Nucleic Acids

 Nucleic Acids history:

Nucleic acid was first isolated in 1868 by a Swiss physician Friedrich Miescher from the nuclei of pus cells. He called it nuclein. In 18i4, Oskar Hertwig wrote " nuclein is the substance responsible for the transmission of hereditary characters". Nuclein was renamed nucleic acid by Altman in 1889.He discovered the existence of two types of nucleic acids.

 The other type of macromolecule that one would find in the acid insoluble fraction of any living tissue is the nucleic acid. These are polynueotides. Together with polysaccharides and polypeptides these comprise the true macromolecular fraction of any living tissue or cell. For nucleic acids, the building block is a nucleotide. A nucleotide has three chemically distinct components. One is a heterocyclic compound, the second is a monosaccharide and the third a phosphoric acid or phosphate.

      As you notice in  the heterocyclic compounds in nucleic acids are the nitrogenous bases named adenine, guanine, uracil, cytosine, and thymine. Adenine and Guanine are substituted purines while the rest are substituted pyrimidines. The skeletal heterocyclic ring is called as purine and pyrimidine respectively. The sugar found in polynucleotides is either ribose ( a monosaccharide pentose) or 2' deoxyribose. A nucleic acid containing deoxyribose is called deoxyribonucleic acid ( DNA)while that contains ribose is called Ribonucleic acid ( RNA).

Location of Nucleic Acids:

The nucleic acids are strong acids and were originally found in the nucleus, hence their name. They are now known to occur in the mitochondria, plastids and cytoplasm as well.They also occur in bacteria that lack nucleus and in viruses too.

Composition of Nucleic Acids:

The nucleic acid are giant molecules having complex structure and very high molecular weights. The nucleic acid molecule is composed of a large number of nucleotide molecules joined into a linear, unbranched chain. The nucleic acids are, thus, nucleotide polymers or polynucleotides. In a nucleic acid, the phosphate components of one nucleotide is joined by a phosphodiester bond to the sugar component of the adjacent nucleotide at the 3' _ carbon atom.

The alternating sugar and phosphate components form the " backbone" of the nucleic acid molecule, and the nitrogenous base components project on one side from the sugar components.

Nucleic acids have two types of nitrogenous bases: double __ ringed purines and single__ ringed Pyrimidines

and two types of pentose sugars: ribose and deoxyribose.

Function of Nucleic acid :

The nucleic acids store and transfer information needed for the synthesis of specific proteins that determine the structure and functions of the cells.

Types of nucleic Acids:

There are two types of nucleic acids: 
1)Deoxyribonucleic acid or DNA 

2): Ribonucleic acid, or RNA .

Each type has subtypes:

1): Deoxyribonucleic Acid (DNA):

DNA is the genetic material and forms molecular basis of heredity in all organisms. In certain viruses, such as tobacco mosaic virus ( TMV), RNA is the genetic material. 

● Location: In prokaryotic cells, DNA occurs in the cytoplasm and is the only component of the prochromosome and plasmids.In eukaryotic cells, DNA is largely confined to the nucleus and is the main component of the chromosomes. It is called nuclear DNA.Each chromosome contains a single DNA molecule extending from one end of the chromosome to the other. The DNA is associated with histone proteins in about 1: 1 ratio to form deoxyribonucleoprotein ( DNP) or chromatin. A small quantity of DNA also occurs in some cytoplasmic organelles, such as  mitochondria and plastids. This is called extracellular or organeller DNA. DNA is present in the chromosome of some viruses too.

● Quantity : The DNA content is fairly constant in all the cells of a given species. Just before cell division, however, the amount of  DNA is doubled. The gametes have half the amount of DNA as they contain half the number of chromosomes. 

