Lecture Molecular biology (Fifth Edition): Chapter 2 - Robert F. Weaver

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Before we begin to study in detail the structure and activities of genes, and the experimental evidence underlying those concepts, we need a fuller outline of the adventure that lies before us. Thus, in this chapter and in chapter 3, we will fl esh out the brief history of molecular biology presented in chapter 1. In this chapter we will begin this task by considering the behavior of genes as molecules.

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Nội dung Text: Lecture Molecular biology (Fifth Edition): Chapter 2 - Robert F. Weaver

Lecture PowerPoint to accompany Molecular Biology Fifth Edition Robert F. Weaver Chapter 2 The Molecular Nature of Genes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Nature of Genetic Material Historical Background • Miescher isolated nuclei from pus (white blood cells) in 1869 – Found a novel phosphorus-bearing substance = nuclein – Nuclein is mostly chromatin, a complex of DNA and chromosomal proteins • End of 19th century – DNA and RNA separated from proteins • Levene, Jacobs, et al. characterized basic composition of DNA and RNA 2-2
Molecular Foundation: Early experiments that explored the question: What is the genetic material? • Key experiments performed by Frederick Griffith in 1928 • Observed change in Streptococcus pneumoniae — from avirulent (R) rough colonies, bacteria without capsules, to virulent (S) smooth colonies, bacteria that had capsules • Result: Heat-killed virulent bacteria could transform avirulent bacteria into virulent bacteria 2-3
Outline of Griffith’s Transformation Experiments 2-4
DNA: The Transforming Material In 1944 Avery, Macleod and McCarty used a transformation test similar to Griffith’s procedure taking care to define the chemical nature of the transforming substance – Techniques used excluded both protein and RNA as the chemical agent of transformation – Exclusion of DNA verified that DNA is the chemical agent of transformation of S. pneumoniae from avirulent to virulent 2-5
Analytical Tools Physical-chemical analysis has often used: 1. Ultracentrifugation Used to estimate size of material 1. Electrophoresis Indicated high charge-to-mass ratio 1. Ultraviolet Absorption Spectrophotometry Absorbance of UV light matched that of DNA 1. Elementary Chemical Analysis Nitrogen-to-phosphorus ratio of 1.67, expected for DNA but lower than expected for protein 2-6
Confirmation for DNA as the genetic material • In the 1940s geneticists doubted the use of DNA as the genetic material as it appeared to be monotonous repeats of 4 bases • By 1953 Watson & Crick published the double- helical model of DNA structure and Chargaff demonstrated that the 4 bases were not present in equal proportions • In 1952 Hershey and Chase demonstrated that bacteriophage infection comes from DNA, adding more evidence to support that DNA is the genetic material 2-7
Outline of Hershey and Chase’s Experiment 2-8
Summary • The classic molecular biology experiments performed by Griffith, Avery, MacLeod, Mccarty, Hershey and Chase combined revealed that DNA is the genetic element 2-9
The Chemical Nature of Polynucleotides • Biochemists determined the components of nucleotides during the 1940s • The component parts of DNA – Nitrogenous bases: • Adenine (A) • Cytosine (C) • Guanine (G) • Thymine (T) – Phosphoric acid – Deoxyribose sugar 2-10
Nucleosides and Deoxyribose • RNA component parts – Nitrogenous bases • Like DNA except Uracil (U) replaces Thymine – Phosphoric acid – Ribose sugar • Bases use ordinary numbers • Carbons in sugars are noted as primed numbers • Nucleotides contain phosphoric acid • Deoxyribose lacks a • Nucleosides lack the hydroxyl group (OH) at phosphoric acid the 2-position 2-11
Purines and Pyrimidines • Adenine and guanine are related structurally to the parent molecule purine • Cytosine, thymine and uracil resemble the parent molecule pyrimidine 2-12
DNA Linkage • Nucleotides are nucleosides with a phosphate group attached through a phosphodiester bond • Nucleotides may contain one, two, or even three phosphate groups linked in a chain 2-13
A Trinucleotide The example trinucleotide has polarity – The top of molecule has a free 5’-phosphate group = 5’ end – The bottom has a free 3’- hydroxyl group = 3’ end 2-14
Summary • DNA and RNA are chain-like molecules composed of subunits called nucleotides • Nucleotides contain a base linked to the 1’-position of a sugar and a phosphate group • The phosphate joins the sugars in a DNA or RNA chain through their 5’- and 3’- hydroxyl groups by phosphodiester bonds 2-15
DNA Structure The Double Helix • Rosalind Franklin’s x-ray diffraction data suggested that DNA had a helical shape • The data also indicated a regular, repeating structure • Chargaff’s data revealed that the content of purines was always roughly equal to pyrimidines • Watson and Crick proposed a double helix with sugar-phosphate backbones on the outside and bases aligned on the interior 2-16
DNA Helix • Structure compared to a twisted ladder – Curving sides of the ladder represent the sugar- phosphate backbone – Ladder rungs are the base pairs – There are about 10 base pairs per turn • Arrows indicate that the two strands are antiparallel 2-17
Summary • The DNA molecule is a double helix, with sugar-phosphate backbones on the outside and base pairs on the inside • The bases pair in a specific way: – Adenine (A) with thymine (T) – Guanine (G) with cytosine (C) 2-18
Genes Made of RNA Viruses are a package of genes – No metabolic activity of their own – When a virus infects a host cell, the cellular machinery is diverted and begins to make viral proteins – Viral genes are replicated and used for the production of viral protein that assemble into virus particles Viruses contain nucleic acid, some viruses use DNA genes, but some viruses have RNA genes, either double- or single-stranded 2-19
Physical Chemistry of Nucleic Acids DNA and RNA molecules can appear in several different structural variants – Changes in relative humidity will cause variation in DNA molecular structure – The twist of the DNA molecule is normally shown to be right-handed, but left-handed DNA also exists and was identified in 1979 2-20