4.2Biological Molecules
Structure of DNA — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 8 parts Lesson 2 of 3 1 quiz
Lesson summary
4.2 Structure of DNA — IGCSE BIOLOGY notes: Describe DNA as a double helix of nucleotides held together by complementary base pairing (Supplement).DNA = deoxyribonucleic acid, the molecule that contains the instructions for the growth and development of all organisms.Location — D…
- Genetic code
- Bonds between strands
- Four bases
- Predicting a strand
- Hydrogen bond
- Location — DNA is found in chromosomes inside the nucleus of eukaryotic cells
Part 1 of 6What DNA is
DNA
DNA deoxyribonucleic acid, the molecule that contains the instructions for the growth and development of all organisms.
Location
- DNA is found in chromosomes inside the nucleus of eukaryotic cells.
Gene
- A length of DNA that codes for one protein.
Genetic code
- The sequence of bases holds the code for the formation of proteins.
Part 2 of 6Double helix
Strands
- A DNA molecule consists of two strands wound around each other.
Double helix
- The shape of two coiled strands, like a twisted ladder.
Bonds between strands
- The two strands are held together by hydrogen bonds between the bases.
Part 3 of 6Nucleotides
Nucleotide
- The individual unit of DNA.
- Every nucleotide contains the same phosphate and the same deoxyribose sugar.
- Nucleotides differ only in the base attached.
Four bases
- Adenine (A), Cytosine (C), Thymine (T) and Guanine (G).
- Only the letters of the bases need to be learned, not the names.
Part 4 of 6Complementary base pairing
| Base | Always pairs with |
|---|---|
| Adenine (A) | Thymine (T) |
| Cytosine (C) | Guanine (G) |
Part 5 of 6Ladder model
Backbone
- The phosphate and sugar parts of the nucleotides form the sides of the ladder.
Rungs
- The bases of the two strands pair up to form the rungs.
Predicting a strand
- If the sequence on one strand is known, the other strand follows from A-T and C-G pairing.
Part 6 of 6More from the specification
1Biological sample protein coding
- Biological sample protein coding analyzing an extracted cellular sample reveals base sequences that determine polypeptide amino acids assemblies.
2Amino acid
- The unit from which proteins are made; the sequence decides the protein's shape and function.
3Glucose
- A simple sugar; the unit that starch, glycogen and cellulose are built from.
4Starch
- An insoluble polysaccharide of glucose used by plants for energy storage.
5Lipids
- Concentrated energy, insulation, cell membranes.
6Carbon
- Element with four valence electrons that forms the backbone of organic molecules.
7Iodine
- Element needed to make thyroxine.
8Carbohydrate
- A molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.
9Glycogen
- The polysaccharide in which glucose is stored in animals.
10Cellulose
- A structural polysaccharide of β-glucose.
Key terms
| Term | Meaning |
|---|---|
| DNA | Deoxyribonucleic acid, the molecule carrying the instructions for growth and development. |
| Double helix | Two DNA strands wound around each other. |
| Nucleotide | The unit of DNA made of a phosphate, a deoxyribose sugar and a base. |
| Base | One of A, C, T or G, attached to the sugar of a nucleotide. |
| Complementary base pairing | The fixed pairing of A with T and C with G. |
| Hydrogen bond | The bond between paired bases that holds the two strands together. |
| Gene | A length of DNA that codes for a protein. |
Avoid these mistakes
1
Pairing A with C or G with T — A always pairs with T and C always pairs with G.
2
Describing the strands as joined by sugar-phosphate bonds — hydrogen bonds between bases join the strands.
3
Thinking each strand is a separate chromosome — one DNA molecule has two strands in a double helix.
4
Learning base names but not letters — the letters A, C, G and T are what the syllabus requires.
Marked practiceQuizzes for this lesson
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