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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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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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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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