CHAPTER 16 THE MOLECULE BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/16A-DNATheGeneticMaterial.pdf · depends on the precise replication of DNA and its ... DNA Copyright

CHAPTER 16THE MOLECULE BASIS OF

INHERITANCE

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section A: DNA as the Genetic Material1. The search for the genetic material lead to DNA2. Watson and Crick discovered the double helix by building models to

conform to X-ray data

• In April 1953, James Watson and Francis Crickshook the scientific world with an elegant double-helical model for the structure of deoxyribonucleicacid or DNA.

• Your genetic endowment is the DNA you inheritedfrom your parents.

• Nucleic acids are unique in their ability to direct theirown replication.

• The resemblance of offspring to their parentsdepends on the precise replication of DNA and itstransmission from one generation to the next.

Introduction


• Once T.H. Morgan’s group showed that genes arelocated on chromosomes, the two constituents ofchromosomes - proteins and DNA - were thecandidates for the genetic material.

• Until the 1940s, the great heterogeneity andspecificity of function of proteins seemed to indicatethat proteins were the genetic material.

• However, this was not consistent with experimentswith microorganisms, like bacteria and viruses.

1. The search for genetic material lead toDNA


• The discovery of the genetic role of DNA beganwith research by Frederick Griffith in 1928.

• He studied Streptococcus pneumoniae, a bacteriumthat causes pneumonia in mammals.• One strain, the R strain, was harmless.

• The other strain, the S strain, was pathogenic.

• In an experiment Griffith mixed heat-killed S strainwith live R strain bacteria and injected this into amouse.

• The mouse died and he recovered the pathogenicstrain from the mouse’s blood.


• Griffith called this phenomenon transformation, achange in genotype and phenotype due to theassimilation of a foreign substance (now known tobe DNA) by a cell.


Fig. 16.1

• For the next 14 years scientists tried to identify thetransforming substance.

• Finally in 1944, Oswald Avery, Maclyn McCartyand Colin MacLeod announced that thetransforming substance was DNA.

• Still, many biologists were skeptical.• In part, this reflected a belief that the genes of bacteria

could not be similar in composition and function tothose of more complex organisms.


• Further evidence that DNA was the geneticmaterial was derived from studies that tracked theinfection of bacteria by viruses.

• Viruses consist of a DNA (sometimes RNA)enclosed by a protective coat of protein.

• To replicate, a virus infects a host cell and takesover the cell’s metabolic machinery.

• Viruses that specifically attack bacteria are calledbacteriophages or just phages.


• In 1952, Alfred Hershey and Martha Chase showedthat DNA was the genetic material of the phage T2.

• The T2 phage, consisting almost entirely of DNAand protein, attacks Escherichia coli (E. coli), acommon intestinal bacteria of mammals.

• This phage can quicklyturn an E. coli cell intoa T2-producing factorythat releases phageswhen the cell ruptures.


Fig. 16.2a

• To determine the source of genetic material in thephage, Hershey and Chase designed an experimentwhere they could label protein or DNA and thentrack which entered the E. coli cell during infection.• They grew one batch of T2 phage in the presence of

radioactive sulfur, marking the proteins but not DNA.• They grew another batch in the presence of radioactive

phosphorus, marking the DNA but not proteins.• They allowed each batch to infect separate E. coli

cultures.• Shortly after the onset of infection, they spun the cultured

infected cells in a blender, shaking loose any parts of thephage that remained outside the bacteria.


• The mixtures were spun in a centrifuge which separatedthe heavier bacterial cells in the pellet from lighter freephages and parts of phage in the liquid supernatant.

• They then tested the pellet and supernatant of the separatetreatments for the presence of radioactivity.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 16.2b

• Hershey and Chase found that when the bacteria hadbeen infected with T2 phages that contained radio-labeled proteins, most of the radioactivity was in thesupernatant, not in the pellet.

• When they examined the bacterial cultures with T2phage that had radio-labeled DNA, most of theradioactivity was in the pellet with the bacteria.

• Hershey and Chase concluded that the injected DNAof the phage provides the genetic information thatmakes the infected cells produce new viral DNA andproteins, which assemble into new viruses.


