.

• Both ends of the transcript are altered:

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

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

• Functions of these modifications:

  • Facilitate export of mRNA to the cytoplasm

  • Protect mRNA from degradation

  • Help ribosomes attach to the 5' end

• UTRs (Untranslated Regions): Regions of mRNA not translated into protein, but important for regulation.