Biomolecules: Structure of Nucleic Acids and Mechanism of Enzyme Action (For CBSE, ICSE, IAS, NET, NRA 2022)

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Structure of Nucleic Acids

Like all-natural molecules, nucleic acids are linear polymeric molecules. They are chain like polymers of thousands of nucleotide units; hence they are also called polynucleotides. A nucleotide consists of three subunits: a nitrogen containing heterocyclic aromatic compound (called base) , a pentose sugar and a molecule of phosphoric acid.

Structure of Nucleic Acids
  • In DNA molecules, the sugar moiety is 2-deoxyribose, whereas in RNA molecules it is ribose. In DNA, four bases have been found.
  • They are adenine (A) , guanine (G) , cytosine (C) and thymine (T) .
  • The first three of these bases are found in RNA also but the fourth is uracil (U) .


  • Vitamins are small organic molecules which are taken in diet and there are required in trace amounts for proper growth.
  • Vitamins can be fat soluble or water soluble.

A list of important vitamins and their deficiency symptoms is given below.

A List of Important Vitamins and Their Deficiency Symptoms is Given Below
VitaminDeficiency Symptom
A. Water Soluble
a) Ascorbic Acid (Vitamin C)Bleeding gums, Bruising
b) Thiamin (Vitamin B1)Fatigue, Depression
c) Riboflavin (Vitamin B2)Cracked lips, Scaly skin
d) Pyridoxine (Vitamin B6)Anemia, Irritability
e) NiacinDermatitis, Dementias
f) Folic acid (Vitamin M)Megaloblastic Anemia
B. Fat Soluble
a) Vitamin ANight blindness, dry skin
b) Vitamin DRickets, osteocalcin
c) Vitamin EHemolysis of RBCs
d) Vitamin KHemorrhage, delayed blood clotting


In a living system, many complex reactions occur at the temperature of about 310K. An example of this is the digestion of food, during which stepwise oxidation to and water and energy production. These reactions are carried out under such mild conditions due to presence of certain chemicals which are called enzymes.

Mechanism of Enzyme Action

There is a particular enzyme for each substrate, and they are said to have lock and key arrangement. It is said that first the substrate molecule binds to the active site of the enzyme which results in the formation of an enzyme-substrate complex. In this complex, the substrate is placed in the right orientation to facilitate a given reaction. This complex then breaks to give the molecule of the product and regenerates the enzyme for the next molecule of the substrate.

Mechanism of Enzyme Action

Characteristics of Enzymes

  • Enzyme catalysed reactions rapidly attain equilibrium.
  • Enzymes function in dilute aqueous solutions, at moderate temperatures and at a specific pH.
  • They are very specific and selective in their action on substrates.
  • Enzymes are highly efficient and are needed in small amounts only.
  • Enzymes speed up biochemical reactions up to ten million times compared to the uncatalyzed reaction.
  • In addition to the protein structure, most active enzymes are associated with some non-protein component required for their activity, called coenzymes.


Hormones are chemical messengers which are secreted by endocrine glands. They are carried through the blood stream to the target tissues.

  • The two important classes of steroid hormones are sex hormones and adrenocortical hormones. The sex hormones control maturation, tissue growth and reproduction whereas the adreno-cortical hormones regulate various metabolic processes.
  • Two most important male sex hormones or androgens are testosterone and androsterone.
  • Another important female hormone is progesterone which prepares the lining of the uterus for the implantation of the fertilized ovum.
  • Another hormone called hydrocortisone is involved in the regulation of glucose metabolism and in the control of inflammation.

Two important hormones which are polypeptides in nature are:

  • Oxytocin occurs in females only and stimulates uterine contraction during childbirth.
  • Vasopressin occurs both in males and females and causes contraction of peripheral blood vessels and an increase in blood pressure.

Insulin, another important polypeptide hormone, is secreted from pancreases. It regulates the metabolism of glucose in the body. The deficiency of insulin in human causes diabetes mellitus.

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