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  • Cell Structure and Functions
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Nucleic Acids

  • Nucleic acids are long chain macromolecules which are formed by end to end polymerization of large number of repeated units called nucleotides.

  • Nucleic acids show a wide variety of secondary structures.

  • There are two types of nucleic acids- deoxyribonucleic acid or DNA and ribonucleic acid or RNA.


  • DNA or deoxyribose nucleic acid is a helically twisted double chain polydeoxyribonucleotide macromolecule which constitutes the genetic material of all organisms with the exception of riboviruses.  

  • In prokaryotes it occurs in nucleoid and plasmids.this DNA is usually circular. In eucaryotes, most of the DNA is found in chromatin of nucleus.

  • It is linear.

  • Smaller quantities of DNA are found in mitochondria and plastids (organelle DNA).

  • It may be circular or linear. Single-stranded DNA occurs as a genetic material in some viruses.

Sense and antisense strands

  • Both the strands of DNA do not take part in controlling heredity and metabolism. Only one of them does so.

  • The DNA strand which functions as template for RNA synthesis is known as template strand, minus (-) strand or antisense strand.

  • Its complementary strand is named nontemplate strand, plus (+) strand, sense or coding strand.

  • The latter name is given because by convention DNA genetic code is written according to its sequence.

(5`) G C A T T C C G G C T A G T A A C (3’)

DNA Nontemplate, Sense (+) or coding Strand

(3’) C G T A A G C C G A T C A T T G (5’)

DNA Template, Antisense, or Noncoding or (-) Strand

(5`) G C A U U C G G C U A G U A AC(3’)

RNA Transcript

  • RNA is transcribed on 3’ -> 5’ (-) strand (template/antistrand) of DNA in 5←3 direction.

  • The (+) strand of DNA is that coding strand which carries genetic information but is non-template.

Ribonucleic acid (RNA)

  • RNA is a single strand or chain (ds RNA is reported in wound tumour virus, Rice Dwarf virus) which is formed by end to end polymerization of a number of ribonucleotides or ribotides.

  • The polynucleotide is characterized by the presence of ribose sugar and uracil instead of thymine.

  • It is formed through transcription where one strand of DNA acts as a template.

  • This is followed by processing to produce different types of RNAs.

  • A strand of RNA contains 70-12000 ribonucleotides.

  • At places it may appear partially double stranded due to folding of single strand.

  • Four types of ribonucleotides occur in RNA. They are adenosine monophosphate, guanosine monophosphate, uridine monophosphate and cytidine monophosphate.

(1) Ribosomal RNA (rRNA). It is the most abundant RNa (70-80% of total) which has 3-4 types. Some of its types (23S,28S, are the longest of all RNAs. As the name indicates, r RNA is a constitutuent of ribosomes. Here it lies coiled in between and over the protein molecules.


(i) r RNAs bind protein molecules and give rise to ribosomes.

3’  end of 18S r RNA (16S in prokaryotes) has nucleotides complementary to those of cap region of m RNA.

(iii) 5S r RNA and surrounding protein complex provide binding site for tRNA.

(2) Transfer RNA (t RNa). It is also called soluble or sRNA. There are over 100 types of tRNAs.  Transfer RNA constitutesabout15% of the total RNA. tRN is the smallest RNA with 70-85 nucleotides and sedimentationcoefficient of 4S.


(i) tRNA is adapter molecule which is meant for transferring amino acids to ribosomes for synthesis of polypeptides. There are different tRNAs for different amino acids. Some amino acids can be picked up by 2-6 tRNAs. tRNAs carry specific amino acids at particular points during polypeptide synthesis as per cidons of mRNA. Codons are recognized by anticodons of tRNAs. Specific amino acids are recognized by particular activating or aminoacyl synthetase enzymes.

(3) Messenger RNA (mRNA). It is a long RNA which constitutes 2-5% of the total RNA content. it brings instructions from the DNA for the formation of particular type of polypeptide. The instructionsare present in the base sequence of its nucleotides. It is called genetic code. Three adjacent nitrogen bases specify a particular amino acid.

