AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The structure of molecules of messenger RNA (mRNA) and of transfer RNA…
Full specification wording
DNA and protein synthesis: The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA).
Both are single-stranded polynucleotides built from RNA nucleotides: ribose, a phosphate group and one of adenine, uracil, cytosine or guanine, joined by phosphodiester bonds. mRNA is a long,…
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 2 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Both are single-stranded polynucleotides built from RNA nucleotides: ribose, a phosphate group and one of adenine, uracil, cytosine or guanine, joined by phosphodiester bonds. mRNA is a long, linear molecule transcribed from one gene, so its length varies with the gene. It has no hydrogen bonding along its length, so its bases are exposed and can be read in threes as codons. It is short-lived, broken down after translation so that a protein is not made indefinitely. tRNA is far shorter, around eighty nucleotides, and folds back on itself. Hydrogen bonds between complementary bases within the same strand pull it into a clover-leaf shape, with one loop carrying three exposed bases called the anticodon and the opposite end carrying an amino acid binding site.
What earns the marks
- mRNA being linear with exposed bases read as triplets called codons, one mRNA carrying the code for one polypeptide.
- tRNA being folded into a clover-leaf shape held by hydrogen bonds between complementary bases in the same strand.
- tRNA having an anticodon of three exposed bases and a binding site that attaches a specific amino acid.
- A stated comparison such as tRNA containing few nucleotides while mRNA contains many.
Common mistakes and how to put them right
Writing deoxyribose or thymine into an RNA answer instead of ribose and uracil.
Calling tRNA double-stranded because it contains hydrogen bonds, when it is one strand folded back on itself.
Saying the anticodon is identical to the codon rather than complementary to it.
Stating that tRNA carries 'an amino acid' without saying a specific amino acid determined by its anticodon.
Placing the amino acid binding site on the anticodon loop.
Exam tips
- In a comparison table write both sides of every difference, such as 'DNA is double-stranded whereas tRNA is single-stranded'; a one-sided statement scores nothing.
- AQA applies the list rule here, so give exactly the number of differences asked for and no more.
- Bank three secure differences you can always reach for: the sugar and base, the number of strands, and the overall shape.
Try the lesson
The start of the DNA and protein synthesis: The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA). lesson, made from checked teaching notes for this specification point.
Have a go first
Which sugar is found in both mRNA and tRNA?
Listen
Both are single-stranded polynucleotides built from RNA nucleotides: ribose, a phosphate group and one of adenine, uracil, cytosine or guanine, joined by phosphodiester bonds. mRNA is a long, linear molecule transcribed from one gene, so its length varies with the gene. It has no hydrogen bonding along its length, so its bases are exposed and can be read in threes as codons. It is short-lived, broken down after translation so that a protein is not made indefinitely. tRNA is far shorter, around eighty nucleotides, and folds back on itself. Hydrogen bonds between complementary bases within the same strand pull it into a clover-leaf shape, with one loop carrying three exposed bases called the anticodon and the opposite end carrying an amino acid binding site. A strong answer earns marks for these points. mRNA being linear with exposed bases read as triplets called codons, one mRNA carrying the code for one polypeptide. tRNA being folded into a clover-leaf shape held by hydrogen bonds between complementary bases in the same strand. tRNA having an anticodon of three exposed bases and a binding site that attaches a specific amino acid. a stated comparison such as tRNA containing few nucleotides while mRNA contains many. Exam tip: In a comparison table write both sides of every difference, such as 'DNA is double-stranded whereas tRNA is single-stranded'; a one-sided statement scores nothing.
Key words
Say what each word means before you turn the card.
codons and anticodons
Try answering aloud or in your head before turning the card.
clover-leaf shape
Try answering aloud or in your head before turning the card.
amino acid binding site
Try answering aloud or in your head before turning the card.
See every definition
- codons and anticodons
- Codons are exposed bases read in threes on mRNA, while the anticodon is three exposed bases carried on one loop of tRNA.
- clover-leaf shape
- The form tRNA takes when hydrogen bonds between complementary bases within the same strand pull it into a folded structure.
- amino acid binding site
- The site at the opposite end of tRNA from the anticodon loop, which attaches a specific amino acid.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of DNA and protein synthesis: The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA).
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge