Structure of DNA and RNA
AQA
A-Level
DNA and RNA are nucleic acids, polymers of nucleotides, and the information they carry lies in the sequence of bases along the strand. In DNA a sequence of three bases, a triplet, codes for one amino acid, so the base sequence of a gene determines the sequence of amino acids in a polypeptide, which determines its tertiary structure and therefore its function. Because the genetic code is universal, the same triplets code for the same amino acids in almost all organisms, which is why a human gene inserted into a bacterium still yields a human protein. DNA is the long-term store: it is stable, the bases are protected on the inside of the double helix by the sugar-phosphate backbone, and it can be copied exactly.
21
Objectives
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Exam Tips
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Pitfalls
40
Key Terms
36
Mark Points
Subtopics in this area
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules.
In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes.
Ribosomes are formed from RNA and proteins.
Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. A condensation reaction between two nucleotides forms a phosphodiester bond.
A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs.
An RNA molecule is a relatively short polynucleotide chain.
Students should be able to appreciate that the relative simplicity of DNA led many scientists to doubt that it carried the genetic code.
Structure of DNA and RNA Revision Guide
Learning Objectives
What you need to know and understand
- Explain how a sequence of bases can carry information, referring to triplets and to the amino acid sequence of a polypeptide.
- Justify why a human gene transferred into a bacterium still produces a functional human protein.
- Distinguish the roles of DNA and of RNA in a eukaryotic cell, naming where each is found.
- Describe the route taken by the information in one gene from the DNA to a polypeptide at the ribosome.
- Explain why a copy of a gene, rather than the DNA itself, is used to transfer information to the ribosome.
- Name the types of RNA and state the role of each in producing a polypeptide.
- State the two components of a ribosome and name the part of the nucleus where their subunits are assembled.
- Explain the selective toxicity of antibiotics targeting 70S ribosomes, noting the distinction between prokaryotic, chloroplast and mammalian ribosomes.
- Distinguish the fate of proteins made on free ribosomes from those made on rough endoplasmic reticulum.
- Draw and label a single nucleotide, naming the pentose and the base for both DNA and RNA.
- Describe the condensation reaction that joins two nucleotides and name the bond and the molecule released.
- Compare a DNA nucleotide with an RNA nucleotide, giving two differences in named components.
- Describe how the two strands of a DNA molecule are held together, naming the bonds and the base pairs.
- Explain why DNA is stable enough to store information yet can still be separated for replication and transcription.
- Calculate the percentage of each base in a sample given the percentage of one, and justify the calculation using complementary base pairing.
- Give three structural differences between a DNA molecule and a transfer RNA molecule, writing both sides of each comparison.
- Describe the structure of transfer RNA and identify the two ends that carry out its function.
- Explain why messenger RNA is used to carry the genetic code from the nucleus to the ribosome.
- Explain why the small number of different bases in DNA once led scientists to doubt that it carried the genetic code.
- Contrast the chemical complexity of proteins with the relative simplicity of DNA.
- Describe how scientific consensus regarding the genetic code shifted in response to new experimental evidence.
Marking Points
Key points examiners look for in your answers
- one mark for the information being carried in the sequence of bases
- one mark for a triplet of three bases coding for one amino acid
- one mark for the base sequence determining the amino acid sequence, and so the primary structure, of a polypeptide
- one mark for DNA storing the information while RNA carries a copy of it to the ribosome
- one mark for the genetic code being universal, so the same triplets code for the same amino acids in different species
- DNA holds genetic information, acting as the code for the sequence of amino acids in polypeptides.
- RNA transfers genetic information from the DNA to the ribosomes for protein synthesis.
- In eukaryotes, messenger RNA carries the genetic code from nuclear DNA through a nuclear pore to ribosomes in the cytoplasm.
- Transfer RNA brings specific amino acids to the ribosome, matching its anticodon to the messenger RNA codon.
- In prokaryotes, transcription and translation occur in the cytoplasm as they lack a nucleus.
- A ribosome consists of ribosomal RNA and protein, arranged in a large and a small subunit.
