Anatomy and Function of a Gene: Dissection Through Mutation (cont)

Chapter 7: Anatomy and Function of a Gene: Dissection Through Mutation (cont) Chapter of the textbook: Genetics: From Genes to Genomes, 4th edition (2011), Hartwell H et al CHAPTER OUTLINE: 7.3 What Mutations Tell Us About Gene Function 7.4 A Comprehensive Example: Mutations That Affect Vision VNU-University of Science - DNThai What mutations tell us about gene function • Mendel’s experiments established that an individual gene can control a visible characteristic, but his laws not explain how genes actually govern the appearance of traits • Investigators working in the first half of the twentieth century carefully studied the biochemical changes caused by mutations in an effort to understand the genotype–phenotype connection • In one of the first of these studies, conducted in 1902, the British physician Dr Archibald Garrod showed that a human genetic disorder known as alkaptonuria is determined by the recessive allele of an autosomal gene VNU-University of Science - DNThai Alkaptonuria: An inborn error of metabolism The biochemical pathway in humans that degrades phenylalanine and tyrosine via homogentisic acid (HA) In alkaptonuria patients, the enzyme HA hydroxylase is not functional so it does not catalyze the conversion of HA to maleylacetoacetic acid As a result, HA, which oxidizes to a black compound, accumulates in the urine VNU-University of Science - DNThai Fig 7.22 Beadle and Tatum (1940s) – "the one gene, one enzyme" hypothesis • Screened for X-ray induced mutations in Neurospora that disrupted synthesis of arginine (arg) – Prototroph – wild-type strain that grows in minimal media without nutritional supplements – Auxotroph – mutant strain that cannot grow in minimal media • Recombination analysis used to map mutations to four different regions of genome • Each region contained a different complementation group • Four genes for arg biosynthesis – ARG-E, ARG-F, ARG-G, and ARG-H VNU-University of Science - DNThai Experimental support for the “one gene, one enzyme” hypothesis Isolation of arginine auxotrophs: Mated an X-raymutagenized strain of Neurospora with another strain, and then isolated haploid ascospores that grew on complete medium Cultures that failed to grow on minimal medium were nutritional mutants Nutritional mutants that could grow on minimal medium plus arginine were arg– auxotrophs VNU-University of Science - DNThai Fig 7.23a Experimental support for the “one gene, one enzyme” hypothesis (2) The ability of wildtype and mutant strains to grow on minimal medium supplemented with intermediates in the arginine pathway Each of the four ARG genes encodes an enzyme needed to convert one intermediate to the next in the pathway VNU-University of Science - DNThai Fig 7.23 b,c Genes specify the identity and order of amino acids in polypeptide chains • 20 different amino acids • R group is the side chain that is unique to each amino acid • Four groups of amino acids based on R group properties (Fig 7.24b) • –COOH group and –NH2 group of adjacent amino acids are joined in covalent peptide bond • Polypeptides have "N terminus" and "C terminus" VNU-University of Science - DNThai Fig 7.24a, c Amino acids with nonpolar R groups R groups Backbone R groups Backbone Part of Fig 7.24b VNU-University of Science - DNThai Amino acids with uncharged R groups R groups VNU-University of Science - DNThai Backbone R groups Backbone Fig 7.24b (cont) Amino acids with charged R groups R groups Backbone R groups Backbone R groups Backbone R groups Backbone Basic R group Acidic R group Fig 7.24b (cont) VNU-University of Science - DNThai 10 The molecular basis of sickle-cell and other anemias GluVal substitution at sixth amino acid affects the three-dimensional structure of the hemoglobin b chain Abnormal protein aggregates cause sickle shape of red blood cells Fig 7.25a VNU-University of Science - DNThai 11 Sickle-cell anemia is pleiotropic Fig 7.25b VNU-University of Science - DNThai 12 Levels of polypeptide structure • Interactions that determine the structure conformation of a polypeptide VNU-University of Science - DNThai Fig 7.26a 13 Levels of polypeptide structure (2) 1o structure is the amino acid sequence 2o structure is the characteristic geometry of localized regions VNU-University of Science - DNThai Fig 7.26b, c 14 Levels of polypeptide structure (3) • The tertiary (3°) structure is the complete threedimensional arrangement of a polypeptide In this portrait of myoglobin, the iron-containing heme group, which carries oxygen, is red, while the polypeptide itself is green VNU-University of Science - DNThai Fig 7.26d 15 Multimeric proteins are complexes of polypeptide subunits Identical subunits VNU-University of Science - DNThai Non-identical subunits Fig 7.27a, b 