Chapter 1The Science of Genetics

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Chapter OutlineAn InvitationThree Great Milestones in GeneticsDNA as the Genetic MaterialGenetics and EvolutionLevels of Genetic AnalysisGenetics in the World: Applications of

Genetics to Human endeavours

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Three Great Milestones in Genetics

Gregor Mendel: Genes and the rules of inheritanceJames Watson and Francis Crick: the structure of

DNAThe Human Genome Project: sequencing DNA and

cataloguing genes

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Mendel: Genes and theRules of Inheritance (1866)

Genes—hereditary factors responsible for traits

Alleles—different forms of genesRules of Inheritance

– Alleles of the same gene separate during gamete formation

– Alleles of different genes are inherited independently

What is a Gene?Genes are made of nucleic acidsNucleic acids are made of building

blocks called nucleotidesNucleotides have three

components– Sugar molecule (ribose or

deoxyribose)– Phosphate molecule– Nitrogen-containing molecule

(adenine, guanine, cytosine, thymine, uracil)

RNA is ribonucleic acidDNA is deoxyribonucleic acid

Watson and Crick:The Structure of DNA (1953)

Nucleotides are linked in a chain through sugar-phosphate interactions

DNA molecules are made of two chains of nucleotides wound around each other in a helix

Base pairs hold the chains together– A pairs with T– G pairs with C

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The Human Genome Project:Sequencing DNA and Cataloguing Genes

Genome—the collection of DNA molecules that is characteristic of an organism

Genomics is the analysis of DNA sequences that make up a genome

Genomics involves DNA sequencing technology, robotics, and computer science

The Human Genome Projectdetermined the sequence of nucleotides in the DNA of the human genome

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Key Points

Gregor Mendel postulated the existence of particular factors—now called genes—to explain how traits are inherited.Alleles, the alternate forms of genes, account

for heritable differences among individuals.James Watson and Francis Crick elucidated

the structure of DNA, a macromolecule composed of two complementary chains of nucleotides.

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Key Points

DNA is the hereditary material of all life forms except some types of viruses, in which RNA is the hereditary material.The Human Genome Project determined the

sequence of nucleotides in the DNA of the human genome.Sequencing the DNA of a genome provides

the data to identify and catalogue all the genes of an organism.

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DNA as the Genetic Material Information flows –Central dogma

from DNA to RNA to protein. In all cellular organisms, the genetic material is DNA.The genetic material

– Must be able to replicate– Must contain information– Must be able to change

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DNA Replication

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DNA ReplicationBased on the complementary nature of the two

strands of duplex DNA molecules.When the two parental strands are separated, the

separated strands can serve as template for the synthesis of new strands.

New strands are assembled by incorporating nucleotides according to base-pairing rules.

At the end of replication, each template strand is paired with a newly synthesized partner strand.

DNA replication is catalyzed by enzymes.

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Gene Expression:Using Genetic Information

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Gene Expression

During transcription, an RNA molecule is synthesized from a DNA template.

This messenger RNA (mRNA) molecules contains the information needed to synthesize a polypeptide.

During translation, the triplet codons in the RNA specify the incorporation of particular amino acids into a polypeptide chain.

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The ProteomeProteome—the collection of all the different proteins

in an organism.Humans have between 20,000 and 25,000 genes in

the genome and hundreds of thousands of proteins in the proteome.

Proteomics—the study of all the proteins in cells.

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The Central Dogma of Molecular Biology

The flow of information is DNA RNA protein.Some viruses can use RNA as a template for the

synthesis of DNA in reverse transcription.Many genes do not encode polypeptides; their end-

products are RNA molecules.© John Wiley & Sons, Inc.

Mutation:Changing Genetic Information

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Key PointsWhen DNA replicates, each strand of a duplex

molecule serves as the template for the synthesis of a complementary strand.When genetic information is expressed, one

strand of a gene’s DNA duplex is used as a template for the synthesis of a complementary strand of DNA.For most genes, RNA synthesis (transcription)

generates a molecule (the RNA transcript) that becomes a messenger RNA (mRNA).

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Key Points

Coded information in an mRNA is translated into a sequence of amino acids in a polypeptide.Mutations can alter the DNA sequence of a

gene.The genetic variability created by mutation is

the basis for biological evolution.

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Genetics and Evolution

Variation in the DNA sequence makes it possible for species to evolve over time.Organisms with similar DNA sequences

are descended from a common ancestor.

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A phylogenetic tree

A phylogenetic tree, or phylogeny, represents the historical relationships among organisms.

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Key Points

Evolution depends on the occurrence, transmission, and spread of mutant genes in groups of organisms.DNA sequence data provide a way of

studying the historical process of evolution.

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Levels of Genetic Analysis

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Geneticists approach their science from different points of view—from that of a gene, a DNA molecule, or a population of organisms.

Classical Genetics

Based on analysis of the outcomes of crosses between different strains of organisms.

Can be coordinated with studies of the structure and behaviour of chromosomes.

Encompasses transmission genetics and studies of the nature of the genetic material

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Molecular Genetics

Studies the replication, expression, and mutation of genes at the molecular level.

Rooted in the study of DNA sequences and the manipulation of DNA molecules.

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Population Genetics Individuals within a population may carry different

alleles of genes.Population genetics is based on analyzing allele

frequencies in a population and determining whether these frequencies changes over time.

Population genetics includes evolution and the inheritance of complex traits.

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Key PointsIn classical genetic analysis, genes are

studied by following the inheritance of traits in crosses between different strains of an organism.In molecular genetic analysis, genes are

studied by isolating, sequencing, and manipulating DNA and by examining the products of gene expression.In population genetic analysis, genes are

studied by assessing the variability among individuals in a group of organisms.

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Genetics in the World:Applications of Genetics to

Human Endeavors

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Genetics is relevant in many venues outside the research laboratory.

Genetics in Agriculture:Selective Breeding

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Genetics in Agriculture:Genetically Modified Organisms

Genetically Modified Organisms (GMOs) are have been altered by the introduction of foreign genes.

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Genetics in Medicine Inborn Errors of Metabolism are metabolic

abnormalities caused to mutant alleles.Genetic Counselors advise people about inheritance

of genetic diseases.Molecular genetics

– new ways to detect mutant alleles.– new ways to treat diseases.

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Genetics in Society

Economic impact—biotechnology industry, pharmaceutical industry.

Legal impact—paternity testing, forensics, identificationPhilosophical impact

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Key Points

Discoveries in genetics are changing procedures and practices in agriculture and medicine.Advances in genetics are raising ethical, legal,

political, social, and philosophical questions.

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