Side by side
| DNA | RNA | |
|---|---|---|
| Sugar | Deoxyribose, which carries hydrogen at the 2′ position | Ribose, which carries a hydroxyl group at the 2′ position |
| Bases | Adenine, thymine, guanine and cytosine | Adenine, uracil, guanine and cytosine; uracil replaces thymine |
| Strands | A double helix whose two strands run antiparallel, 5′ to 3′ against 3′ to 5′ | Usually single stranded |
| Base pairing | A pairs with T through two hydrogen bonds, G with C through three | Pairs with a DNA template during transcription, with A on DNA matched by U on RNA |
| Location in a eukaryotic cell | Stays in the nucleus and does not leave it | Made in the nucleus, then carried out to the cytoplasm to be used |
| Main role | Carries the genetic information for the development and functioning of the organism | Mostly involved in protein synthesis, under the direction of DNA |
| Types | One molecule type, organized into chromosomes | Several, including mRNA, rRNA, tRNA and miRNA |
Two differences in the monomer, several in the consequence
Every nucleotide in either polymer has the same three parts: a nitrogenous base, a five-carbon sugar and a phosphate group. Only two things differ between DNA and RNA at that level. The sugar is deoxyribose in DNA and ribose in RNA, and the difference is a single hydroxyl group at the 2′ position. The base set is A, T, G, C in DNA and A, U, G, C in RNA, with uracil standing in for thymine. Everything else follows from those two substitutions plus the fact that DNA is normally double stranded and RNA normally is not, including where each one is found and what it can do.
Where students go wrong
The first error is thinking uracil pairs with something different. Uracil occupies thymine's position and pairs with adenine, so a DNA strand reading AATTGCGC transcribes to UUAACGCG. The second is calling RNA single stranded as if it never pairs at all; it is normally a single chain, but it base pairs with a DNA template during transcription and folds back on itself in tRNA and rRNA. The third is writing a complementary strand without minding direction. The strands of a double helix are antiparallel, so the 5′ end of one faces the 3′ end of the other, and ignoring that inverts the answer.
Where this leads next
The reason RNA exists in this form is the central dogma: DNA in the nucleus is transcribed into mRNA, which travels to the cytoplasm and is translated into protein. That is the transcription and translation page. Once the mRNA is in the cytoplasm, reading it means grouping its bases into triplets and looking each one up, which is the codon chart page. Both depend on the base and sugar facts above, and in particular on uracil, since every codon a student ever looks up is written in RNA rather than DNA.
Common mistakes
- Pairing uracil with guanine or cytosine. Uracil takes thymine's place opposite adenine.
- Saying RNA is never double stranded. It is usually single stranded, but it pairs with a DNA template during transcription and folds on itself in tRNA and rRNA.
- Ignoring the antiparallel arrangement when writing a complementary strand, which reverses the answer.
- Treating RNA as one molecule. mRNA, rRNA, tRNA and miRNA have different jobs in protein synthesis and its regulation.
Related concepts
- Codon chart: A codon chart maps each three-nucleotide mRNA codon to the amino acid it specifies.
- Transcription vs translation: Transcription copies a gene's DNA sequence into messenger RNA, one nucleotide at a time.
- Prokaryotic vs eukaryotic cells: All cells share four components: a plasma membrane, cytoplasm, DNA and ribosomes.