chp. 16
Nucleic acids are assembled in the _____ direction.
5' to 3'
In Synthesizing a New DNA Strand - Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork. - Most DNA polymerases require a primer and a DNA template strand. - The rate of elongation is about
500 nucleotides per second in bacteria and 50 per second in human cells
Cytosine makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?
8%
In an analysis of the nucleotide composition of a molecule of DNA, which of the following combinations of base pairs will be found?
A + C = G + T
In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules?
A + G = C + T
Watson and Crick reasoned that the pairing was more specific, dictated by the base structures. They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C). The Watson-Crick model explains Chargaff's rules: in any organism the amount of
A = T, and the amount of G = C
supercoils
A coil of coils; a circular molecule of DNA can coil upon itself to form a supercoil.
Which of these is(are) pyrimidines?
C, D, and E
What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized?
DNA polymerase can join new nucleotides only to the 3 end of a pre-existing strand, and the strands are antiparallel.
In Synthesizing a New DNA Strand - Enzymes called ___________ catalyze the synthesis of new DNA at a replication fork
DNA polymerases
___________ require a primer to which they can add nucleotides
DNA polymerases
Synthesizing a New DNA Strand
DNA polymerases require a primer to which they can add nucleotides. The initial nucleotide strand is a short RNA primer. This is synthesized by the enzyme primase
DNA is copied during _________ , and cells can repair their DNA
DNA replication
The copying of DNA is remarkable in its speed and accuracy. More than a dozen enzymes and other proteins participate in
DNA replication
Many proteins work together in
DNA replication and repair
Ligase joins
DNA segments into a continuous strand.
Which of the following statements accurately describes the structure of a eukaryotic chromosome?
It is a single linear molecule of double-stranded DNA plus proteins.
In the early 1950s, many researchers were racing to describe the structure of DNA using different approaches. Which of the following statements is true?
Jim Watson and Francis Crick built theoretical models, incorporating current knowledge about chemical bonding and X-ray data.
- Watson and Crick's semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or "conserved" from the parent molecule) and one newly made strand. - Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new) - Experiments by _________ and ___________ supported the semiconservative model
Matthew Meselson and Franklin Stahl
_____________ bind to and stabilize single-stranded DNA
Single-strand binding proteins
True or false? Single-stranded DNA molecules are said to be antiparallel when they are lined up next to each other but oriented in opposite directions.
T
In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage known as
T2
In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage known as T2. They designed an experiment showing that only one of the two components of
T2 (DNA or protein) enters an E. coli cell during infection
Griffith's experiments with S. pneumoniae were significant because they showed that traits could be transferred from one organism to another. What else did he find that was significant?
The transferred traits were heritable.
Griffith's experiments with S. pneumoniae were significant because they showed that traits could be transferred from one organism to another. What else did he find that was significant?
The transferred traits were heritable. Submit
letter A indicates
a DNA double helix
Replication begins at particular sites called ____________ , where the two DNA strands are separated, opening up a replication "bubble"
origins of replication
This is an image of a _____.
phage This is a T2 phage, a type of phage that infects E. coli.
This is an image of a(n) _____. Letter A indicates:
phosphate group
A heat-killed, phosphorescent (light-emitting) strain of bacteria is mixed with a living, non-phosphorescent strain. Further observations of the mixture show that some of the living cells are now phosphorescent. Which of the following observations would provide the best evidence that the ability to phosphoresce is a heritable trait?
phosphorescence in descendants of the living cells
This is synthesized by the enzyme
primase
The initial nucleotide strand is a short RNA
primer
At the end of each replication bubble is a __________ , a Y-shaped region where new DNA strands are elongating
replication fork
The action of helicase creates _____.
replication forks and replication bubbles
Watson and Crick's __________ of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or "conserved" from the parent molecule) and one newly made strand
semiconservative model
DNA replication is
semiconservative.
It became apparent to Watson and Crick after completion of their model that the DNA molecule could carry a vast amount of hereditary information. Which of the following characteristics of DNA is responsible for this?
sequence of bases
Griffith worked with two strains of a bacterium, one pathogenic and one harmless. When Griffith mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain,
some living cells became pathogenic
In his work with pneumonia-causing bacteria and mice, Griffith found that
some substance from pathogenic cells was transferred to nonpathogenic cells, making them pathogenic.
