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Nucleic Acids
The structure of nucleic acid molecules allows for the coding and transmission of
information.
The best-known nucleic acid, DNA, occurs in the form of a double helix.
The DNA double helix is made up of two intertwined strands of nucleotides. The nucleotide strands have a sugarphosphate backbone, and each nucleotide contains one of four bases, adenine, cytosine, guanine, or thymine.
© 2014 Nature Education All rights reserved.
Topics Covered in this Module
Structure of Nucleic Acids
Functions of Nucleic Acids
Major Objectives of this Module
Describe the structure of nucleotides.
Explain how nucleotides assemble into polymers.
Distinguish between DNA and RNA.
Explain the roles of nucleic acids in transmission and expression of hereditary information.
page 59 of 989
4 pages left in this module
Principles of Biology
12 Nucleic Acids
Structure of Nucleic Acids
The human body contains an amazing variety of different types of cells that
originate from just one fertilized egg. But how does that single cell manage to
grow and differentiate into the diverse cells that make up a human being? All
the information needed for this amazing transformation from a single cell to a
complex organism comes from one type of biological molecule called
deoxyribonucleic acid (DNA). DNA encodes all the information needed to
create life's diversity. It is responsible for the large variety of cells within an
organism as well as the diversity among organisms of all species. How is it
that a simple DNA molecule encodes the complexity of life? What are the
building blocks of this amazing molecule?
Nucleic acids are polymers of nucleotides.
DNA is a nucleic acid. Along with lipids, carbohydrates, and proteins, nucleic
acids are one of the four classes of large biological molecules that are
essential to cellular structure and function. Nucleic acids include both DNA,
which encodes genetic information, and ribonucleic acid (RNA), a
macromolecule that has multiple roles, the most well studied of which is to
deliver information from DNA to sites of protein synthesis. Like
carbohydrates and proteins, nucleic acids are polymers, meaning they
consist of multiple repeating monomers. For both DNA and RNA, the
repeating monomers are nucleotides that are arranged in a wide variety of
sequences to make up polynucleotides.
The structure of the nucleotide.
Both DNA and RNA are made up of nucleotide monomers, each of which
contains three parts: a phosphate functional group (–PO42-), a five-carbon
sugar, or pentose, and a nitrogenous base (often referred to simply as the
"base") (Figure 1a). When a sugar is linked to a nitrogenous base but the
phosphate group is absent, this is called a nucleoside. Five different types
of nitrogenous base occur in nucleic acids: guanine, thymine, adenine,
cytosine, and uracil.
Because the nucleotide is made up of two different carbon-based units (the
nitrogenous base and the pentose), a special notation has been developed
to distinguish the carbons in the two structures. The carbons in the
nitrogenous base are numbered 1, 2, 3, etc., while the carbons in the
pentose are numbered 1′, 2′, 3′, etc. The symbol that looks similar to an
apostrophe is referred to as a prime, so the pentose number 1′ is written
"one-prime." Notice that in a nucleic acid, the phosphate group is attached to
the 5′ (five-prime) carbon. This numbering becomes important when we learn
in the next section about the linkages that connect nucleotides into nucleic
acids.
DNA and RNA differ in two different ways. First, the chemical structure of the
nucleotides found in these two classes of nucleic acids differs. DNA contains
guanine, thymine, adenine, and cytosine. RNA contains uracil in place of
thymine. Second, a different type of sugar is present in DNA than in RNA.
The sugar found in RNA is ribose, and the sugar found in DNA is
deoxyribose. Ribose and deoxyribose are nearly identical: both are
pentoses that are closed into a ring in the nucleotide. The only difference is
that ribose has a hydroxyl group attached to the 2′ carbon and deoxyribose
does not. As the oxygen atom is missing from the second carbon in
deoxyribose, the sugar is more accurately called 2-deoxyribose.
contents
Figure 1: The structure of the
nucleotide.
a) The three components of a
nucleotide include a phosphate
functional group, a pentose, and a
nitrogenous base. b) The pentose
sugar is a key feature that distinguishes
RNA from DNA. The sole difference
between ribose (found in RNA) and
deoxyribose (used in DNA) is the 2′
carbon. In ribose, the 2′ carbon is
bound to a hydroxyl group. In
deoxyribose, the –OH is absent, and a
hydrogen is in its place. This small
difference makes RNA much more
chemically reactive than DNA.
