DNA & RNA — A-Level Biology Revision
Revise DNA & RNA for A-Level Biology. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to CarbohydratesWhat is DNA & RNA?
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids, essential for all known forms of life. DNA carries the genetic instructions for the development, functioning, growth, and reproduction of all organisms and many viruses, in the form of a double helix structure. RNA has various biological roles, including the synthesis of proteins (mRNA, tRNA, rRNA) and the regulation of gene expression.
Board notes: The structure of DNA and RNA, DNA replication, and protein synthesis are core topics in all A-Level Biology specifications (AQA, Edexcel, OCR). The specific details of transcription and translation, and the roles of different types of RNA, are covered in depth.
Step-by-step explanationWorked example
If a DNA molecule has 20% adenine (A), then it must also have 20% thymine (T) due to base pairing. This accounts for 40% of the bases. The remaining 60% must be split equally between guanine (G) and cytosine (C), so there will be 30% G and 30% C.
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Common mistakes
- 1Confusing the structures of DNA and RNA. DNA is double-stranded, has deoxyribose sugar, and contains the base thymine (T). RNA is single-stranded, has ribose sugar, and contains uracil (U) instead of thymine.
- 2Not understanding the base pairing rules. In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). In RNA, adenine (A) pairs with uracil (U).
- 3Forgetting that the two DNA strands are anti-parallel. One strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction, which is crucial for DNA replication and transcription.
DNA & RNA exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for DNA & RNA
Core concept
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids, essential for all known forms of life. DNA carries the genetic instructions for the development, functioning, growth, and repr…
Frequently asked questions
What is the function of messenger RNA (mRNA)?
mRNA carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm, where it is used as a template for protein synthesis.
What is a nucleotide?
A nucleotide is the basic building block of nucleic acids (DNA and RNA). It consists of a sugar molecule (either ribose in RNA or deoxyribose in DNA) attached to a phosphate group and a nitrogen-containing base.