Nucleic acids are the substances found in the nucleus of every living cells and it is the linear polymers of nucleotides attached together and determine their role as informational macromolecules. The two major types of nucleic acids are DNA and RNA. Both of them are compose of sugar, phosphate and bases. DNA primarily serve as a repository of genetic information where as RNA plays many roles in expression of the information given by DNA. According to the theory of central dogma given by Francis Crick, the information encoded in DNA is transfer to mRNA by the process of transcription. And through the process of translation the protein is manufactured by various RNAs in the ribosome within the cell cytoplasms.
DNA mRNA Proteins
Move to cytoplasm through pores (Within nucleus) (In nucleus) (Inside ribosome)
( Becker et al, 2007)
DNA is a nucleic acid; it is in long polynucleotide chain and is usually a hereditary
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Material where the genetic information of all living cell/organisms is stored.However, in
Some case RNA act like a genetic material where DNA material is completely absent,
E.g. in tobaccos mosaic various (Gupta, 2001).
The DNA in all living cell is mostly existed in double stranded form and rarely observed
in single stranded form, example-single stranded DNA in Parvovirus (Gupta,2001).
The DNA in prokaryotic cell occurred highly twisted, circular and associated with
inner side of plasma membrane, free of protein complex. In eukaryote it is found in
chromosome of nucleus, mitochondria ,cell cytoplasm and chloroplast (plants)(Conn &Stump, 1976).
Fig.1 (Nelson & Cox, 2000).
The DNA (nucleic acid) is however build up of four different types of building blocks
Called nucleotides. Each of the nucleotide is composed of: a) phosphate and
b) Nucleoside moiety.This nucleoside consists of bases-a cyclic nitrogen containing
Compound (provide stable) either double ring purine base (adenine and guanine) or single
ring pyrimidine base (cytosine and thymine) and pentose sugar (2'deoxy ribose)
(Deoxyribonucleic Acid).The base thymine is normally in methylated form and
it is stable base which provide protection to DNA from enzyme nuclease.
Structure of four bases.
The sugar of DNA is 2'deoxyribose (pentose), hydroxyl group (OH) is present only at carbon 3â€²
Position but lack hydroxyl groups at carbon 2â€²position, unlike in RNA .In DNA a number of
monomer nucleotide united to give long polynucleotide chain/macromolecule unit by the
formation of bridge called phosphodiester bond between two nucleotide at carbon 3â€²and carbon
5â€²end.The bases are side group anneal to the pentose sugar with the formation of glucosidic
linkage bond. Obviously in DNA molecule phosphate and sugar are the two primary back bonds
which are exposed exterior of the helix, joining together alternately and its hydrophilic in
nature. The side group (base pair) are hydrophobic in nature, which is hinder interior of the back
bond and form hydrogen bond between specific base pairing of the two opposite strand, so the
base themselves become to strong (Gardner et al, 2005).
Fig 3 DNA double helix.
(Interactive Concepts in Biochemistry, 2002).
Figure: 4. The chemical structure of DNA.
Deoxyribonucleic Acid (n.d)
According to Watson and Crick (1953) fig 3, DNA exists as a double helical structure,
Where one of the polynucleotide chain is complementary to another and it is twisted
together around a common axis and form a right handed helical structure. This double
helix of two complementary DNA chains are arrange in opposition direction (i.e.
antiparallel) along the helix ,where one of the strand running in downward direction from
5â€²carbonâ†’3â€²carbon and other complementary strand run in 3â€²carbon â†’5â€²carbon
Direction (Becker, 2007) .The two opposite strands are held together by the formation of
hydrogen bond between the specific base pairing and the base pairing is always specific i.e.
adenine always pair with thymine (A=T) by forming two hydrogen bonds and guanine with
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Cytosine (Gâ‰¡C) by forming three hydrogen bonds. (Gardner et al.2005).The base pairing will be
not happened until and unless the base is not specific, so the DNA sequences will be not in
spontaneous and if it is specific sequence will be in spontaneous manner. Antiparallel in
nature made DNA so stable. The base pairs in DNA are stacked 3.4Çº distance with 10 bases
pairs per turn (360) of double helix and bases are perpendicular to each other. The double helical
Structure is quite rigid, viscous with immense length and small diameter consisting of minor and
major groove, with 34A. The major groove is deep and wide for the site where most protein
DNA interactions occur, whereas the minor grooves are quite narrow and shallow. Diameter of
double helix is 20Çº. (Gardner. et al, 2005).
