International A Level Biology / Proteins and DNA

Nucleotides, DNA and RNA

Edexcel International A Level Biology (YBI11), chapter 5: Proteins and DNA, specification point 2.9. Free lesson.

Read the revision notes · International A Level Biology past papers

DNA and RNA are polynucleotides made from nucleotides. Knowing how their parts join, how DNA strands pair, and how DNA differs from RNA helps you explain their structures clearly.

What a nucleotide contains

A nucleotide, also called a mononucleotide, has three parts. Its sugar is a pentose, which means it has five carbon atoms. DNA contains the sugar deoxyribose, while RNA contains ribose. Deoxyribose has one fewer oxygen atom than ribose. The base contains nitrogen and is the part that varies between nucleotides in DNA. A nucleotide has a phosphate group, a pentose sugar and a base. To draw one, show a single nucleotide and name its sugar and base. Do not draw two nucleotides joined by base pairing.

Which bases are found in DNA and RNA

There are five bases, which belong to two groups. Purines have two rings of atoms. Pyrimidines have one ring. DNA and RNA share three bases, but RNA has uracil instead of thymine. Adenine (A) and guanine (G) are purines found in both DNA and RNA. Cytosine (C) is a pyrimidine found in both DNA and RNA. Thymine (T) is a pyrimidine found only in DNA. Uracil (U) is a pyrimidine found only in RNA. DNA contains A, T, C and G. RNA contains A, U, C and G.

How nucleotides form a strand

DNA and RNA are polynucleotides, meaning chains of many nucleotides. Nucleotides join by a condensation reaction, which removes a molecule of water as a bond forms. That bond is a strong covalent phosphodiester bond. It joins the sugar of one nucleotide to the phosphate of the next. Repeating this creates a sugar-phosphate backbone, with bases sticking out. When describing the bond, say what it joins: the sugar of one nucleotide to the phosphate of the next.

How DNA strands pair

DNA has two strands. Their sugar-phosphate backbones are on the outside, while the bases point inwards and form pairs. This is complementary base pairing: adenine pairs with thymine, and cytosine pairs with guanine. Hydrogen bonds hold the paired bases, and therefore the two strands, together. A pairs with T using two hydrogen bonds. C pairs with G using three. Each pair contains one purine and one pyrimidine. This keeps each rung the same width and the backbones the same distance apart. The strands run in opposite directions, so they are antiparallel. Within each strand, strong…

How the DNA double helix forms

In a cell, the DNA ladder twists into a double helix. The sugar-phosphate backbones wind around the outside, and the base pairs sit inside like ladder rungs. Hydrogen bonds hold the two antiparallel strands together along the molecule.

How DNA compares with RNA

DNA and RNA are both polynucleotides, with nucleotides joined by phosphodiester bonds. They differ in their sugar, bases, number of strands and length. A comparison should give a similarity as well as clear differences. DNA has deoxyribose; RNA has ribose. DNA has adenine, thymine, cytosine and guanine. RNA has adenine, uracil, cytosine and guanine. DNA has two strands twisted into a double helix. RNA is usually a single strand. DNA molecules are very long; RNA molecules are much shorter.

More lessons in Proteins and DNA