© 2014 Nature Education All rights
reserved.
Nitrogenous bases include purines and pyrimidines.
The five nucleotides found in DNA and RNA are divided into two separate
families — pyrimidine or purine — based on their structure (Figure 2). A
pyrimidine has a six-member ring containing four carbons and two
nitrogens. The pyrimidines include cytosine (C), thymine (T), and uracil (U).
Cytosine and thymine are found in DNA, and cytosine and uracil (U) are
found in RNA. A handy mnemonic to remember the pyrimidine bases is
"CUT the py" — the letters in CUT represent the three pyrimidines: cytosine,
uracil, and thymine. A purine molecule consists of a five-member ring
attached to a six-member ring. Each ring has two nitrogens. The purines
include adenine (A) and guanine (G), which are found in both DNA and RNA
(Figure 2).
Figure 2: Two types of nitrogenous bases are found in DNA and RNA.
a) Pyrimidines consist of a single ring of four carbon atoms and two
nitrogen atoms. The three pyrimidines, cytosine, thymine, and uracil, differ
in the presence of amine (–NH2) and methyl (–CH3) functional groups. b)
Purines consist of a five-member ring connected to a six-member ring.
The rings contain four nitrogens. The two purines, adenine and guanine,
differ in the number and position of amine and carbonyl functional groups.
© 2012 Nature Education All rights reserved.
Test Yourself
Compare the main features of a pyrimidine molecule with a purine molecule.
Submit
Nucleic acids form when phosphodiester linkages connect nucleotide
monomers.
A phosphodiester linkage bonds two nucleotides together. A
phosphodiester linkage (also called a phosphodiester bond) forms when the
phosphate group of one nucleotide becomes covalently bound to the
hydroxyl group at the 3′ carbon of another nucleotide (Figure 3). Note in
Figure 3 that the entire polymer molecule has a 5′ end and a 3′ end. The 3′
end typically is a free –OH, and the 5′ end typically is a phosphate group
(although it could be an –OH). Thus, a nucleic acid molecule has polarity.
This concept of polarity is important and will appear again in this module
when we talk about the sequence of DNA and about DNA replication and
transcription. Together, the repeating phosphate and pentose units form the
backbone of the nucleic acid. Because the phosphate groups carry a
negative charge, they face outward, toward water. The hydrophobic
nitrogenous bases face inward, where they are sequestered from water.
Figure 3: Section of a strand of DNA.
DNA is a repeating polymer of nucleotides held together by
phosphodiester linkages (also called phosphodiester bonds). In this
illustration, one of each of the four types of nucleotides found in DNA is
included.
© 2014 Nature Education All rights reserved.
Test Yourself
Describe a phosphodiester linkage. In what molecules is this linkage found?
Submit
Controversy.
Who discovered the structure of DNA? Most people would answer, "Watson
and Crick." But the populist version of the discovery of DNA omits some key
players. One of these players was Maurice Wilkins who, along with Crick and
Watson, was awarded the Nobel Prize for the 1953 discovery of the DNA
double helix structure. Rosalind Franklin, however, was not so honored.
During the early 1950s, several research groups were racing to solve the
puzzle of DNA's structure. Francis Crick and James Watson proposed one
idea, based on a triple helix. At a scientific meeting, Rosalind Franklin
(Figure 4), working in another group, pointed out that such a model did not fit
with the data she had seen using X-ray crystallography. She instead
proposed that DNA forms a double helix with the phosphate backbone on the
outside, but she wanted to get better data before publishing. While Franklin
was toiling away, Crick and Watson obtained Franklin's unpublished X-ray
diffraction patterns of DNA from Maurice Wilkins, without Franklin's
permission or knowledge. They then used this information and Franklin's
interpretation of the data to propose their new model, the subject of their
landmark paper published in 1953. Franklin died in 1958 at the age of 38 and
did not share in the Nobel Prize, which is not awarded posthumously. There
is no doubt, however, that Franklin's work was a key to unlocking the puzzle
of DNA structure.