• The fact that cells double the amount of DNA in acell prior to mitosis and then distribute the DNAequally to each daughter cell provided somecircumstantial evidence that DNA was the geneticmaterial in eukaryotes.

• Similar circumstantial evidence came from theobservation that diploid sets of chromosomes havetwice as much DNA as the haploid sets in gametesof the same organism.


• By 1947, Erwin Chargaff had developed a series ofrules based on a survey of DNA composition inorganisms.• He already knew that DNA was a polymer of nucleotides

consisting of a nitrogenous base, deoxyribose, and aphosphate group.

• The bases could be adenine (A), thymine (T), guanine(G), or cytosine (C).

• Chargaff noted that the DNA composition variesfrom species to species.

• In any one species, the four bases are found incharacteristic, but not necessarily equal, ratios.


• He also found a peculiar regularity in the ratios ofnucleotide bases which are known as Chargaff’srules.

• The number of adenines was approximately equal tothe number of thymines (%T = %A).

• The number of guanines was approximately equal tothe number of cytosines (%G = %C).• Human DNA is 30.9% adenine, 29.4% thymine, 19.9%

guanine and 19.8% cytosine.


• By the beginnings of the 1950’s, the race was on tomove from the structure of a single DNA strand tothe three-dimensional structure of DNA.• Among the scientists working on the problem were Linus

Pauling, in California, and Maurice Wilkins and RosalindFranklin, in London.

2. Watson and Crick discovered the doublehelix by building models to conform to X-ray data



Fig. 16.3

• The phosphategroup of onenucleotide isattached to the sugarof the nextnucleotide in line.

• The result is a“backbone” ofalternatingphosphates andsugars, from whichthe bases project.

• Maurice Wilkins and Rosalind Franklin used X-raycrystallography to study the structure of DNA.• In this technique, X-rays are diffracted as they passed

through aligned fibers of purified DNA.

• The diffraction pattern can be used to deduce the three-dimensional shape of molecules.

• James Watson learnedfrom their researchthat DNA was helicalin shape and he deducedthe width of the helixand the spacing of bases.


Fig. 16.4

• Watson and his colleague Francis Crick began towork on a model of DNA with two strands, thedouble helix.

• Using molecular models made of wire, they firsttried to place the sugar-phosphate chains on theinside.

• However, this did not fit the X-ray measurementsand other information on the chemistry of DNA.


• The key breakthrough came when Watson put thesugar-phosphate chain on the outside and thenitrogen bases on the inside of the double helix.• The sugar-phosphate chains of each strand are like the

side ropes of a rope ladder.

• Pairs of nitrogen bases, one from each strand, form rungs.

• The ladder forms a twist every ten bases.



Fig. 16.5

• The nitrogenous bases are paired in specificcombinations: adenine with thymine and guaninewith cytosine.

• Pairing like nucleotides did not fit the uniformdiameter indicated by the X-ray data.• A purine-purine pair would be too wide and a pyrimidine-

pyrimidine pairing would be too short.

• Only a pyrimidine-purine pairing wouldproduce the 2-nmdiameter indicatedby the X-ray data.


• In addition, Watson and Crick determined thatchemical side groups off the nitrogen bases wouldform hydrogen bonds, connecting the two strands.• Based on details of their

structure, adenine wouldform two hydrogen bondsonly with thymine andguanine would form threehydrogen bonds only withcytosine.

• This finding explainedChargaff’s rules.


Fig. 16.6

• The base-pairing rules dictate the combinations ofnitrogenous bases that form the “rungs” of DNA.

• However, this does not restrict the sequence ofnucleotides along each DNA strand.

• The linear sequence of the four bases can be variedin countless ways.

• Each gene has a unique order of nitrogen bases.

• In April 1953, Watson and Crick published asuccinct, one-page paper in Nature reporting theirdouble helix model of DNA.


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CHAPTER 16 THE MOLECULE BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/16A-DNATheGeneticMaterial.pdf · depends on the precise replication of DNA and its ... DNA Copyright