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Chapter 2
Replication of Genetic Information
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Part I
Relationship between Cells and Genetic Information
Chapter 2
Replication of Genetic Information
As the smallest units of organisms, cells share the single basic property/function
of dividing and multiplying to create progeny cells. All cellular components must
be doubled in number before a cell can divide, but DNA, a genetic material, has
the distinctive characteristic of existing as a single molecule in each cell. Since
DNA molecules carry the full genetic information of the organisms to which they
belong, a DNA molecule from the parent cell must be accurately replicated (i.e.,
doubled), and the two molecules must be distributed equally to the two daughter
cells without fail. In this chapter, we will first learn how genetic-informationcarrying DNA consists of two long polymers of small low-molecular units called
nucleotides (i.e., double-stranded DNA). We will then learn how DNA has an
ingenious mechanism to accurately replicate itself using a strand from the parent
DNA as a template – a mechanism not used in the synthesis of other polymers.
I. Cell Growth and DNA Replication
Cell Growth – the Most Basic Cell Function
All organisms on earth have one thing in common: they are made of cells.
Surrounded by a phospholipid bilayer, cells contain proteins as functional polymers
and have DNA as genetic information that dictates cell structures and functions. As
the smallest units of organisms, all cells divide and multiply to create progeny cells.
Although multicellular organisms produce offspring as a function on the level of
individual organisms, this function is supported by the multiplication of component
cells, and cell multiplication is a basic function of the process by which a fertilized
egg develops into an individual organism. Cell multiplication is the most basic and
common of cell functions, and has survived a long process of evolution.
Special Characteristics of DNA Replication
Cells multiply by binary fission. Before cell division, all cellular components must
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be doubled (Fig. 2-1). However, cellular components consist of a great number
of molecules, and the concept of “doubling” here is applied loosely; components
are not necessarily equally divided and distributed precisely into two cells (i.e.,
daughter cells).
DNA, which consists of genetic information, is quite different. Prokaryotes have
only one DNA molecule per cell. Although eukaryotes have a structure slightly
Figure 2-1
Schematic diagram of cell division
more complex than that of prokaryotes, they also have only one molecule of the
same DNA type per cell. Since DNA molecules contain all the genetic information
of the organism to which they belong, a DNA molecule identical to that of the
parent cell must be replicated during cell multiplication, and the two resulting
identical copies of the DNA must be equally distributed to the two daughter cells.
This phenomenon, which involves individual molecules, presents a rigorous
condition not found in other cellular molecules. First, let’s start by looking at
exactly what DNA is.
II. What Kind of Molecule is DNA?
Nucleic Acids as a Unit
DNA is a type of nucleic acid. A nucleic acid is a compound consisting of a
base, a pentose and a phosphate (Fig. 2-2A). As shown in Figure 2-2B, bases
are aromatic ring (heterocyclic) compounds containing nitrogen, and are roughly
divided into purines and pyrimidines. There are five main bases in nucleic acids
(Fig. 2-2B). Their names and one-letter abbreviations are frequently referred to
throughout this book, and should therefore be remembered. There are two types
of pentose: ribose and 2-deoxyribose (Fig. 2-2C). Compounds consisting of a
base and a pentose are collectively called nucleosides (Fig. 2-3). In nucleosides,
the carbon numbers of a sugar are expressed as “number’.” Compounds in
which a phosphate or phosphates are linked to the hydroxyl group of their sugar
in nucleosides are called nucleotides or nucleic acids (Fig. 2-4). The number of
phosphates added is not necessarily one, and in fact many nucleotides have
three phosphates (Fig. 2-5). The position of a phosphate is not limited to 5’, but
many of the phosphates found in living bodies are 5’-phosphates. There are
many types of functional nucleotides. Typical examples include ATP (adenosine
5'-triphosphate), which supplies energy to enzymatic reactions that require it,
and cAMP (adenosine 3', 5'-cyclic monophosphate), which works in the signal
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(A)
(B)
(C)
Figure 2-2 Nucleic acids, bases and pentoses
transduction pathway. When bases are not specified, they are called NMP
(ribonucleoside monophosphate) or dNTP (deoxyribonucleoside triphosphate).
Nucleic acids are roughly classified into DNA (deoxyribonucleic acid) and RNA
(ribonucleic acid). The difference between the two lies in whether the pentose is
2-deoxyribose (in this case, DNA) or ribose (in this case, RNA). There is also a
base-level difference between DNA and RNA: A, C and G are common, but T
is found only in DNA and U is found only in RNA (Fig. 2-5).