- Ribosomal subunits are assembled at the nucleolus, then combine around mRNA in the cytoplasm to form the intact ribosome.
- Ribosomes have no membrane, unlike most other organelles.
- Eukaryotic cells contain 80S ribosomes, which are larger than the 70S ribosomes found in prokaryotes and chloroplasts.
- The ribosome is the site of translation, located either free in the cytoplasm or attached to the rough endoplasmic reticulum.
- Antibiotics targeting 70S ribosomes are selectively toxic because human cytosolic ribosomes are 80S.
- one mark for DNA or RNA being a polymer of nucleotides, for which polynucleotide is accepted
- one mark for a nucleotide consisting of a pentose sugar, a phosphate group and a nitrogen-containing organic base, with phosphoric acid accepted for phosphate
- one mark for naming deoxyribose and thymine in DNA against ribose and uracil in RNA
- one mark for a condensation reaction between the phosphate group of one nucleotide and the sugar of the next, releasing water
- one mark for naming the bond formed between nucleotides as a phosphodiester bond
- one mark for two polynucleotide strands wound into a double helix
- one mark for the strands being held together by hydrogen bonds between the bases
- one mark for specific complementary base pairing, adenine with thymine and cytosine with guanine
- one mark for two hydrogen bonds between adenine and thymine and three between cytosine and guanine
- one mark for the sugar-phosphate backbone lying on the outside with the bases on the inside
- RNA is a single polynucleotide strand, whereas DNA is double-stranded.
- RNA contains ribose, whereas DNA contains deoxyribose.
- RNA contains uracil, whereas DNA contains thymine.
- RNA is much shorter, with far fewer nucleotides, than a DNA molecule.
- Transfer RNA is a folded molecule with an anticodon and an amino acid binding site, unlike the DNA double helix.
- Messenger RNA is a copy of one gene that leaves the nucleus through a nuclear pore for translation at a ribosome.
- State that DNA contains only four different bases, whereas proteins are built from twenty different amino acids.
- Explain that scientists believed DNA lacked the chemical diversity to carry complex genetic information.
- Identify that proteins were initially favoured as the genetic material due to their limitless structural variety.
- Describe how accumulating experimental evidence shifted scientific consensus to accept DNA as the genetic code.
Examiner Tips
Expert advice for maximising your marks
- 💡Whenever you write 'information', say what form it takes: a sequence of bases.
- 💡One gene, one polypeptide. Keep that phrase, because 'codes for a protein' loses marks where a protein has several polypeptides.
- 💡In recombinant DNA questions, the mark is for the universal genetic code, so write that phrase in full.
- 💡When discussing the role of DNA, specify that it codes for polypeptides or the amino acid sequence, rather than just saying it holds 'information'.
- 💡Always qualify statements about transcription occurring in the nucleus by specifying that this applies to eukaryotic cells, as prokaryotes lack a nucleus.
- 💡If a question asks for evidence that a cell is eukaryotic, 'larger ribosomes' or '80S ribosomes' is creditworthy, but 'ribosomes' alone is not, because all cells have them.
- 💡Write rough endoplasmic reticulum in full at least once in any answer where it earns a mark.
- 💡Link free and bound ribosomes to the destination of the protein, not just to their position.
- 💡When asked to describe the structure of DNA, work logically through nucleotide components, phosphodiester bonds, the double helix, hydrogen bonds and specific base pairing.
- 💡A clearly annotated diagram earns the equivalent marking points, so draw the three components and label them if you are short of words.
- 💡Always name bases in full; the abbreviations cost you nothing but gain you nothing either.
- 💡If a question mentions stability, write both halves: each hydrogen bond is weak, but there are very many of them.
- 💡If a question mentions strand separation, the same fact works the other way: individually weak bonds are easy to break.
- 💡State the specific base pairs clearly (adenine with thymine, cytosine with guanine) rather than just saying 'complementary base pairing', to ensure you secure the mark.
- 💡In a table of differences, complete both columns for every row; an empty cell scores nothing even if the other cell is right.
- 💡The list rule applies, so give exactly the number of differences asked for and no more.
