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2.2.3Topic 2 · Genes and Health

RNA- Structure — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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2.2.3 RNA- Structure — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the structure of RNA, compare it with DNA, and explain the roles of messenger, transfer and ribosomal RNA.Ribonucleic acid (RNA) is a polynucleotide that carries information from DNA to the ribosomes and…

  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Like DNA, it is built from nucleotides joined by phosphodiester bonds
  • However, its nucleotides contain the sugar ribose, and the base uracil replaces thymine
  • Uracil pairs with adenine

Section 1 of 9What is RNA?

Ribonucleic acid (RNA) is a polynucleotide that carries information from DNA to the ribosomes and takes part in protein synthesis.

Like DNA, it is built from nucleotides joined by phosphodiester bonds.
Ribose
However, its nucleotides contain the sugar ribose, and the base uracil replaces thymine.
Uracil pairs with adenine.
Cells make RNA by copying sections of DNA in a process called transcription.

Section 2 of 9Comparing RNA with DNA

FeatureDNARNA
SugarDeoxyriboseRibose
BasesA, T, C, GA, U, C, G
StrandsTwo, in a double helixOne (but may fold)
LengthVery longRelatively short
StabilityVery stableLess stable, short-lived
LocationNucleus (eukaryotes), mitochondria, chloroplastsMade in the nucleus; found in nucleus, cytoplasm, ribosomes
FunctionLong-term storage of genetic informationCarries and uses information to make proteins

Its single strand and shorter length make RNA able to leave the nucleus through nuclear pores, whereas DNA cannot.

Section 3 of 9Messenger RNA (mRNA)

mRNA is a single-stranded, linear molecule that is a complementary copy of a gene.

It is made in the nucleus by transcription.
In eukaryotes the first copy (pre-mRNA) has non-coding sections called introns removed and the coding exons joined, a process called splicing, before it leaves through a nuclear pore.
In the cytoplasm it attaches to a ribosome where it is translated.
Codons
Its bases are read in triplets, called codons, each specifying an amino acid or a stop signal.

mRNA is unstable and degrades fairly quickly. This allows a cell to control how much of a protein is produced, since the protein can only be made while mRNA is present.

Section 4 of 9Transfer RNA (tRNA)

tRNA is a single polynucleotide chain, about 75 to 90 nucleotides long, folded by hydrogen bonding between complementary regions into a clover-leaf shape.

Anticodon
One end carries an attached amino acid, and a loop contains an anticodon, a triplet of bases complementary to a particular mRNA codon.
Each tRNA is specific to one amino acid.
An enzyme attaches the correct amino acid to its tRNA using ATP.
The anticodon ensures that amino acids are brought to the ribosome in the order specified by the mRNA.

Section 5 of 9Ribosomal RNA (rRNA)

rRNA combines with proteins to form ribosomes, made of a large and a small subunit.

1
Ribosomes are about half rRNA by mass. rRNA forms the structure of the ribosome and has a catalytic role in forming peptide bonds.
2
The small subunit binds mRNA, and the large subunit has binding sites for two tRNA molecules at a time.
3
Ribosomes are synthesised in the nucleolus.
4
Eukaryotic ribosomes (80S) are larger than those of prokaryotes (70S).

Section 6 of 9Links and interpretation

The relationship between DNA, mRNA and protein is called the central dogma: DNA is transcribed to RNA, which is translated to protein.

Links and interpretation
Exam questions often supply a DNA template strand and ask for the mRNA sequence: replace each base with its complement, using U instead of T.
For example, the DNA template TAC GGA gives mRNA AUG CCU.
Then tRNA anticodons would be UAC GGA, the same as the DNA template but with U for T.

The structural differences between DNA and RNA explain their different jobs: DNA is double-stranded and protected for long-term storage, whereas RNA is a disposable working copy. Some viruses, such as HIV and influenza, use RNA as their genetic material.

Finally, consider RNA's wider roles.

1
Some RNA molecules act as enzymes, called ribozymes, and rRNA in the ribosome is one example,
  • which supports the idea that RNA may have been the first genetic material in an 'RNA world'.
2
Small regulatory RNAs can bind to mRNA and block its translation.
3
Retroviruses use reverse transcriptase to make DNA from their RNA genome.
4
Questions on these topics usually only need the basic point: RNA is more versatile than the storage-only role of DNA suggests.

Remember to write answers about location and function together. For example, state that mRNA is made in the nucleus and travels to the cytoplasm, that tRNA works at the ribosome in the cytoplasm, and that rRNA is a component of the ribosome. That level of detail is expected for full marks at this level.

Section 7 of 9Exam-style questions

  1. Give three differences between the structure of DNA and mRNA. [3]3 marks

  2. The DNA template strand has the base sequence TACGGATTC. State the mRNA sequence and the tRNA anticodon for the second codon. [3]3 marks

  3. Describe the roles of tRNA in protein synthesis. [3]3 marks

Section 8 of 9Key terms

TermMeaning
RNAA single-stranded polynucleotide containing ribose and uracil.
mRNAMessenger RNA: a copy of a gene carrying the code to a ribosome.
tRNATransfer RNA: carries a specific amino acid and has an anticodon.
rRNARibosomal RNA: a structural and catalytic component of ribosomes.
CodonA triplet of bases on mRNA coding for an amino acid.
AnticodonA triplet of bases on tRNA complementary to a codon.
UracilA pyrimidine base found in RNA that pairs with adenine.
RibosomeOrganelle made of rRNA and protein where translation occurs.

Section 9 of 9Exam technique

1
Saying that RNA is double-stranded. It is single-stranded, though tRNA folds on itself.
2
Describing anticodons as being on mRNA. Codons are on mRNA; anticodons are on tRNA.
3
Writing thymine in an mRNA sequence. mRNA contains uracil.
4
Forgetting that mRNA is complementary to the template strand, and so matches the coding strand with U for T.

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