High-molecular Nucleic Acids
DNA is polydeoxyribonucleotide in which nucleotides are polymerized. 5’ and
3’ of 2-deoxyribose are joined by phosphodiester linkage (Fig. 2-6). Highmolecular RNA is required when genes are expressed, and as shown in Figure
2-6, high-molecular DNA and high-molecular RNA have very similar structures.
Both are long strings of molecules with the linkage of pentoses in a certain
direction (in Fig. 2-6, the upward part is the 5’ direction or the 5’ end, and the
downward part is the 3’ direction or the 3’ end).
From the specification of whether a high-molecular nucleic acid is DNA or RNA,
its structure can be simply expressed as a sequence of single letters (a base
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Figure 2-3 Nucleosides
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Figure 2-4 Nucleotides
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Figure 2-5 Nucleotide types
Figure 2-6
Structure of high-molecular nucleic acids
sequence). By convention, the 5’ end is written on the left-hand side and the 3’
end on the right-hand side, unless otherwise specified.
DNA – a Double Strand
All high-molecular DNA found in nature (excluding that of viruses) is doublestranded (Fig. 2-7). DNA takes a shape called B-form, and as shown in Figure
2-8, two bases – A and T – are linked by two hydrogen bonds while C and G
are linked by three hydrogen bonds. These respectively form base pairs, and
create a right-handed helix with a diameter of approximately 2 nm (Fig. 2-7).
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Figure 2-7
Double-stranded structure of DNA (B-form)
Since a particular pairing rule exists, once the base sequence of one DNA strand
is known, that of the other strand is automatically determined; these two are
called complementary strands. The length of DNA is often expressed as the
number of base pairs (bp).
The directions of the two strands (5’ → 3’) run opposite to each other in an
orientation referred to as antiparallel. The “spiral stairs” formed by the bases are
not situated in the center of the helix structure; rather, they are slightly deviated
from the center, creating wider grooves (major grooves) and narrower grooves
(minor grooves) (Fig. 2-7). These grooves play important roles when proteins that
control the expression of genes recognize base sequences and attach to them.
RNA – a Single Strand
All high-molecular RNA found in nature (except that of viruses) is single-stranded. In
many cases, however, RNA is partially double-stranded due to the pairing of bases
Figure 2-8
Hydrogen bonds forming WatsonCrick base pairs
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within the strand. This structure is called A-form, and is characterized by a minimal
difference between wider and narrower grooves. Like DNA-DNA and RNA-RNA
pairings, DNA and RNA form pairs by creating antiparallel double strands.
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Circular Strands for Prokaryotes and Linear Double Strands for Eukaryotes
Many prokaryotes have closed-circular double-stranded DNA. In other words,
their DNA has no ends. Its structure forms a twisted shape (Type I) (Fig. 2-9). Type
II is a form in which the twist of Type I is uncoiled as a result of nicking in the
DNA strands, but this form is rare in nature. On the other hand, all nuclear DNA
in eukaryotes is linear double-stranded (Type III) and has ends. This difference in
form between prokaryotes and eukaryotes is a key characteristic of DNA.
Figure 2-9 Circular and straightchain structures of DNA
Column
Denaturation and Renaturation of DNA
Double strands of DNA are separated into single strands in an alkali
environment with a pH value of 12 or more or by heating to 90˚C or higher.
This is called the denaturation of DNA. The phenomenon of proteins losing
their higher-order structure is also called denaturation; however, organic
media and acids that denature proteins precipitate but do not denature
DNA. The transformation of single-stranded DNA back to double strands is
called renaturation or annealing. During this process, base pairs are formed
between two single strands. Annealing between heterogeneous DNA, or
between DNA and RNA, is called hybridization. These techniques are
frequently used in genetic engineering.
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Column
DNA — a Long, Thin Thread
DNA is the thread of life. E. coli has circular double-stranded DNA with an
approximate length of 2 mm. A human somatic cell contains linear doublestranded DNA of approximately 2 m, consisting of 6 x 109 base pairs. To
help visualize this, if they were magnified to 500,000 times, the diameter
of the DNA would be 1 mm and its length would be 1,000 km. Since DNA
in human somatic cells is distributed to 46 threads, the length per thread
would be approximately 22 km. This 1,000-km DNA would be housed in
a nucleus with a diameter of 5 m. The number of DNA threads is doubled
to 96 before cell division, and these are distributed equally to the two
daughter cells without fail.
