85% From NCERT AIPMT - 2008
1. Thymine, Uracil - Pyrimidines
2. Uracil, Cytosine - Pyrimidines
3. Guanine, Adenine - Purines
4. Adenine, Thymine - Purines
Ans:4. Adenine, Thymine - Purines
- 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
polynucleotides. 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 Figure 9.1, 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 which contains ribose is called
ribonucleic acid (RNA).
- PLANT GROWTH REGULATORS
The plant growth regulators (PGRs) are small, simple molecules of diverse
chemical composition. They could be indole compounds (indole-3-acetic
acid, IAA); adenine derivatives (N6-furfurylamino purine, kinetin),
derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid,
GA3) or gases (ethylene, C2H4). Plant growth regulators are variously
described as plant growth substances, plant hormones or phytohormones
in literature.
- Cytokinins
Cytokinins have specific effects on cytokinesis, and were discovered as
kinetin (a modified form of adenine, a purine) from the autoclaved herring
sperm DNA. Kinetin does not occur naturally in plants. Search for natural
substances with cytokinin-like activities led to the isolation of zeatin from
corn-kernels and coconut milk. Since the discovery of zeatin, several
naturally occurring cytokinins, and some synthetic compounds with cell
division promoting activity, have been identified. Natural cytokinins are
synthesised in regions where rapid cell division occurs, for example, root
apices, developing shoot buds, young fruits etc. It helps to produce new
leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot
formation. Cytokinins help overcome the apical dominance. They promote
nutrient mobilisation which helps in the delay of leaf senescence.
- The salient features of the Double-helix structure of DNA are as follows:
(i) It is made of two polynucleotide chains, where the backbone is
constituted by sugar-phosphate, and the bases project inside.
(ii) The two chains have anti-parallel polarity. It means, if one
chain has the polarity 5'à3', the other has 3'à5' .
(iii) The bases in two strands are paired through hydrogen bond
(H-bonds) forming base pairs (bp). Adenine forms two hydrogen
bonds with Thymine from opposite strand and vice-versa.
Similarly, Guanine is bonded with Cytosine with three H-bonds.
As a result, always a purine comes opposite to a pyrimidine. This
generates approximately uniform distance between the two
strands of the helix (Figure 6.2).
(iv) The two chains are coiled in a right-handed fashion. The pitch
of the helix is 3.4 nm (a nanometre is one billionth of a
metre, that is 10^-9 m) and there are roughly 10 bp in each