● Chemical Composition:  Chemical structure of DNA was explained by P.A. Levene. DNA is the largest macromolecule in the organisms. It is a long, double* chain of deoxyribonucleotide, or deoxyribotide, units.The two deoxy__ ribonucletide chains are twisted around a common axis to form a righthanded double helix ( spiral) that encloses a cylindrical space in it. Each deoxyribonucleotide unit, in turn,consists of three different molecules: phosphate, ( PO³-4), a 5_ carbon deoxyribose sugar ( C5H10O4) and double __ ringed purine, i.e., adenine ( A) or guanine ( G) ; or a 6__ membered, single _ ringed pyrimidine, viz.,thymine (T) or cytosine (C).

Thus, there are 4 types of deoxyribonucleotides in DNA, namely, adenine __ deoxyribose_ phosphate, or adenosine monophosphate ( AMP) ; guanine__ deoxyribose _ phosphate, or guanosine monophosphate ( GMP); cytosine_ deoxyribose_ phosphate, or cytidine monophosphate ( CMP); and thymine_ deoxyribose _ phosphate, or thymidine monophosphate ( TMP). In each chain, the phosphate component carried by the carbon atom at position 5' of the sugar in one nucleotide unit is joined by phosphodiester bond to the hydroxyl component of the carbon atom at positions 3' of the sugar in the next nucleotide unit.These 3', 5'_ phosphodiester bonds provide a considerable stiffness to the polynucleotide.The alternating sugar__  phosphate _ sugar components form the " backbones" of the DNA double helix.The phosphate groups provide acidity to the nucleic acid.The nitrogenous  base molecules are joined to the sugar molecules at the 1' _ carbon position by glycosidic bonds and project into the space enclosed in the helix at about 90% to the long axis of the helix.The nitrogen 9 of a purine and the nitrogen 1 of a pyrimidine join the 1' carbon of the deoxyribose sugar. The two deoxyribonucleotide chains are held together by hydrogen bonds between paired bases, and by van der Waals attractions between the stacked bases.

Adenine of one chain is always joined to thymine of the other chain by 2 hydrogen bonds ( A= T). Cytosine of one chain is always linked to guanine of the other chain by 3 hydrogen bonds ( C= G) . Thus, there are only four possible base pairs: A__ T, T__ A, C__ G and G__ C in the DNA molecule. 

● Levels of Structure:  A DNA molecule shows primary, secondary and tertiary structure like a protein molecule, being coiled in three orders for accommodation in a small space.

● Polarity:  The polynucleotide chains show polarity ( direction).One end of each DNA strand is called roman fat 5' end. The last deoxyribose unit at this end has the carbon at position 5 free. The other end of the strand is termed 3' end. The last deoxyribose unit at this end has the carbon at position 3 free.

● Variety: Although only four types of base pairs are involved in the formation of DNA molecule, these base pairs may occur in any sequence, and there may be any number of sequences in a molecule. This gives an infinite variety to the DNA molecule. However, each individual has a specific sequence of base pairs in its DNA.

Types of DNA:

DNA molecules are of 2 types:

1):Linear  Linear DNA is found in the nuclei of eukaryotic cells. It is associated with proteins in about 1: 1 ratio. 

2): Circular: Circular DNA is found in many viruses, all prokaryotic cells, and in the mitochondria and chloroplasts ( plastids) of eukaryotic cell.It is associated with very little protein.

● Complementarity of Base Pairs: 

The two nucleotide chains of DNA molecule are not identical, but complementary to each other with respect to base pairs. This is due to restrictions on base pairing. A base pair must consist of a purine and a pyrimidine. There are two reasons for this limitation.

(i) The space available between the two sugar__ phosphate chains of DNA, i.e., 20°A (2nm), can accommodate one purine and one pyrimidine, but not two purines, which would be too large, and not two Pyrimidines, which would not be close enough to form proper hydrogen bonds.It may be noted that purine bases, with carbon rings, are about  twice as wide as Pyrimidines bases.

(ii) A specific purine pairs with a specific pyrimidine because of a perfect match between hydrogen donor and acceptor sites on the two bases. Only adenine and thymine and also guanine and cytosine have the proper spatial arrangement to form correct hydrogen binding. Adenine and cytosine, and also guanine and thymine do not form proper hydrogen bonds and cannot pair.