(4) Genetic RNA. It is found in some viruses called riboviruses. genetic RNA may be single stranded (e.g., Tobacco Mosaic Virus or TMV) or double stranded(e.g., Rheovirus). Genetic RNA acts as a hereditary material. It may, however, not replicate directly, but form DNA in the host cell to produce RNA of its own types.

Differences Between DNA and RNA



It usually occurs inside nucleus and some cell organelles.

Very little RNA occurs inside nucleus. Most of it is found in the cytoplasm.


DNA is the genetic material.

RNAis not the genetic material except in certain viruses,e.g.,Reovirus.


It isdouble stranded with the exception of some viruses (e.g., phage f × 174).

RNA is single stranded with the exception of some viruses (e.g., double stranded inReovirus).


DNA contains over a million nucleotides.

Depending upon the type, RNA contains 70-12000 nucleotides.


Molecular weight ranges from 3-4 million in Escherichia coli to 263 million in chromosome 1 of human beings.

Molecular weight ranges from 2500-2,000,000.


It is fuelgen positive.

RNa is Fuelgen negative.


It contains deoxyribose sugar.

It contains ribose sugar.


Nitrogen base thymine occurs in DNa alongwith three others – adenine, cytosine and guanine.

Thymine is replaced by uracil in RNA. The other three are similar – adenine, cytosine and guanine.


It replicate to form new DNA molecules.

It can not normally replicate itself.


DNAconrols metabolism and genetics including variations.

RNA translates the transcribed message for forming polypeptides.


Its quantity is fixed for cell.

The quantity of RNA of a cell is variable.


DNA controls metabolism and genetics including variations.

It only controls metabolism under instructios from DNA.


Purine and pyrimidine bases are in equal number.

There is no proportionality between number of purines and purimidine bases.


It is long lived.

Some RNAs are very short lived while others have somewhat longer life.

Q.1- DNA is a polymer of    [CPMT 1998]    
(a)    Nucleotide    (b)    Nucleoside    (c)    Amino acids    (d)    All of the above    
Q.2- Purines (bases) of DNA are represented by    [CPMT 1973; MP PMT 1986]    
(a)    Uracil and guanine    (b)    Guanine and adenine    
(c)    Adenine and cytosine    (d)    None of these
Q.3- In DNA guanine pairs with    [CPMT 1980; AFMC 1988]    
(a)    Cytosine    (b)    Thymine    (c)    Uracil    (d)    Adenine
Q.4- The number of hydrogen that bound guanine and cytosine    [BHU 1989]    
(a)    1    (b)    2    (c)    3    (d)    4
Q.5- Strands of DNA are bonded by    [CPMT 1980; AIIMS 1989]    
(a)    Hydrogen     (b)    Carbon    (c)    Oxygen    (d)    Nitrogen
Q.6- DNA is concentrated in    [CPMT 1979, 86]    
(a)    Chromatin as DNA-protein complex    (b)    Ribosomes    
(c)    Golgi bodies            (d)    Plastids
Q.7- The similarity between DNA and RNA is that both are    [KCET (Med.) 1999]    
(a)    Double stranded    (b)    Having similar sugars    
(c)    Polymers of nucleotides    (d)    Having similar pyrimidines
Q.8- DNA differs from RNA    [ST 1977; BHU 2003]    
(a)    In the nature of sugar alone        (b)    In the nature of purine alone    
(c)    In the nature of sugar and pyrimidines    (d)    None of these
Q.9- DNA differs from RNA    [MP PMT 1998]    
(a)    In having cytosine but no guanine    (b)    In having thymine but no cytosine    
(c)    In having uracil but no thymine        (d)    In having thymine but no uracil
Q.10- Which one of the following is widely distributed in a cell    [CPMT 1977]    
(a)    DNA    (b)    RNA    (c)    Chloroplast    (d)    Chromoplast





















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