AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed…
Full specification wording
Structure of DNA and RNA: Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. A condensation reaction between two nucleotides forms a phosphodiester bond.
A nucleotide consists of three parts: a pentose (five-carbon sugar), a phosphate group and a nitrogen-containing organic base.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A nucleotide consists of three parts: a pentose (five-carbon sugar), a phosphate group and a nitrogen-containing organic base. In DNA, the pentose is deoxyribose and bases are adenine, thymine, cytosine or guanine. In RNA, the pentose is ribose and uracil replaces thymine. Adenine and guanine are double-ringed purines; thymine, cytosine and uracil are single-ringed pyrimidines. Nucleotides join via condensation: the phosphate of one nucleotide reacts with the hydroxyl group on carbon three of the next, releasing water and forming a phosphodiester bond. This builds the sugar-phosphate backbone of a polynucleotide. Hydrolysis reverses this, using water to break phosphodiester bonds.
What earns the marks
- DNA or RNA being a polymer of nucleotides, for which polynucleotide is accepted.
- A nucleotide consisting of a pentose sugar, a phosphate group and a nitrogen-containing organic base, with phosphoric acid accepted for phosphate.
- Naming deoxyribose and thymine in DNA against ribose and uracil in RNA.
- A condensation reaction between the phosphate group of one nucleotide and the sugar of the next, releasing water.
- Naming the bond formed between nucleotides as a phosphodiester bond.
Common mistakes and how to put them right
Writing adenosine for adenine or cysteine for cytosine.
Put it right by learning the exact names of the bases.
Giving the bases as the letters A, T, C and G when asked to name them.
Put it right by writing the full names.
Calling the bond between nucleotides glycosidic, peptide or hydrogen.
Put it right by stating it is a phosphodiester bond.
Writing 'sugar' without naming it.
Put it right by specifying deoxyribose or ribose.
Putting uracil in DNA or thymine in RNA.
Put it right by remembering uracil is only in RNA.
Exam tips
- When asked to describe the structure of DNA, work logically through nucleotide components, phosphodiester bonds, the double helix, hydrogen bonds and specific base pairing.
- A clearly annotated diagram earns the equivalent marking points, so draw the three components and label them if you are short of words.
- Always name bases in full; the abbreviations cost you nothing but gain you nothing either.
Try the lesson
The start of the Structure of DNA and RNA: Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. A condensation reaction between two nucleotides forms a phosphodiester bond. lesson, made from checked teaching notes for this specification point.
Have a go first
Which pentose sugar is found in a DNA nucleotide?
Listen
A nucleotide consists of three parts: a pentose (five-carbon sugar), a phosphate group and a nitrogen-containing organic base. In DNA, the pentose is deoxyribose and bases are adenine, thymine, cytosine or guanine. In RNA, the pentose is ribose and uracil replaces thymine. Adenine and guanine are double-ringed purines; thymine, cytosine and uracil are single-ringed pyrimidines. Nucleotides join via condensation: the phosphate of one nucleotide reacts with the hydroxyl group on carbon three of the next, releasing water and forming a phosphodiester bond. This builds the sugar-phosphate backbone of a polynucleotide. Hydrolysis reverses this, using water to break phosphodiester bonds. A strong answer earns marks for these points. DNA or RNA being a polymer of nucleotides, for which polynucleotide is accepted. a nucleotide consisting of a pentose sugar, a phosphate group and a nitrogen-containing organic base, with phosphoric acid accepted for phosphate. naming deoxyribose and thymine in DNA against ribose and uracil in RNA. a condensation reaction between the phosphate group of one nucleotide and the sugar of the next, releasing water. A common slip: writing adenosine for adenine or cysteine for cytosine. Instead, learning the exact names of the bases. Exam tip: When asked to describe the structure of DNA, work logically through nucleotide components, phosphodiester bonds, the double helix, hydrogen bonds and specific base pairing.
See it
Key words
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nucleotide structure
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pentose sugar
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phosphodiester bond
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condensation reaction
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See every definition
- nucleotide structure
- Formed from a pentose, a nitrogen-containing organic base and a phosphate group.
- pentose sugar
- In DNA it is deoxyribose and in RNA it is ribose.
- phosphodiester bond
- Formed by a condensation reaction between two nucleotides.
- condensation reaction
- Between two nucleotides it forms a phosphodiester bond.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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Test your understanding of Structure of DNA and RNA: Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. A condensation reaction between two nucleotides forms a phosphodiester bond.
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