  The concept of complementary base pairing provides that an adenine in one chain must be matched with a thymine in the other chain ; and a guanine in one chain must have cytosine opposite to it in the other chain.Thus, the two chains are complementary to each other. Hence,from the base sequence in one DNA chain, the bass sequence in its complementary chain can be easily predicted. For example, if the base sequence in one chain in A,T,C,G,G,T, A that in the other chain will be T,A,G,C,C,G,A,T.

   Base Pair: The term " base pair" refers to two bases, one in each chain of DNA molecule, joined together by hydrogen bonds.Each base pair consists of one 2__ ringed purine and one 1_ ringed pyrimidine. Therefore, all the base pairs have equal width, and the DNA helix has a constant diameter. 

● Antiparallel Direction: 

The two chains of DNA molecule run in opposite or antiparallel directions. This means that the carbon atom at position 5 in the sugar component is in one direction in one chain and in the opposite direction in the other chain between paired bases, and  by van der Waals attractions between the stacked bases.

Thus,the two chains are 
parallel but their 5 prime~ symbol ➡️~ roman3' directions are opposite. This is analogous to a 2__ lane road, where the lanes run parallel but carry traffic in opposite directions.

Basis For DNA Replication:

The complementarity of bases in DNA molecules and antiparallel directions of the two chains of DNA molecule provide the basis for the precise replication of DNA.

● Chargaff's Rules:In 1950, E.E. Chargaff formulated important generalizations about DNA structure. These generalizations are called Chargaff's rules in his honour. They are summarized below__ 

(i) The DNA molecule, irrespective of its source, always has the A__ T base pairs equal in number to the G__ C base pairs.

(ii) The purines and pyrimidines are always  in equal amounts, i.e., A+ G= T+ C

(iii) The amount of adenine is always equal to equal to that of thymine, and the amount of guanine is always equal to that of cytosine, i.e., A= T and G= C.

(iv) The base ratio A+ T/ G+ C may vary from one species to another, but is constant for a given species. This ratio can be used to identify the source of DNA, and can help in classification.

(v) The deoxyribose sugar and phosphate components occur in equal proportions. 

● Denaturation and Renaturation: 

If DNA molecule is exposed to high temperature or titration with an acid or an alkali, the two strands unwind and separate by breakdown of hydrogen bonds between the base pairs.This process is called denaturation ¹,or melting. When denatured DNA is incubated at a low temperature, the two separated strands reassociate to form a DNA duplex. This process is termed renaturation².

● Physical Structure:

Astbury, Wilkins and Franklin have suggested 3__ dimensional, helical configuration for DNA molecule by X__ ray diffraction studies. The purines and Pyrimidines are flat rings stacked one above the other,and arranged at right angles to the sugar _ phosphate backbones.The adenine_thymine and guanine _ cytosin base pairs are precisely of the same size and shape. Therefore, the double helix has a constant diameter of 20 A° along its entire length. Its  one complete turn is 34 A° long and has 10 helix has a constant diameter of 20 A° along its entire length.Its one complete turn is 34 A° long and has 10 base pairs. The successive base pairs are 3__ 4 A° apart. These investigations helped Watson and Crick to design a model of DNA molecule. 

Watson __ Crick Model:

J.D.Watson, an American biologist, and F.H.C. Crick , and English chemist, in 1953 suggested a model of  DNA molecule to explain its structure. Their paper consisted of just 2 pages but it became a cornerstone of the molecular biology. Their model got them the 1962 Nobel Prize. According to Watson _ Crick model, the DNA molecule consists of two long, parallel chains ( strands) which are joined together by short crossbars at regular intervals. The two chains are spirally coiled around a common axis in a regular manner to form a double helix. The double helix is of  constant diameter and has a major groove about 22 A° wide alternately. The bases face the interior of the double helix whereas the sugar and phosphate components form backbones on the outside.The helix is generally right_ handed, that is, the turns run clockwise looking along the helical axis.