Figure 4: British scientist Rosalind Franklin provided data that were
key to discovering the structure of DNA.
Rosalind Franklin provided exceptional quality data from X-ray diffraction
patterns of purified DNA. Her data allowed Crick and Watson to derive the
molecule's double helix structure.
Science Source.
DNA and RNA form complex secondary structures.
As often happens in science, Watson and Crick built their model on a
foundation laid by earlier scientists. By the 1950s, it was already known
through the work of Oswald Avery and his colleagues that DNA is the
molecule of heredity that makes up genes and chromosomes. Around the
same time, Austrian chemist Erwin Chargaff established what is now known
as Chargaff's Rule: the amount of cytosine in cellular DNA always equals the
amount of guanine, and the amount of thymine always equals the amount of
adenine.
Based on this information and Rosalind Franklin's X-ray crystal structure,
Crick and Watson created a three-dimensional model and proposed that
DNA is a double helix consisting of two intertwined strands of DNA, with a
phosphate-sugar backbone on the exterior. On the interior, hydrogen bonds
form between adenine and thymine and between guanine and cytosine
(Figure 5). This hydrogen bonding is referred to as base pairing. The
specificity of base pairing accounts for Chargaff's rule. Furthermore, the
hydrogen bonds that form between the base pair provide the "glue" that
holds two strands of DNA together. The beauty of this model is that it
suggested, in the words of Crick and Watson, "a possible copying
mechanism for genetic material." The Watson-Crick model confirmed
Franklin's inference from her X-ray diffraction data that the phosphate groups
were on the outside of the helix. It also corresponded with the measurements
Franklin provided on the dimensions of the DNA molecule.
Crick and Watson's model also showed that each strand in the double helix
had directionality. That is, the 5′ to 3′ direction of one strand was opposite to
the complementary strand, oriented in a 3′ to 5′ direction. This distinction is
crucial because a DNA sequence of bases running in one direction is not the
same as the identical sequence running in the other direction. For example,
the sequence of bases 5′-ATCGTG-3′ is not the same as 3′-ATCGTG-5′. Two
complementary strands of DNA running in opposite directions are said to be
antiparallel. Again, by convention, the sequence of DNA is always written in
the 5′-to-3′ direction. So the second sequence above would properly be
written as follows: 5′-GTGCTA-3′.
Figure 5: DNA is antiparallel and forms hydrogen bonds between
specific pairs of bases.
In a double-stranded molecule of DNA, the two strands are antiparallel —
they run in opposite directions. The arrows indicate the "forward" direction
of DNA from the 5′ carbons of the deoxyribose sugars in the backbone to
the 3′ carbons. In double-stranded DNA, guanine (G) forms three
hydrogen bonds with cytosine (C), and adenine (A) forms two hydrogen
bonds with thymine (T). The specific base pairing between G–C and A–T
explains Chargaff's observations and suggests that if the sequence of one
strand is known, the sequence of the other strand can be inferred.
© 2014 Nature Education All rights reserved.
BIOSKILL
Determine Whether a DNA Sample Is from a Bacterium or Virus
Examine the data in Table 1, which shows the proportions of each of the four
bases present in DNA samples from various organisms. Notice that in most
cases, there is an approximately equal ratio of A:T and G:C. What does this
signify? It means that the DNA is likely double-stranded. This is because any
time there is an A on one strand, there will be a T on the complementary
strand, and vice versa. The same is true for a C or G; every time there is a C
on one strand, there is a G on the complementary strand, and vice versa.
However, a virus can be a single strand of DNA, so these ratios would not
apply.
Relative proportions (%) of bases in
DNA
Sample
A
T
G
C
A
30.9
29.4
19.9
19.8
B
28.8
29.2
20.5
21.5
C
29.3
29.3
20.5
20.7
D
19.1
30.2
18.9
31.7
E
32.8
32.1
17.7
17.3
F
27.3
27.1
22.7
22.8
G
31.3
32.9
18.7
17.1
H
24.7
23.6
26.0
25.7
Table 1: Base ratios in DNA
samples.
Based on the relative proportions of the
different bases in DNA, it is possible to
determine whether a sample of DNA
comes from a single-stranded DNA
virus or an organism whose DNA is
double-stranded.
