Lecture Molecular biology (Fifth Edition): Chapter 1 - Robert F. Weaver

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Chapter 1 - A brief history. This chapter introduce to a brief history of molecular biology. Molecular biology grew out of the disciplines of genetics and biochemistry. In this chapter we will review the major early developments in the history of this hybrid discipline, beginning with the earliest genetic experiments performed by Gregor Mendel in the mid-nineteenth century.

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Nội dung Text: Lecture Molecular biology (Fifth Edition): Chapter 1 - Robert F. Weaver

Lecture PowerPoint to accompany Molecular Biology Fifth Edition Robert F. Weaver Chapter 1 A Brief History Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A Brief History • What is molecular biology? – The attempt to understand biological phenomena in molecular terms – The study of gene structure and function at the molecular level • Molecular biology is a melding of aspects of genetics and biochemistry 1-2
1.1 Transmission Genetics • Transmission genetics deals with the transmission of traits from parental organisms to their offspring • The chemical composition of genes was not known until 1944 – Gene - genetic units – Phenotype - observable characteristics 1-3
Mendel’s Laws of Inheritance • A gene can exist in different forms called alleles • One allele can be dominant over the other, recessive, allele • The first filial generation (F1) contains offspring of the original parents • If each parent carries two copies of a gene, the parents are diploid for that gene 1-4
Mendel’s Laws of Inheritance • Homoozygotes have two copies of the same allele • Heterozygotes have one copy of each allele • Parents in 1st mating are homozygotes, having 2 copies of one allele • Sex cells, or gametes, are haploid, containing only 1 copy of each gene • Heterozygotes produce gametes having either allele • Homozygotes produce gametes having only one allele 1-5
Summary • Genes can exist in several different forms or alleles • One allele can be dominant over the other, so heterozygotes having two different alleles of one gene will generally exhibit the characteristic dictated by the dominant allele • The recessive allele is not lost; it can still exert its influence when paired with another recessive allele in a homozygote 1-6
The Chromosome Theory of Inheritance • Chromosomes are discrete physical entities that carry genes • Thomas Hunt Morgan used the fruit fly, Drosophila melanogaster, to study genetics • Autosomes occur in pairs in a given individual (not the X or the Y chromosome) • Sex chromosomes are identified as X and Y – Females have two X chromosomes – Males have one X and one Y chromosome 1-7
Location of Genes on a Chromosome • Every gene has its place, or locus, on a chromosome • Genotype is the combination of alleles found in an organism • Phenotype is the visible expression of the genotype – Wild-type phenotype is the most common or generally accepted standard – Mutant alleles are usually recessive 1-8
Genetic Recombination and Mapping • In early experiments genes on separate chromosomes behaved independently • Genes on the same chromosome behaved as if they were linked • This genetic linkage is not absolute • Offspring show new combinations of alleles not seen in the parents when recombination occurs 1-9
Recombination • During meiosis, gamete formation, crossing over can occur resulting in the exchange of genes between the two homologous chromosomes • The result of the crossing-over event produces a new combination of alleles • This process is called recombination 1-10
Genetic Mapping • Morgan proposed that the farther apart two genes are on a chromosome, the more likely they are to recombine • If two loci recombine with a frequency of 1%, they are said to be separated by a map distance of one centimorgan (named for Morgan) • This mapping observation applies both to prokaryotes and to eukaryotes 1-11
Physical Evidence for Recombination • Microscopic examination of the maize chromosome provided direct physical observation of recombination using easily identifiable features of one chromosome • Similar observations were made in Drosophila • Recombination was detected both physically and genetically in both animals and plants 1-12
Summary • The chromosome theory of inheritance holds that genes are arranged in linear fashion on chromosomes • Certain traits tend to be inherited together when the genes for those traits are on the same chromosome • Recombination between two homologous chromosomes during meiosis can scramble the parental alleles to yield nonparental combinations • The farther apart two genes are on a chromosome the more likely it is that recombination will occur 1-13
1.2 Molecular Genetics • The Discovery of DNA: The general structure of nucleic acids was discovered by the end of the 19th century – Long polymers or chains of nucleotides – Nucleotides are linked by sugars through phosphate groups • Composition of Genes: DNA? RNA? Protein? In 1944, Avery and his colleagues demonstrated that genes are composed of DNA 1-14
The Relationship between Genes and Proteins • Experiments have shown that a defective gene gives a defective or absent enzyme • This lead to the proposal that one gene is responsible for making one enzyme • Proposal not quite correct for 3 reasons: 1. One enzyme may be composed of several polypeptides, each gene codes for only one polypeptide 2. Many genes code for non-enzyme proteins 3. End products of some genes are not polypeptides (i.e. tRNA, rRNA) 1-15
Activities of Genes Genes perform three major roles • Replicated faithfully • Direct the production of RNAs and proteins • Accumulate mutations thereby allowing for evolution 1-16
Replication • Franklin and Wilkins produced x-ray diffraction data on DNA, Watson and Crick proposed that DNA is double helix – Two DNA strands wound around each other – Strands are complementary – if you know the sequence of one strand, you automatically know the sequence of the other strand • Semiconservative replication keeps one strand of the parental double helix conserved in each of the daughter double helices 1-17
Genes Direct the Production of Polypeptides • Gene expression is the process by which a gene product is made • Two steps are required – 1. Transcription: DNA is transcribed into RNA – 2. Translation: the mRNA is read or translated to assemble a protein – Codon: a sequence of 3 nucleic acid bases that code for one amino acid within the mRNA 1-18
Genes Accumulate Mutations Genes change in several ways • Change one base to another • Deletions of one base up to a large segment • Insertions of one base up to a large segment • The more drastic the change, the more likely it is that the gene or genes involved will be totally inactivated 1-19
Summary • All cellular genes are made of DNA arranged in a double helix • This structure explains how genes replicate, carry information and collect mutations • The sequence of nucleotides in a gene is a genetic code that carries the information for making an RNA • A change in the sequence of bases constitutes and mutation, which can change the sequence of amino acids in the genes polypeptide product 1-20