III. Genes and DNA
Definitions of Genes
It is often said that genes are DNA, or conversely, that DNA consists of genes;
however, genes are not spread throughout high-molecular DNA from one end to
the other. A gene is defined as a region of high-molecular DNA containing
information that determines the primary structure of proteins (amino acid sequence)
or the structure of non-coding RNA (base sequence) (explained in Chapter 3).
Generally in prokaryotes, genes are densely located with very narrow intervals
between them. In eukaryotes, they are sparsely located throughout the DNA with
wide intervals between them.
Genomes
The overall DNA contained in one cell is called a genome. In prokaryotes, cells
contain one thread (i.e., one molecule) of DNA, whereas human somatic cells
have 46 threads of DNA per cell (i.e., per nucleus), of which 23 are derived
from the mother (the ovum) and 23 from father (the sperm). The somatic cells of
eukaryotes commonly have two sets of genes derived from both parents, and
these cells are called diploids. Cells that have only one set of genes are called
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haploids. Most prokaryotes are haploids, as are the germ cells of eukaryotes.
The whole DNA of diploid cells is called a genome; however, in some cases,
from a functional viewpoint, the DNA of haploid cells is called a genome and
that of diploid cells is referred to as two sets (copies) of the genome.
DNA Content of Organisms
DNA content greatly varies among organisms. Figure 2-10 shows the amount of
DNA per haploid in several organisms. Generally, DNA content per cell is larger
in eukaryotes than in prokaryotes. Human somatic cells contain approximately
1,000 times as much DNA as those of E. coli (per haploid). For diploid cells, the
amount per cell is 6 pg. Generally among eukaryotes, the higher an organism,
the more DNA content there is, although there can be great variations among
organisms of the same group. Among vertebrates, such variations can be very
distinctive in fish and amphibians, with some species having more DNA content
than humans. Some higher plant species also have more DNA than humans, so
it does not necessarily hold true that higher DNA content represents a higher
organism. In other words, humans are not the highest species with the largest
amount of DNA.
Figure 2-10 Distribution of DNA content per cell
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The Number of Genes in Organisms
The Human Genome Project, which aims to determine the sequence of all the
chemical base pairs in human DNA, is now almost complete, and the genome
sequences of many other organisms are also being increasingly identified.
Contrary to predictions, the number of genes in humans is now estimated to be
only six times as many as that in E. coli (approx. 26,000 in humans and 4,300
in E. coli ). The number of genes also does not differ greatly among fruit flies,
Arabidopsis thaliana and humans.
Despite the less-than-significant difference in the number of genes between
humans and E. coli, in eukaryotes (including humans), one gene can synthesize
multiple types of protein with different amino acid sequences, and the number of
protein types produced in humans is estimated to be around 100,000. This
mechanism is discussed later (Chapter 3).
Eukaryotes – Characterized by Many Non-gene DNA Regions
The DNA of eukaryotes has a much larger proportion of regions that are not
genes (amino-acid-coding sequences) than prokaryotes. As an example, humans
have a much higher DNA content than E. coli, but have only slightly more genes.
As shown in Figure 2-11, in mammals, only 3% of the whole DNA sequence
codes for amino acids. One of the characteristics of eukaryotic DNA is that –
unlike DNA in prokaryotes – it has a large number of repetitive sequences, which
represent over half the total DNA in some species. Short repetitive sequences
may be located at the same sites or be scattered throughout the genome, and
little is known about the meaning and function of their existence.
Only a small part of the gene that determines the structure of a protein in
eukaryotes has an amino-acid-coding sequence for protein synthesis. Figure 2-12
shows a schematic diagram of a eukaryotic gene. Introns do not contain an
amino-acid-coding sequence. Based on the classical definition, therefore, introns
are not genes, but by convention in eukaryotes, genes include introns and exons.
Some genes have introns that are 10 to 100 times as long as exons.
In prokaryotes, regions that regulate gene expression are short (from tens of bp
to a hundred bp), whereas in eukaryotes they are much longer (dozens of kbp).
This is another major difference between prokaryotes and eukaryotes.
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Figure 2-11 What types of DNA do mammals have?
Figure 2-12 Exons and introns
IV. Replication of DNA
Outline of DNA Replication
The replication of DNA involves the production of high-molecular DNA by
polymerizing deoxyribonucleotides, which are units of DNA.
