Chapter 13 test questions 1

As a result of proofreading by DNA polymerases, the overall error rate in the completed DNA molecule is approximately __________. a. 1 error per 100 nucleotides b. 1 error per 1,000 nucleotides c. 1 error per 1,000,000 nucleotides d. 1 error per 1,000,000,000 nucleotides e. 1 error per 10,000,000,000 nucleotides

1 error per 10,000,000,000 nucleotides The rate of initial pairing errors during replication is about 1 in 100,000. This level of accuracy is then dramatically enhanced by proofreading mechanisms.

DNA polymerase adds nucleotides to the __________ of the leading strands, and to the __________ of the lagging strands (Okazaki fragments). a. 5′ end ... 5′ end b. sugar group ... phosphate group c. 5′ end ... 3′ end d. 3′ end ... 5′ end e. 3′ end ... 3′ end

3′ end ... 3′ end Although the leading strand and lagging strand are synthesized in opposing directions with respect to the movement of the replication fork, the DNA polymerase enzyme can only add nucleotides to the 3′ end of a growing DNA strand.

In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules? a. A + T = G + T b. A + G = C + T c. A = C d. A = G e. G = T

A + G = C + T

Which one of the following statements is correct? a. Adenine forms three hydrogen bonds with thymine; guanine forms two hydrogen bonds with cytosine. b. Adenine forms three covalent bonds with thymine; guanine forms two covalent bonds with cytosine. c. Adenine forms two covalent bonds with thymine; guanine forms three covalent bonds with cytosine. d. Adenine forms two hydrogen bonds with guanine; thymine forms three hydrogen bonds with cytosine. e. Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine.

Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine. This is a consequence of the characteristics of the bases.

Which of the following statements about replication origins is/are correct? a. Bacterial chromosomes have a single origin of replication, but eukaryotic chromosomes have many origins. b. The DNA sequence at the origin of replication is recognized by specific proteins that bind to the origin. c. Replication proceeds in both directions from each origin. d. The two strands of DNA at the origin of replication are separated, creating a replication bubble. e. All of the listed responses are correct.

All of the listed responses are correct. All of these statements are correct.

Which of the following considerations was/were important in the choice of viruses and bacteria for early experiments on DNA? a. Their chromosomes have a simpler structure. b. They typically have relatively small genomes. c. They can interact with each other. d. They have short generation times. e. All of the listed responses are correct.

All of the listed responses are correct. Experiments on DNA were, and still are, carried out on these subjects for all of these reasons.

Which of the following statements is/are correct with regard to individuals with the disorder xeroderma pigmentosum? a. These individuals have high rates of skin cancer. b. Cells in these individuals have difficulty repairing thymine dimers. c. These individuals are extremely sensitive to sunlight. d. These individuals usually have inherited defects in the nucleotide excision repair system. e. All of the listed responses are correct.

All of the listed responses are correct. Individuals with this disorder are unusually sensitive to sunlight because they cannot repair the thymine dimers that can result from exposure to ultraviolet light.

What is the major difference between bacterial chromosomes and eukaryotic chromosomes? a. Bacterial chromosomes have much more protein associated with the DNA than eukaryotes. b. There is no difference between bacterial and eukaryotic chromosomes. c. The DNA molecules of bacterial chromosomes have a slightly different structure than those of eukaryotic chromosomes. d. Eukaryotes have a single circular chromosome whereas bacteria have several linear chromosomes. e. Bacteria usually have a single circular chromosome whereas eukaryotes have several linear chromosomes.

Bacteria usually have a single circular chromosome whereas eukaryotes have several linear chromosomes. The arrangement of DNA and its association with proteins is different for prokaryotes and eukaryotes.

