Nucleic Acids

This lesson covers:

  1. The structure of nucleotides 
  2. The reactions that synthesise and breakdown nucleic acids 
  3. The role and structures of DNA and RNA
  4. The similarities and differences between DNA and RNA

Nucleotide structure

Nucleotides are the building blocks of nucleic acids such as DNA and RNA. Nucleotides are monomers and can join together to form dimers (dinucleotides) and polymers (polynucleotides or nucleic acids).

Diagram showing the structure of a nucleotide with a phosphate group, pentose sugar, and nitrogenous base.

Nucleotides are made up of three components:

  1. A pentose sugar - Contains 5 carbon atoms.
  2. A nitrogenous base - Contains carbon and nitrogen.
  3. A phosphate group - Contains phosphate.

Polynucleotides

Nucleotides are joined together via condensation reactions to form a polynucleotide. The phosphate group of one nucleotide forms a covalent bond with the sugar of another. This forms a phosphodiester bond.

Diagram showing the structure of a polynucleotide with phosphodiester bonds and sugar-phosphate backbone.

Many nucleotides can join in this way to create a chain of phosphates and sugars known as the sugar-phosphate backbone. 


Phosphodiester bonds can be broken via hydrolysis reactions, releasing the nucleotide monomers.

DNA

Deoxyribonucleic acid (DNA) is a type of nucleic acid that contains the instructions needed to make proteins.

Diagram showing the structure of a DNA nucleotide with deoxyribose, a phosphate group, and a base A, T, G, or C.

Each DNA nucleotide is made up of three components:

  1. Deoxyribose - A pentose sugar.
  2. A, T, G, or C base - Adenine, thymine, guanine, or cytosine.
  3. A phosphate group

DNA structure

In 1953, two scientists, James Watson and Francis Crick were credited with working out the structure of DNA.


With the help of other scientists like Rosalind Franklin, they found that DNA is made up of two polynucleotide strands wound around each other to form a double helix.

Diagram showing the DNA double helix structure with sugar-phosphate backbone and bases

The following features allow DNA to pass genetic information from one generation to another: 

  1. Sugar-phosphate backbone - This protects coding bases on the inside of the helix.
  2. Double stranded - This allows strands to act as templates in DNA replication.
  3. Large molecule - It stores lots of information.
  4. Double helix - This makes the molecule compact.
  5. Complementary base pairing - This allows accurate DNA replication.
  6. Weak hydrogen bonds - This allows strands to separate in DNA replication.

Purines and pyrimidines

There are four nitrogenous bases found in DNA: adenine (A), guanine (G), thymine (T), and cytosine (C). These bases can be grouped into two categories: purines and pyrimidines.


Differences between purines and pyrimidines:

  • Purines - These are larger bases that contain two carbon ring structures (A and G).
  • Pyrimidines - These are smaller bases that contain one carbon ring structure (T and C).
Diagram showing the structures of purines and pyrimidines with adenine and guanine as purines and thymine and cytosine as pyrimidines.

Complementary base pairing

The two DNA strands are held together via hydrogen bonding between bases.


Each base only joins with one other specific base:

  • Adenine pairs with thymine via 2 hydrogen bonds.
  • Cytosine pairs with guanine via 3 hydrogen bonds.
Diagram showing hydrogen bonding between complementary bases in DNA, with adenine pairing with thymine and cytosine pairing with guanine.

This is known as complementary base pairing.


A smaller pyrimidine base always binds to a larger purine base. This arrangement maintains a constant distance between the two sugar-phosphate backbones.

Antiparallel strands

Each DNA strand has a 5' end and a 3' end. The 5' end has carbon 5 in the deoxyribose attached to a phosphate whereas the 3' end has carbon 3 of the deoxyribose exposed. 

Diagram showing antiparallel DNA strands with 5' and 3' ends, hydrogen bonds between A-T and C-G base pairs.

The DNA strands in a molecule are described as being antiparallel, meaning they run in opposite directions. One strand runs from 5' to 3', while the other strand runs from 3' to 5'. 

RNA

Ribonucleic acid (RNA) is a type of nucleic acid that uses information from DNA to synthesise proteins.

Diagram showing the structure of an RNA nucleotide with ribose sugar, phosphate group, and a base A, U, G, or C.

Each RNA nucleotide is made up of three components: 

  1. Ribose - A pentose sugar.
  2. A, U, G, or C base - Adenine, uracil, guanine, or cytosine. 
  3. A phosphate group

RNA structure

Unlike DNA, RNA nucleotides contain the sugar ribose rather than deoxyribose. They also contain the base uracil in place of thymine (uracil pairs with adenine).

Illustration showing the structure of RNA with a sugar-phosphate backbone and bases adenine, uracil, guanine, and cytosine.

RNA is a single stranded molecule made up of just one polynucleotide strand. These strands are much shorter than DNA strands.

Comparing DNA and RNA

Table comparing DNA and RNA with details on pentose sugar, bases, size, number of strands, and diagrams of their structures.