Test Yourself
Which of the samples listed in Table 1 is likely a single-stranded DNA virus? Explain your
answer.
Submit
BIOSKILL
Other structural characteristics of DNA.
Examine Figure 6 to see the dimensions of a molecule of DNA.
Figure 6: Molecular representations of DNA.
(a) The DNA double helix contains two polynucleotide chains arranged
antiparallel to one another. The sugar-phosphate backbones are on the
outside. On the inside, A-T and G-C base pairs hold the two strands of the
nucleic acid together. (b) A molecular model of DNA showing much of the
same information as the ribbon model in panel (a). nm = nanometers.
© 2012 Nature Education http://www.ks.uiuc.edu/Research/vmd/
Used with permission. All rights reserved.
Figure Detail
DNA in the helix can be twisted in one of several forms, or conformations.
There are three DNA helix conformations thought to be biologically active.
The most common shape in living cells is called B-DNA form, which closely
aligns with the Watson and Crick model of DNA structure. A-DNA is similar to
B-DNA but is shorter and more compact in nature. Both A-DNA and B-DNA
take the shape of a right-handed double helix, which twists to the right-hand
side. A third type is Z-DNA, which resembles B-DNA except the helix is
left-handed in directionality.
RNA structure differs dramatically from DNA structure.
Like the bases in DNA, the bases in RNA can form base pairs. However,
while base pairing in DNA usually occurs between two different nucleic acid
molecules, base pairing in RNA typically occurs within a single RNA
molecule. The RNA molecule folds over, and bases on one part of the strand
form base pairs with complementary bases on another part. This feature of
RNA results in a wide variety of so-called secondary structures. One
common structure is the stem-loop, which forms a secondary structure called
a hairpin (Figure 7).
Most often, DNA is double-stranded and serves as the genetic material,
while RNA is single-stranded and serves many functions, including shuttling
genetic instructions to the protein-making machinery of the cell. However,
there are exceptions, particularly in viruses. In viruses, RNA can serve as the
genetic material, and the RNA can be double-stranded. For example, the
rotavirus, which causes diarrhea in children, is a double-stranded RNA virus.
In other viruses, DNA serves as the genetic material, but the DNA may be
double-stranded or single-stranded.
Figure 7: A stem-loop hairpin occurs in RNA when a sequence folds
back on itself.
When a strand of RNA folds over itself, the two folded strands may
become joined due to hydrogen bonding between complementary base
pairs. An unpaired loop may arise in an area where base pairing between
the folded strands cannot occur.
© 2014 Nature Education All rights reserved.
Figure Detail
Test Yourself
What is the main difference between the double helix formed in a DNA molecule compared
with that formed in the RNA molecule?
Submit
IN THIS MODULE
Structure of Nucleic Acids
Functions of Nucleic Acids
Summary
Test Your Knowledge
PRIMARY LITERATURE
Classic paper: The discovery of
the neutron (1932)
Possible existence of a neutron.
View | Download
Classic paper: The idea of the DNA
double helix (1953)
Molecular structure of nucleic acids.
View | Download
Classic paper: X-ray diffraction
reveals helical structure of DNA
(1953)
Molecular configuration in sodium
thymonucleate.
View | Download
Classic paper: The first
sequencing of a complete DNA
genome (1977)
Nucleotide sequence of bacteriophage
ΦX174 DNA.
View | Download
Classic paper: X-rays reveal the
structure of myoglobin (1958)
A three-dimensional model of the myoglobin
molecule obtained by X-ray analysis.
View | Download
SCIENCE ON THE WEB
Chargaff In His Own Words
Listen to an interview with Erwin Chargaff
and how he discovered base pairing
Who Discovered the Shape of DNA?
Read a short bio on a woman who made a
large contribution to discovering helical
structure of DNA
DNA Structure and Function
Read about the discovery of DNA
How Fast Can You Build DNA?