Forms of DNA: Three major forms of DNA are mention below base on their umber of
base pairs, relative humidities, structure ,etc.
10 (common in human) 12
(Strickberger, 1976 ).
Functions of DNA related to its structure.
The double helical structure and specific base pair present in DNA severs as a mechanism for
the accurate replication/transmission of the genetic information. Due to the complementary
strand of the double helical structure, each strand serve as a template to specified the base
Sequence of a newly synthesized complementary strand thus, lead to the production of two
daughter molecules of DNA, which are identical to that of parent DNA (Conn &Stumpf, 1976).
Double helix of DNA also contains two types of grooves; they are major and minor grooves.
The major grooves are the site /space for the binding of other regulatory proteins/non histone as
well as space for other strands of nucleic acids and minor grooves are the sites for histone
In double helix DNA, the bases are hinder/shield inside the helical structure which is
hydrophobic in nature and many hydrogen bonds (weaker than normal chemical bond) are
form between the specific bases pairing of the complementary strand along the length of double
helix, at least two hydrogen bond for each base pair, thus it maintain the high degree of
stability and rigidity to the molecule (Strickberger,1985). At the same time sugar and phosphate
Backboned are also arranged alternately from outside and keep connected each other by
phosphodiester bond .Therefore the base and back bond maintain DNA stability and stored the
genetic information for long times (Nelson & Cox, 2000). However, in DNA due to
absence of OH (present only H -made stable) group at C2' position, it prevent the formation of
cyclic phosphate ester thus, less susceptible in hydrolysis at room temperature and will not attack
by the alkaline, since, hydrogen cannot attack unlike in RNA ribose at C2'position hydroxyl
group is presence which act as nucleophile attacking the phosphodiester bond and lead in less
Chance of storing genetic information (Lodish, 2008).In DNA molecule it contain
thymine base instead of uracil for genetic information storage. This is very important in DNA
because it contain methyl group which keep neutral and cell ensured to repair the damage DNA
cause by the eradication of cytosine with out damaged the other DNA. It can also keep the DNA
structure stability for long time ,since the thymine base is no reactive , thus genetic information
is stored for long time and occur exact runs of genetic information .The methylated thymine
even prevent DNA from chemical attracts/enzyme nuclease .If uracil is present instead of
thymine than it will prevent repairing of damaged DNA , more over error will take place during
the flow of genetic information and protein synthesis.(Becker,2006).In DNA, the base
and sugar are bounded each other by the formation of N-glycosidic bond, which aid in
maintaining the DNA structure stability at extreme pH (Conn & Stumpf,1976).
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Therefore DNAs main function is to stored and transmit the information /it act like "instruction
manual" in its sequences and provide all the information needed (Lodish,2008).
RNA: Ribonucleic acid
RNA is also one of the nucleic acid found in many organisms including plants animals, viruses
and bacteria (Gupta,2001).RNA molecule is mostly in single stranded form, double stranded is
not common. However double stranded is observed in wound tumors (plant virus) and
Reovirus (animal virus).Unlike DNA,RNA is rarely genetic in nature and so its main task is to
carry/simply translating the message encoded in DNA into proteins.RNA in most organism is
non-genetic in nature (do not serve as genetic material even the information is presence
in).Despite occur mostly in non-genetic form ,it also occur as genetic in nature only in the
specific system where DNA(usual genetic material) is invariably absent ,so RNA function as
genetic material like DNA. Example in tobacco mosaic virus(Gupta,2001).If RNA is in double
stranded ,the base pairing will be specific as that of DNA .