In a comparison between asexually reproducing bacteria and sexually reproducing multicellular eukaryotes, uncorrected errors in replication are more likely to be transmitted to subsequent generations in bacteria than in multicellular eukaryotes. Which of the following provides the best evidence-based explanation for this difference? a. Although the genome for each is represented as pairs of homologous chromosomes, errors in bacteria affect both homologs, but only one is affected in eukaryotes. b. Unlike multicellular eukaryotes, bacteria lack DNA repair enzymes, so there are just more uncorrected errors at play. c. Because they are asexual and single-celled, all uncorrected errors of replication in bacteria are transmitted to subsequent generations. Multicellular eukaryotes typically reproduce sexually, so uncorrected errors are transmitted only if they occur in germ cells that meiotically divide to produce gametes. d. Only errors that lead to faster growing strains are left uncorrected in bacteria, whereas all errors are detected and repaired in multicellular eukaryotes. e. The third and fourth listed responses can explain this phenomenon.

Because they are asexual and single-celled, all uncorrected errors of replication in bacteria are transmitted to subsequent generations. Multicellular eukaryotes typically reproduce sexually, so uncorrected errors are transmitted only if they occur in germ cells that meiotically divide to produce gametes. Uncorrected errors in dividing somatic cells of multicellular eukaryotes may lead to problems in individuals, but they will not be transmitted to offspring. In bacteria, however, all uncorrected errors are transmitted to subsequent generations because cell division equates to reproduction in single-celled organisms.

A biochemist isolates, purifies, and combines in a test tube a variety of molecules needed for DNA replication. When she adds some DNA to the mixture, replication occurs, but each DNA molecule consists of a normal strand paired with numerous segments of DNA a few hundred nucleotides long. What has she probably left out of the mixture? a. nucleotides b. primase c. DNA polymerase d. DNA ligase e. Okazaki fragments

DNA ligase

What enzyme joins Okazaki fragments? a. topoisomerase b. primase c. DNA ligase d. DNA polymerase e. helicase

DNA ligase Helicases unwind the DNA; polymerase synthesizes the complementary strands; ligase joins the Okazaki fragments.

The removal of the RNA primer and addition of DNA nucleotides to the 3' end of Okazaki fragments in its place is carried out by __________. a. ligase b. primase c. DNA polymerase III d. DNA polymerase I e. nuclease

DNA polymerase I Upon encountering the RNA primer, DNA polymerase III falls off the DNA and is replaced by DNA polymerase I.

What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized? a. Polymerase can work on only one strand at a time. b. Helicases and single-strand binding proteins work at the 59 end. c. The origins of replication occur only at the 59 end. d. DNA ligase works only in the 39 S59 direction. e. DNA polymerase can join new nucleotides only to the 39 end of a growing strand.

DNA polymerase can join new nucleotides only to the 39 end of a growing strand.

Which one of the following statements regarding DNA replication is correct? a. The two strands of DNA separate, and nuclease enzymes digest one strand. Then, DNA polymerase synthesizes two new strands out of the old ones. b. Ligase separates the two strands of the DNA double helix. Then, DNA polymerase synthesizes the leading strand and primase synthesizes the lagging strand. c. Helicases separate the two strands of the double helix, and DNA polymerases then construct two new strands using each of the original strands as templates. d. The two strands separate, and each one receives a complementary strand of RNA. Then this RNA serves as a template for the assembly of many new strands of DNA. e. Ligase assembles nucleotides into Okazaki fragments. Then polymerase joins these fragments together into a DNA strand.

Helicases separate the two strands of the double helix, and DNA polymerases then construct two new strands using each of the original strands as templates. In the semiconservative model of DNA replication, the two strands of the parental molecule separate, and each functions as a template for the synthesis of a new complementary strand.

Which of the following components is required for DNA replication? a. messenger RNA b. RNA primer c. sucrases d. proteases e. ribosomes

RNA primer In the cell, the preexisting chain, the primer needed to initiate DNA elongation, is RNA, not DNA.

Once the DNA at the replication fork is unwound by helicases, what prevents the two strands from coming back together to re-form a double helix? a. The helicase pushes the two strands so far apart that they have no chance of finding each other. b. Single-strand binding proteins bind the unwound DNA and prevent the double helix from re-forming. c. One of the strands is rapidly degraded, preventing the double helix from re-forming. d. The helicase modifies the DNA in such a way as to eliminate the affinity between the two strands. e. DNA polymerase follows the helicase so closely that there is no chance for the strands to come back together.