Match the base pairs to build a DNA double
helix.
page 60 of 989
3 pages left in this module
Principles of Biology
12 Nucleic Acids
Functions of Nucleic Acids
Why were Crick, Watson, Franklin, and other teams so motivated to discover
the structure of DNA? From cellular studies, scientists knew that hereditary
material was located in the nucleus of the cell. They also believed that DNA
was the molecule most likely to convey hereditary information. They believed
that revealing the structure of DNA was perhaps the most significant
contribution in biology since Charles Darwin wrote On the Origin of Species. When
Crick and Watson wrote that the structure "suggests a possible copying
mechanism for the genetic material," they ushered in a new era of biology.
Their publication was the beginning of a transformation in genetics, cell
biology, and biochemistry and was arguably the birth of molecular biology.
Replication of DNA copies hereditary information.
So why did Crick and Watson think that DNA could copy genetic information?
The key, they realized, was the base-pairing rules. When the two DNA
strands were separated, each would be able to form another helix identical to
the original. While the details took years of subsequent research to work out,
the basic principle is beautifully simple. The double helix unravels,
separating the two strands. Enzymes then use each parent strand as a
template to make a new, complementary strand by attaching the
complementary nucleotides. One of the enzymes aids in the formation of the
phosphodiester bonds that make up the backbones of the two new strands.
The result is two DNA double helices identical to the original (Figure 8).
contents
Figure 8: Two identical DNA molecules can be produced from a
single original molecule.
During DNA synthesis, the DNA double helix partly unravels, and each
parent strand is used as a template to form identical daughter strands.
© 2014 Nature Education All rights reserved.
The ordering of nucleotide bases encodes hereditary information.
Although Crick and Watson deduced the structure of DNA and speculated as
to how it could replicate, they did not know how the bases stored hereditary
information. How did the sequence of bases on the DNA molecule store
information that could be passed on from one generation to the next? The
key is in the order of the bases. The order is preserved from the DNA
template strand, or parent molecule, to the new daughter molecules. In this
way, the daughter molecules preserve the hereditary information encoded in
the parent DNA.
Scientists conducting the earliest genetic studies were aware that units of
heredity are discrete. That is, traits are passed on as units, individual parts of
a whole. Gregor Mendel showed that traits of pea plants were inherited as
units. In the late 19th century, geneticist Hugo de Vries coined the term
gene: the smallest unit of heredity in organisms. But what were these units?
How did the structure of DNA relate to genes? That was the challenge for
scientists after Crick and Watson published the structure of DNA.
Although the details that related DNA to genes are complex, the principle is
straightforward. A sequence of nucleotide bases in DNA makes up a gene.
With its ordered sequence of bases, a gene contains the instructions to
make proteins. The type of proteins present in a cell determines the
particular properties or functions of that cell. Thus, despite being a relatively
simple molecule, DNA directs the synthesis of the proteins that create
complex structures and perform diverse cellular functions. We will learn
much more about these processes in later modules.
RNA has many roles in a cell.
The roles of DNA in a cell are few. DNA is the keeper of the genes that
encode hereditary information. DNA provides the blueprint for what cells
form and how they function. In contrast, RNA plays multiple roles in a cell
and an organism. RNA molecules come in many different forms, and their
functions vary depending on the type of RNA. The most famous RNA is
messenger RNA (mRNA), which encodes the information from a gene and
carries it to the machinery that decodes the mRNA and makes proteins.
Associated with the process of making proteins are two other well-studied
types of RNA: ribosomal RNA (rRNA) and transfer RNA (tRNA). More
recently, many other types of RNA molecules have been identified that
regulate key cellular processes, such as the making of proteins, cell
differentiation, and chromosome stability. For now, let's focus only on the
RNA molecules involved in protein synthesis. We will look at all of these RNA
molecules in more detail in later modules.
How does DNA communicate the information it contains in its genes to
synthesize proteins? Again, the key is in the base pair rules that govern how
complementary polynucleotide strands form from a template. The process of
RNA synthesis is known as transcription. During transcription, the DNA
double helix partly unwinds so that RNA can be synthesized from the
complementary strand. In eukaryotes, a messenger RNA molecule typically
encodes a single gene, while in prokaryotes, the mRNA can contain multiple
genes in a row. In eukaryotes, the mRNA migrates out of the nucleus to
ribosomes in the cytoplasm. Ribosomes are large complex structures made
from a combination of rRNA and over 50 proteins. The ribosomes decode
the RNA sequence and synthesize the appropriate protein, linking amino
acids together. The process of converting the "message," that is, the
sequence of nucleotides in mRNA, into a sequence of amino acids requires
tRNA as the adaptor molecule.
