1. Definition of Nucleic Acids
Nucleic acids are biopolymers that carry genetic information in cells and are essential for the functioning and replication of all living organisms. The two main types of nucleic acids are Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).
- DNA (Deoxyribonucleic Acid): DNA is the hereditary material in humans and almost all other organisms. It is the blueprint that carries the genetic instructions used in the growth, development, functioning, and reproduction of organisms.
- RNA (Ribonucleic Acid): RNA plays a role in various biological roles, including the transmission of genetic information from DNA to protein (through processes like transcription and translation). RNA can also have enzymatic functions and participate in gene regulation.
Both DNA and RNA are composed of long chains (polymers) of nucleotides, which are the basic structural units of nucleic acids.
2. Structure of Nucleotides
A nucleotide consists of three components:
- A nitrogenous base
- A pentose sugar (deoxyribose in DNA and ribose in RNA)
- A phosphate group
The nitrogenous bases are the most critical components because they carry genetic information.
3. Nitrogenous Bases: Purines and Pyrimidines
The nitrogenous bases of nucleic acids are divided into two categories based on their structure: purines and pyrimidines.
A. Purines
Purines have a two-ring structure: one six-membered ring and one five-membered ring. The purine bases are:
- Adenine (A):
- Found in both DNA and RNA.
- In DNA, it pairs with thymine (T), while in RNA, it pairs with uracil (U).
- Guanine (G):
- Found in both DNA and RNA.
- In both DNA and RNA, guanine pairs with cytosine (C).
B. Pyrimidines
Pyrimidines have a single-ring structure. The pyrimidine bases are:
- Cytosine (C):
- Found in both DNA and RNA.
- In both DNA and RNA, cytosine pairs with guanine (G).
- Thymine (T):
- Found only in DNA.
- In DNA, thymine pairs with adenine (A).
- Uracil (U):
- Found only in RNA.
- In RNA, uracil pairs with adenine (A).
4. Base Pairing in DNA and RNA
- In DNA, the base pairs are:
- Adenine (A) pairs with Thymine (T) (through two hydrogen bonds)
- Cytosine (C) pairs with Guanine (G) (through three hydrogen bonds)
- In RNA, the base pairs are:
- Adenine (A) pairs with Uracil (U) (instead of thymine)
- Cytosine (C) pairs with Guanine (G)
5. Importance of Purine and Pyrimidine Bases
- Genetic Information: The sequence of purine and pyrimidine bases in a DNA or RNA strand encodes the genetic instructions needed to build and maintain an organism. This sequence is read during transcription and translation to produce proteins.
- DNA Replication: In DNA, the complementary base pairing between adenine-thymine and cytosine-guanine ensures that DNA can replicate accurately, with one strand serving as a template for the synthesis of the other.
- RNA Functions: In RNA, the uracil replaces thymine, allowing RNA to carry genetic information from DNA to ribosomes for protein synthesis.
Summary Table of Purine and Pyrimidine Bases
| Type of Base | Base | Found In | Pairs With | Structure |
| Purine | Adenine (A) | DNA, RNA | Thymine (T) in DNA, Uracil (U) in RNA | Two-ring structure |
| Guanine (G) | DNA, RNA | Cytosine (C) | Two-ring structure | |
| Pyrimidine | Cytosine (C) | DNA, RNA | Guanine (G) | Single-ring structure |
| Thymine (T) | DNA | Adenine (A) | Single-ring structure | |
| Uracil (U) | RNA | Adenine (A) | Single-ring structure |