Nucleic acids

Nucleic acids are biopolymers that are essential for all forms of life. RNA and DNA are the two types of nucleic acids that exist in living organisms.

Both DNA and RNA are composed of nucleotides, joined via condensation reactions to form a long strand.  Nucleotides have three components:

  • a phosphate
  • a pentose sugar (deoxyribose for DNA and ribose for RNA)
  • a nitrogenous base

There are four types of nitrogenous bases:

  • Adenine
  • Thymine (which is substituted for Uracil in RNA)
  • Cytosine
  • Guanine

Below is a RNA nucleotide from VCAA’s 2006 Exam 1 Examination report.

nucl (Click to enlarge)

When drawing nucleotides be wary of where each component joins.  When drawing a DNA or RNA nucleotide make sure you specify the sugar (deoxyribose or ribose).

DNA structure

DNA is composed of two strands of nucleotides.  The phosphate end of the nucleotide is called the 5′ end (pronounced 5-prime), and the sugar end is the 3′ end.  In DNA, the two chains are anti-parallel, which means they run in opposite directions – one chain has 5′ at the top and 3′ at the bottom, while the other chain runs from 3′ to 5′.

The two strands are held together by hydrogen bonds between nitrogenous bases.  Due to a rule called complementary base pairing, only C can bind with G (with three hydrogen bonds), and only A can bind with T (with two hydrogen bonds).

The double-stranded anti-parallel chain then twists into a structure called a double helix.

Comparing DNA and RNA

Differences between DNA and RNA include:

  • DNA is double-stranded, RNA is single-stranded
  • DNA contains deoxyribose, RNA contains ribose
  • DNA has the base Thymine, RNA has the base Uracil

Types of RNA

The three main types of RNA are:

  • mRNA (messenger RNA): carries information from the nucleus to the ribosome so that the protein can be synthesised
  • tRNA (transfer RNA): carries specific amino acids to the ribosome for protein synthesis
  • rRNA (ribosomal RNA): a structural component of the ribosomes

You’ll learn about their roles when you learn about gene expression in Unit 4.