Nucleic acids: DNA and RNA

In a nutshell

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are nucleic acids that are found in living cells. They are both polymers of nucleotides but differ in the sugar they contain. DNA stores genetic information whereas RNA transfers genetic information from DNA to the ribosomes.​ This summary will explain the different nucleic acids and their structures.

Nucleotides

Definition

A nucleotide is a molecule that contains a nitrogen-containing base, a phosphate group and a pentose sugar (a sugar with five carbons). The general structure can be seen below.

The nitrogen-containing bases can either be purines or pyrimidines. Purines, like adenine (A) and guanine (G) have two nitrogen-containing rings. Pyrimidines, like thymine (T), cytosine (C) and uracil (U) only have one nitrogen-containing ring. The structure of these can be seen below. 

​

DNA

The DNA nucleotide consists of a phosphate group, a pentose sugar called deoxyribose and a base. The base can be adenine (A), thymine (T), cytosine (C) or guanine (G). 

​

RNA

The RNA nucleotide consists of a phosphate group, a pentose sugar called ribose and a base. The base can be adenine (A), uracil (U), cytosine (C) or guanine (G).

​

​

Polynucleotides

DNA and RNA are polynucleotides. These are polymers of nucleotides. Adjacent nucleotides are joined through a condensation reaction between the phosphate group of one nucleotide and the sugar of another nucleotide. The bond between nucleotides is called a phosphodiester bond. 

​

DNA

Hydrogen bonds form between complementary bases to join two DNA polynucleotides. Adenine pairs with thymine using two hydrogen bonds. Cytosine pairs with guanine using three hydrogen bonds.

Note: Due to the pairing described above, there will always be equal amounts of adenine and thymine in a molecule of DNA. The same applies to cytosine and guanine.

The double helix structure was discovered by Watson and Crick in $$1953$$​. In the double helix structure, two polynucleotide strands run antiparallel (opposite directions) to one another and twist into a helical shape.

The sugar and phosphate molecules alternate which gives the DNA double helix a sugar-phosphate backbone. Each DNA strand will have a $$5'$$​ ($$5$$ prime) and a $$3'$$ ($$3$$ prime) end. The $$5'$$end is where the phosphate group is attached to the fifth carbon atom on the deoxyribose sugar whereas the $$3'$$ end is where the phosphate group is attached to the third carbon atom on the deoxyribose sugar. 

The two strands will run in opposite directions to ensure that the bases fit together and the deoxyribose molecules point in opposite directions. This is shown below.

RNA

RNA forms a single-stranded polynucleotide chain which is shorter chain than DNA.

Examples

• Messenger RNA (mRNA) carries a copy of a gene from the nucleus to the ribosome where it can be translated to produce a protein.
  
• Transfer RNA (tRNA) carries amino acids to the ribosomes during translation.
  
• Ribosomal RNA (rRNA) is what the ribosomal subunits are made from.