Step by Step
Tx
Transcription — DNA to mRNA, in the nucleus
Transcription begins with DNA unwinding, followed by RNA polymerase reading the template strand and building a complementary mRNA strand (pairing A with U, T with A, G with C, and C with G). This entire process happens inside the nucleus.
Proc
mRNA processing — preparing for export
Before mRNA can leave the nucleus, it's processed: introns (non-coding intervening sequences) are removed, exons (the actual coding sequences) are spliced together, and a 5' cap plus a poly-A tail are added. Only after this processing does the mature mRNA exit through the nuclear pores.
Tl
Translation — mRNA to protein, at the ribosome
At the ribosome, mRNA codons (each a sequence of 3 bases) are read one at a time. tRNA molecules bring the matching amino acid, matching their anticodon to the mRNA's codon. As each amino acid is added, peptide bonds form, building the growing chain, until the finished polypeptide is released.
Cod
Codons and mutations
There are 64 possible codons: 61 code for amino acids (with the genetic code being redundant, since 20 amino acids need to be covered), and 3 are stop codons (UAA, UAG, UGA) that signal translation to end. Mutations — substitutions, insertions, or deletions in the DNA sequence — can alter the resulting protein's structure and function.
A single DNA substitution mutation might change one codon into a different codon that still specifies the same amino acid (due to the genetic code's redundancy), producing no functional change at all — illustrating why not every mutation actually alters the resulting protein.
Applied Walkthrough
1
A researcher identifies a single-base mutation in a patient's DNA but finds the resulting protein is completely unchanged and functions normally.
2
Ask: how is this possible, given that a mutation clearly did occur? Because the genetic code is redundant — with 61 codons coding for only 20 amino acids, several different codons can specify the exact same amino acid. If the mutation happened to change a codon into a different codon that still codes for the identical amino acid, the resulting protein sequence would be completely unaffected.
3
Contrast: if the same single-base change had instead altered a codon into one of the 3 stop codons (UAA, UAG, UGA), that would prematurely terminate translation entirely, very likely producing a severely truncated, non-functional protein instead — a dramatically different outcome from the same type of single mutation.
4
This contrast — a 'silent' mutation with no functional consequence versus a mutation that creates a premature stop codon — illustrates why not all DNA mutations are equally significant, and why understanding codon redundancy matters for predicting a mutation's actual impact.
Exam Application
Exams test the correct sequence of transcription and translation (DNA → mRNA in the nucleus → processed mRNA exits → protein built at the ribosome), the specific base-pairing rules in transcription, the role of introns/exons in mRNA processing, and the codon/anticodon matching system along with the concept of stop codons.
⚠ Common Trap
The most common trap is assuming every DNA mutation necessarily changes the resulting protein. Due to the redundancy of the genetic code (61 codons for just 20 amino acids), some mutations are silent, producing no change in the resulting amino acid sequence at all — while others, like those creating a premature stop codon, can have a dramatic effect.
✓ Quick Self-Check
1. Where does transcription take place, and what enzyme carries it out?
In the nucleus; RNA polymerase carries it out.
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2. What happens to mRNA during processing, before it exits the nucleus?
Introns are removed, exons are spliced together, and a 5' cap and poly-A tail are added.
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3. Where does translation take place, and what role does tRNA play?
At the ribosome; tRNA brings the amino acid matching its anticodon to the mRNA's codon.
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4. How many total codons exist, and how many code for amino acids versus serving as stop codons?
64 total codons; 61 code for amino acids, and 3 (UAA, UAG, UGA) are stop codons.
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5. Why might a single DNA mutation sometimes have no effect on the resulting protein?
Because the genetic code is redundant — multiple different codons can specify the same amino acid, so some mutations don't actually change the resulting amino acid sequence.
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