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Chapter 1 The Science of Genetics

© John Wiley & Sons, Inc.

Chapter Outline! An Invitation! Three Great Milestones in Genetics! DNA as the Genetic Material! Genetics and Evolution! Levels of Genetic Analysis! Genetics in the World: Applications of

Genetics to Human Endeavors


Three Great Milestones in Genetics

! Gregor Mendel: Genes and the rules of inheritance

! James Watson and Francis Crick: the structure of DNA

! The Human Genome Project: sequencing DNA and cataloguing genes


Mendel: Genes and the Rules of Inheritance (1866)

! Genes—hereditary factors responsible for traits

! Alleles—different forms of genes

! Rules 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 acids! Nucleic acids are made of building

blocks called nucleotides! Nucleotides have three components

–  Sugar molecule (ribose or deoxyribose)

–  Phosphate molecule–  Nitrogen-containing molecule

(adenine, guanine, cytosine, thymine, uracil)

! RNA is ribonucleic acid! DNA 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


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 Project determined the sequence of nucleotides in the DNA of the human genome


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.


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.


DNA as the Genetic Material! Information flows 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


DNA Replication


DNA Replication! Based 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


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.


The Proteome! Proteome—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.


The Central Dogma of Molecular Biology

! The flow of information is DNA è RNAè 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


Key Points! When 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).


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.


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.


A phylogenetic tree

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


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.


Levels of Genetic Analysis


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 behavior of chromosomes.! Encompasses transmission genetics and

studies of the nature of the genetic material


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.


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.


Key Points! In 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.


Genetics in the World:Applications of Genetics to

Human Endeavors


Genetics is relevant in many venues outside the research

laboratory.

Genetics in Agriculture:Selective Breeding


Genetics in Agriculture:Genetically Modified Organisms

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


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.


Genetics in Society

! Economic impact—biotechnology industry, pharmaceutical industry.

! Legal impact—paternity testing, forensics, identification

! Philosophical impact


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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