Gene Expression and mRNA Structure

Structure of Eukaryotic mRNA

Eukaryotic messenger RNA (mRNA) molecules are essential intermediates in gene expression, carrying genetic information from DNA to the ribosome for protein synthesis. Their structure includes untranslated regions (UTRs) that play regulatory roles.

• 5'-UTR (Untranslated Region): Located upstream of the coding sequence; involved in regulation of translation initiation.

• Coding Region: Begins with the AUG start codon and ends with a stop codon; encodes the amino acid sequence of the protein.

• 3'-UTR (Untranslated Region): Located downstream of the coding sequence; influences mRNA stability and translation efficiency.

• Poly(A) Tail: A stretch of adenine nucleotides added to the 3' end; enhances mRNA stability and export from the nucleus.

Example: The presence of 5'- and 3'-UTRs allows for post-transcriptional regulation of gene expression, affecting how much protein is produced from a given mRNA.

Linking Genes to Proteins

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein. Genes encode proteins, which perform cellular functions.

• Transcription: DNA is transcribed into mRNA.

• Translation: mRNA is translated into a polypeptide chain (protein).

• Proteins: Complex molecules with diverse structures and functions, including enzymes, structural components, and signaling molecules.

Example: Hemoglobin is a protein encoded by multiple genes, each specifying a different globin subunit.

Amino Acids: The Building Blocks of Proteins

Classification of Amino Acids

Amino acids are organic molecules that serve as the monomers of proteins. Each amino acid has a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group) that determines its properties.

• Nonpolar (Hydrophobic): Alanine (Ala, A), Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I), Proline (Pro, P), Methionine (Met, M), Phenylalanine (Phe, F), Tryptophan (Trp, W)

• Polar (Hydrophilic): Glycine (Gly, G), Serine (Ser, S), Threonine (Thr, T), Cysteine (Cys, C), Tyrosine (Tyr, Y), Asparagine (Asn, N), Glutamine (Gln, Q)

• Polar, Positively Charged (Basic): Lysine (Lys, K), Arginine (Arg, R), Histidine (His, H)

• Polar, Negatively Charged (Acidic): Aspartic acid (Asp, D), Glutamic acid (Glu, E)

Example: The side chain of each amino acid determines its chemical reactivity and role in protein structure.

Peptide Bond Formation

Linking Amino Acids

Amino acids are joined together by peptide bonds to form polypeptides. The peptide bond forms between the carboxyl group of one amino acid and the amino group of another, releasing water.

• Amino (N) Terminus: The end of the polypeptide with a free amino group.

• Carboxyl (C) Terminus: The end with a free carboxyl group.

Equation:

Example: The sequence of amino acids in a polypeptide determines its structure and function.

Levels of Protein Structure

Primary, Secondary, Tertiary, and Quaternary Structure

Proteins have four levels of structural organization, each contributing to their final shape and function.

• Primary Structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding patterns stabilized by hydrogen bonds, such as alpha helices and beta-pleated sheets.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, formed by interactions among side chains.

• Quaternary Structure: Association of multiple polypeptide subunits into a functional protein complex (e.g., hemoglobin).

Example: Hemoglobin consists of two alpha and two beta subunits, each with its own tertiary structure, assembled into a quaternary structure.

Connecting Genes to Enzyme Function: Historical Experiments

Metabolic Disorders and Gene Function

Studies of human metabolic disorders, such as phenylketonuria (PKU) and alkaptonuria, provided early evidence that genes encode enzymes.

• PKU: Caused by a deficiency in the enzyme that converts phenylalanine to tyrosine, leading to accumulation of phenylpyruvic acid.

• Inheritance: PKU is autosomal recessive; affected individuals must inherit two defective alleles.

• Symptoms: Intellectual disability can result if not treated with a low-phenylalanine diet.

Equation:

Example: Early diagnosis and dietary management can prevent symptoms in PKU patients.

Beadle and Tatum: One Gene–One Enzyme Hypothesis

Neurospora crassa Experiments

Beadle and Tatum used the bread mold Neurospora crassa to demonstrate that each gene encodes a specific enzyme, leading to the "one gene–one enzyme" hypothesis.

• Prototrophs: Wild-type strains that can grow on minimal media.

• Auxotrophs: Mutant strains that require specific supplements (e.g., arginine) to grow.

• Genetic Analysis: Mutants were used to map the order of biochemical steps in arginine biosynthesis.

Example: An auxotrophic mutant unable to synthesize tyrosine can grow only when tyrosine is added to the medium.

Genetic Mapping of Biochemical Pathways

By supplementing minimal media with pathway intermediates, researchers determined the order of enzymatic steps and the genes responsible for each.

• Arginine Biosynthesis Pathway: Involves conversion of precursors to ornithine, citrulline, and finally arginine.

• Mutant Rescue: A mutant blocked at a specific step can be rescued by providing intermediates downstream of the block.

Example: An arg2 mutant cannot convert ornithine to citrulline, but can be rescued by adding citrulline or arginine to the medium.

Additional info: The use of haploid spores in Neurospora genetics simplifies analysis, as each mutant expresses its phenotype directly without dominance effects.