Ribonucleic Acid (RNA): MDCAT Biology notes
Ribonucleic acid (RNA) MDCAT notes: nucleotides, RNA vs DNA, mRNA, tRNA and rRNA, transcription, translation, codons and amino acid counts.
Nucleotides: the building blocks
A nucleoside is a nitrogenous base joined to a pentose sugar, for example adenosine. A nucleotide is a nucleoside with a phosphate group added. Nucleotides link into polynucleotide chains through phosphodiester bonds (C–O–P–O–C), which form a sugar-phosphate backbone.
- ATP is a nucleotide with three phosphates. It can be called a nucleoside triphosphate: adenosine plus three phosphates.
- NAD is a dinucleotide, two nucleotides joined together.
- Adenine and guanine on their own are bases, not nucleotides.
RNA compared with DNA
| Feature | RNA | DNA |
|---|---|---|
| Sugar | Ribose | Deoxyribose |
| Bases | A, G, C, uracil | A, G, C, thymine |
| Strands | Usually single | Double helix |
| Backbone | Sugar-phosphate | Sugar-phosphate |
Types of RNA
| Type | Share of cell RNA | Role |
|---|---|---|
| rRNA (ribosomal) | Largest, about 80% | Combines with proteins to form ribosomes, the site of protein synthesis |
| tRNA (transfer) | About 15% | Brings specific amino acids to the ribosome; its anticodon pairs with the mRNA codon |
| mRNA (messenger) | Smallest, a few percent | Carries the genetic information copied from DNA to the ribosome |
So the order of abundance among the RNAs is rRNA > tRNA > mRNA.
Transcription
Transcription is making an RNA copy of a gene's DNA sequence. In eukaryotes it happens in the nucleus. The enzyme is RNA polymerase. It uses one DNA strand as a template and pairs bases by complementarity, with uracil in place of thymine:
- DNA A pairs with RNA U; DNA T pairs with RNA A; G pairs with C.
- Worked example: for the template DNA 3′-TACGGA-5′, the mRNA is 5′-AUGCCU-3′.
Transcription produces mRNA (and tRNA and rRNA). It does not make amino acids and it is not semi-conservative; that term describes DNA replication.
Translation and the genetic code
Translation uses the mRNA sequence to build a polypeptide at the ribosomes. The code is read in triplets (codons): three nucleotides specify one amino acid.
- Start codon: AUG (methionine).
- Stop codons: UAA, UAG, UGA. They code for no amino acid.
Counting amino acids
$$\text{amino acids} = \frac{\text{coding nucleotides}}{3}\ (-1 \text{ if the stop codon is included})$$
Worked example: A reading frame of 600 nucleotides that includes a stop codon has 200 codons. The stop codon adds no amino acid, so the polypeptide has 199 amino acids. If a question gives a plain nucleotide count with no mention of a stop codon, simply divide by 3.
Common MDCAT traps
- RNA has ribose and uracil. Thymine and deoxyribose belong to DNA.
- rRNA is the most abundant RNA, while mRNA carries the message.
- Transcription uses RNA polymerase, not DNA polymerase or a "transcriptase".
- If the stop codon is included, subtract one codon before counting amino acids.
- When writing mRNA from DNA, every A on the template gives U, never T.
Quick revision
- Nucleotide = base + pentose + phosphate.
- ATP is a nucleotide; NAD is a dinucleotide.
- Transcription goes DNA → RNA; translation goes mRNA → protein.
- UAA is a stop codon and AUG is the start codon.
- One codon = three nucleotides = one amino acid.