Single-strand binding proteins bind the unwound DNA and prevent the double helix from re-forming. As soon as the helicase passes, the single-strand binding proteins rapidly coat the unwound DNA and prevent the strands from coming back together.

One strand of a DNA molecule has the base sequence 5′-ATAGGT-3′. The complementary base sequence on the other strand of DNA will be 3′- __________-5′. a. ATAGGT b. UAUCCA c. TGGATA d. TGGAUA e. TATCCA

TATCCA A always pairs with T, and G with C.

Who is credited with explaining the structure of the DNA double helix? a. Rosalind Franklin b. Watson and Crick c. Griffith d. Avery, McCarty, and MacLeod e. Hershey and Chase

Watson and Crick In April 1953, Watson and Crick published a one-page paper in the journal Nature elucidating their molecular model for the DNA double helix.

Which of the following techniques was most helpful to Watson and Crick in developing their model for the structure of DNA? a. X-ray crystallography b. cloned DNA c. radioactive labeling d. electrophoresis e. transgenic animals

X-ray crystallography Watson and Crick based their model of DNA on insights they were able to gain from Franklin's X-ray diffraction photo.

Replication Fork

a Y-shaped region on a replicating DNA molecule where the parental strands are being unwound and new strands are being synthesized

Lagging Strand

a discontinuously synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in a 5′ ? 3′ direction away from the replication fork

DNA Ligase

a linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3′ end of one DNA fragment (such as an Okazaki fragment) to the 5′ end of another DNA fragment (such as a growing DNA chain)

Single-Strand Binding Protein

a protein that binds to the unpaired DNA strands during DNA replication, stabilizing them and holding them apart while they serve as templates for the synthesis of complementary strands of DNA

Topoisomerase

a protein that breaks, swivels, and rejoins DNA strands; during DNA replication, topoisomerase helps to relieve strain in the double helix ahead of the replication fork

Nucleotide Excision Repair

a repair system that removes and then correctly replaces a damaged segment of DNA using the undamaged strand as a guide

At each end of a DNA replication bubble is __________. a. an origin of replication b. a replication fork c. a telomere d. a gene e. a ribosome

a replication fork Replication forks are found at the ends of replication bubbles.

Okazaki Fragment

a short segment of DNA synthesized away from the replication fork on a template strand during DNA replication; many such segments are joined together to make up the lagging strand of newly synthesized DNA

Primer

a short stretch of RNA with a free 3′ end, bound by complementary base pairing to the template strand and elongated with DNA nucleotides during DNA replication

Histone

a small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in chromatin structure

Phage

a virus that infects bacteria; also called a bacteriophage

Bacteriophage(s)

a virus that infects bacteria; also called a phage

In DNA, the two purines are __________, and the two pyrimidines are __________. a. adenine and cytosine ... guanine and thymine b. adenine and guanine ... cytosine and thymine c. adenine and thymine ... cytosine and guanine d. cytosine and guanine ... adenine and thymine e. cytosine and thymine ... adenine and guanine

adenine and guanine ... cytosine and thymine Purines have two rings, and pyrimidines have one.

DNA Polymerase

an enzyme that catalyzes the elongation of new DNA (for example, at a replication fork) by the addition of nucleotides to the 3′ end of an existing chain; there are several different DNA polymerases; DNA polymerase III and DNA polymerase I play major roles in DNA replication in E. coli

Nuclease

an enzyme that cuts DNA or RNA, either removing one or a few bases or hydrolyzing the DNA or RNA completely into its component nucleotides

Helicase

an enzyme that untwists the double helix of DNA at replication forks, separating the two strands and making them available as template strands

Virus

an infectious particle incapable of replicating outside of a cell, consisting of an RNA or DNA genome surrounded by a protein coat (capsid) and, for some viruses, a membranous envelope

The role of DNA polymerases in DNA replication is to __________. a. attach free nucleotides to the new DNA strand b. synthesize an RNA primer to initiate DNA strand synthesis c. separate the two strands of DNA d. link together short strands of DNA e. All of the listed responses are correct.

attach free nucleotides to the new DNA strand Elongation of new DNA at a replication fork is catalyzed by enzymes called DNA polymerases.

