Lesson 1 of 6 / DNA replication and gene expression
DNA and the three RNA roles
How does nucleic-acid structure store and use information?
In this lesson: Describe DNA, mRNA, tRNA and rRNA structure and roles.
About 7 min
The key ideaA nucleotide sequence stores information; complementary base pairing enables copying and recognition.
Explore the idea
Zoom from one unit to paired strands
Adjacent nucleotides join through phosphodiester bonds in the sugar-phosphate backbone. The variable base carries sequence information.
Compare mRNA, tRNA and rRNA
Explore the idea
Match the RNA to its job
The message carries codons. A ribosome reads them in the 5'-to-3' direction.
Original organisation models. RNA uses ribose and uracil and can fold through internal base pairing. Subunit shapes are illustrative.
DNA uses deoxyribose and thymine; RNA uses ribose and uracil. In a tRNA, the amino acid attaches at the 3-prime end. The anticodon lies in a different region.
Explanation
A nucleotide contains a pentose sugar, phosphate and nitrogenous base. DNA uses deoxyribose and A, T, C and G; RNA uses ribose and U instead of T. Covalent phosphodiester bonds join sugars and phosphates into a directional backbone with 5-prime and 3-prime ends.
DNA normally has two antiparallel strands in a double helix. Complementary bases pair through hydrogen bonds: A with T and C with G. The sequence can vary while the backbone remains regular, allowing information storage; complementarity allows one strand to guide synthesis of another.
mRNA is a usually single-stranded transcript carrying codons used during translation. A tRNA folds by internal base pairing into a specific three-dimensional shape, with an anticodon recognising an mRNA codon and an amino-acid attachment site at its 3-prime end. A charged tRNA carries its corresponding amino acid.
rRNA combines with proteins to form ribosomes and has structural and catalytic roles, including peptide-bond formation. A ribosome is therefore not made solely of protein. RNA can have extensive folded regions even though it is usually described as single-stranded.
Step by step
- 1
Identify the monomer
Name sugar, phosphate and base.
- 2
Distinguish backbone and pairing
Phosphodiester bonds link a strand; hydrogen bonds join complementary bases.
- 3
Connect RNA shape to role
Locate a tRNA anticodon and amino-acid attachment site.
Worked example
Work through the evidence
A DNA strand is 5-prime ATGC 3-prime. Write its complementary strand with directions.
One way to explain it
3-prime TACG 5-prime. If written in the usual 5-prime to 3-prime direction, it is 5-prime GCAT 3-prime.
Why this answer works
- Pair A with T and G with C.
- Reverse orientation because the strands are antiparallel.
Is this true? "The two DNA strands run in the same direction because the bases match."
Complementarity concerns base identity; the backbones are antiparallel.