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Chapter 1The Science of Genetics
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
DNA Replication
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
Gene Expression:Using Genetic Information
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
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).
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
A phylogenetic tree
A phylogenetic tree, or phylogeny, represents the historical relationships among organisms.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
Levels of Genetic Analysis
© John Wiley & Sons, Inc.
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
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
Genetics in the World:Applications of Genetics to
Human Endeavors
© John Wiley & Sons, Inc.
Genetics is relevant in many venues outside the research laboratory.
Genetics in Agriculture:Selective Breeding
© John Wiley & Sons, Inc.
Genetics in Agriculture:Genetically Modified Organisms
Genetically Modified Organisms (GMOs) are have been altered by the introduction of foreign genes.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.
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.
© John Wiley & Sons, Inc.