16 Multimeric proteins are complexes of polypeptide subunits (2) • (c) Three distinct protein receptors for the immune-system molecules called interleukins (ILs; purple) All contain a common gamma ( ) chain ( yellow), plus other receptor-specifi c polypeptides (green) A mutant chain blocks the function of all three receptors, leading to X-linked severe combined immune deficiency (XSCID) One polypeptide in different proteins VNU-University of Science - DNThai Fig 7.27c 17 Multimeric proteins are complexes of polypeptide subunits (3) • (d) One α-tubulin and one -tubulin polypeptide associate to form a tubulin dimer Many tubulin dimers form a single microtubule The mitotic spindle is an assembly of many microtubules VNU-University of Science - DNThai Fig 7.27d 18 How genotypes and phenotypes correlate? "One gene, one enzyme" concept is not broad enough • Not all proteins are enzymes • Some proteins are multimeric and subunits are encoded by different genes Complex pathways can be dissected through genetic analysis Different mutations in a single gene can produce different phenotypes • Different amino acid substitutions can have different effects • on protein function Mutations can affect protein function by altering the amount of normal protein made VNU-University of Science - DNThai 19 7.4 A comprehensive example: Mutations that affect vision The cellular basis of vision VNU-University of Science - DNThai The molecular basis of vision Fig 7.28 20 How mutations modulate light and color perception • (a) Amino acid substitutions (black dots) that disrupt rhodopsin’s three-dimensional structure result in retinitis pigmentosa Other substitutions diminishing rhodopsin’s sensitivity to light cause night blindness • (b) Substitutions in the blue pigment can produce tritanopia (blue colorblindness) • (c) Red colorblindness can result from particular mutations that destabilize the red photoreceptor VNU-University of Science - DNThai Fig 7.29a - c 21 How mutations modulate light and color perception (2) • Unequal crossing-over between the red and green genes can change gene number and create genes encoding hybrid photoreceptor proteins VNU-University of Science - DNThai Fig 7.29d 22 Essential Concepts Mutations are alterations in the nucleotide sequence of the DNA molecule that occur by chance and modify the genome at random Mutations can be transmitted from generation to generation when DNA replicates Mutations that affect phenotype occur naturally at a very low rate Forward mutations usually occur more often than reversions The agents of spontaneously occurring mutations include chemical hydrolysis, radiation, and mistakes during DNA replication Mutagens raise the frequency of mutation above the spontaneous rate The Ames test screens for mutagenic chemicals Cells have evolved a number of enzyme systems that repair DNA and thus minimize mutations Mutations are the raw material of evolution Although some mutations may confer a selective advantage, most are harmful Somatic mutations can cause cancer and other illnesses in individuals VNU-University of Science - DNThai 23 Essential Concepts Mutations within a single gene usually fail to complement each other The concept of a complementation group thus defines the gene as a unit of function A gene is composed of a linear sequence of nucleotide pairs in a discrete, localized region of a chromosome Recombination can occur within a gene, and even between adjacent nucleotide pairs The function of most genes is to specify the linear sequence of amino acids in a particular polypeptide (one gene, one polypeptide) The sequence determines the polypeptide’s three-dimensional structure, which, in turn, determines its function Mutations can alter amino acid sequence and thus change protein function in many ways Each protein consists of one, two, or more polypeptides Proteins composed of two or more different subunits are encoded by two or more genes 10 The rhodopsin gene family provides an example of how the processes of gene duplication followed by gene divergence mutation can lead to evolution of functional refinements, such as the emergence of accurate systems for color vision VNU-University of Science - DNThai 24 ... mutagenic chemicals Cells have evolved a number of enzyme systems that repair DNA and thus minimize mutations Mutations are the raw material of evolution Although some mutations may confer a selective... of an autosomal gene VNU-University of Science - DNThai Alkaptonuria: An inborn error of metabolism The biochemical pathway in humans that degrades phenylalanine and tyrosine via homogentisic acid... acid (HA) In alkaptonuria patients, the enzyme HA hydroxylase is not functional so it does not catalyze the conversion of HA to maleylacetoacetic acid As a result, HA, which oxidizes to a black