Structure of DNA and RNAAQA A-Level Biology: Revision notes
Section 1
Information-carrying molecules
DNA and RNA are information-carrying molecules. In all living cells DNA holds the genetic information. RNA transfers genetic information from DNA to the ribosomes, where it is used to make proteins. Ribosomes are themselves made of RNA and protein.
Both DNA and RNA are polymers of nucleotides, which makes them polynucleotides.
Section 2
Nucleotide structure
Each nucleotide is formed from three components:
- a pentose (five-carbon) sugar
- a nitrogen-containing organic base
- a phosphate group
In DNA the sugar is deoxyribose and the base is adenine (A), cytosine (C), guanine (G) or thymine (T). In RNA the sugar is ribose and the base is adenine, cytosine, guanine or uracil (U) in place of thymine.
RNA has uracil, not thymine, and ribose, not deoxyribose. Do not mix up the two.
Section 3
The phosphodiester bond
A condensation reaction between two nucleotides joins the phosphate group of one to the pentose sugar of the other, forming a phosphodiester bond and releasing a molecule of water. Repeating this builds a polynucleotide chain with a sugar-phosphate backbone and the bases projecting from it.
A chain of n nucleotides contains n - 1 phosphodiester bonds. For example, a strand of 10 nucleotides has 9.
Section 4
The DNA double helix
A DNA molecule is a double helix of two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs:
- adenine pairs with thymine (two hydrogen bonds)
- cytosine pairs with guanine (three hydrogen bonds)
The sugar-phosphate backbones are on the outside and the bases on the inside. In double-stranded DNA the amount of A equals T and the amount of C equals G. For example, if 22 % of bases are G, then C is 22 %, and A and T are each (100 - 44) / 2 = 28 %.
Many hydrogen bonds make DNA stable, but each is weak, so the strands can be separated when needed.
Section 5
RNA and the history of the code
An RNA molecule is a relatively short polynucleotide chain, usually single-stranded, in contrast with the very long, double-stranded DNA. Messenger RNA is a short copy of a gene, small enough to leave the nucleus.
In the early twentieth century many scientists doubted that DNA carried the genetic code, because it seemed relatively simple, with only four different bases, whereas proteins are made from about 20 different amino acids and seemed far more varied. Later evidence showed that the sequence of bases is the code.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Structure of DNA and RNA
- Analysis of a sample of double-stranded DNA from a bacterium showed that 22 % of its bases were guanine.Explain why the percentage of adenine in the sample is equal to the percentage of thymine.2 marks
- In a eukaryotic cell, DNA is held in the nucleus. A gene is copied into a molecule of messenger RNA (mRNA), which then moves to the ribosomes in the cytoplasm.Explain how the structure of mRNA makes it suitable for carrying genetic information from the nucleus to the ribosomes.2 marks
- In the early twentieth century many scientists doubted that DNA carried the genetic code. A synthetic double-stranded DNA fragment, 10 base pairs long, contains 3 adenine-thymine base pairs and 7 cytosine-guanine base pairs.Suggest why many scientists doubted that DNA carried the genetic code.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).