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

Chapter 7: Anatomy and Function of a Gene: Dissection Through Mutation cont CHAPTER OUTLINE: 7.3 What Mutations Tell Us About Gene Function 7.4 A Comprehensive Example: Mutations Tha

Chapter 7: Anatomy and Function

of a Gene: Dissection Through

Mutation (cont)

CHAPTER OUTLINE:

7.3 What Mutations Tell Us About Gene Function

7.4 A Comprehensive Example: Mutations That Affect Vision

Chapter 7 of the textbook: Genetics:

From Genes to Genomes, 4th edition

(2011), Hartwell H et al

What mutations tell us about gene

function

• Mendel’s experiments established that an individual gene can

control a visible characteristic, but his laws do 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

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Alkaptonuria: An inborn error of

metabolism

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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

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

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auxotrophs: Mated an

X-ray-mutagenized 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

Fig 7.23a

Experimental support for the “one gene, one enzyme” hypothesis (2)

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The ability of

wild-type and mutant strains

to grow on minimal

medium supplemented

with intermediates in

the arginine pathway

Each of the four ARG

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

Amino acids with uncharged R groups

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VNU-University of Science - DNThai Fig 7.24b (cont)

R groups Backbone R groups Backbone

Amino acids with charged R groups

R groups Backbone R groups Backbone

R groups Backbone R groups Backbone

The molecular

basis of sickle-cell

and other anemias

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Fig 7.25a

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

Sickle-cell anemia is pleiotropic

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Fig 7.25b

Levels of polypeptide structure

Levels of polypeptide structure (2)

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1o structure is the amino acid sequence

2o structure is the characteristic geometry of localized regions

Fig 7.26b, c

• The tertiary (3°) structure

is the complete

three-dimensional 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.

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Levels of polypeptide structure (3)

Fig 7.26d

Multimeric proteins are complexes of

polypeptide subunits

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Identical subunits Non-identical subunits

Fig 7.27a, b

• (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)

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Multimeric proteins are complexes of

• (d) One α-tubulin and

one -tubulin

polypeptide associate

to form a tubulin

dimer Many tubulin

dimers form a single

Multimeric proteins are complexes of

polypeptide subunits (3)

How do genotypes and phenotypes

correlate?

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"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

7.4 A comprehensive example:

Mutations that affect vision

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VNU-University of Science - DNThai Fig 7.28

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

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VNU-University of Science - DNThai Fig 7.29a - c

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

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Essential Concepts

1 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

2 Mutations that affect phenotype occur naturally at a very low rate

Forward mutations usually occur more often than reversions

3 The agents of spontaneously occurring mutations include chemical

hydrolysis, radiation, and mistakes during DNA replication

4 Mutagens raise the frequency of mutation above the spontaneous rate The Ames test screens for mutagenic chemicals

5 Cells have evolved a number of enzyme systems that repair DNA and thus minimize mutations

6 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

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7 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

8 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

9 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

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Essential Concepts