*1
Generally, this process is expressed as follows:
[dNMP] n + dNTP → [dNMP] n+1 + PPi
(PPi: pyrophosphate)
dNMP – a compound generated as a result of the detachment of pyrophosphate*1
from dNTP – is added to the 3’-OH of [dNMP]n. This indicates that DNA synthesis
occurs in the direction from 5’ to 3’. This is also the case for RNA synthesis, and highmolecular nucleic acids are always synthesized from 5’ toward 3’.
Deoxyribonucleotides are linked by DNA polymerase. In E. coli, DNA polymerase I,
II, III, etc. are known, and III is the main replicative enzyme. In mammals, several types,
including α, β, γ, δ and ε, have been identified. Of these, α, δ and ε are the
main replicative enzymes, while the others serve mainly to repair damage to DNA.
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Column
DNA Damage and Repair
DNA is continually subjected to damage. There are many natural or artificial
chemicals that bond with DNA bases, form base-base bonds, or cut DNA
strands. Radiation such as ultraviolet rays and cosmic rays also modifies
bases or cuts strands. DNA faces such threats from the birth of the organism
that carries it, and all organisms are equipped with a range of functions to
detect and repair DNA damage. As an example, damage to bases caused
by ultraviolet rays or certain chemicals is fixed by a mechanism called
excision repair, in which the damaged sites (including the surrounding
regions) are cut out and removed, and the resulting gaps are filled with
DNA polymerase (Column Fig. 2-1). Defects in the genes coding for the
enzymes involved in this mechanism result in a hereditary disease called
xeroderma pigmentosum, which makes sufferers prone to developing cancer.
Many other hereditary diseases associated with repair-enzyme defects are
known; these often cause cancer and, in rare cases, accelerated senescence
Column Figure 2-1
Excision repair of DNA damages
(progeria). In short, many repair-enzyme systems are in place to continually
repair gene damage, thus minimizing the accumulation of defects.
The Need for a Template in Replication
During the process of replication, the original double strands are unwound, and
new nucleotides are added to each single strand in a way that forms base pairs
(Fig. 2-13). This diagram shows the DNA double-helix structure published in
1953 by James Watson and Francis Crick, which suggested the possibility that
DNA was replicated using templates. In fact, during the replication process, base
pairs (C-G and A-T) are formed using each original strand as a template. As a
result, once the process is complete, two new double strands of DNA with the
same base sequence as the original are created. One strand of the new double
strand is the original template (i.e., the parent strand), and the other is a new
strand (i.e., the daughter strand). This method of replication is called
semiconservative replication. While many high molecules such as proteins and
sugar chains exist in the living body, this template-based semiconservative
synthetic method is unique to DNA.
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Figure 2-13
Semiconservative replication using a template
Column
Accuracy of Replication
DNA replication must be accurate. If DNA were synthesized using
nucleotides that do not follow the pairing rule, changes in the sequence
would occur in the newly created strand to form what is known as a mutation.
A mutation that occurs within the region of an important gene, if the effect is
significant, results in the death of the cell. Alternatively, in some cases, the
cell may become cancerous. In humans, one cell contains 6 x 109 base
pairs, and the division of 1011 – 1012 cells is thought to occur each day.
When DNA polymerase extends a new strand, the possibility of its inserting
incorrect nucleotides is said to be 10-6 – 10-4. DNA polymerase has a
proofreading function by which incorrectly inserted nucleotides are removed
and replaced with the correct ones. Additionally, errors missed by the
proofreading function are detected and replaced with the correct nucleotides
by a mismatch repair mechanism. There are multiple mismatch repair
systems, and the final frequency of errors is in the range of 10 -11 – 10 -10. To
artificially build a reaction system that achieves such low error rates is
difficult, even in the field of precision engineering.
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Replication – a Discontinuous Process
Double strands of DNA always run in opposite directions. This is the case for
completed DNA as well as for DNA during the process of replication. When
DNA synthesis is considered based on the Watson-Crick model (Fig. 2-14), one
of the daughter strands must be synthesized in the 3’→5’ direction. However,
DNA polymerase always synthesizes in the 5’→3’ direction. Let’s look at this in
more detail.
