Lesson 1 of 6 / DNA replication and gene expression
DNA and the three RNA roles
How does nucleic-acid structure store and use information?
I can describe DNA, mRNA, tRNA and rRNA structure and roles.
I can explain DNA structure and the roles and structures of mRNA, tRNA and rRNA.Syllabus
Syllabus 9477, Core 2(a). Explain DNA structure and the roles and structures of mRNA, tRNA and rRNA.
About 7 min
Make a guess. You are not marked.
Ribosomes make proteins. Are ribosomes themselves made only of protein?
Show the answer
No. They are largely rRNA, which helps catalyse peptide bonds.
Ribosomes combine rRNA with proteins. The rRNA has structural and catalytic roles.
The key idea
A 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.
Watch out for this
A student says: "The two DNA strands run in the same direction because the bases match." What is wrong with this?
Show the answer
Complementarity concerns base identity; the backbones are antiparallel.
Check your understanding
Which RNA directly matches an anticodon to an mRNA codon?
Show the answer
tRNA
A charged tRNA links codon recognition to delivery of an amino acid.
Quick recall
How does nucleic-acid structure store and use information?