Why is the code a triplet and not a doublet?

A doublet gives only 16 combinations, fewer than the 20 amino acids; a triplet gives 64.

What does universal mean for the genetic code?

The same triplet codes for the same amino acid in all organisms.

What does non-overlapping mean?

Each base is part of only one triplet; bases are read in order, three at a time.

What does degenerate mean?

Most amino acids are coded for by more than one triplet.

The complete set of genes in a cell.

The full range of proteins that a cell is able to produce.

Describe the structure of mRNA.

A single-stranded, linear polynucleotide of nucleotides with ribose and the bases A, U, G, C, arranged in codons.

Describe the structure of tRNA.

A single polynucleotide strand folded into a clover-leaf, held by hydrogen bonds, with an anticodon and an amino acid binding site.

What is an anticodon?

Three bases on tRNA that pair with a complementary codon on mRNA.

The genetic code and mRNAAQA A-Level Biology: Flashcards

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What is a triplet?

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What is a triplet?: A sequence of three DNA bases that codes for a specific amino acid.
What is a codon?: A sequence of three bases on mRNA that codes for an amino acid or a stop signal.
Why is the code a triplet and not a doublet?: A doublet gives only 16 combinations, fewer than the 20 amino acids; a triplet gives 64.
What does universal mean for the genetic code?: The same triplet codes for the same amino acid in all organisms.
What does non-overlapping mean?: Each base is part of only one triplet; bases are read in order, three at a time.
What does degenerate mean?: Most amino acids are coded for by more than one triplet.
Define the genome.: The complete set of genes in a cell.
Define the proteome.: The full range of proteins that a cell is able to produce.
Describe the structure of mRNA.: A single-stranded, linear polynucleotide of nucleotides with ribose and the bases A, U, G, C, arranged in codons.
Describe the structure of tRNA.: A single polynucleotide strand folded into a clover-leaf, held by hydrogen bonds, with an anticodon and an amino acid binding site.
What is an anticodon?: Three bases on tRNA that pair with a complementary codon on mRNA.
How many bases code for a polypeptide of 100 amino acids (without the stop codon)?
Which base does RNA have in place of thymine?: Uracil.

Exam questions on The genetic code and mRNA

Bacteria have been genetically modified to make human insulin by inserting the human insulin gene into their DNA. Mature insulin consists of two polypeptide chains: the A chain has 21 amino acids and the B chain has 30 amino acids.
Explain why a code of two bases per amino acid could not code for all of the amino acids used to make proteins, and why a triplet code can.2 marks
A short region of mRNA has the base sequence AUG GCU UCC AAG UAA. Part of the genetic code is as follows: AUG codes for methionine (and starts translation); GCU and GCC both code for alanine; UCC and UCU both code for serine; AAG and AAA both code for lysine; UAA is a stop codon.
Give the base sequence of the DNA template strand that was transcribed to make the first two codons (AUG GCU), written in the same order.2 marks
A student is comparing the two types of RNA that take part in making a polypeptide from the genetic information in a gene: messenger RNA (mRNA) and transfer RNA (tRNA).
Describe the structure of a molecule of mRNA.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).