  In other words, the DNA molecule has the form of a twisted ladder.The vertical bars of the ladder are formed of alternating phosphate and deoxyribose sugar components. The horizontal rungs of the ladder are joined to the sugar components of the vertical bars, and are composed of purines and Pyrimidines linked by hydrogen atoms.

● DNA Forms: Five forms of DNA have been reported: A, B, C, D and Z. The A, B, C, and D forms are right _ handed double helices. The B_ DNA occurs under the physiological conditions prevailing in the living cells. It has the specification given in the foregoing description of DNA. The A_ DNA has 11 base pairs per turn of the helic. The C_ DNA has 9 base pairs per turn of the helix, and D_ DNA has only 8 base pairs. The Z_ DNA is left _ handed, contains 12 base pairs per turn of the helix, each turn is 45 A° long, and its sugar _ phosphate backbone follow a zig_ zag path along the helix. The last feature gives the Z_ DNA its name.

Prokaryotic DNA Versus Eukaryotic DNA  

Prokaryotic DNA 

1): Occurs in  the cytoplasm. 

2): Much less in amount than in eukaryotic cells.

3): Circular in form.

4): Has little protein associated with it.

5): Can code for fewer proteins.

6): Denatures into a tangled mass.

7): There is little nonfunctional DNA.

8): No noncoding introns exist.

9): No organeller DNA. 

10): Exist as a single molecule ( chromosome).

11): Additional DNA may occur as plasmids.

Eukaryotic DNA 

1): Occurs in the nucleus, mitochondria and plastids.

2): Much more in amount than in prokaryotic cells.

3): Linear in form in the nucleus, circular in mitochondria and plastids.

4): Nuclear DNA has proteins associated with it, extranuclear DNA is without proteins. 

5): Can code for far more proteins. 

6): Nuclear DNA denatures into two distinct strands, extranuclear DNA denatures into a tangled mass.

7): Greater part of DN is nonfunctional. 

8): Noncoding introns occur between coding exons.

9): Organeller DNA present in mitochondria and plastids. 

10): Exists as 2 to many molecules ( chromosomes).

11): No plasmids occur. 

Genetic Code: 

The particular sequence of bases in each polynucleotide chain is known as the primary sequence of DNA. This primary sequence forms the genetic code, i.e.,the information required by  the cell to synthesize all the specific proteins it needs. The primary sequence of DNA shows unlimited variation, and this is the basis of huge variety seen in the living systems.

  A DNA molecule include many genes, each with a specific sequence of nucleotides. A sequence AGGTAACCT codes for one protein and the sequence CGCCTTAAC codes for a different protein ( real genes are very long, may have hundreds to thousands of nucleotides).

● Sense and Missense DNA Chains: The genetic information exists in the base sequence of one of the two chains of DNA molecule. This chain is often called sense chain.Its complementary chain is termed missense chain, or antisense chain. The missense chain is important in the replication of DNA molecule in cell division, but does not take part directly in transcription. It is copied from the sense chain.

● Noncoding DNA : Greater part of DNA in eukaryotic cells does not code for RNAs. This "extra",or  noncoding DNA seems to have no function. It has two special forms: 

(i) Repetitious DNA  The noncoding  DNA has many base repeated several times. The repeated sequences are collectively called repetitius DNA.

(ii) Jumping Genes  Some repetitive DNA sequences are not found at fixed sites in the DNA of different individuals of the same species. Such " mobile" DNA segments are often referred to as " jumping genes". They cause mutations and, thus, have a role in evolution.  However, they usually have no role in the life of the individual. 

    The existence of nonfunctional DNA indicates that the eukaryotes use only a small fraction of their total DNA.

  Bacteria have little nonfunctional DNA. 

● Satellite DNA : Part of DNA having long stretches of repetitive base pairs is called satellite DNA. A satellite DNA having a few ( 1__ 6) base pair repeats is termed microsatellite, and that with more ( 10__60) base pair repeats is known as  minisatellite. The minisatellites are highly variable and are specific for each individual. These help in DNA matching for identification of persons. Minisatellites were first discovered by Jaffreys et al in 1985.

