15.10 Nucleic Acids

Nucleic acids are naturally occurring biomolecules that serve as the primary information-carrying molecules in cells, storing and transmitting genetic information.

Types of Nucleic Acids

There are two main types of nucleic acids:

1. Deoxyribonucleic Acid (DNA)
2. Ribonucleic Acid (RNA)

15.10.1 Structural Components of DNA and RNA

Both DNA and RNA are polymers composed of repeating monomer units called nucleotides. In DNA, there are four different types of nucleotides, which differ only by the nitrogenous base attached to the pentose sugar.

Components of a Nucleotide

Each nucleotide consists of three fundamental components:

1. Pentose Sugar: A five-carbon sugar.

   • In RNA, the sugar is ribose.
   • In DNA, the sugar is deoxyribose, where the hydroxyl group ($-OH$) at carbon number 2 of ribose is replaced by a hydrogen atom ($-H$).

2. Nitrogenous Base: These are heterocyclic aromatic compounds.

   • Purines: Contain a two-carbon nitrogen ring structure.
     • Adenine ($A$)
     • Guanine ($G$)
   • Pyrimidines: Contain a one-carbon nitrogen ring structure.
     • Cytosine ($C$)
     • Thymine ($T$) - Found only in DNA
     • Uracil ($U$) - Found only in RNA, replaces Thymine

3. Phosphate Unit: A phosphate group ($P{O}_4^{3-}$) that links the nucleotides together. It is attached to the C-5 position of the deoxyribose sugar in DNA and the ribose sugar in RNA, forming a sugar-phosphate backbone.

15.10.2 Difference between RNA and DNA

The key distinctions between DNA and RNA are summarized in the table below.

15.10.3 Structure of DNA

The structure of DNA was famously discovered by J. Watson and Francis Crick in 1953, a discovery that earned them the 1962 Nobel Prize and initiated the field of molecular biology.

• Double Helix: DNA exists as a spiral structure of two strands twisted around each other, known as a double helix.

• Hydrogen Bonding: The two strands are held together by hydrogen bonds formed between complementary pairs of bases:

  • Adenine ($A$) pairs with Thymine ($T$)
  • Guanine ($G$) pairs with Cytosine ($C$)

• Genetic Code: The specific sequence of base pairs along a strand constitutes the genetic code, which stores the information needed to synthesize proteins. A segment of DNA that carries this information is called a gene.

• Self-Replication: DNA is capable of self-replication, ensuring that an exact copy of the genetic information is passed on to each daughter cell during cell division.

15.10.4 Structure of Ribonucleic Acid (RNA)

RNA typically exists as a single-stranded molecule. It is synthesized by DNA to transport the genetic code from the nucleus to the cytoplasm for protein synthesis.

• Function: RNA is responsible for controlling the synthesis of new proteins by receiving, reading, decoding, and using the genetic information from DNA.
• Protein Synthesis Process:
  1. DNA copies the genetic code into messenger RNA (mRNA).
  2. The mRNA carries this code into the cytoplasm.
  3. Ribosomal RNA (rRNA) and transfer RNA (tRNA) translate the information in the mRNA.
  4. This process synthesizes a chain of amino acids called a polypeptide.
  5. The polypeptide is sent to the Golgi bodies to be processed into a functional protein.

Possible Questions/Answers

Q: Write the names of the structural components of DNA and RNA. A: Both DNA and RNA are made of three components:

1. A pentose sugar (deoxyribose in DNA, ribose in RNA)
2. A nitrogenous base (A, G, C, T in DNA; A, G, C, U in RNA)
3. A phosphate unit

Q: Which type of sugar is present in DNA? A: Deoxyribose.

Q: What is the sugar unit in RNA? A: Ribose.