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11.4 DNA

GCSE Chemistry · AQA · Chapter 11: Polymers

A free GCSE Chemistry lesson for AQA (8462) from OBA School: dNA as a natural polymer: nucleotides, two polymer chains and the double helix. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe DNA as a large molecule that carries genetic instructions
  • Describe DNA as two polymer chains made from four different nucleotides, joined by condensation polymerisation
  • Describe the double helix and the pairing of bases
  • Compare DNA with other natural polymers
  • Use base pairing to work out sequences and percentages of bases

Key words

  • DNA: Deoxyribonucleic acid: a large natural polymer that carries the genetic instructions for living organisms and viruses.
  • Nucleotide: The monomer of DNA: a phosphate group, a sugar (deoxyribose) and one of four bases.
  • Base: The part of a nucleotide that varies: A (adenine), T (thymine), C (cytosine) or G (guanine).
  • Polynucleotide: A polymer made from many nucleotides joined by condensation polymerisation – DNA is a polynucleotide.
  • Double helix: The shape of DNA: two polymer chains twisted around each other like a spiral ladder.
  • Complementary base pairs: The bases that always pair across the two strands: A with T, and C with G.

What DNA is and what it does

DNA (deoxyribonucleic acid) is a very large molecule that is essential for life. It encodes the genetic instructions for the development and functioning of living organisms and viruses. In plant and animal cells it is found in the chromosomes in the nucleus.

The order of the bases along DNA is a code. It tells the cell the order in which to join amino acids to make each protein.

Nucleotides: the monomers of DNA

Like proteins and starch, DNA is a natural polymer. Its monomers are called nucleotides. Each nucleotide contains a phosphate group, a sugar called deoxyribose and a base. There are four different bases, so there are four different nucleotides.

Nucleotides join together, phosphate to sugar, to make a long chain. The phosphates and sugars form a sugar–phosphate backbone, with the bases sticking out to one side.

Each new link forms in a condensation reaction: a molecule of water is released every time two nucleotides join. So DNA is made by condensation polymerisation, like proteins and starch, and is called a polynucleotide.

many nucleotides → DNA (a polynucleotide) + water

Two chains and a double helix

Most DNA molecules are made of two polymer chains (strands) that run in opposite directions. The bases on one chain pair up with bases on the other: A always pairs with T, and C always pairs with G.

Within each strand the atoms are held by strong covalent bonds. Between the two strands, the base pairs are held by weak intermolecular forces, so the two strands can separate when DNA is copied, without breaking the backbones.

Key fact: In a DNA molecule, the amount of A equals the amount of T, and the amount of C equals the amount of G. This is because of base pairing.

Using base pairing

Worked example: the complementary strand

One strand has the sequence A T G C C G T A. What is the sequence on the other strand?
Pair each base: A→T, T→A, G→C, C→G.

Answer: T A C G G C A T

Worked example: percentages of bases

A sample of DNA contains 21% adenine (A). Find the percentages of the other bases.
T pairs with A, so T = 21%. A + T = 42%.
C + G = 100 − 42 = 58%, and C = G, so each is 58 ÷ 2 = 29%.

Answer: T 21%, C 29%, G 29%

  • DNA; Monomers nucleotides; Number of different monomers four
  • Protein; Monomers amino acids; Number of different monomers about 20
  • Starch / cellulose; Monomers glucose; Number of different monomers one

Common mistakes

  • Saying DNA is made of amino acids. Its monomers are nucleotides; amino acids make proteins.
  • Pairing A with G or C with T. The pairs are A–T and C–G.
  • Saying DNA is one chain. Most DNA is two polymer chains twisted into a double helix.
  • Assuming all four bases are present in equal amounts. Only A = T and C = G.
  • Saying the two strands are joined by covalent bonds. The backbones are covalent, but the strands are held together by weak intermolecular forces between base pairs.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the monomers of DNA called? (Choose from: Amino acids, Nucleotides, Glucose molecules, Esters.)

Answer: Nucleotides

Working: DNA is made from four different nucleotides. Amino acids make proteins; glucose makes starch and cellulose.

Question 2. True or false? Most DNA molecules consist of two polymer chains twisted into a double helix.

Answer: True

Working: True. Two strands are joined by base pairs and twisted into a double helix.

Question 3. One DNA strand has the sequence A C G T T. Write the sequence of the other strand (letters only).

Answer: TGCAA

Working: A→T, C→G, G→C, T→A, T→A: TGCAA.

Related lessons

Continue in the full course

This lesson is section 11.4 (AGCCH1104) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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