During the replication of DNA, __________. a. the cell undergoes mitosis b. the reaction is catalyzed by RNA polymerase c. errors never occur d. both strands of a molecule act as templates e. only one strand of the molecule acts as a template

both strands of a molecule act as templates When a cell copies a DNA molecule, each strand serves as a template for ordering nucleotides into a new complementary strand.

The incorporation of an incorrect base into the DNA during replication __________. a. will trigger the cell to destroy the new strand, and replication will begin again b. cannot be repaired, and a new mutation will invariably result c. will almost certainly lead to the death of the cell d. can be repaired by the mismatch repair system e. is virtually impossible, as the accuracy of DNA polymerase is such that errors almost never occur

can be repaired by the mismatch repair system The mismatch repair system can detect and rectify incorrectly incorporated bases.

Unlike prokaryotic DNA replication, replication of eukaryotic chromosomes __________. a. involves two leading strands and no lagging strands b. is semiconservative c. has a single origin d. is error-free e. cannot be completed by DNA polymerase

cannot be completed by DNA polymerase This is the case because eukaryotic chromosomes are linear, and DNA polymerase cannot replicate the extreme 3′ end of the template strands.

The elongation of the leading strand during DNA synthesis a. depends on the action of DNA polymerase. b. produces Okazaki fragments. c. does not require a template strand. d. progresses away from the replication fork. e. occurs in the 39 S59 direction.

depends on the action of DNA polymerase.

Heterochromatin

eukaryotic chromatin that remains highly compacted during interphase and is generally not transcribed

Telomeres __________. a. get longer with each round of DNA replication b. are shorter for younger individuals c. are found in both prokaryotic and eukaryotic cells d. remain the same regardless of the frequency of DNA replication e. get shorter with each round of DNA replication

get shorter with each round of DNA replication However, the enzyme telomerase, which is not present in most cells of multicellular organisms, can lengthen the telomeres.

During Griffith's experiments with Streptococcus pneumoniae in mice, material from __________ bacteria transformed __________ bacteria. a. heat-killed pathogenic ... living nonpathogenic b. living nonpathogenic ... heat-killed pathogenic c. heat-killed nonpathogenic ... living pathogenic d. living pathogenic ... heat-killed nonpathogenic e. living nonpathogenic ... living pathogenic

heat-killed pathogenic ... living nonpathogenic Griffith found that some of the living nonpathogenic cells were converted to the pathogenic form.

After the formation of a replication bubble, which of the following is the correct sequence of enzymes used for the synthesis of the lagging DNA strand? a. primase, helicase, DNA polymerase, ligase b. helicase, DNA polymerase, primase, ligase c. helicase, primase, DNA polymerase, ligase d. ligase, primase, DNA polymerase, helicase e. helicase, primase, ligase, DNA polymerase

helicase, primase, DNA polymerase, ligase First the double helix is unwound; primase makes the RNA primer; DNA polymerases elongate the growing strand and replace the RNA primer with DNA; and DNA ligase joins the Okazaki fragments.

In a nucleosome, the DNA is wrapped around a. ribosomes. b. satellite DNA. c. histones. d. a thymine dimer. e. polymerase molecules.

histones.

The two antiparallel strands of nucleotides that form the DNA double helix are held together by __________. a. covalent bonds between nitrogen atoms in adenine and in thymine b. hydrogen bonds between nucleotide bases c. covalent bonds between carbon atoms in deoxyribose molecules d. ionic bonds between guanine and cytosine e. 5' deoxyribose and phosphate bonds

hydrogen bonds between nucleotide bases The two strands of a DNA molecule are held together by hydrogen bonding between the nitrogenous bases, which are paired in the interior of the helix.

Telomerase __________. a. is an enzyme that lengthens telomeres b. slows the rate of cancer cell growth c. prevents the loss of centromeric DNA d. splits telomeres e. speeds cell aging

is an enzyme that lengthens telomeres Telomerase is found in germ cells and in many human cancers, increasing the longevity of these cells.