● Palindromic DNA: It is a part of DNA in which the base sequence of one strand is opposite to that of the other strand when read opposite directions.  DNA regions that transcribe rRNA are often palindromic. However,  the true Significanc of palindromic DNA is not clear.

  3' __C __G__G__A__A__T__T__C__C__G__5'

  5' __G__C__C__T__T__A__A__G__G__C__3'

     Human body cells normally have 46 chromosomes with a total of 1.6 billion base pairs. The DNA of 46 chromosomes of a single cell has a total length of about 2 meters. The chromosomes of a dividing cell are ten thousand times shorter this. The proteins cause the packing of DNA molecules so tightly. 

● In Vitro Synthesis of DNA  In 1961, Kornberg and his coworkers synthesised DNA from a mixture of all the four types of deoxyribonucleoside triphosphates, DNA polymerase enzyme, metal ions and a single strand of viral DNA. The latter was separated from  its complementary strand by denaturation. It  acted as a template.

●Function of DNA:

The DNA plays a multiple role__

1): It is the genetic material and carrier hereditary characters from parents to the young ones. This is achieved through its unique property of  replication.

2): It enables the cell to maintain, grow and divide by directing  by synthesis of structural proteins. 

3): It controls metabolism in the cell by directing the formation of necessary enzymatic proteins.

4): It produces RNAs by transcription for use in protein synthesis. 

5) It creates variety in population by causing recommendations through crossing over.

6): It contributes to the evolution of the organisms by undergoing gene mutations ( changes in the base pairs).

7): It brings about differentiation of cells during development. Only certain genes remain functional in particular cells. This enables the cells having similar genes to assume different structure and function. 

8): It controls the postnatal development through adulthood to death by its " internal clock".
Thus, DNA is the  very basis of life .

Summary of DNA Characteristics:

1): DNA molecule is a double chain of deoxyribonucleotide units and has a uniform diameter. 

2): The successive units are joined by phosphodiester bonds in each chain ( strand)

3): The sugar __ phosphate __ sugar __ phosphate backbones are located on the outside of the molecule, and nitrogenous bases project inward.

4): The bases project inward at planes approximately perpendicular to the long axis of the molecule, and are, therefore, stacked one above the other.

5): The two chains are spirally coiled around a common axis to form a regular, right __ handed double helix.

6): The helix has a major groove and a minor groove alternately.

7): The helix is 20 A° wide; it's complete turn is 34 A° long, and has 10 base pairs; and the successive base pairs are 3.4 A° apart.

8): The two chains are complementary to each other with respect to base sequence. 

9): The two chains are hydrogen bonded: A on one chain is joined to T on the other chain by 2 hydrogen bonds; C on one chain is linked to G on the other chain by 3hydrogen  bonds.

10) The two chains are antiparallel. One aligned in 5' ➡️3' direction, the other in 3'➡️5' direction. 

11): The amount of A+ G = the amount of T+C; the amount of A= the amount of T; and the amount of G= the amount of C. Sugar and phosphate group occur in equal proportion. 

12): The DNA molecule is remarkable stable due to hydrogen bonding and hydrophobic reactions. 

13): The DNA molecule can replicate itself and can also transcribe RNAs.

14): The base sequence of one chain ( sense chain) of DNA serves as the genetic code.

15): The amount of DNA per nucleus is constant in all the body cells of a given species.

16): DNA can easily undergo denaturation and renaturation. 

17): DNA can be synthesized in vitro.

18): Only a small fraction of DNA is functional in eukaryotes.

19): Eukaryotic DNA has many repeated base sequences, some of which are " mobile".

20): DNA is dextrorotatory. 

21): DNA code is discontinuous, having non__ coding segments ( introns) between coding segments ( exons).

22): Each strand of DNA molecule has polarity with 3' and 5' ends.

