How Genes Work: The Molecular Basis of Gene Expression and Mutation

Chapter 16: How Genes Work

Introduction to How Genes Work

Understanding how genes function at the molecular level is central to modern biology. Genes, composed of DNA, serve as the blueprint for life, but the process by which genetic information is converted into functional molecules—gene expression—was not fully understood until the mid-20th century.

• Gene Expression: The process of converting information in DNA into functioning molecules within the cell.

• Key Question: How do genes work at the molecular level?

What Do Genes Do?

Early experiments by Beadle and Tatum revealed that genes control specific biochemical reactions by encoding enzymes. By inducing mutations in bread mold (Neurospora crassa), they observed the effects on phenotype and established the concept of null or loss-of-function alleles.

• Null Alleles: Nonfunctioning alleles resulting from gene damage.

• One-Gene, One-Enzyme Hypothesis: Each gene contains information to make a specific enzyme.

• One-Gene, One-Polypeptide Hypothesis: Most genes contain instructions for making proteins, not just enzymes.

• Genetic Screen: Technique to identify mutants deficient in specific metabolic steps.

The Genetic Code Hypothesis

Francis Crick proposed that the sequence of bases in DNA acts as a code, specifying the amino acid sequence of proteins. The information in DNA is not directly translated into proteins but is mediated by RNA.

• DNA: Information storage molecule.

• RNA: Acts as an intermediary between genes and proteins.

• Messenger RNA (mRNA): Carries genetic information from DNA to the site of protein synthesis.

• RNA Polymerase: Enzyme that synthesizes RNA using a DNA template.

The Central Dogma of Molecular Biology

The central dogma summarizes the flow of genetic information in cells: DNA codes for RNA, which codes for proteins. Genes are stretches of DNA that code for proteins, and the sequence of DNA determines the sequence of RNA, which in turn determines the sequence of amino acids in proteins.

• Transcription: Process of using a DNA template to make complementary RNA.

• Translation: Process of using information in mRNA to synthesize proteins.

• Genotype: Determined by the sequence of bases in DNA.

• Phenotype: Observable traits produced by proteins.

Modifications to the Central Dogma

Not all genes code for mRNAs that are translated into proteins; some RNAs have other functions. Additionally, some viruses use reverse transcriptase to synthesize DNA from an RNA template, demonstrating that information flow can be bidirectional.

• Non-coding RNAs: RNAs that perform important cellular functions without being translated.

• Reverse Transcriptase: Enzyme in some viruses that synthesizes DNA from RNA.

The Genetic Code

The genetic code specifies how a sequence of nucleotides codes for a sequence of amino acids. The code is composed of triplets called codons, each specifying a particular amino acid.

• Codon: Group of three bases that specifies an amino acid.

• Triplet Code: Three-base code is the minimum needed to specify 20 amino acids.

• Start Codon (AUG): Signals where protein synthesis starts; codes for methionine.

• Stop Codons (UAA, UAG, UGA): Signal the end of the protein-coding sequence.

Properties of the Genetic Code

The genetic code has several important properties that ensure accurate and efficient translation of genetic information.

• Redundant: Most amino acids are encoded by more than one codon.

• Unambiguous: Each codon specifies only one amino acid.

• Non-overlapping: Codons are read one at a time.

• Nearly Universal: The code is conserved across almost all organisms.

• Conservative: Codons specifying the same amino acid often share the first two bases.

Applications of the Genetic Code

Knowledge of the genetic code allows biologists to predict amino acid sequences from DNA sequences and vice versa. Due to redundancy, multiple DNA sequences can code for the same protein.

• Predicting Protein Sequence: Determining the amino acid sequence from a given DNA or mRNA sequence.

• Reverse Translation: Inferring possible DNA or mRNA sequences from a known protein sequence.

Types and Consequences of Mutation

Mutations are permanent changes in an organism’s DNA that can alter genotype and produce new alleles. They are classified by their scale and impact.

• Point Mutations: Changes affecting one or a few bases.

• Chromosome-level Mutations: Larger changes affecting chromosome structure or number.

Point Mutations

Point mutations can have various effects on protein function and organismal fitness.

• Missense Mutation: Changes an amino acid in the protein.

• Silent Mutation: Does not change the amino acid sequence due to code redundancy.

• Frameshift Mutation: Alters the reading frame, changing all subsequent codons.

• Nonsense Mutation: Converts a codon specifying an amino acid into a stop codon.

• Fitness Impact: Mutations can be beneficial, neutral, or deleterious.

Chromosome Mutations

Chromosome mutations can change chromosome number or structure, with significant effects on phenotype and disease.

• Polyploidy: Increase in the number of chromosome sets.

• Aneuploidy: Addition or loss of individual chromosomes.

• Inversion: Segment of chromosome breaks off, flips, and rejoins.

• Translocation: Segment of chromosome attaches to another chromosome.

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Karyotype: Complete set of chromosomes in a cell, used to visualize mutations.

Table: Consequences of Point Mutations That Alter Codons

Type of Mutation	Effect on Protein	Effect on Fitness
Missense	Changes one amino acid	Can be beneficial, neutral, or deleterious
Silent	No change in amino acid sequence	Usually neutral
Frameshift	Alters reading frame, changes many amino acids	Usually deleterious
Nonsense	Introduces premature stop codon	Usually deleterious

*Additional info: The above notes expand on the original content by providing definitions, examples, and context for key terms and processes, ensuring a self-contained study guide suitable for exam preparation.*