During the synthesis of the daughter strands following the uncoiling of the parent
strand, three double strands of DNA appear in a structure called the replication
fork (Fig. 2-14). Looking closely at this fork structure, at the point where DNA
synthesis occurs (i.e., the replication point), one of the daughter strands (i.e., the
leading strand) is synthesized in the same direction as that in which the replication
fork runs. The other daughter strand (i.e., the lagging strand) is synthesized in the
direction opposite to that of the replication fork, because DNA is synthesized in
the 5’→3’ direction (Fig. 2-14). Along the lagging strand, short DNA fragments
of approximately 100 nucleotides are continually synthesized, and are
Figure 2-14 Discontinuous replication of the lagging strand
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subsequently linked with each other. These short strands are called Okazaki
fragments after Reiji Okazaki, the molecular biologist who discovered them. This
type of replication is called discontinuous replication.
DNA polymerase follows the reaction below:
[dNMP] n + dNTP → [dNMP] n+1 + PPi
However, this reaction does not occur when n = 1. At least a fragment of two or
more nucleotides (called a primer) is needed so that new nucleotides can be
added there. On the other hand, RNA polymerase can synthesize RNA from n =1
using DNA as a template. RNA primers are synthesized by RNA polymerase
prior to DNA synthesis, and DNA synthesis starts from there using DNA polymerase
(Fig. 2-15). This mechanism was also discovered by Okazaki et al.
DNA synthesis proceeds along the lagging strand – removing RNA primers that
have performed their roles on the way – and the gaps between the short DNA
fragments are subsequently bonded by DNA ligase*2.
Figure 2-15
*2
DNA ligase: An enzyme that links together breaks
between 3’-OH and 5’-phosphate on one of the
double strands of DNA. It cannot link a break if
even one base is missing.
DNA synthesis – the need for RNA primers
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Column
The Many Enzymes Involved in DNA Replication
In fact, replication reactions are even more complicated than replication itself (Column
Fig. 2-2). At the tip of the replication fork, helicase unwinds the parental double
strand. There are single-strand binding proteins that stabilize the single strands
exposed by the helicase. RNA primers are synthesized by primase, and DNA
polymerase extends the primers to form new DNA strands. Along the lagging strand,
as previously mentioned, DNA is synthesized while RNA primers are being removed,
and DNA ligase subsequently joins the deoxyribonucleotides together. Ahead of the
replication point of the fork, topoisomerase (DNA gyrase) cuts the DNA strand to
release the tension held by the parental strand and links it again. Various enzymes
and proteins with such a function form a large replication complex; similar mechanisms
are essentially at work in organisms from bacteria through to humans.
Column Figure 2-2 Overview of DNA replication
Replication Origin and Endpoint
In prokaryotes, the long DNA strand has a single replication origin from which replication
forks proceed in both directions. Since prokaryotic DNA is circular, the two replication
points meet at the opposite side of the circle at a location known as the replication
endpoint. The replication origin and endpoint have characteristic base sequences, and
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particular proteins lead the start and the end of DNA synthesis. A DNA unit that replicates
from a single origin of replication is called a replicon. Prokaryotic DNA consists of one
replicon, and in E. coli the replication of the replicon takes approximately 40 minutes.
Eukaryotes have more DNA content than prokaryotes, and have multiple replication
origins on their DNA. In other words, eukaryotic DNA consists of multireplicons. In this
case, replication forks run in both directions from each replication origin, and the
replication of each replicon takes approximately one hour.
Column
PCR (Polymerase Chain Reaction)
PCR is a technique by which a particular fragment of DNA with a known base
sequence is amplified in tubes. Template DNA, DNA primers (fragments of 10 – 20
nucleotides are chemically synthesized in advance) and DNA polymerase are needed
for this process. As shown in Column Figure 2-3, a particular segment of DNA can
be amplified without limit for analysis. In theory, DNA can be amplified even from a
single cell. Fragmented DNA can also be amplified as long as the target segment is
not fragmented. RNA can also be used as a template, and PCR has a wide range of
application areas such as criminal investigations and court evidence as well as gene
cloning and the amplification and cloning of particular DNA segments.
Column Figure 2-3 Schematic diagram of PCR
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Summary
Chapter 2
• All organisms have high-molecular DNA that carries genetic information. The use of this
information involves high-molecular RNA.
• DNA and RNA are both organic compounds called nucleic acids (nucleotides).
• A nucleotide (a structural unit of DNA and RNA) consists of three components – a base, a
pentose and a phosphate. High-molecular DNA and RNA are polynucleotides composed of
nucleotides.
• DNA consists of four base types – adenine (A), cytosine (C), guanine (G) and thymine (T) – and
RNA consists of four base types – adenine (A), cytosine (C), guanine (G) and uracil (U).
• The pentose that constitutes DNA is 2-deoxyribose, while that in RNA is ribose.