Ribonucleic Acid (RNA):

● Location: In a prokaryotic cell, whole of the RNA is found in the cytoplasm because there is no nucleus. In a eukaryotic cell, bulk of RNA occurs in the cytoplasm, and a small amount in the nucleus. It is mainly confined to the ribosomes in  the cytoplasm and to the nucleolus in the nucleus.

● Quantity:  The amount of RNA in a cell varies with nutritional states, types of tissue, etc.

● Composition:  RNA molecule is a single chain of ribonucleotide, or robotide ,units.A ribonucleotide unit further consists of 3 different molecules: phosphate, ribose sugar and nitrogenous base.The nitrogenous base may be a purine ( adenine or guanine) or a pyrimidine ( cytosine or uracil).Thus, there are four types of ribonucleotides in RNA, namely adenine _ ribosephosphate, or adenosine monophosphate ( AMP); guanine _ ribose__ phosphate, or guanosine monophosphate ( GMP); cytosine _ ribosephosphate, or cytidine monophosphate ( CMP)  and uracil_ ribose _ phosphate, or uridine monophosphate ( UMP). In an RNA chain, the phosphate group at 5' position of sugar molecule of the next ribonucleotide at 3' position by phosphodiester bond. This forms a " backbone" of alternating sugar _ phosphate _ sugar molecules. The nitrogenous base molecules are joined to the sugar molecules at carbon 1' position of this " backbone " by glycosidic bonds and project on its  one side .purines and Pyrimidines do not occur in equal amounts in the RNA.

● Variety:  The four nitrogenous bases ( A,U,C,G) found in RNA may occur in any sequence, and there may be any number of sequences in a molecule. This gives an infinite variety to RNA molecule.

Types of RNA:

There are three major types of RNA in every cell: messenger RNA ,or mRNA;ribosomal RNA,or rRNA; and transfer RNA, or tRNA. The three types of RNAs are synthesized from different regions of the DNA template. Only one DNA strand, called the sense strands, is apparently for transcription. The basis for selection is not known.In eukaryotic cells, the RNAs are formed in the nucleus and pass into the cytoplasm through nuclear pores.In prokaryotic cells, RNA is released by the nucleoid directly into the cytoplasm after synthesis. All the three types of RNAs are transcribed in both prokaryotes and eukaryotes as precursor molecules,which must undergo changes, called processing, to become fit for their function. 

  RNA chain is complementary to the DNA sense strand which produces it. Thus, from the base sequence in DNA strand, the base sequence in RNA chain can be easily predicted. For example, if the base sequence in DNA strand is ATCGGCTA, the base sequence in RNA will be UAGCCGAU.

(a) Ribosomal RNA: 

The rRNA molecule is greatly coiled. In combination with proteins, it forms the small and large subunits of the ribosomes, hence its name. It forms about 80% of the total RNA of a cell.Protein synthesis occurs on the ribosomes

 (b) Messenger RNA:

The mRNA molecule is linear. It carries message ( information) from DNA about the sequence of particular amino acids to be joined to form a polypeptide, hence its name. It is also called informational RNA, or template RNA. It forms about 5% of the total RNA of  a cell. Its molecule is the longest of all the RNA types. Its length is related to the size of the polypeptide to be synthesized with its information. There is a specific mRNA for each polypeptide. Because of the variation in size in mRNA population in a cell, the mRNA is often called heterogeneous nuclear RNA, or heRNA.One mRNA molecule normally specifies a single polypeptide chain and is said to be monocistronic. It occurs in the eukaryotes. In some case,mostly prokaryotes, one mRNA can specify more than one polypeptide, and is described as polycistronic.

  An mRNA has at its 5' end a methylated cap, a small noncoding region and an initiation  codon and carries at its 3' end a termination coden, a small noncoding region and a polyA tail.

