Question

What are Nucleic acids . Write -
1> Structure of Nucleic acid
2> Chemical formula of nucleic acid.

Also write a note on Galactose , lactose and maltose ?

Answer 1

Nucleic acids are complex organic substances present in living cells, especially DNA or RNA, whose molecules consist of many nucleotides linked in a long chain.

Nucleic acids are of two types namely, DNA and RNA. DNA is Deoxy Ribonucleic acid, it contains deoxy ribose sugar, phosphate and bases like adenine, guanine, thymine and cytosine whereas RNA, ribonucleic acid, contains ribose sugar, phosphate and bases like
Adenine, guanine, uracil and cytosine.
STRUCTURE OF DNA:
Structure of DNA studied under four different headings. They are
i) Primary structure
ii) Secondary structure
iii) Tertiary structure
iv) Quaternary structure

i) Primary structure:

primary structure of DNA is formed by the covalent backbone consisting of deoxyribo nucleotides linked to each other by phosphodiester bonds. DNAs are long chains of nucleotide units or polydeoxyribonucleotides. The substrates for polymerization are nucleoside triphosphates, but the repeating unit or monomer, of a nucleic acid is a monophosphate. During polymerization, the 3’-OH group of the terminal nucleotide residue in the existing chain makes a nucleophilic attack upon the alpha phosphate of the incoming nucleoside triphosphate to form 5’à3’ phosphodiesterbond. This reaction is catalyzed by DNA polymerase. Serial polymerization generates long polymers variously called chains or strands, containing an invariant sugar-phosphate backbone with 5’à3’ polarity and projecting nitrogenous base.
The most importamt clue to the DNA structure came from the discovery made by Erwin charagaff and etal in 1940s. series of experiments conducted by them lead to postulate a series of results are called as charagaffs’ rule. They are
a) DNA specimens isolated from different tissues of the same species have the same base composition.
b) Dna base composition varies from one species to another species.
c) In a given species, the base composition of DNA does not change with age, nutritional states or change in environoment
d) The number of adenine residues in all DNAs irrespective of species is equal to the number of thymine residues(A=T) and the number of guanine residues equal to the cytosine residues(G=C).
From this, the following relation derived
Purines = Pyrimideines;
A+G = T+C;
ii) SECONDARY STRUCTURE:

James Watspm and Francis Crick postulated the secondary structure of DNA in 1953 in their double helical model. They studied the x-ray diffraction pattern of DNA made by Rosalind Franklin and Maurice Wilkins and arrived at the double helical structure DNA. Features of double helical structure is as follows:
i) DNA molecule present as an alpha helical structure and if it is double stranded structure. Here, the relatively hydrophobic bases are present inside the molecule which polar sugar and phosphoric acid molecules are present on the outside. This shields hydrophobic bases from water and made hydrophilic sugar and phosphoric acid molecu
le to interact with water.

i) The two polynucleotide strands can be separated froom each other only by unbounding or uncoiling such a coiling of the two strands is called plateneomic coiling.
ii) Two polynucleotide chains are associated by hydrogen bonds between bases. The double helical structures is stablised by hydrophobic interactions between adjacent bases brought about by electrons in pi rings. There are two hydrogen bonds between adenine and thymine. Three hydrogen bonds present between guanosine and cyotosine.
Pairing between two purines and two pyrimidine bases does not occurs. The diameter of the strands only 2nm. Two purines would be too large to be accommodate within the space. Two pyrimidines would be too small and the hydrogen bonding between two pyrimidines would be too weak.
iii) Co planarity of the bases:
Bases in the same plane are straggled one above the other perpendicular to the central axis.
iv) Complementarities of the bases:
The two strands are exactly complementary with each other. Thus if the sequence of bases of one strand is known, the sequence of the other strand can be easily written down.
v) Periodicity:
Ten mononucleotides are present in one turn of the helix giving periodicity of 3.4nm along the central axis of the DNA. The distance between two nucleotide is 0.34nm. the spatial relationship between the two strands create the major groove and minor groove.
vi) anti parallel polarity:
Two strands are antiparallel to each other and run in opposite direction i.e. 3’à5’ and 5’à3’



Answer 2

Nucleic acids are large polymers formed by the repetition of monomers called nucleotide, bound by phosphodiesterase bonds. Long chains are formed; Some nucleic acid molecules reach giant sizes of millions of nucleotide in chains. There are two basic types, DNA and RNA.

The discovery of nucleic acids is due to Johann Friedrich Miescher, who in 1869 isolated nuclei from cells an acidic substance called nuclein, a name that was later changed to nucleic acid. Subsequently, in 1953, James Watson and Francis Crick discovered the structure of DNA, using the X-ray diffraction technique.


The nucleic acid structure refers to the morphology of nucleic acids such as DNA and RNA. Details of the structure of the nucleic acids allowed to reveal the genetic code. Usually, this structure developed by the model of James Watson and Francis Crick is divided into four different levels:

♦ The primary structure, which is the sequence of nitrogenous bases of each of the strands that make up the DNA.
♦ The secondary structure, which is the set of interactions between the nitrogenous bases, ie, which parts of the chains are linked to each other.
♦ The tertiary structure, the location of atoms in three-dimensional space, taking into account the geometric and steric constraints.
♦ The quaternary structure, which is the highest-level organization of DNA in chromatin, or interactions between RNA units in the ribosome or splice.

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Simple sugar is known as Galacatose. Galacatose are from the simple carbohydrates. Its Chemical Formula is C6H12O6. 

Lactose is any of a class of sugars whose molecules contain two monosaccharide residues composed of glucose and galacatose. Its chemical Formula is C12H22O11.

Maltose is a sugar produced by the breakdown of starch, e.g., by enzymes found in malt and saliva. It is a disaccharide consisting of two linked glucose units. Its Chemical Formula is C12H22O11.