The structural diversity of RNA molecules, which is exemplified by mRNA,
rRNA and tRNA, is the basis for RNA's many functional roles in cells. RNA
even functions as an enzyme in certain cellular processes. The discovery
that RNA could function as an enzyme was a surprise because people long
thought enzymatic activity was the sole province of proteins.
The functions of both DNA and RNA are directly related to their structures.
The double-stranded helix of DNA provides a way for the genome to be
replicated and passed on to progeny. RNA, in contrast, plays a multitude of
roles and comes in various forms, including mRNA, tRNA, and rRNA. Each
form of RNA serves a specialized function. Without RNA, the cell would have
no way to make proteins, which serve as enzymes, structural components,
and membrane components. Without DNA, RNA would have no information
with which to produce these important cellular components.
Test Yourself
How does DNA communicate the hereditary information it contains to synthesize proteins?
Submit
Figure 9 reviews the correct structure of a nucleotide.
Figure 9: Creating a nucleotide.
A nucleotide contains a nitrogenous base, a five-carbon sugar, and a
phosphate group.
© 2012 Nature Education All rights reserved.
Transcript
CAREERS
Knowledge of Nucleic Acids Applied to Biotechnology
One of the top challenges humankind faces is how to feed Earth's
burgeoning population. Many scientists believe that biotechnology holds the
answers. Private companies, governments, and research foundations are
pumping funds into biotechnology to develop high-yielding, disease resistant
and nutritionally enhanced crop varieties. These organizations need experts
who understand the biochemistry of nucleic acids and the techniques to
study their structure and function. Other growing fields include
biotechnological approaches to treating human disease and green
approaches to remediation of waste and pollution, such as the use of
genetically altered bacteria to consume oil spilled into the environment.
There is no doubt that demand for expertise in biotechnology will outstrip
supply. There are many opportunities in these fields that apply knowledge of
nucleic acids, their structure, and their composition. By all accounts,
biotechnology is a growing industry that will have an impact on our Earth's
future.
CAREERS
IN THIS MODULE
Structure of Nucleic Acids
Functions of Nucleic Acids
Summary
Test Your Knowledge
PRIMARY LITERATURE
Classic paper: The discovery of
the neutron (1932)
Possible existence of a neutron.
View | Download
Classic paper: The idea of the DNA
double helix (1953)
Molecular structure of nucleic acids.
View | Download
Classic paper: X-ray diffraction
reveals helical structure of DNA
(1953)
Molecular configuration in sodium
thymonucleate.
View | Download
Classic paper: The first
sequencing of a complete DNA
genome (1977)
Nucleotide sequence of bacteriophage
ΦX174 DNA.
View | Download
Classic paper: X-rays reveal the
structure of myoglobin (1958)
A three-dimensional model of the myoglobin
molecule obtained by X-ray analysis.
View | Download
SCIENCE ON THE WEB
Chargaff In His Own Words
Listen to an interview with Erwin Chargaff
and how he discovered base pairing
Who Discovered the Shape of DNA?
Read a short bio on a woman who made a
large contribution to discovering helical
structure of DNA
DNA Structure and Function
Read about the discovery of DNA
How Fast Can You Build DNA?
Match the base pairs to build a DNA double
helix.
page 61 of 989
2 pages left in this module
Principles of Biology
12 Nucleic Acids
Summary
Describe the structure of nucleotides.
A nucleotide is made up of three components: a phosphate group, a ribose
sugar with five carbon atoms in a ring shape, and a nitrogenous base.
OBJECTIVE
Explain how nucleotides assemble into polymers.
Nucleotides assemble into polymers via the formation of phosphodiester
bonds between the 5′ carbon of one nucleotide's sugar and the 3′ carbon of
another nucleotide's sugar. The phosphate functional group is part of this
linkage. These covalent bonds hold together the sugar-phosphate backbone
of the polynucleotide.
OBJECTIVE
OBJECTIVE
Distinguish between DNA and RNA.
