Gene Expression and Point Mutations

Summary of Eukaryotic Gene Expression

• Transcription and RNA Processing in the Nucleus:     * DNA and RNA Polymerase: The process begins in the nucleus where DNA serves as a template for RNA polymerase to synthesize an RNA transcript.     * RNA transcript (pre-mRNA): The initial transcript contains both exons (coding regions) and introns (non-coding regions).     * RNA Processing: Before leaving the nucleus, the pre-mRNA undergoes several modifications:         * 5' Cap: A modified guanine nucleotide is added to the $5'$ end.         * Poly-A Tail: A sequence of adenine nucleotides is added to the $3'$ end.         * Splicing: Introns are removed, and exons are joined together.

• Export to Cytoplasm: The processed mRNA (messenger RNA) moves from the nucleus into the cytoplasm through nuclear pores.

• Amino Acid Activation:     * Aminoacyl-tRNA Synthetase: This enzyme is responsible for "charging" tRNA molecules by attaching the correct amino acid (aa) to them.     * Activated Amino Acid: The resulting structure is an amino acid covalently bonded to a tRNA with a specific anticodon (e.g., $A CC$).

• Translation and the Initiation Complex:     * Ribosome Structure: The ribosome consists of subunits and three primary sites: E (Exit), P (Peptidyl), and A (Aminoacyl).     * The Process:         * The mRNA is read in the $5'$ to $3'$ direction.         * Codons on the mRNA (e.g., $AAA$, $UGG$, $UUU$, $AUG$, $U$) pair with complementary anticodons on the tRNA molecules.         * A polypeptide chain is synthesized as the ribosome moves along the mRNA.

Linear Nature of Genetic Information

• Sequence Parallelism: There is a direct linear relationship between the sequences of DNA, mRNA, and the resulting protein:     * The sequence of the non-template DNA strand ($5'$ to $3'$) parallels the $5'$ to $3'$ mRNA sequence.     * The mRNA sequence parallels the N-terminus to C-terminus ($N$ to $C$) amino acid (aa) sequence of the polypeptide.

• Structural and Functional Impact:     * Any change in the DNA sequence can alter the amino acid sequence of the polypeptide.     * Changes in the amino acid sequence can disrupt the protein's structure and, consequently, its biological function.     * Molecular Interactions in Proteins: Protein structure is maintained by various bonds and interactions within the polypeptide backbone and side chains:         * $H$-bonds (Hydrogen bonds).         * Ionic bonds.         * Disulfide bridges (formed between $S$ atoms in cysteine residues).         * Hydrophobic interactions.

Point Mutations: Definitions and Origins

• Definition: Point mutations are chemical changes in the DNA that affect a single nucleotide pair.

• Causes of Point Mutations:     * Replication/Repair Errors: Mistakes made during the process of DNA replication or during the subsequent repair of damaged DNA.     * Mutagens: External physical or chemical agents that cause damage to the DNA molecule.

Nucleotide-pair Substitutions

• Definition: The replacement of one nucleotide pair with another.

• Alternative Name: Also referred to as a Single Nucleotide Polymorphism (SNP).

• Outcomes in Protein Coding Regions:     * Silent Mutation: A change in the DNA that does not result in a change to the amino acid sequence (often due to the redundancy of the genetic code).     * Nonsense Mutation: A mutation that changes an amino acid codon into a premature stop codon, which shortens the protein and usually renders it non-functional.     * Missense Mutation: A mutation that results in the substitution of one amino acid for another in the polypeptide chain.

Frameshift Mutations

• Definition: Mutations caused by the insertion or deletion (indel) of nucleotide pairs.

• Mechanism: These mutations shift the "reading frame" of the genetic message downstream from the mutation site.

• Exceptions: If an insertion or deletion occurs in a multiple of $3$ nucleotides, the reading frame remains intact, though specific amino acids will be added or lost.

• Consequences:     * Extensive Missense: A shift in the reading frame typically changes every amino acid following the mutation.     * Nonsense Mutations: Frameshifts frequently create premature stop codons shortly after the insertion or deletion.

Effects of Mutations on Phenotypes

• Determinants of Effect: The phenotypic impact depends on the type of mutation and its specific position within the gene, which includes the protein-coding sequence and regulatory sequences.

• Range of Functional Effects:     * Neutral Mutation: Has no effect on the protein's function (e.g., silent mutations or substitutions in non-critical regions).     * Partial Loss of Function: The protein retains some activity but is less efficient than the wild type.     * Complete Loss of Function: The mutation results in the total absence of the enzyme or protein activity (example: the allele for white flowers vs. purple flowers where the enzyme to synthesize pigment is absent).     * Gain of Function: The mutation creates a protein with a new or enhanced function.         * Example: Lactase persistence in adult humans, which allows for lactose tolerance.

• Organismal Impact: Functional changes (Loss/Gain) can be beneficial or harmful depending on:     * Genetic Context: How the mutation interacts with other genes in the organism.     * Environment: The external conditions in which the organism lives.

Case Study: Hemoglobin (Hb) and Malaria

• Hemoglobin Structure: A protein composed of four subunits: two alpha ($\alpha$) globin chains and two beta ($\beta$) globin chains.

• Mutations at Amino Acid 6 of the Beta Chain:     * Sickle Cell Anemia (Hb S): A specific missense mutation. If an individual is homozygous for this allele, they suffer from severe blood disease. However, heterozygotes are resistant to malaria.     * Hb C: Results in mild anemia.     * Hb Makassar: Results in no discernible effect on the individual.

• Malaria and Genetic Selection: Malaria is caused by a parasitic unicellular eukaryote. The distribution of the sickle-cell allele correlates with areas where malaria is prevalent.

• Frequencies of the Sickle-Cell Allele:     * $0-2.5\%$     * $2.5-5.0\%$     * $5.0-7.5\%$     * $7.5-10.0\%$     * $10.0-12.5\%$     * $>12.5\%$

Reflection Questions

• Neutral Mutations vs. Loss/Gain of Function: Users should be able to describe the different specific locations within a gene where a point mutation could occur to be classified as a neutral mutation. This should be contrasted with DNA changes leading to partial loss of function, complete loss of function, or gain of function mutations.

• Discussion: Write down answers and discuss with peers before moving to Chapter 18 content.