Which of the following attributes of DNA is most crucial to its accurate duplication? a. its helical nature and hydrogen bonding b. its deoxyribose sugar and phosphate groups c. its specific sequence of bases d. its phosphodiester linkages and complementary strands e. its specific base pairing through hydrogen bonds

its specific base pairing through hydrogen bonds Hydrogen bonding makes it easy to separate the two strands. Specific complementary base pairing ensures that an accurate strand will be constructed on each template strand.

In prokaryotes, the rate of elongation during DNA replication is __________ the rate in eukaryotes. a. much slower than b. much faster than c. The rates are not comparable, because elongation only occurs in prokaryotes. d. about the same as e. sometimes faster and sometimes slower than

much faster than The rate of elongation is about 500 nucleotides per second in bacteria and about 50 nucleotides per second in human cells.

The spontaneous loss of amino groups from adenine in DNA results in hypoxanthine, an uncommon base, opposite thymine. What combination of proteins could repair such damage? a. nuclease, telomerase, primase b. nuclease, DNA polymerase, DNA ligase c. DNA ligase, replication fork proteins, adenylyl cyclase d. telomerase, primase, DNA polymerase e. telomerase, helicase, single-strand binding protein

nuclease, DNA polymerase, DNA ligase

Which set of enzymes is involved in nucleotide excision repair? a. nuclease, DNA polymerase, primase b. hydrolase, nuclease, and ligase c. ligase, nuclease, and primase d. nuclease, DNA polymerase, and ligase e. DNA polymerase, helicase, primase

nuclease, DNA polymerase, and ligase Nucleotide excision repair involves nucleases, DNA polymerase, and ligase.

In the "beads on a string" structure of unfolded chromatin, the "beads" are __________. a. looped domains b. nucleoids c. heterochromatin d. genes e. nucleosomes

nucleosomes Nucleosomes are complexes of DNA wrapped around eight histone molecules.

Monomers for the synthesis of DNA are called __________. a. nucleotides b. disaccharides c. amino acids d. monosaccharides e. fatty acids

nucleotides Each nucleotide unit of DNA consists of a nitrogenous base (A, G, C, or T), the sugar deoxyribose, and a phosphate group.

E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment? a. one intermediate-density band b. one low-density and one intermediate-density band c. one high-density and one intermediate-density band d. one high-density and one low-density band e. one low-density band

one low-density and one intermediate-density band

The two strands of a DNA double helix are antiparallel. This means that __________. a. the two strands are mirror images b. one strand is actually composed of RNA c. only one of the two strands can be used as a template for replication, because DNA polymerase only works in one direction d. one strand runs in the 5' to 3' direction, and the other runs in the 3' to 5' direction e. they both run in the 3' to 5' direction

one strand runs in the 5' to 3' direction, and the other runs in the 3' to 5' direction This allows the two strands to fit together properly.

Until Hershey and Chase showed that DNA was the genetic molecule of the phages they studied, what class of molecules was considered the best candidate for carrying genetic information and why? a. nucleoside triphosphates because of the ability to add and remove phosphate groups b. sterols because of the different variations on their ring structure c. proteins because they were thought to be the only molecule with both the variety and specificity of function to account for the array of heritable traits observed d. amino acids because of all the ways they can join together e. carbohydrates because they are found in abundance in all organisms

proteins because they were thought to be the only molecule with both the variety and specificity of function to account for the array of heritable traits observed Many scientists believed that proteins must be the chemical carriers of genetic information.

Antiparallel

referring to the arrangement of the sugar-phosphate backbones in a DNA double helix (they run in opposite 5′ ? 3′ directions)

The experiments of Meselson and Stahl showed that DNA __________. a. contains complementary base pairing b. is the genetic material c. replicates in a semiconservative fashion d. is composed of nucleotides e. codes for the amino acid sequences of proteins

replicates in a semiconservative fashion In the semiconservative model of DNA replication, the two strands of the parental molecule separate. Each functions as a template for the synthesis of a new complementary strand.

Origin of Replication

site where the replication of a DNA molecule begins, consisting of a specific sequence of nucleotides

In his work with pneumonia-causing bacteria and mice, Griffith found that a. the protein coat from pathogenic cells was able to transform nonpathogenic cells. b. bacteriophages injected DNA into bacteria. c. heat-killed pathogenic cells caused pneumonia. d. some substance from pathogenic cells was transferred to nonpathogenic cells, making them pathogenic. e. the polysaccharide coat of bacteria caused pneumonia.

some substance from pathogenic cells was transferred to nonpathogenic cells, making them pathogenic.

