A gene is a stretch of DNA that codes for one polypeptide. The cell copies it into mRNA in the nucleus, edits out the introns, and then a ribosome in the cytoplasm reads the mRNA three bases at a time, while tRNA molecules deliver the matching amino acids.
A polypeptide is coded for by a gene, a sequence of nucleotides forming part of a DNA molecule. The genetic code is a triplet code: each group of three bases codes for one amino acid, or acts as a start or stop signal, and it is universal (the same in almost all organisms). In transcription, in the nucleus, RNA polymerase separates the strands of the gene and builds mRNA complementary to the template (transcribed) strand; the other is the non-transcribed strand. In eukaryotes the primary transcript is modified: introns (non-coding) are removed and exons (coding) are joined. In translation, at a ribosome in the cytoplasm, each tRNA carries a specific amino acid and has an anticodon complementary to an mRNA codon. Starting at AUG, amino acids are joined by peptide bonds until a stop codon is reached.
Fig. 1: the first three codons of the gene in the lab, from DNA to mRNA to tRNA to amino acids
Section 01
Transcription: a gene copied into mRNA.
Press Transcribe and watch RNA polymerase (green) move along the gene, opening the DNA just around itself. The sequence panel fills in the mRNA codon by codon: compare it with the coding and template strands printed above it.
Gene: a sequence of nucleotides that forms part of a DNA molecule and codes for one polypeptide.
RNA polymerase: binds at the start of the gene, breaks the hydrogen bonds so a short section of DNA unwinds, and moves along the gene.
Template (transcribed) strand: the one DNA strand whose bases are read. Free RNA nucleotides pair with it: A with U, T with A, C with G, G with C. The other strand is the non-transcribed (coding) strand.
Joining: RNA polymerase links the RNA nucleotides with phosphodiester bonds, so the mRNA has the same base sequence as the non-transcribed strand, but with U in place of T.
Behind the enzyme: the DNA rewinds and the mRNA peels away. It leaves the nucleus through a nuclear pore.
Strand in the lab
First three triplets
non-transcribed (coding) DNA, 5′ to 3′
ATG GCC TTC
template (transcribed) DNA, 3′ to 5′
TAC CGG AAG
mRNA, 5′ to 3′
AUG GCC UUC
Examiner trap
mRNA is complementary to the template strand, not to the coding strand. So the mRNA reads the same as the coding (non-transcribed) strand, with U instead of T. If you are given the template strand, pair every base and swap T for U.
Section 02
Translation: codons meet anticodons.
Press Translate. For each codon the panel shows the CODON, the tRNA ANTICODON, the AMINO ACID it carries and the growing POLYPEPTIDE. Watch what happens at the last codon, UAA.
Ribosome: the mRNA attaches to a ribosome in the cytoplasm (free or on rough ER). The ribosome holds two codons at a time and moves along the mRNA one codon at a time.
tRNA: each tRNA molecule carries one specific amino acid at one end and has an anticodon, three unpaired bases complementary to an mRNA codon, at the other.
Start: translation begins at the start codon AUG, which codes for methionine.
Elongation: a tRNA with the complementary anticodon pairs with the next codon by hydrogen bonds; a peptide bond forms between neighbouring amino acids, and the empty tRNA leaves to collect another amino acid.
Stop: at a stop codon (UAA, UAG or UGA) no tRNA binds, so the polypeptide is released.
Feature of the code
Meaning
triplet
three bases code for one amino acid; 43 = 64 triplets for 20 amino acids
start and stop
some triplets do not code for an amino acid in the chain but mark where to stop (and AUG marks where to start)
degenerate
most amino acids have more than one codon
non-overlapping
each base is read as part of one codon only
universal
the same triplets code for the same amino acids in almost all organisms
Section 03
Introns out, exons joined.
Press Remove the introns. The three grey introns lift away and fade, and the four coloured exons slide together to form the mRNA.
Primary transcript: in eukaryotes, the RNA made by transcription is a full copy of the gene, including non-coding sequences.
Introns: non-coding sequences within the gene; they are cut out of the primary transcript in the nucleus.
Exons: coding sequences; they are joined together, in order, to form the mature mRNA that leaves the nucleus.
Prokaryotes: their genes have no introns, so their mRNA can be translated as it is made.
Examiner trap
Remember which is which: exons are expressed (they stay in the mRNA); introns are in the way (they are removed). It is the RNA that is edited, not the DNA: the introns stay in the gene.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the transcription simulator, the coding strand starts 5′ ATG GCC. How does the mRNA start?
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In the translation simulator, which anticodon is shown for the codon GCC?
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What does the ANTICODON readout show when the ribosome reaches UAA?
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In the introns and exons simulator, which segments remain in the mature mRNA?
Stage 2 · Exam
Exam-style questions
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Finish the four checks above to unlock the exam questions
Exam style+20 XP
A template DNA strand reads 3′ TAC GGT CTA 5′. What is the sequence of the tRNA anticodons used to translate the mRNA made from it?
Exam style+20 XP
Which statement describes the genetic code as universal?
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A primary transcript has 1500 nucleotides. Its introns total 600 nucleotides. The mRNA has a start codon and a stop codon. What is the maximum number of amino acids in the polypeptide?
Exam style+20 XP
What is the role of RNA polymerase?
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Transcription and translation, mastered.
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