DNA is a double helical structure that transfers the genetic information from one generation to another. it consists of two strands with the four nucleotide basis .The four nucleotides contains adenine, cytosine, guanine, thymine .These four nuclic basis are such arranged and coiled with the help of hydrogen bonds and forms the helical structure of DNA. In RNA the thymine is replaced with uracil. Here you will learn the replication ,transcription and translation process in DNA.

1. DNA, The Genetic Material
DNA, Deoxyribonucleic Acid, is the genetic material in your cells. It
was passed on to you from your parents and determines your
characteristics. The discovery that DNA is the genetic material was
another important milestone in molecular biology.
Griffith Searches for the Genetic
Material:
In the 1920's, Frederick Griffith made an important discovery. He was
studying two different strains of a bacterium, called R (rough) strain
and S (smooth) strain. He injected the two strains into mice.The S
strain killed (virulent) the mice, but R did not (non-virulent).Griffith also
injected mice with S-strain bacteria that had been killed by heat. As
expected, the killed bacteria did not harm the mice. However, when
the dead S-strain bacteria were mixed with the live R-strain bacteria
and injected, the mice died.
Griffith showed that a substance could be transferred to harmless
bacteria and make them deadly. He deducted that something in the
killed strain was transferred to the previously harmless R-strain,
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2. making the R-strain deadly. He called this process ​transformation​, as
something was "​transforming" ​the bacteria from one strain into other
strain.
● What was that something?
● What type of substance could change the characteristics of
the organism that received it?
Avery's Team Makes a Major
Contribution:
In 1940's, a team of scientists led by Oswald Avery tried to answer
the questions raised by Griffith's results. They inactivated various
substances in the S-strain bacteria. They then killed the S-strain
bacteria and mixed the remains with live R-strain bacteria (keep in
mind, the R-strain bacteria usually did not harm the mice.) When they
inactivated proteins, the R-strain was deadly to the injected mice. ​This
ruled out proteins as the genetic material​. ​Even without the
S-strain proteins, the R-strain was changed or transformed, into the
deadly strain. However, when the researchers inactivated the DNA in
the S-strain, the R-strain remained harmless. This led to the
conclusion that ​DNA is the substance that controls the characteristics
of organisms​. In other words ​DNA is the genetic material​.
Hershey and Chase Seal the Deal:
The conclusion that DNA is the genetic material was not widely
accepted at first. It had to be confirmed by other research. In the
1950s, Alfred Hashey and Martha Chase did experiments with viruses
and bacteria. Viruses are not made of cells. They are basically the
DNA inside a protein coat. To reproduce, a virus must insert its own
genetic material into a cell (such as bacterium). Then it uses a cell's
machinery to make more viruses. The researchers used different
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3. radioactive elements to label the DNA and proteins in viruses. This
allowed them to identify which molecule the viruses inserted into
bacteria. DNA is the molecule that is identified. ​This confirmed that
DNA is the genetic material​.
Summary
● The work of several researchers led to the discovery that
DNA is the genetic material.
● Along the way, Griffith discovered the process of
transformation.
The
Double Helix Model:
​Two kinds of molecules participate in protein synthesis. Both are
based on a similar building block, the nucleotide, giving them their
name "​nucleic acids". ​One of these molecules, ​Deoxyribonucleic
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4. acid ​or ​DNA​, is the genetic material, and the other, ​Ribonucleic acid
or ​RNA​, is produced in the nucleus and moves to the cytoplasm,
where it participates in protein synthesis. The study of how the
information stored in DNA codes for RNA and protein is ​molecular
genetics.
The Double Helix is a description of the molecular shape of a
double-stranded DNA molecule. In​ 1953​, ​Francis Crick ​and ​James
Watson ​first described the molecular structure of DNA, which they
called a "​double helix".
The Double Helix describes the appearance of double stranded DNA,
which is composed of two linear strands that run opposite to each
other or antiparallel and twist together. Each DNA strand within the
double helix is a long, linear molecule made of smaller units called
"​nucleotides​"​ ​that form a chain. Each strand has a backbone made of
alternating groups of ​sugar(deoxyribose)​ and ​phosphate groups​.
Attached to each sugar is one of four bases:​adenine(A), cytosine(C),
guanine (G) and thymine (T). ​The two strands are held together by
bonds between the bases, adenine forming a base pair with thymine,
and cytosine forming a base pair with guanine.
Due to base pairing, the DNA strands are complementary to each
other, run in opposite directions, and are called as ​antiparallel
strands​.The nitrogenous bases are stacked like the steps of
staircase. The pairs are bound to each other by ​hydrogen bonds​.
The two strands of a helix run in opposite directions, so that the 5'
carbon end of one strand faces the ​3'​ carbon end of its matching
strand.
Base pairs:
Only certain types of base pairing are allowed. For example, a certain
purine can only pair with pyrimidine. This means Adenine pairs with
Thymine, and Guanine pairs with Cytosine. This is known as the base
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5. complementary rule because the DNA strands are complementary to
each other. If the sequence of one strand is AATTGGCC, the
complementary strand would have the sequence TTAACCGG.
DNA Replication:
DNA Replication is the process by which a double-stranded DNA
molecule is copied to produce two identical DNA molecules.
Replication is an essential process because, whenever a cell divides,
the two new daughter cells must contain the same genetic information,
or DNA, as the parent cell.
