RNA Structure — mRNA, tRNA and rRNA

While DNA is the long-term store of genetic information, RNA is the molecule that actually gets things done in the cell. RNA molecules carry genetic information to the ribosome, deliver amino acids during protein synthesis, and form part of the structure of the ribosomes themselves. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (c) — the structure of RNA — with particular reference to the three main classes: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

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1. Key Differences Between RNA and DNA

Feature	DNA	RNA
Pentose sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G (uracil replaces thymine)
Typical strand number	Double-stranded	Single-stranded (but may fold)
Typical length	Millions–billions of nucleotides	Tens–tens of thousands
Location	Mostly in the nucleus (also mitochondria and chloroplasts)	Nucleus + cytoplasm (made in nucleus, used in cytoplasm)
Stability	High (long-lived)	Low (short-lived, degraded by RNases)
Function	Long-term store of genetic information	Expression and translation of genetic information

Key Definition — RNA: A polymer of ribonucleotides containing ribose sugar, the bases A, U, C, G and a phosphate group; normally single-stranded and involved in protein synthesis.

The two chemical differences — ribose instead of deoxyribose, and uracil instead of thymine — are small, but they matter. The 2'-OH in ribose makes RNA more chemically reactive and less stable than DNA: a feature, not a bug, for a molecule whose job is to be made, used and destroyed quickly.

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2. Messenger RNA (mRNA)

Messenger RNA is the single-stranded copy of a gene that carries the genetic information from the nucleus to the ribosomes in the cytoplasm, where it is translated into a polypeptide.

Key features:

• Single-stranded, linear polynucleotide
• Made in the nucleus by transcription (Lesson 6) using a gene as the template
• Length is typically a few hundred to a few thousand nucleotides — it corresponds to the length of the gene it encodes
• Codons: successive triplets of bases that each code for a specific amino acid (e.g., AUG = methionine, UAA = stop)
• Shorter-lived than DNA — degraded by the cell after it has been used a few times for translation

Role

mRNA acts as a working copy of the gene. Rather than transporting the precious DNA out of the nucleus (where it could be damaged), the cell makes disposable RNA copies of individual genes as they are needed. Each mRNA carries a set of codons that specifies a particular amino acid sequence in a polypeptide.

5' — A U G — U U U — G G C — U A A — 3'
     Met   Phe   Gly   STOP

In this example, the mRNA encodes a three-amino-acid polypeptide (Met-Phe-Gly) followed by a stop codon.

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3. Transfer RNA (tRNA)

Transfer RNA is the adaptor molecule of protein synthesis. Each tRNA molecule carries a specific amino acid and matches it to the correct codon on the mRNA using its own anticodon.

Key features:

• A short RNA molecule, typically 75–95 nucleotides long
• A single RNA strand that folds back on itself, forming internal base pairs that give it a clover-leaf shape in 2D and an L-shape in 3D
• Contains an anticodon — three unpaired bases at one end that are complementary to a codon on mRNA
• Contains an amino acid attachment site (the CCA sequence at the 3' end) — this is where a specific amino acid is covalently attached
• Each amino acid has at least one specific tRNA (some have several, because the code is degenerate — see Lesson 5)

         Amino acid
            |
            A
            C
            C — 3' end
            |
      [clover-leaf]
            |
        Anticodon
         X Y Z