Insight on dna replication

2. Introduction
Presentation Outline:
Origin and Structure of DNA
DNA Replication
DNA Recombination
Gene Mutation
Conclusion

3. Introduction
• Deoxyribonucleic acid (DNA) was discovered in 1868 by Friedrich Miescher, a
Swiss physician when he isolated a novel compound from the nuclei of
leucocytes [1].
• It is composed of four types of nucleotide bases (adenine, cytosine, guanine
and thymine) which are arranged in a given sequence.
• DNA carries the traits of species of living organism, and a little distortion in
its structure can lead to mutation in offspring, causing defects in properties
and character coordination.

4. DNA; Definition and Origin
Definition; DNA is a large organic biomolecule of complex structure found in living
cells (eukaryotes, prokaryotes and in some viruses), which codes genetic
information for the transmission of inherited traits [1].
Origin; All cellular organisms possess double stranded DNA genomes; this
complex structure is attained from specific folding of two single helical
chains in anti-direction. DNA originated from Ribonucleic acid (RNA) in a
protein universe. RNA is a single-stranded molecule with its backbone
comprising of alternating sugar and phosphate groups. A close examination
of Fig. 1., exposes the fact that DNA structure is a replica of RNA in a double-
strand fashion. One can say that the ribose sugar in RNA is changed to
deoxyribose through reduction reaction in DNA, also the simple base
uracil is methylated into thymine. DNA is obtained from RNA through a
series of reduction reactions and chain entanglement.

5. Fig. 1. DNA vs RNA; A Schematic Supporting the Origin of DNA [2]

6. Structure of DNA
Structurally, DNA exist as a double
helix polymer, a spiral consisting of
two DNA strands wound around each
other. Each strand of the molecule is a
long chain polymer made up of
nucleotide monomers.
Fig. 2. A cross section of DNA Strand showing the linkage of
Deoxyribose to Phosphate units and the four nitrogenous bases [1].

7. DNA Replication
Definition; DNA replication is process during cell division at which DNA makes a copy of itself, making
available DNA duplicate that is passed on to offspring. This process is virtually identical no matter which
species of organism, or location in the cell it occur. Replication occurs in severally steps; and each stage
and sub-stages are carried out by a specific enzyme.
Step 1. Initiation
(helicase enzyme)
The double helix
structure of DNA is
unzipped; this involves
the breakage of the H-
bonds holding
complementary
nitrogenous bases along
the strand chains.
Separation begins from
one end forming a Y
shaped replication fork)
as it proceeds.
Fig. 3. Schematic of DNA Replication Fork Showing the Leading and Lagging Strands [2]

8. Step 2. Elongation (primase and
polymerase);
primase synthesizes short pieces of
RNA referred to as a primer, which
binds along to the end of the leading
strand to synthesize its
complementary DNA. The
polymerase adds new
complementary nucleotide bases as
it works along the leading strand in
the 5’ to 3’ direction.
Contrarily, numerous RNA primer
molecules are produced by the
primase enzyme along the lagging
strand. They bind to various points,
adding chunks of DNA (referred to as
the Okazaki fragments) to the lagging
strand in a 5’ to 3’ fashion. Okazaki
fragments joins before the end of the
process to form a full DNA strand.
Nitrogenous bases are matched ,
before the primer molecules are
stripped-off by exonuclease. Fig. 4. Schematic showing the activities of DNA replication Enzymes [3]
Step 3. Proofreading and Seal-up of the new DNA strands (DNA ligase); The
synthesized DNA strands are proofread to check for possible errors in the
new sequencing, before they are sealed up under the supervision of the
enzyme DNA ligase. DNA ligase seals the new sequences into two new
continuous strands

9. N/B: a counter sequencing occurs for the new constructed strand paying attention to the format
of the existing strand. For instance, if the strand to be replicated (leading or lagging) is having a
base sequence 5’-AGAACCTGA-3’, then the new strands obtained as its complement would only
follow a 3’-TCTTGGACT-5’ base sequence.
The two obtained daughter DNAs are pure replica of the parent, and their information content
are same.
Fig. 5. Synthesis of complementary strand from template strand [2]

10. Application of DNA Replication
Polymerase Chain Reaction (PCR)
PCR mimics the cellular replication method in the amplification of DNA in a short period. It essentially copies the target
DNA over and over in a test tube, to produce large amount of the desired DNA. Here, replication is selective, and occurs
in cycles leading to many products in a short time.
Fig. 6. Steps in Polymerase Chain Reaction in PCR
Ingredients required; A template (the
DNA containing the target sequence that
is being copied), Primers (to initiate
synthesis), thermostable DNA
polymerase (to carry out the
synthesis), ​DNA nucleotides (to build the
new DNA strands)

11. DNA Recombination
DNA recombination is a biochemical process
in which specific proteins interact with DNA
to create molecules with altered nitrogenous
base sequencing. Recombination can come
from different processes along the strands of
DNA, which could lead to deletions,
duplications or formation of a new order of
allelic variation. Recombinant DNA is
achieved by combining DNA from two or
more sources, often DNAs from different
organism are combined.
Restriction enzymes are used to cut out DNA
fragments from their normal position in the
chromosome, which is then inserted into
other DNA molecules using the ligases
enzyme. Recombinant DNA is possible
because DNA molecules from all organism are
structurally alike, with differences only in
nucleotide sequence embedded in the overall
identical structure Fig. 10. Schematic showing interactions of DNA vector
(adapted from Encyclopedia Britannica Inc).

12. Gene Mutation
Mutation in DNA occurs when there is
a change in its natural nucleotide
sequence.
This can result from mistake in DNA
copying which is possible during cell
division, exposure to mutagen
chemicals or ionization radiation, or by
infections
There are two forms of mutation in
living organisms; germ line mutation
(which occur in the eggs and sperm
cells of eukaryotes and are possibly
passed on to offspring), and Somatic
mutations (which occurs in body cells
and are not passed on to offspring.
Processes like nucleotide deletion,
insertion, duplication, substitution,
translocation, etc., along the helix
chain are sources of DNA mutation. Fig. 7. Sources of Mutation in DNA molecules.

13. Conclusion
• DNA replication and recombination are basically natural
processes that occur in living organism during
reproduction and gene distribution
• Scientists, having studied the structure of DNA and this
processes developed many technological innovations that
have played crucial roles in medicine, agriculture, food
and chemical industries, etc.
• All developments in the DNA field is based on the
principle that nucleotide sequences in DNA are specific,
and determines the functions of the molecule.

14. References
[1] Ferry, G. (2019). The structure of DNA.
[2] https://www.zmescience.com/medicine/genetic/dna-replication-steps-43264/
[3] https://teachmephysiology.com/biochemistry/cell-growth-death/dna-replication/