The copying of DNA is remarkable in its
speed and accuracy
You can tell that this is an image of a DNA nucleotide and not an RNA nucleotide because you see a _____.
sugar with two, and not three, oxygen atoms Submit
An old DNA strand is used as a _____ for the assembly of a new DNA strand.
template
The Basic Principle: Base Pairing to a Template Strand - Since the two strands of DNA are complementary, each strand acts as a _______ for building a new strand in replication
template
In the polymerization of DNA, a phosphodiester bond is formed between a phosphate group of the nucleotide being added and which of the following atoms or molecules of the last nucleotide in the polymer?
the 3' OH
In Synthesizing a New DNA Strand - Primase can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template. - The primer is short (5-10 nucleotides long), and
the 3′ end serves as the starting point for the new DNA strand
Watson and Crick reasoned that the pairing was more specific, dictated by
the base structures
- Watson and Crick's semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or "conserved" from the parent molecule) and one newly made strand. - Competing models were the conservative model (the two parent strands rejoin) and
the dispersive model (each strand is a mix of old and new)
In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage known as T2. They designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection. They concluded that the injected DNA of the phage provides
the genetic information
Which of these nitrogenous bases is found in DNA but not in RNA?
thymine
In a DNA double helix an adenine of one strand always pairs with a(n) _____ of the complementary strand, and a guanine of one strand always pairs with a(n) _____ of the complementary strand.
thymine ... cytosine
Watson and Crick built models of a double helix to conform
to the X-rays and chemistry of DNA.
some of the living cells from his experiment became pathogenic after Griffith mixed heat that killed the other cells. He called this phenomenon _________ , now defined as a change in genotype and phenotype due to assimilation of foreign DNA
transformation
Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA. Franklin had concluded that there were
two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule's interior
In Synthesizing a New DNA Strand - Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork. - Most DNA polymerases require a primer and a DNA template strand. - The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells. - Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate. - dATP supplies adenine to DNA and is similar to the ATP of energy metabolism. - The difference is in their sugars: dATP has deoxyribose while ATP has ribose. - As each monomer joins the DNA strand, via a dehydration reaction, it loses
two phosphate groups as a molecule of pyrophosphate
antiparallel
The opposite arrangement of the sugar-phosphate backbones in a DNA double helix.
Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA. Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule's interior. Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions). At first, Watson and Crick thought the bases paired like with like
(A with A, and so on), but such pairings did not result in a uniform width
What did the structure of DNA's double helix suggest about DNA's properties?
- DNA can be replicated by making complementary copies of each strand. - DNA stores genetic information in the sequence of its bases. - DNA can change. Errors in copying can result in changes in the DNA sequence that could be inherited by future generations.
Additional Evidence That DNA Is the Genetic Material
- DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group - The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C) - In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next - This evidence of diversity made DNA a more credible candidate for the genetic material
Evidence That Viral DNA Can Program Cells
- More evidence for DNA as the genetic material came from studies of viruses that infect bacteria - Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research - A virus is DNA (sometimes RNA) enclosed by a protective coat, often simply protein
In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next. This evidence of diversity made DNA a more credible candidate for the genetic material. Two findings became known as Chargaff's rules
- The base composition of DNA varies between species - In any species the number of A and T bases is equal and the number of G and C bases is equal
Evidence That DNA Can Transform Bacteria
- The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 - Griffith worked with two strains of a bacterium, one pathogenic and one harmless - When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic - He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA - Later work by Oswald Avery, Maclyn McCarty, and Colin MacLeod identified the transforming substance as DNA - Many biologists remained skeptical, mainly because little was known about DNA
In the 1950s, when Watson and Crick were working on their model of DNA, which concepts were well accepted by the scientific community?
- chromosomes are found in the nucleus. - Genes are located on chromosomes. - Chromosomes are made up of protein and nucleic acid.
In a nucleotide, the nitrogenous base is attached to the sugar's _____ carbon and the phosphate group is attached to the sugar's _____ carbon.
1' ... 5'
What catalyzes DNA synthesis?
DNA polymerase
Where would RNA polymerase attach?
A: RNA polymerase attaches to DNA.
For a science fair project, two students decided to repeat the Hershey and Chase experiment, with modifications. They decided to radioactively label the nitrogen of the DNA, rather than the phosphate. They reasoned that each nucleotide has only one phosphate and two to five nitrogen atoms. Thus, labeling the nitrogen atoms would provide a stronger signal than labeling the phosphates. Why won't this experiment work?
Amino acids (and thus proteins) also have nitrogen atoms; thus, the radioactivity would not distinguish between DNA and proteins.
In the accompanying image, a nucleotide is indicated by the letter _____.
B
Griffith worked with two strains of a bacterium,
one pathogenic and one harmless
A nitrogenous base is indicated by the letter _____.
C
Why is the new DNA strand complementary to the 3' to 5' strands assembled in short segments?
DNA polymerase can assemble DNA only in the 5' to 3' direction
Who conducted the X-ray diffraction studies that were key to the discovery of the structure of DNA?
D. Franklin
After Griffith's transformation experiment later work by Oswald Avery, Maclyn McCarty, and Colin MacLeod identified the transforming substance as _____
DNA
Hershey and Chase set out to determine what molecule served as the unit of inheritance. They completed a series of experiments in which E. coli was infected by a T2 virus. Which molecular component of the T2 virus actually ended up inside the cell?