• In high-molecular DNA, two long polynucleotide strands form a right-handed double helix.
• Bases are connected between the two strands of high-molecular DNA by a hydrogen bond,
creating base pairs. These are formed between adenine (A) and thymine (T) and between
cytosine (C) and guanine (G).
• Although high-molecular RNA is single-stranded, partial double strands are often formed within
the molecule.
• The structure of high-molecular nucleic acids has a particular nucleotide order characterized by
its base sequence. This is called the base sequence of nucleic acids.
• Although one human cell contains approximately 1,000 times as much DNA as E. coli, humans
have approximately 26,000 genes – only six times as many as E. coli’s 4,300.
• Each DNA molecule in a cell has a unique base sequence.
• When a cell multiplies, a DNA molecule identical to that of the parent cell is replicated, and the
two molecules are equally distributed to the two daughter cells.
• DNA synthesis is also called DNA replication because genetic information is replicated during
the process.
• When DNA is replicated using the parent strand as a template, four nucleotide types are
individually added to form base pairs, thus synthesizing daughter strands with a sequence
complimentary to that of the parent strand. This method is called semiconservative replication.
• Newly synthesized double-stranded DNA consists of one parent and one daughter strand.
• Although the main agent in the DNA replication reaction is DNA polymerase, the reaction is
complex and involves many types of enzymes and proteins.
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Problems
[1] 1) Draw the structural formulas for DNA and RNA, and point
i) What is the abundance ratio of DNA with heavy, light and
02
medium specific gravity in E. coli cultured in a medium
out the structural differences between the two.
2) DNA is more stable than RNA. Explain the reasons for this
containing 15N as a nitrogen source?
ii) What is the abundance ratio of DNA with heavy, light and
in terms of the structural differences between them.
3) Explain the implications of DNA being more stable than
medium specific gravity in E. coli transferred to a medium
containing normal nitrogen (14N) and divided once?
RNA for organisms.
iii) W
hat is the abundance ratio of DNA with heavy, light and
[2]The DNA sequence that constitutes human chromosomes has
3 billion base pairs, of which 40 million are within exons and
medium specific gravity in E. coli transferred to a medium
containing normal nitrogen (14N) and divided three times?
1.1 billion are within introns.
[4] C
onsider whether the following statements are true or false
*Answer 1) – 3) to two significant digits.
1) Calculate the percentage of all chromosomes expressed
and explain your decision:
1) Many DNA cleavage enzymes – called restriction enzymes
as mature mRNA.
2) The total number of genes in human chromosomes is
– recognize a palindromic sequence, that is, the sequence
taken as 25,000 here. Assuming that one gene contains
on one strand reads the same in the reverse direction on
8.8 exons, calculate the average DNA sequence length
the complementary strand (e.g., GAATTC/CTTAAG) and
per exon.
cut it.
3) As with 2), assuming that one gene contains 7.8 introns,
2) Restriction enzymes recognize a specific sequence and cut
it; the cut sequences are always located within the amino-
calculate the average DNA sequence length per intron.
acid-coding regions of genes.
3) Some viruses have reverse transcriptase, which transcribes
[3] Respond to the following tasks on DNA replication:
10
1) DNA is a huge molecule with a molecular weight of 10
11
– 10 , and during the process of cell growth must
replicate a molecule that has a structure identical to its
RNA into DNA.
4) D NA reverse-transcribed from mRNA by reverse
transcriptase contains promoter sequences.
own. Describe the characteristics of the DNA replication
5) Even if genome DNA extracted from a cell contains the
mechanism that are not found in the synthesis of polymers
coding regions of a gene, DNA reverse-transcribed from
such as proteins and polysaccharides in terms of their
mRNA extracted from the same cell (complementary DNA
relationship with DNA structure.
or cDNA) does not necessarily contain the gene.
2) E. coli cultured for many hours in a medium containing
15
N as a nitrogen source was transferred to a medium
[5] W
ith regard to a PCR reaction using one molecule of
14
containing normal nitrogen ( N) and divided three times.
genome DNA as a template, list the types of molecules
Double-stranded DNA was then extracted from the E.
generated as the reaction cycles proceed and the number of
coli, and the specific gravity (buoyant density) was
molecules for each type using the number of reaction cycles
measured using equilibrium density gradient centrifugation
(N). Also, calculate the number of molecules for each type
in cesium chloride.
when N is 10, and predict the main molecule when N
becomes large.
(Answers on p.251)
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