STPM Biology Biological Molecules Part 17 Nucleic Acids

1. The 2 nucleic acids in the cells are:

• DNA (deoxyribonucleic acid)
• RNA (ribonucleic acid)

2. Nucleic acids are natural polymers. Nucleic acid monomers are nucleotides.

3. A nucleotide has 3 components:

• 5-carbon sugar (pentose)
• Organic base / nitrogenous base
• Phosphoric acid

Structure of nucleotide

4. The pentose of nucleotides are ribose or deoxyribose. 

5. Nucleotide containing ribose is called ribonucleotides (RNA monomers).

6. Nucleotide containing deoxyribose is called deoxyribonucleotide (DNA monomers).

Structure of ribose and deoxyribose

7. Nucleotide has one of these five organic bases:

• adenine (A)
• guanine (G)
• thymine (T)
• cytosine (C)
• urasil (U)

8. These bases can be divided into 2 group:

• Purines (double-ringed molecule): Adenine, Guanine
• Pyrimidine (single-ringed molecule): Thymine, Cytosine, Uracil

5 organic bases

Formation of nucleotides and nucleic acid

1. In formation of nucleotide, a nitrogenous base is first linked to pentose by condensation reaction to form nucleoside.
2. Phosphate group is then added to the nucleoside to form a nucleotide.
3. Nucleic acids are polynucleotides. Polynucleotides are formed by linking nucleotides together.
4. Two nucleotides are linked together through condensation reaction to form dinucleotides.
5. Further addition of nucleotides to dinucleotides will form polynucleotides.
6. Nucleotides in polynucleotides are linked together in phosphodiester bonds.
   
   

   

   Formation of phosphodiester bond

   
   
7. A nucleotide chain has 5' end and a 3' end. The 5' end of the polynucleotide chain is the end with the free phosphate group.

5' end and 3' end of nucleotide chain