- 💡Name the type of RNA you mean; 'RNA is short' is too vague when transfer, messenger and ribosomal RNA differ in structure.
- 💡When asked why scientists doubted DNA, explicitly contrast the four bases of DNA with the twenty amino acids of proteins.
- 💡Emphasise the phrase 'relative simplicity' when describing early views of DNA's structure.
Common Mistakes
Pitfalls to avoid in your exam answers
- describing DNA as a protein, or saying it is built from amino acids
- writing that 'DNA is universal' when the point being credited is that the genetic code is universal
- saying a gene codes for a characteristic rather than for a polypeptide
- confusing the triplet in DNA with the codon in mRNA and the anticodon in tRNA
- claiming RNA stores genetic information in all organisms, ignoring that in cells it is DNA that does so
- Assuming all eukaryotic DNA is in the nucleus; correction: remember that mitochondria and, in photosynthetic eukaryotes, chloroplasts also contain their own DNA.
- Stating that DNA travels to the ribosome; correction: DNA remains in the nucleus (in eukaryotes) or nucleoid region, and it is messenger RNA that transfers the information to the ribosome.
- Writing that RNA makes the protein; correction: RNA carries the genetic code and brings amino acids, but the ribosome catalyses the formation of peptide bonds.
- Drawing or describing a ribosome as a membrane-bound organelle; correction: state that ribosomes lack a membrane and are not enclosed by one.
- Saying a ribosome is made of DNA, or of protein alone; correction: state that it is made of ribosomal RNA and protein.
- Confusing the nucleolus, which makes ribosomal subunits, with a ribosome itself; correction: distinguish the nucleolus as the site of subunit assembly from the intact ribosome assembled in the cytoplasm.
- Stating that all ribosomes in a eukaryotic cell are 80S; correction: remember that chloroplasts contain 70S ribosomes, and mammalian mitochondria contain ~55S ribosomes, distinct from the 80S ribosomes in the cytosol.
- writing adenosine for adenine or cysteine for cytosine — correct this by learning the exact names of the bases
- giving the bases as the letters A, T, C and G when asked to name them — correct this by writing the full names
- calling the bond between nucleotides glycosidic, peptide or hydrogen — correct this by stating it is a phosphodiester bond
- writing 'sugar' without naming it — correct this by specifying deoxyribose or ribose
- putting uracil in DNA or thymine in RNA — correct this by remembering uracil is only in RNA
- giving base pairs as letters when the question asks you to name the bases — correct this by writing the full names of the bases
- saying the two strands are joined by phosphodiester bonds, which join nucleotides within a strand — correct this by stating that hydrogen bonds join the strands
- swapping the numbers, so adenine and thymine are given three hydrogen bonds and cytosine and guanine two — correct this by remembering A–T has two and C–G has three
- describing hydrogen bonds as strong, instead of explaining that many weak bonds together give stability — correct this by stating that individual hydrogen bonds are weak but numerous
- saying the strands are identical rather than complementary — correct this by stating that the strands are complementary, with A opposite T and C opposite G
- Writing T and U instead of thymine and uracil when the question asks for names; correction: write the full names of the bases.
- Saying RNA is double-stranded because transfer RNA contains internal base pairing; correction: RNA is single-stranded but can fold via internal hydrogen bonds.
- Listing features of DNA and RNA in separate sentences so no comparison is actually made, which loses every comparison mark; correction: write paired statements that directly contrast the two.
- Describing messenger RNA as a clover leaf, or transfer RNA as linear; correction: match each RNA type to its correct shape.
- Giving deoxyribose as the sugar in RNA; correction: RNA contains ribose.
- Claiming mRNA is always destroyed after a few rounds of translation; correction: mRNA lifetimes vary and degradation is regulated.
- Assuming scientists doubted DNA because it had not yet been discovered; correct by noting its chemical composition was known well before its function was accepted.
- Stating that DNA is too short to carry the code; correct by explaining the doubt was based on its chemical simplicity (only four bases), not its physical length.
- Treating the acceptance of DNA as a single sudden discovery; correct by recognising it as a gradual shift in consensus driven by accumulating peer-reviewed evidence.