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1
Examination
MOLECULAR GENETICS
Joni Hendri
NIM.12/336568/PMU/07330
BIOTECHNOLOGY STUDY PROGRAMME
POST-GRADUATE SCHOOL
UNIVERSITY OF GADJAH MADA
YOGYAKARTA
2013
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Several modes of genetic materials replication in viruses
Viruses are a particle that contains the genetic material in the form of DNA or
RNA that is enveloped by protein called capsid. Besides DNA viruses and RNA
viruses, there is a simpler particle of virus called prion. Prions or viroids are small
RNA molecules consisting of 359 nucleotide bases and not enveloped by the protein.
Prions are known as infective agent that causes several diseases in both humans and
animals.
Viruses may contain double-stranded DNA, double-stranded RNA, single-
stranded DNA or single-stranded RNA. In addition, there is the viral genom in the
form of linear and circular nucleic acid moleculs. The mechanism for double-stranded
DNA, single-stranded DNA, double-stranded RNA and single-stranded RNA viral
replication will differ.
Replication of DNA viruses
Some viruses had a genetic material DNA in the form of double-stranded or
single-stranded which is different in mechanism of replication. Mechanism of double-
stranded DNA viral replication is a semiconservative models consisting of several
stages: (1). Denaturation of DNA strands, (2). Initiation of DNA synthesis, (3).
Elongation of DNA strand, (4). Ligation of DNA fragments and (5). Termination of
DNA synthesis. However, if that virus is a single-stranded DNA, replication will
going through three phases: (1). Duplex replicative form, (2). Rolling-circle
replication and (3). Synthesis of strands (+) which is generating a single strand of
molecules. Furthermore, the replication mechanism will vary between linear and
circular DNA, for example, in replication of circular DNA molecule, the replication
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fork will moving into two directions (Bidirectional replication) to form the ɵ (theta)
structure.
Generally, DNA viruses replicate their genetic material by one of three modes:
1. Bidirectional replication from a circular substrate. This process may proceed via a
“theta-form” intermediate (eg, papillomaviruses), or in some cases via a “rolling
circle” mechanism that results in the generation of concatemeric (head-to-tail) viral
genomes.
2. Replication from a linear substrate. In this case, synthesis of new DNA strands is
not simultaneous. Rather, it occurs sequentially (ie, first one strand is made in its
entirety & then the next strand is made). Examples include adenoviruses.
3. Replication via an RNA intermediate. Hepadnaviruses (hepatitis B virus) are
unique since they contain a partially dsDNA genome that must be converted into
an RNA form by the virion enzyme reverse transcriptase during the virus life cycle
.
Replication of RNA virus
RNA viruses have a variety of modes of replication, for example, retroviruses
replication is done by converting single-stranded RNA into double-stranded DNA in
advance using reverse transcriptase enzyme. On the contrary, RNA replication in
TMV (tobacco mosaic virus) is replicated directly from RNA to RNA. Although
commonly RNA replication refers to several important points of replication which are
shown as follows:
1. The viral RNA genome can act as its own message (positive strand viruses) or the
complementary strand can be the mRNA (negative strand viruses).
2. All RNA viruses except retroviruses encode an RNA-dependent RNA polymerase.
In the negative strand RNA viruses this polymerase is part of the virion, and it
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must enter the cytosol along with the viral genome. This is necessary in order for
the virus to generate mRNAs from its genome.
3. All RNA viruses replicate in the cytoplasm except orthomyxoviruses (influenza A
& B), borna disease virus, hepatitis delta virus and retrovirses. Additional feature
of orthomyxoviruses: these viruses have segmented genomes.
4. Retroviruses are unique. These viruses have a positive sense RNA genome which
must be converted into a dsDNA form by the virion enzyme reverse transcriptase.
This double-stranded DNA is then integrated into the host cell chromosome by the
viral integrase enzyme. Upon integration into the host chromosome, the viral DNA
can then be transcribed by cellular RNA polymerase II, to produce new genomic
RNA molecules.
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Interrealtionship betwen DNA replication and genetic expression with the
phenomenon of cell geowth
In the biological sense, cells are the smallest unit of organization which is the
foundation of life. All functions of life are organized and took place inside the cell.
Cell growth and development cannot be separated from the cycle of life which is
experienced by the cell in order to survive. This cycle regulate cell growth by
regulating fission time and cell growth by regulating gene expression or gene
translation amount respectively.
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.
Cells multiply by binary fission. Before cell division, all cellular components
must be doubled. 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
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
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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
Genetic material replication is a process that initiated the cell growth;
however, growth is simultaneous reactions involving many processes related to each
other. Replication of genetic material is followed by forming of progeny that carry
genetic material duplicate from replication results.
The replication of DNA involves the production of high-molecular DNA by
polymerizing deoxyribonucleotides, which are units of DNA.Generally, this process is
expressed as follows: 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, one of the
daughter strands must be synthesized in the 3’→5’ direction. However, DNA
polymerase always synthesizes in the 5’→3’ direction. Let’ 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.
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. Along the
lagging strand, short DNA fragments of approximately 100 nucleotides are
continually synthesized, and are subsequently linked with each other. These short
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strands are called Okazaki fragments after Reiji Okazaki, the molecular biologist who
discovered them. This type of replication is called discontinuous replication. In the
Contrary, synthesis of DNA strand wich has an opposite direction to the direction of
the opening fork geometry replication is done step by step (discontinue synthesis
type). Generally, it can be said that the mechanism of DNA replication takes place in
semi-discontinue type, because of differences in the mechanism synthesis of both
DNA strand synthesis.
Mechanisms of DNA replication requires a variety of proteins and enzymes.
Proteins and enzymes are produced by genetic expression mechanisme. Specifically,
genetic information refers to the nucleotide (base) sequence of DNA strands; mRNA
is synthesized using DNA as a template, by which the genetic information of the DNA
is transcribed as the sequence of the mRNA. The base sequence of mRNA is defined
as a series of genetic codes, and such codes in mRNA are used to synthesize proteins
on cytoplasmic granules called ribosomes. One piece of mRNA code corresponds to
one amino acid, and these amino acids are linked together following the order of the
codes, thus synthesizing proteins.
Therefore, it can be said that the replication process will not occurs without the
process of genetic expression. In other words, there is an interrelationship between
DNA replication and genetic expression with the phenomenon of cell growth
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References :
1. Yuwono, T. (2005). Biologi Molekuler. Penerbit Erlangga. Jakarta
2. Cann, A.J. (2005). Principles of Molecular Virology Fourth Edition. Elsevier
Academic Press. USA.
3. Wagner, E.K and Hawlett, M.J. (2004). Basic virology . Second edition.Balckwall
Publishing. USA