BIO102 Ch16a

Chapter 16: How Genes Work

Introduction to Genetic Information Flow

  • The fundamental flow of genetic information is represented by the central dogma: DNA → RNA → Proteins.

  • This chapter outlines:

    • The central dogma of molecular biology (16.1)

    • The genetic code, which uses a 3-letter coding system (16.2)

    • The role of genes in functionality (16.3)

    • Mechanisms by which mutations can alter genes and genomes (16.4)

The Function of Genes

  • Initial Proposal by Beadle and Tatum:

    • Genes can determine function by creating mutations through chemical or radiation-induced damages.

    • The mutated gene's phenotype is compared with the wild type to assess functionality.

  • Loss-of-Function Alleles:

    • Nonfunctional alleles resulting from mutations are called null or loss-of-function alleles.

Experimentation: Arginine Biosynthesis Pathway

  • Metabolic Pathway Overview:

    • The pathway for arginine synthesis involves several intermediates and enzymes.

    • The experiment by Srb and Horowitz involved mutagenizing Neurospora crassa cells with X-rays and examining their growth capabilities on minimal media.

    • Auxotrophs: Mutants that cannot grow without an added nutrient (arginine) are termed auxotrophs.

  • Experimental Results:

    • Specific mutants (arg1, arg2, arg3) lack different enzymes in the arginine biosynthetic pathway:

      • arg1: lacks enzyme 1 (precursor to ornithine)

      • arg2: lacks enzyme 2 (ornithine to citrulline)

      • arg3: lacks enzyme 3 (citrulline to arginine)

    • Growth of these mutants is significant in determining the nature of their metabolic blockage, supporting the one-gene, one-enzyme hypothesis, now revised to one-gene, one-polypeptide hypothesis.

Biosynthetic Pathways of Other Amino Acids

  • Isoleucine Pathway Analysis:

    • Study of mutant strains highlights their specific blocks in isoleucine biosynthesis.

    • Strains are tested by addition of specific intermediates to identify where the mutation impacts the pathway.

Genetic Disorders Linked to Gene Function

  • Phenylketonuria (PKU):

    • An autosomal recessive disorder caused by a defect in the enzyme phenylalanine hydroxylase.

    • Individuals with the defective gene accumulate toxic substances leading to metabolic disorders.

The Messenger RNA Hypothesis

  • Linking DNA and Protein Synthesis:

    • This hypothesis proposes that mRNA serves as a vital carrier of information from DNA to protein synthesis.

  • Transcription and Translation: Key processes in gene expression.

The Genetic Code

  • Triplet Code:

    • The genetic code consists of triplets (codons) of RNA bases (U, C, A, G) to specify amino acids.

    • Codon Functionality:

      • Sense codons specify an amino acid.

      • Nonsense codons signal termination of translation.

  • Reading the Codon:

    • Codons are read in the 5' to 3' direction without overlap, indicating that a three-base codon is necessary to provide enough combinations for the 20 amino acids (64 possible combinations with triplets).

  • Start and Stop Codons:

    • Protein coding sequences commence with the AUG start codon, coding for Methionine.

    • Sequences terminate with stop codons that do not code for amino acids.

The Central Dogma and Its Implications

  • Understanding Genotype and Phenotype:

    • The central dogma provides a framework for understanding how genetic information determines physical characteristics and functions in organisms.