Four steps of DNA Replication:
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6. 1. Replication Fork Formation(​Before DNA can be Replicated, the
double stranded molecule must be "Unzipped" into two single
strands…​ )
2. Primer Building​(The leading strand is the simplest to replicate… )
3. Elongation​(​DNA polymerases are responsible for creating new
strand by a process of Elongation… ​)
4. Termination​(The gaps between newly synthesized DNA
segments are sealed together with enzymes)
The Replication process relies on the fact that each strand of DNA
can serve as a template for duplication. DNA Replication initiates at
specific points, called origins, where the DNA double helix is
unwound. A short segment of RNA, called a primer, is then
synthesized and acts as a starting point for new DNA synthesis. An
enzyme called as DNA polymerase next begins replicating the DNA by
matching bases to the original strands. Once synthesis is complete,
the RNA primers are replaced with DNA, and any gaps between the
newly synthesized DNA segments are sealed together with enzymes.
DNA Replication is a crucial process; therefore to ensure that
mistakes, or mutations, are not introduced, the cell proofreads the
newly synthesized DNA. Once the DNA in a cell is replicated, the can
divide into two cells,each of which has an identical copy of the original
DNA.
Why does DNA Replication need to occur?
DNA Replication needs to occur because existing cells divide to
produce new cells. Each cell needs a full instruction manual to operate
properly. So the DNA needs to be copied before cell division so that
each new cell receives a full set of instructions.
● ​Where does DNA Replication occur?
DNA Replication occurs in the cytoplasm of prokaryotes and in the
nucleus of eukaryotes.
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7. The Genetic Code:
DNA must code for 20 different amino acids found in all organisms.
The information carrying capabilities of DNA reside in the sequence of
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8. nitrogenous bases. The genetic code is a sequence of three bases-a
triplet code. Each three-base combination is a ​codon​. More than one
codon can specify the same amino acid because there are 64 possible
codons, but only 20 amino acids. This characteristic of the code is
referred to as ​degeneracy​. Note that not all codons code for an amino
acid. The base sequence UAA,UAG,and UGA are all stop signals that
indicate where polypeptide synthesis should end. The base sequence
AUG codes for the amino acid methionine, which is a start signal.
Important points:
● Messenger RNA ​(mRNA) is a linear strand that carries a set of
genetic instructions for synthesizing proteins to the cytoplasm.
● Transfer RNA​ (tRNA) picks up amino acids in the cytoplasm,
carries them to ribosomes,and helps position them for
incorporation into a polypeptide.
● Ribosomal RNA ​(rRNA), along with proteins, makes up
ribosomes.
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9. ​Transcription:
​Transcription is the process by which DNA is copied (transcribed) to
mRNA, which carries the information needed for protein synthesis.
Transcription takes place in two broad steps. First, pre-messenger
RNA is formed, with the involvement of RNA polymerase enzyme. The
process lies on Watson-Crick base pairing,and the resultant single
strand of RNA is the reverse complement of the original DNA
sequence. The pre-messenger RNA is then "edited" to produce the
desired mRNA molecule in a process called ​RNA splicing.
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10. Formation of pre-messenger RNA :
The mechanism of transcription has parallels in that of ​DNA
replication​. As with DNA Replication, partial unwinding of the double
helix must occur before transcription can take place, and it is the RNA
polymerase enzymes that catalyze this process.
Unlike DNA replication, in which both strands are copied, only one
transcribed. The strand that contains the gene is called ​the sense
strand,​ while the complementary strand is ​the antisense strand. ​The
mRNA produced in transcription is the copy of the sense strand, but it
is the antisense strand that is transcribed.
Ribonucleoside triphosphates (NTPs) align along the antisense DNA
strand, with Watson-Crick base pairing (A pairs with U). RNA
polymerase joins the ribonucleotides together to form a
pre-messenger RNA molecule that is complementary to a region of
the antisense DNA strand. Transcription ends when the RNA
polymerase enzyme reaches a triplet of bases that is read as a ​stop
signal. The DNA molecule re-winds to re-form the double helix.
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11. Reverse transcription:
In reverse transcription, RNA is "​reverse transcribed​"into DNA.
This process, catalyzed by reverse transcriptase enzymes, allows
retroviruses, including the human immunodeficiency virus (HIV),
to use RNA as their genetic material. Reverse transcriptase
enzymes have also found applications in biotechnology, allowing
scientists to convert RNA to DNA for techniques such as ​PCR​.
Translation:
The mRNA formed in transcription is transported out of the
nucleus, into the cytoplasm, to the ribosome (to the cell's protein
synthesis factory). Here it directs protein synthesis. Messenger
RNA is not directly involved in protein synthesis-transfer RNA
(tRNA) is required for this. The process by which mRNA directs
protein synthesis with the assistance of tRNA is called
translation​.
The ribosome is a very large complex of RNA and protein
molecules. Each three-base stretch of mRNA (triplet) is known as
codon​, and one codon contains the information for a specific
amino acid. As the mRNA passes through the ribosome. Each
codon interacts with the anticodon of the specific transfer RNA
(tRNA) molecule by Watson-Crick base pairing. This tRNA
molecule carries an amino acid at its 3'-terminus, which is
incorporated into the growing protein chain. The tRNA is then
expelled from the ribosome.
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12. Now, tRNA molecule bind to the two binding sites of of the
ribosome, and by hydrogen bonding to the mRNA. A peptide bond
forms between the two amino acids to make a dipeptide, while the
tRNA is left uncharged. The uncharged tRNA molecule leaves the
ribosomes, while the ribosome moves one codon to the right (the
dipeptide is translated from one binding site to the other ) another
tRNA molecule binds, a peptide forms between the amino acids to
make a tripeptide, the uncharged tRNA molecule leaves the
ribosome.
​The end…
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