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1.2.1Cell Level Systems

DNA Structure — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 13

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision
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Part 1 of 7A polymer of nucleotides

DNA is a polymer made of repeating units called nucleotides.

Sugar
Each nucleotide has three parts: a sugar (deoxyribose), a phosphate group and one of four bases.
The sugar and phosphate groups of neighbouring nucleotides join to form a long backbone, with a base attached to each sugar.
Adenine (A)
The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G).

Part 2 of 7The double helix

A DNA molecule is made up of two strands wound around each other in a double helix, like a twisted ladder.

The double helix
The two sugar-phosphate backbones are the sides of the ladder, and the pairs of bases make the rungs.
The strands are held together by weak bonds between the bases.
The shape of the double helix was worked out in 1953 by Watson and Crick, using X-ray diffraction images produced by Rosalind Franklin and Maurice Wilkins.

Part 3 of 7Complementary base pairing

The bases on the two strands pair in a specific way: A always pairs with T, and C always pairs with G. These are called complementary base pairs. For example, if one strand has the sequence ATTGCA, the other strand must read TAACGT.

BasePairs with
A (adenine)T (thymine)
C (cytosine)G (guanine)

Complementary pairing means that the amount of A in a DNA molecule equals T, and C equals G.

Complementary base pairing
If a sample has 30 percent adenine, it will have 30 percent thymine, and the remaining 40 percent is shared equally between cytosine and guanine,
  • so 20 percent each.
It also allows the molecule to be copied accurately, since each strand can act as a template for a new one.

Part 4 of 7The code and protein synthesis

The sequence of bases along one strand is the genetic code. Three bases in a row (a triplet) code for one amino acid. A gene is a length of DNA with the sequence needed to make one protein. In protein synthesis:

Messenger RNA (mRNA)
The DNA of the gene is copied into a molecule of messenger RNA (mRNA) in the nucleus.
Ribosome
The mRNA moves out into the cytoplasm to a ribosome.
The ribosome reads the mRNA in triplets and carrier molecules bring the matching amino acids.
The amino acids are joined in the right order to form a protein chain.
The chain folds to make a protein with a specific shape.

Some DNA does not code for proteins but regulates genes by switching them on or off.

Part 5 of 7Extracting DNA and links

DNA can be extracted from fruit such as strawberries in the lab by breaking the cells in a mixture of detergent and salt, filtering, and adding cold ethanol, when the DNA appears as white strands. The detergent breaks down cell membranes, the salt helps the DNA to clump, and the cold ethanol makes it precipitate as it is insoluble in alcohol.

This topic links to DNA and the genome (6.1.4), mutations that change a base sequence, and to genetic engineering. It also relates to the structure of other biological polymers such as proteins and carbohydrates.

Part 6 of 7The history of the discovery

Understanding of DNA structure is an example of how scientific ideas develop from the work of many people.

1
Early work showed that DNA was found in the nucleus and that it was the material of inheritance.
2
In the early 1950s Rosalind Franklin and Maurice Wilkins used X-ray diffraction to obtain images of DNA fibres, which suggested a helical shape.
3
Chargaff had already shown that the amounts of A and T were equal, and so were C and G.
4
James Watson and Francis Crick combined these findings to build a model of a double helix with paired bases, published in 1953,
  • and this model immediately suggested how DNA could be copied.

New evidence led scientists to accept the model because it explained the pairing rule and could be tested. This shows that scientific models are accepted when they are supported by evidence and are able to explain observations.

Today, DNA sequencing is automated and the cost has fallen dramatically, which is why whole genomes can be read quickly in hospitals and research laboratories.

Part 7 of 7Exam-style questions

  1. Describe the structure of DNA. [4]4 marks

  2. A DNA sample contains 22% guanine. Calculate the percentage of adenine. [2]2 marks

  3. One DNA strand has the base sequence TACGGT. State the sequence on the complementary strand. [1]1 mark

Key terms

TermMeaning
NucleotideA unit of DNA made of a sugar, a phosphate and a base.
Double helixThe twisted two-strand shape of DNA.
Complementary base pairingA pairs with T and C pairs with G.
PolymerA large molecule made from many similar units.
mRNAMessenger RNA carrying a copy of a gene to a ribosome.
RibosomeStructure in the cytoplasm where proteins are made.
TripletThree bases coding for one amino acid.

Avoid these mistakes

1
Pairing A with C or G with T.
2
Saying DNA is a single strand.
3
Saying a gene codes for a whole organism rather than a protein.
4
Saying that bases are held together by strong bonds; the bonds are weak and break easily during copying.
5
Confusing DNA bases with amino acids.

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