DNA is a double helix made from two strands of deoxyribonucleic acids. In
the double helix, A base pairs with T, and G base pairs with C. RNA is
typically single-stranded and is made from ribonucleic acids. RNA can fold
into hairpins in which A base pairs with U and G base pairs with C. This
stem-loop configuration is a common secondary structure of RNA.
Explain the roles of nucleic acids in transmission and
expression of hereditary information.
The sequence of bases in the DNA molecule encodes hereditary information.
The RNA molecule is a copy of information encoded in the DNA sequence.
RNA can function as a template that transfers information out of the nucleus
to the cytoplasm (in eukaryotes), where ribosomes read the RNA and
synthesize proteins based on the sequence of bases in the RNA molecule.
OBJECTIVE
Key Terms
antiparallel
Describing the opposite orientation of two complementary strands: 5' to 3' and 3'
to 5'.
base pairing
Hydrogen bonding between nitrogenous bases within polynucleotides; specifically
hydrogen bonding between guanine and cytosine and between adenine and
thymine in DNA; in RNA the base pairs are guanine and cytosine, and adenine
and uracil.
conformation
The spatial arrangement or shape of a macromolecule, such as a protein or
nucleic acid.
deoxyribonucleic acid (DNA)
The primary molecule of inheritance in all organisms (with the exception of some
viruses); a double-stranded nucleic acid containing nucleotides that contain
deoxyribose.
deoxyribose
Five-carbon sugar found in DNA nucleotides; differs from ribose in that it lacks a
hydroxyl (OH) group at the 2′-carbon.
DNA
See deoxyribonucleic acid.
double helix
Two antiparallel strands held together by hydrogen bonds coiled into a helical
shape. Sometimes also referred to as a "twisted ladder."
contents
gene
Segment of DNA that encodes a particular protein.
hairpin
Important secondary structure formation in RNA.
nitrogenous base
Component of nucleotides that makes each unique: adenine, guanine, cytosine,
thymine, and uracil; grouped into two families: pyrimidines and purines. Key
element of complementary base-pairing of nucleic acids.
nucleic acid
One of the four classes of large biological molecules; DNA and RNA are nucleic
acids.
nucleoside
Component of a nucleotide: a five-carbon sugar linked to a nitrogenous base
without the phosphate group present.
nucleotide
An organic molecule containing a nitrogenous base, one or more phosphates, and
a five-carbon sugar molecule; the monomer of nucleic acids.
phosphodiester bond
Covalent bond in nucleic acids that links nucleotides together; connects the
5′-carbon on one sugar with the 3′-carbon on the next sugar via the phosphate
group.
polynucleotide
Repeating monomers of nucleotides linked together.
purine
A nitrogenous base containing a five-member ring connected to a six-member
ring. The rings contain four nitrogens; one of the two types of nitrogenous bases in
nucleotides; includes adenine and guanine.
pyrimidine
Composed of a six-member ring containing four carbons and two nitrogens; one of
the two types of nitrogenous bases in nucleotides; includes cytosine, thymine, and
uracil.
ribonucleic acid (RNA)
Nucleic acid containing ribose as the sugar. Used to transmit genetic information
to ribosomes in the cell.
ribose
Five-carbon sugar found in RNA nucleotides.
IN THIS MODULE
Structure of Nucleic Acids
Functions of Nucleic Acids
Summary
Test Your Knowledge
PRIMARY LITERATURE
Classic paper: The discovery of
the neutron (1932)
Possible existence of a neutron.
View | Download
Classic paper: The idea of the DNA
double helix (1953)
Molecular structure of nucleic acids.
View | Download
Classic paper: X-ray diffraction
reveals helical structure of DNA
(1953)
Molecular configuration in sodium
thymonucleate.
View | Download
Classic paper: The first
sequencing of a complete DNA
genome (1977)
Nucleotide sequence of bacteriophage
ΦX174 DNA.
View | Download
Classic paper: X-rays reveal the
structure of myoglobin (1958)
A three-dimensional model of the myoglobin
molecule obtained by X-ray analysis.
View | Download
SCIENCE ON THE WEB
Chargaff In His Own Words
Listen to an interview with Erwin Chargaff
and how he discovered base pairing
Who Discovered the Shape of DNA?
