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Lectures by Kathleen FitzpatrickSimon Fraser University

Copyright 2012 Pearson Education Inc.Mark F. Sanders John L. Bowman

G E N E T I CA N I N T E G R A T E D A P P R O A C H

A N A LY S I SChapter 1

The Molecular Basis of

Heredity, Variation, and

Evolution

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Copyright 2012 Pearson Education Inc.Genetics Analysis: An Integrated Approach

The science of genetics

Genetics is the study of inherited variation, its origin,and transmission

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1.1 Modern Genetics Has Entered Its SecondCentury

Man has long puzzled over the causes of familial tendenciesin people and domesticates

It is only in the last 150 years or so, with the advent and

application of the scientific method, that significant progress

has been made towards solving these mysteries.3

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Basic Genetic Concepts

Alleles: alternative (variant) forms of a gene Phenotype: the appearance or characteristics of an

individual

Genotype: the genetic constitution of an individual Chromosomes: long molecules of double-stranded

DNA and protein that contain genes

Genome: the complete set of genes of an individual

Gametes: specialized reproductive cells that convey

genetic information from parent to offspring

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7/39Copyright 2012 Pearson Education Inc.Genetics Analysis: An Integrated Approach

Modern Genetics Has Three Major Branches

Transmission genetics (Mendelian genetics) isthe study of the transmission of traits in successivegenerations

Evolutionary genetics studies the origins of andgenetic relationships between organisms, andevolution of genes and genomes

Molecular genetics studies inheritance andvariation at a molecular level

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Progress in Understanding DNA Function

During the 1950s the structure and replication ofDNA were elucidated

In the 1960s, the genetic code and mechanisms of

transcription and translation were deciphered

By the 1970s, cloning and the development of

recombinant DNA technology had begun

In the 1980s, scientists began to study and compare

entire genomes of microbes

In 2001, a consortium of scientific groups published

a first draft of the human genome

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The Three Domains of Life

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DNA Is the Hereditary Material

Deoxyribonucleic acid (DNA) is the primaryhereditary material for most organisms

Ribonucleic acid (RNA) is used by some viruses

DNA has a double-stranded structure, a DNA

double helix, or DNA duplex

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Plastids: Mitochondria and Chloroplasts

Plant and animal cells contain mitochondria

Plant cells contain chloroplasts

These organelles contain their own DNA on single

circular chromosomes

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1.2 The Structure of DNA Suggests a Mechanismfor Replication

Identification of DNA as the hereditary material wasthe foundation of new molecular-focused

approaches in biological research

The molecular structure of DNA was key to

understanding:

How DNA could carry genetic information How the molecule replicated

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The DNA Double Helix

Combining data from multiple sources, includingRosalind Franklins X-ray diffraction data & ErwinChargaffs rules of nucleotide abundance, Watsonand Crick deduced the structure of DNA in 1953

The structure was described as a double helix withsugar phosphate backbones on the outsides andnucleotide bases arrayed in complementary pairstoward the center

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Chargaffs Rule - %A = %T & %C = %G

Watson and Crick used Chargaffs rule to deduce that nucleotidesare arranged in complementary pairs

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Determining the structure of a biological macromolecule rarely constitutesa major scientific breakthrough. The discovery of the structure of DNA

was a rare exception, because it provided immediate insights into majorbiological questions, including:

1.) how complex information is encoded by

a chemically simple substance

2.) how information is copied andtransmitted

The paper is also celebrated as a superb

example of (now-extinct) clear, concise, and

broadly accessible scientific writing.

It has not escaped our notice that the specific pairing we have postulated

immediately suggests a possible copying mechanism for the genetic material.

Watson & Crick. 1953 Nature 171:737-738

see also Cricks letter to his 12-year-old son: www.thehistoryblog.com/archives/25193

The DNA Double Helix

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DNA Nucleotides

DNA nucleotides are composed of a deoxyribose

(5-carbon) sugar, a phosphate group, and one of

four nitrogenous bases designated:

Adenine (A)

Guanine (G)

Thymine (T)

Cytosine (C)

Nucleotides are linked together by aphosphodiester bond between the 5 phosphate

group of one nucleotide and the 3 hydroxyl of

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Complementary Base Pairing

Complementary base pairing occurs between an Aon one strand and a T on the other, or a G on onestrand and a C on the other

Hydrogen bonds form between the complementarybase pairs

The 5 and 3 designations of the phosphate andhydroxyl at the ends of the DNA strands establishpolarity; the two strands are antiparallel

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DNA Replication

Each single strand of DNA contains the informationneeded to generate its complementary strand

Meselson and Stahl demonstrated that DNAreplication was semiconservative about 5 yearsafter DNA structure was elucidated

Semiconservative replication creates two newduplexes, each composed of one parental(original) strand and one newly made daughterstrand

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1.3 Transcription and Translation Express Genes

The central dogma of biology describes the flow ofhereditary information; the original was proposed byFrancis Crick

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1.3 Transcription and Translation Express Genes

The central dogma of biology describes the flow ofhereditary information; the original was proposed byFrancis Crick

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Transcription

RNA Polymerase uses one strand of DNA to directsynthesis of a single-stranded RNA transcript

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Regulation of Transcription

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Translation

Translation converts the genetic messagecarried by mRNA into a sequence of amino acids

joined together by covalent peptide bonds at

the ribosome

The resulting polypeptide, upon folding, makes

up all or part of a protein

Each amino acid is specified by a codon, three

consecutive nucleotides on the mRNA

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Translation

Translation begins when mRNA attaches to the

ribosome in a manner that places the start codon(AUG) in the correct position, and moves 5 to 3 until

a stop codon is reached.

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Translation

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The Genetic Code

mRNA encodes an amino acid sequence via thegenetic code

There are 64 possible triplet codons, read in the5 to 3 direction; each specifies one amino acid

There are 20 common amino acids; some aminoacids are specified by one codon and others by

up to six different codons

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1.4 Evolution Has a Molecular Basis

Life is not static or uniform; it evolves as DNAacquires mutational changes

The biochemical processes that replicate DNA andexpress genetic information are universal

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N t l S l ti

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Natural Selection

Natural selection operates on phenotypic variation,which is influenced by underlying genetic variation

As one phenotype is favored over another, thefrequencies of associated alleles are altered

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F P Th t L d t Ch i

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Four Processes That Lead to Changes inAllele Frequencies in a Population Over Time

Mutation: the addition of allelic variants that serve asraw material of evolutionary change

Selection: differential survival and/or reproductive

success of members of a population due to

phenotypic variation

Migration: the movement of individuals from one

population to another

Drift: the random change of allele frequencies due to

chance

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M d S th i f E l ti

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Modern Synthesis of Evolution

The modern synthesis of evolution mergesevolutionary theory with population genetics and

transmission genetics

It provides a unifying view of the genetic processes

and mechanisms that produce evolutionary change

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Ch01 Lecture v01