Structure of RNA
RNA is very similar to DNA ,which is made up of four different building blocks the
nucleotide. Nucleotide is composed of phosphate, sugar (ribose) and base. In this nucleic
acid the bases are purine(adenine and guanine) ,which is double ring and pyrimidine
(uracil and cytosine),which is single ring. Sugar and phosphate are the backbone. Unlike
DNA , RNA contains hydroxyl group at both C2 and C3 of the ribose ring. It has also got
uracil base instead of thymine .RNA can form 3-D structures by pairing of
complementary base within a linear sequence, G pairs with C and A with U forming
different structures .Base stacking interaction play vital role in stabilizing the RNA
structure-weak interaction. Phosphodiester bond is also present and the helix runs from 5'
â†’3'.The four bases of RNA are given below.
Fig5.Sugar Adenine Guanine Cytosine Uracil
Fig6.Bases of RNA (Kratz,2009).
Figure 7 : Chemical structure of RNA (Kratz,2009).
Non-genetic RNA synthesis on DNA template and forms three major RNA. They are
differ from one another base on their structure and functional role in the cells. They are
mention be love:
Fig 8.Structure of mRNA. (Interactive Concepts in Biochemistry, 2002)
Is principally single stranded and complementary with the sequence of DNA.
It is formed on chromosomes in the nucleolus and transported out into the cell
Cytoplasm (Gupta, 2001).It is heterogeneous class of molecules.This mRNA presents in linear
Polymers form a codon for the manufacture of protein since, it is a template for protein
synthesis. In 5'/N-terminus , methionine, AUG codon act as start/switch on codon and at 3'/C-
terminus normally with UAA,UGA,UAG of this three codon function as stop/switch off
codon. Between this two codon (start and stop) present a reading frame, where sequencing of
amino occurs.( Becker,2007).This RNA contribute small mass of the total RNA present in the
cell. It is less stable , short lived. In bacterial cells mRNAs are highly unstable they are
constantly being degraded and resynthesised but in eukaryotic cells the turn over rate is much
less.( Conn & Stump, 1976).
Function of mRNA related to its structure.
During the cores of transcription,mRNA is synthesis from the gene segment of DNA
which ultimately carries genetic information contain in DNA into a codon (three letters
genetic code /triplets) to ribosome in cytoplasm.The translation initiates from 5'end
,where codon runs from particular switch on codon to switch off codon at 3'end/ C-
terminus of polypeptide chain term "Reading frame".Each of the amino acids is encode by one/three codons in this mRNA and each codon specifics one amino acid.The start codon like methionine ,AUG specified amino acids only at N-terminus but the three stop codon present at C-terminus did not specified any amino acids ,so translation is terminated . The reading frame ,which is located in between start and stop codon is remain as uninterrupted codon sequence in
mRNA and is translated into a linear sequence of amino acids in turn synthesis of protein occur.Therefore it served as intermediate between DNA and protein molecule.( Lehninger,1995).
Transfer RNA (tRNA)/Soluble RNA (sRNA)
It is a smallest polymer RNA species ,single stranded, helica structure,form on chromosome and found in cytoplasm located in ribosome (Conn & Stump, 1976 )but is independent of ribosome that is charge with an amino acids at one end and coated with anticodon at another end. This tRNA exist in three levels:
Fig 9 .Secondary structure of tRNA. ( Secondary Structure of tRNA, n.d.)
1/Primary level (linear shape),2/.Secondary level(stem shape) and 3/.Tertiary structure (3-D shape).All this three tRNA shares a common secondary structure and 3-D shape. The secondary structure is represented by clover leaf shape and 3-D resemblance an inverted 'L'.They possesses four base pair stems having three stem loops- a) anticodon loop at the bottom of secondary structure along with three letters of anticodon, b) dihydrouracil loop(D-loop) on left side and finally T loop the sequences of three bases.The above mention anticodon gets pair with mRNA codon ,thus activated amino acids (the amino acids encoded by mRNA codons) can added to growing polypeptide molecules.The process is specific ,it is maintain by aminoacyl-tRNA synthetase enzyme.In this process pairing of base in between two RNA usually occurs in 3'base in mRNA codon 5'in tRNA codon.If the amino acid is so specific to tRNA than correct synthesis of ptotein occurs, but if the wrong amino acid is attached there will be immediate removable of that amino acid out of tRNA.(Lodish,etal,2008).