Read a short bio on a woman who made a
large contribution to discovering helical
structure of DNA
DNA Structure and Function
Read about the discovery of DNA
How Fast Can You Build DNA?
Match the base pairs to build a DNA double
helix.
page 62 of 989
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Principles of Biology
12 Nucleic Acids
Test Your Knowledge
1. How does the base composition of DNA and RNA differ?
Bases in RNA include adenine, cytosine, guanine and thymine. DNA bases include
adenine, cytosine, guanine and uracil.
Bases in DNA include adenine, cytosine, guanine and thymine. RNA bases include
adenine, cytosine, guanine, and uracil.
Only purines are found in DNA and only pyrimidines are found in RNA.
The nucleotide compositions of DNA and RNA do not differ except that RNA has a
ribose sugar and DNA has a deoxyribose sugar.
Only purines are found in RNA, whereas only pyrimidines are found in DNA.
2. Which of the following correctly describes the sugar component of a nucleotide?
It is a disaccharide.
It is a hexose.
It includes an amine functional group.
It is connected by the 1-position carbon to a phosphate functional group.
None of the answers are correct.
3. Why is the directionality of a nucleic acid specified as running from 5′ to 3′ ?
The orientation of the phosphodiester bond on the 5′ and 3′ carbons of linked
sugar molecules gives the polynucleotide directionality.
The orientation of the phosphodiester bond on the 5′ and 3′ carbons of linked
phosphate functional groups gives the polynucleotide directionality.
The orientation of the phosphodiester bond on the 5′ and 3′ carbons of linked
amine functional groups gives the polynucleotide directionality.
There is no significance of the directionality because 3′ to 5′ directionality is for
notational convenience.
None of the answers are correct.
4. What kind of nucleotide base has a two-ring structure with an amine functional
group?
cytosine
pyrimidine
There is not enough information to determine the answer.
purine
either guanine or thymine
5. Which of the following sequences could theoretically base pair in a short segment
of DNA?
5′-AACTTGA-3′
3′-GGTCCAG-5′
5′-AACGGTA-3′
3′-TTGCCAT-5′
5′-AACGGTA-3′
5′-TTGCCAT-3′
5′-AACGGTA-3′
3′-AACGGTA-5′
5′-AACGGUA-3′
3′-UUGCCAU-5′
contents
6. Complete the following sentence: After a eukaryotic mRNA strand...
migrates to the nucleus, it is copied by the DNA strand, which is then "read" by
ribosomes to synthesize proteins.
is synthesized in the nucleus using DNA as the template, the mRNA migrates to
the cytoplasm, where ribosomes use mRNA as a template to direct the synthesis
of proteins.
copies a sequence of bases from DNA in the nucleus, it rearranges the base
sequence to form genes that code for specific proteins.
replicates from a parent strand, two daughter strands form that are different from
the parent strand.
is transcribed from a sequence of bases in DNA in the cytoplasm, it migrates to the
nucleus, where ribosomes use the RNA as a template to direct the synthesis of
proteins.
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IN THIS MODULE
Structure of Nucleic Acids
Functions of Nucleic Acids
Summary
Test Your Knowledge
PRIMARY LITERATURE
Classic paper: The discovery of
the neutron (1932)
Possible existence of a neutron.
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Classic paper: The idea of the DNA
double helix (1953)
Molecular structure of nucleic acids.
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Classic paper: X-ray diffraction
reveals helical structure of DNA
(1953)
Molecular configuration in sodium
thymonucleate.
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Classic paper: The first
sequencing of a complete DNA
genome (1977)
Nucleotide sequence of bacteriophage
ΦX174 DNA.
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Classic paper: X-rays reveal the
structure of myoglobin (1958)
A three-dimensional model of the myoglobin
molecule obtained by X-ray analysis.
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SCIENCE ON THE WEB
Chargaff In His Own Words
Listen to an interview with Erwin Chargaff
and how he discovered base pairing
Who Discovered the Shape of DNA?
Read a short bio on a woman who made a
large contribution to discovering helical
structure of DNA
DNA Structure and Function
Read about the discovery of DNA
How Fast Can You Build DNA?
Match the base pairs to build a DNA double
helix.
page 63 of 989