Nucleosome

the basic, bead-like unit of DNA packing in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone

Mismatch Repair

the cellular process that uses specific enzymes to remove and replace incorrectly paired nucleotides

Chromatin

the complex of DNA and proteins that makes up eukaryotic chromosomes; when the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope

Double Helix

the form of native DNA, referring to its two adjacent antiparallel polynucleotide strands wound around an imaginary axis into a spiral shape

In an important experiment, a radioactively labeled bacteriophage was allowed to infect bacteria. In a first trial, the phage contained radioactive DNA, and radioactivity was detected inside the bacteria. Next, phage-containing radioactive protein was used, and the radioactivity was not detected inside the bacteria. These experiments led to the conclusion that __________. a. the genetic material of the phage is protein b. DNA is made of nucleotides c. bacteriophages can infect bacteria d. the genetic material of the phage is DNA e. genes are on chromosomes

the genetic material of the phage is DNA In order to replicate, viral material must enter a bacterial cell. This experiment showed that the viral DNA had entered the bacterial cell.

Euchromatin

the less condensed form of eukaryotic chromatin that is available for transcription

Leading Strand

the new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5′ ? 3′ direction

DNA Replication

the process by which a DNA molecule is copied; also called DNA synthesis

A scientist assembles a bacteriophage with the protein coat of phage T2 and the DNA of phage T4. If this composite phage were allowed to infect a bacterium, the phages produced in the host cell would have __________. a. the protein and DNA of T2 b. the protein of T2 and the DNA of T4 c. a mixture of the DNA and proteins of both phages d. the protein and DNA of T4 e. the protein of T4 and the DNA of T2

the protein and DNA of T4 The protein and DNA would match that of the phage whose DNA was used.

Chargaff found that for DNA __________. a. A + T = G + C b. the ratio of A to C is close to 1:1 and the ratio of G to T is close to 1:1 c. the ratio of A to T is close to 1:1 and the ratio of G to C is close to 1:1 d. A + T = 50% of the total bases e. the ratio of A to G is close to 1:1 and the ratio of T to C is close to 1:1

the ratio of A to T is close to 1:1 and the ratio of G to C is close to 1:1 This is a result of specific base pairing and of DNA's double-helical structure.

The information in DNA is contained in __________. a. the variation in the structure of nucleotides that make up the DNA molecule b. the types of sugars used in making the DNA molecule c. the sequence of amino acids that makes up the DNA molecule d. the sequence of nucleotides along the length of the two strands of the DNA molecule e. All of the listed responses are correct.

the sequence of nucleotides along the length of the two strands of the DNA molecule Although base-pairing rules dictate the combinations of nitrogenous bases forming the rungs of the double helix, they do not restrict the sequence of nucleotides along each strand, and the linear sequence of bases can be varied in countless ways.

Telomere

the tandemly repetitive DNA at the end of a eukaryotic chromosome's DNA molecule; telomeres protect the organism's genes from being eroded during successive rounds of replication; see also repetitive DNA

The unwinding of DNA at the replication fork causes twisting and strain in the DNA ahead of the fork, which is relieved by an enzyme called __________. a. helicase b. topoisomerase c. ligase d. relievase e. primase

topoisomerase Topoisomerase cuts the DNA and allows it to spin around its central axis, which relieves the strain caused by twisting.

Semiconservative Model

type of DNA replication in which the replicated double helix consists of one old strand, derived from the parental molecule, and one newly made strand

Avery and his colleagues' 1944 experiment showed that DNA __________. a. consists of sugars, phosphate groups, and bases b. was the substance that transformed the bacteria in Griffith's experiment c. uses three bases to code for one amino acid d. contains adenine, guanine, thymine, and cytosine e. has two strands held together with hydrogen bonds

was the substance that transformed the bacteria in Griffith's experiment Avery and his colleagues announced that the transforming agent was DNA.