DNA
The radioactive isotope 32P labels the T2 phage's _____.
DNA
DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group. The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C). In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next. This evidence of diversity made
DNA a more credible candidate for the genetic material
Hereditary information is encoded in
DNA and reproduced in all cells of the body
DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group. The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C). In 1950, Erwin Chargaff reported that
DNA composition varies from one species to the next
Which of the following facts did Hershey and Chase make use of in trying to determine whether DNA or protein is the genetic material?
DNA contains phosphorus, whereas protein does not.
What did Rosalind Franklin's famous photo 51 show?
DNA is a helix.
What are the chemical components of a DNA molecule?
DNA is composed of a sequence of subunits, each containing a phosphate, a deoxyribose sugar, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). Summary: sugars, phosphate groups, nitrogenous bases
Early, flawed DNA models proposed by Watson and Crick and by Linus Pauling correctly described which property of DNA?
DNA is composed of sugars, phosphates, and bases.
Which of these is a difference between a DNA and an RNA molecule?
DNA is usually double-stranded, whereas RNA is usually single-stranded.
Which of the following investigators was (were) responsible for the discovery that in DNA from any species, the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine?
Erwin Chargaff
Which of the following statements describes the process of transformation in bacteria?
External DNA is taken into a cell, becoming part of the cell's genome.
The discovery of the genetic role of DNA began with research by
Frederick Griffith in 1928
______ are enzymes that untwist the double helix at the replication forks
Helicases
Who demonstrated that DNA is the genetic material of the T2 phage?
Hershey and Chase
Hershey and Chase used _____ to radioactively label the T2 phage's proteins.
Hershey and Chase used radioactive sulfur to label the phage's proteins. 35S
Which of the following statements correctly describes the structure of chromatin?
Heterochromatin is highly condensed, whereas euchromatin is less compact.
Which of the following statements accurately describes one characteristic of histones?
Histone H1 is not present in the nucleosome bead; instead, it draws the nucleosomes together.
Which of the following molecular characteristics cause histones to bind tightly to DNA?
Histones are positively charged, and DNA is negatively charged.
In his transformation experiments, what phenomenon did Griffith observe?
Mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain can convert some of the living cells into the pathogenic form.
Which part of a deoxynucleoside triphosphate (dNTP) molecule provides the energy for DNA synthesis?
Phosphate groups
Which of the following enzymes creates a primer for DNA polymerase?
Primase
Which of these is responsible for catalyzing the formation of an RNA primer?
Primase catalyzes the formation of an RNA primer. D
Which of the following statements about DNA synthesis is true?
Primers are short sequences that allow the initiation of DNA synthesis.
The synthesis of a new strand begins with the synthesis of a(n) _____.
RNA primer complementary to a preexisting DNA strand
In the Hershey and Chase experiment that helped confirm that DNA, not protein, was the hereditary material, what was the key finding?
Radioactively labeled phosphorus was present inside the infected bacteria.
loops
The computational concept of running the same sequence multiple times.
Erwin Chargaff observed that the proportions of adenine (A) and thymine (T) bases were always equal, as were the proportion of guanine (G) and cytosine (C). Chargaff's observation suggests which of the following statements?
The data suggest that A would always pair with T and G would always pair with C in a DNA molecule.
Which of the following statements about Okazaki fragments in E. coli is true?
They are formed on the lagging strand of DNA.
______ relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands
Topoisomerase
Which of the following enzymes is important for relieving the tension in a helix as it unwinds during DNA synthesis?
Topoisomerase`
Building a Structural Model of DNA: Scientific Inquiry - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity. -Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure. - Franklin produced a picture of the DNA molecule using this technique. - Franklin's X-ray crystallographic images of DNA enabled
Watson to deduce that DNA was helical
Building a Structural Model of DNA: Scientific Inquiry - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity. - Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure. - Franklin produced a picture of the DNA molecule using this technique. - Franklin's X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical. - The X-ray images also enabled
Watson to deduce the width of the helix and the spacing of the nitrogenous bases
Hershey and Chase used a DNA-based virus for their work. How might the results have been different if they had used an RNA virus?
With an RNA virus, radioactive RNA would have been in the final pellet.