(c) Transfer RNA :

It has many varieties. Each variety carries a specific amino acid to the messenger RNA on the ribosomes to form a polypeptide. The tRNA forms the remaining 15% of the total RNA of a cell. Its molecule is the smallest of all the RNA types, and is folded to assume the form of a clover leaf. It contains some nucleotides having unusual bases. A tRNA molecule has four regions:

 ( i) carrier end (3' end) where amino acid joins for incorporation into a polypeptide, it carries CCA_ OH group;

(ii) Recognition end which is opposite to the carrier end and bears an antipodean that binds to a matching codon of mRNA;

(iii) Enzyme site on one side for the binding of an enzyme; and 

(iv) Ribosome site on the other side for attachment to  ribosomes. 

● Other  RNAs: There are three other forms of RNA from their functional point of view : 

1)Genetic or genomic 
2): Small nuclear RNA or snRNA 

3): Small cytoplasmic RNA or scRNA .

1): Genetic ( Genomic ) RNA: It occurs in certain viruses known as riboviruses.It may be single_ stranded ( tobacco mosaic virus or TMV) or double _ stranded ( rheovirus). Genomic RNA may form a DNA copy of it which then transcribe RNA copies of it's kind ( human immunodeficiency virus or HIV), or synthesize new genomic RNA on itself ( TMV).

2): Small Nuclear RNAs ( snRNAs):  These stay in the nucleus and held in processing RNAs.

3): Small Cytoplasmic RNAs ( scRNAs):These come out into the cytoplasm and join  protein molecules to form signal recognition particles. The latter help in the synthesis of secretory proteins. 

● Functions: RNA plays a multiple role in cells:

(i) It brings about protein synthesis in the cell. All the three types of RNAs play a role in this process.

(ii) RNA is the genetic material of certain viruses. 

(iii) The rRNA is a component of 

ribosomes,the site of protein synthesis. 

(iv) Some RNA is associated with chromatin 

fibres during  interphase. It initiates replication of DNA.

(v)RNA primer is essential for starting replication of DNA.

( vi) Folding of prokaryotic DNA into a nucleoid needs the help of RNA.

( vii) Certains RNAs act as enzymes, e.g., ribonuclease.

Differences Between RNA and DNA ?


1): It contains ribose sugar.

2): It lacks thymine. 

3): It has uracil.

4): It is a single chain of polynucleotides.

5): Its quantity varies in different cells.

6): It mainly occurs in the cytoplasm.  A small quantity is found in the nicleus.

7): It is formed by DNA and can not replicate itself.

8): It is mainly a component of ribosomes.

9): It guides protein synthesis. It also initiates replication. 

10)  It is of 3 main types: mRNA, tRNA and rRNA.

11): Its molecule is relatively short ( 70 _ 12000 nucleotides) and molecular weight is relatively low.

12): It is a genetic material in certain viruses. 

13): It may contain unusual bases in addition to the normal ones.

14): Adenine and uracil are not necessarily in equal amounts,  nor are cytosine and guanine necessarily in equal amounts. 

15): No primer is needed for transcription of RNA.

16): RNA forms ribosomes. 

17): It is hydrolysed by enzyme RNA _ ase.

18): The mRNA translates its information into polypeptides. 

19) RNA does not show helical coiling.

20): No primer is needed for its transcription. 


1): It contains deoxyribose sugar.

2): It has thymine. 

3): It lacks uracil.

4): It is a double chain of polynucleotides. 

5): Its quantity is fixed in each cell of a species, except gametes. 

6): It mainly occurs in the nucleus. A small quantity is found in mitochondria and plastids.

7): It can replicate itself. 

8): It is the main constituent of chromosomes. 

9): It controls structure, metabolism, differentiation, heredity and evolution. 

10): It is of 2 types: linear intranuclear and circular extranuclear. 

11) Its molecule is very long ( millions of nucleotide pairs) and molecular weight is very high.

12) It is a genetic material in all organisms. 

13): It does not contain unusual bases.

14): Amount of adenine is equal to that of thymine, also amount of cytosine is equal to that of guanine.

15): A primer is needed for replication of DNA.

16): DNA forms chromosome or chromosomes. 

17): DNA is hydrolysed by enzyme DNA__ ase.

18): DNA transcribes its genetic information to mRNA.

19): DNA shows regular helical coiling.

20): A primer is needed for its replication. 


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