2.6 Structure of DNA and RNA

2.6 The structure of DNA

Lesson one: The structure of DNA

Objective: Understand the nature of nucleotides, and how they can be assembled to form DNA and also RNA

  • Keywords:
  • Nucleotide: a building block for polynucleotides (eg. DNA and RNA)
  • Deoxyribose: the pentose sugar found in DNA (carbon 2 is missing an oxygen atom)
  • Ribose: the pentose sugar found in RNA (carbon 2 has a hydroxide group including an oxygen atom)
  • Nitrogenous base: part of a nucleotide, may be a purine (2 ring base such as Adenine and Guanine, or a pyrimidine (single ring base such as cytosine and thymine)

introductory video (note some details are not required at IB level eg. B-form DNA)

DNA is made from the sub-units nucleotides. One nucleotide consists of a ribose sugar (pentose), a phosphate group, and a nitrogenous base.

To join the nucleotides together into a chain, strong covalent bonds are formed between the phosphate and ribose of adjacent nucleotides.

image credit: mrothery.co.uk

image credit: mrothery.co.uk

Skill: Draw a nucleotide, showing the detail of the carbon atoms in the ribose, and the phosphate group.

Skill: Identify the 3′ and the 5′ end of a nucleotide, based on what you know of the numbering system for carbon atoms in a sugar. Be able to identify the 3′ and 5′ end of a chain of nucleotides.

Assembling a DNA molecule:

There are four nitrogenous bases found in DNA:

  • Adenine (A)
  • Guanine (G)
  • Thymine (T)
  • Cytosine (C)

These bases are paired in DNA structure, according to the following base pair rules:

1) A-T

2) G-C

Weak hydrogen bonds form between the nitrogenous bases, assembling an anti-parallel double helix.

Strong covalent bonds join the 3′ end of a deoxyribose sugar to the phosphate of another nucleotide. These bonds are called phosphodiester linkage)

image credit: socratic

image credit: socratic


The complete is often shown as a 2-D flat formation merely to simplify the structure, although in reality this shape is twisted into a double helix.



The above molecule is twisted to form a double helix:

DNA_helix 2


A gene may contain as many as 27,000 base pairs.  Typically only a section of about 1,000 base pairs is copied to make single stranded RNA.

Ribose Nucleic acid is a single stranded copy of the sense strand of DNA (one of the two strands, that codes for a protein). The rest of the DNA is important for regulation of gene expression eg. promoter genes (which activate genes).


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