Function of tRNA related to its structure.
The 5 ï€ˆ-P term terminus is always base paired and it is thought that this contribute to the stability of the tRNA. On the other hand the 3 ï€ˆ terminus always a four base - single stranded region and it is the acceptor end. ( Freifelter,1986).In this RNA the folded structure promotes its decoding function and modified base aid in binding of right amino acids. The L-shaped structure extended the length of its base pairing stem by stacking them in two sets forming long base paired region, and so each stem are band slight due to imperfect aligned.Thus,such aligned provide base paired to stacked on each other and provide stability(Weaver,2002).In tRNA the tertiary structure having anticodon base stack are projecting out away from the backbone of tRNA, this places them in position to interact with base of codon in mRNA. And twisted nature of anticodon backbone into particular helix shape, which facilitates base pairing with corresponding codons (Weaver,2002).
RibosomalRNA (rRNA) structure.
It exist in single stranded form and most essential components of important part of protein synthesis mechainery are rRNA, ribosome(actual sites of protein synthesis),enzyme etc.It is more stable kind of RNA. This species makes about 80% of total RNA in the cell. The rRNA is formed from only a small section of DNA molecule thus, no definite base relation ship between rRNA and DNA as a hole. Ribosomal RNA has helical structure which is result from folding back of a single stranded polymer at places where hydrogen bonding is possible due to short lengths of complementary structure. However, rRNA does not occur as a double stranded polymer. More over rRNA does not have extremely rigid and stable double helical structure of DNA,it may exist in several conformations.And this RNA served as important structural components in ribosomes.( Conn & Stumpf,1976).Three kinds of rRNA found in prokaryotes are 16s, 5s and 23s and ribosome are bind to the mRNA close to translation start site. In eukaryotes 18s, 5s and 28s are found and ribosome are bind at 5'end of the mRNA(Conn&Stumpf,1976).
Function of rRNA related to its structure.
It involves in building up of ribosomal structure,where the complex structure runs along
the instruction of mRNA molecule.The ribosome is being bind to mRNA at 5'end,thus
check the suitable codon of mRNA.This also catalyzes the assembly of amino acids in
polypeptide chains.It even binds with tRNA and the accessory protein for protein
manufactures according to the information present by mRNA(rRNA).(:Lodish,2008).
General functions of RNA
In RNA it contains uracil base , which is not stable as compare to thymine ,so RNA
easily involed in folding thus ,result in forming secondary structure(single stranded helix).when
it is folding ,uracil gets pair with adenine and so stabilized the secondary structure thus, helps in
smooth flow of genetic information in the correct sequences of amino acid during protein
synthesis.It also prevent the renaturation of helix due to highly reactive of uracil and abscent of
methyl group.so living cell got immediate information(Weaver,2002). RNA being Ss polymer it
bend /fold back on itself to form weak bond with another part of same molecule due its
flexibility of back bond,then recognized particular shape base on exact sequence.Than selective
binding occur ,thus result in flow of genetic information during protein synthesis.In RNA, OH
group is present on C-2' which makes chemically laible by hydrolysing the phosphodiester bond
at neutral pH.This lead to mononucleotides.OH group also provide chemical reactive group in
catalysis.Such catalytic roles are play by rRNA in peptide bond formation especially during
protein synthesis, finally accurate flow of information(Lodish,2008). The main task for RNA material is to manufacture the protein through two process called transcription and translation involving all three types of RNA to do the job differently but they function cooperatively towards common goal-protein synthesis (Becker,etal,2007).mRNA carries the genetic information from DNA for it orderly and specific sequences of amino to new protein. rRNA serves as structural component in ribosome and tRNA carry the activated amino acids to specified sites on the mRNA template(Conn &Stumpf,1976).