Building a Structural Model of DNA: Scientific Inquiry - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity. - Maurice Wilkins and Rosalind Franklin were using a technique called
X-ray crystallography to study molecular structure
Building a Structural Model of DNA: Scientific Inquiry - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity. - Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure. - Franklin produced a picture of the DNA molecule using this technique. - Franklin's X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical. - The X- ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases - The pattern in the photo suggested that the DNA molecule was made up of two strands, forming
a double helix
Early in the 20th century, the identification of the molecules of inheritance loomed as
a major challenge to biologists
DNA is a polymer of nucleotides, each consisting of
a nitrogenous base, a sugar, and a phosphate group
In Synthesizing a New DNA Strand - Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork. - Most DNA polymerases require a primer and a DNA template strand. - The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells. - Each nucleotide that is added to a growing DNA strand is
a nucleoside triphosphate
Building a Structural Model of DNA: Scientific Inquiry - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity. - Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure. - Franklin produced
a picture of the DNA molecule using this technique
In Synthesizing a New DNA Strand - Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork. - Most DNA polymerases require
a primer and a DNA template strand
Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA. Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule's interior. Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions). At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width. Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in
a uniform width consistent with the X-ray data
In Synthesizing a New DNA Strand - Primase can start an RNA chain from scratch and
adds RNA nucleotides one at a time using the parental DNA as a template
Watson and Crick reasoned that the pairing was more specific, dictated by the base structures. They determined that
adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group. The nitrogenous bases can be
adenine (A), thymine (T), guanine (G), or cytosine (C)
In Synthesizing a New DNA Strand - Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork. - Most DNA polymerases require a primer and a DNA template strand. - The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells. - Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate. - dATP supplies
adenine to DNA and is similar to the ATP of energy metabolism
DNA is the genetic material for
all living organisms
The synthesis of a DNA strand begins with the formation of
an RNA primer.
Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA. Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule's interior. Watson built a model in which the backbones were
antiparallel (their subunits run in opposite directions).
Since DNA polymerase can assemble DNA only in the 5' to 3' direction, the new strand complementary to the 3' to 5' strand must be assembled either in short 5' to 3' segments, which
are later joined together by ligase, or be assembled continuously.
The Search for the Genetic Material: Scientific Inquiry - The role of DNA in heredity was first discovered by studying
bacteria and the viruses that infect them
Such viruses, called __________ , are widely used in molecular genetics research
bacteriophages (or phages)
The Basic Principle: Base Pairing to a Template Strand - Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication. - In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on
base-pairing rules
In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA. Hereditary information is encoded in DNA and reproduced in all cells of the body. This DNA program directs the development of
biochemical, anatomical, physiological, and (to some extent) behavioral traits
Enzymes called DNA polymerases
catalyze the synthesis of new DNA at a replication fork
The Search for the Genetic Material: Scientific Inquiry - When T. H. Morgan's group showed that genes are located on
chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material
An old DNA strand is used as a template for the synthesis of a
complementary new strand.
Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble". A eukaryotic chromosome may have hundreds or even thousands of origins of replication. Replication proceeds in both directions from each origin, until the entire molecule is
copied
In Synthesizing a New DNA Strand - Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork. - Most DNA polymerases require a primer and a DNA template strand. - The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells. - Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate. - dATP supplies adenine to DNA and is similar to the ATP of energy metabolism. - The difference is in their sugars:
dATP has deoxyribose while ATP has ribose
The elongation of the leading strand during DNA synthesis
depends on the action of DNA polymerase.
The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a
double helix
The relationship between structure and function is manifest in the
double helix
In 1953, James Watson and Francis Crick introduced an elegant
double-helical model for the structure of deoxyribonucleic acid, or DNA
After DNA replication is completed, _____.
each new DNA double helix consists of one old DNA strand and one new DNA strand
The relationship between structure and function is manifest in the double helix. Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for
genetic material
The first step in the replication of DNA is catalyzed by _____.
helicase
In a nucleosome, the DNA is wrapped around
histones
Letter C indicate
histones
Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble". A eukaryotic chromosome may have
hundreds or even thousands of origins of replication
Building a Structural Model of DNA: Scientific Inquiry - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for
its role in heredity
After allowing phages grown with bacteria in a medium that contained 32P and 35S, Hershey and Chase used a centrifuge to separate the phage ghosts from the infected cell. They then examined the infected cells and found that they contained _____, which demonstrated that _____ is the phage's genetic material.
labeled DNA ... DNA
Short segments of newly synthesized DNA are joined into a continuous strand by _____.
ligase
After Griffith's transformation experiment later work by Oswald Avery, Maclyn McCarty, and Colin MacLeod identified the transforming substance as DNA. & Many biologists remained skeptical, mainly because
little was known about DNA
Which of the following lists represents the order of increasingly higher levels of organization of chromatin?
nucleosome, 30-nm chromatin fiber, looped domain
This is an image of a(n) _____.
nucleotide
In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next. This evidence of diversity made DNA a more credible candidate for the genetic material. Two findings became known as Chargaff's rules - The base composition of DNA varies between species - In any species the number of A and T bases is equal and the number of G and C bases is equal. The basis for these rules was not
understood until the discovery of the double helix
A _____ is DNA (sometimes RNA) enclosed by a protective coat, often simply protein
virus
More evidence for DNA as the genetic material came from studies of
viruses that infect bacteria