AP Biology · Unit 6 · Topic 6.3
Pick the wrong strand as the template and you get the wrong message — so pick it, and see what you get. Then cut the introns out and count how many proteins one gene can specify.
Learning objective 6.3.A Describe the mechanisms by which genetic information flows from DNA to RNA.
A gene is written as its coding strand, but that is not the strand RNA polymerase reads — it reads the other one. Choose a strand and see what comes out. Getting this wrong is not a near miss.
Read the correct mRNA and the coding strand side by side in the drawing. They are the same sequence, letter for letter, with U wherever the DNA has T. That is not a coincidence and it is not a rule to memorise separately — the RNA is complementary to the template, and so is the coding strand, so the two must match.
Switch between the two options and watch the mismatch count. It is not one base in ten or one base in three — it is every base, because the wrong RNA is the complement of the right one. A protein built from it would have no relationship at all to the intended one.
The choice is not made base by base. The promoter sits at one end of the gene and can only be bound one way round, so once RNA polymerase is in place the strand and the direction are both already settled. Different genes on the same chromosome can use different strands as their template, because they have their own promoters pointing their own way.
The primary transcript is a copy of the whole gene, introns and all. Splicing removes the introns and joins the exons; a cap and a tail are added at the ends. Switch the optional exons in and out and count the different mRNAs this one gene can make.
Each optional exon is either in or out, and the choice for one says nothing about the choice for another. Independent binary choices multiply, so n of them give 2n outcomes — which is exactly the argument that gave 223 gametes from independent assortment.
Look at the readout: the mature mRNA is longer than the exons it was assembled from, even though splicing only ever removes material. The extra length is the poly-A tail, which is added afterwards. Nothing in the DNA sequence corresponds to it, which is the clearest evidence that processing is not just editing.
The definitions are about fate rather than sequence. An optional exon in one splicing pattern is spliced out like an intron in another — so “exon” is not a permanent property of a stretch of DNA, it is a description of what happened to it in a particular transcript.
Both copy DNA using base pairing, and there the resemblance stops. Almost every row of this table differs — and the one that does not is worth as much attention as the ones that do.
| Replication | Transcription |
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This one difference explains a great deal. DNA polymerase's inability to start a chain is what makes primase necessary, what makes the lagging strand discontinuous, and what makes ligase necessary. RNA polymerase has none of those problems, so transcription has no equivalent of Okazaki fragments at all.
Replication is all-or-nothing and happens once per cell cycle: every base of both strands, exactly once. Transcription is selective and repeatable: one gene, one strand, as many times as the cell needs the product. That difference is the whole of Topic 6.5 in advance — regulation is possible because transcription is optional in a way replication is not.
Both build 5′ to 3′. It was also the one row DNA and RNA shared in Topic 6.1. Every nucleic acid strand a cell makes is built by adding to a 3′ end, without exception, and a surprising number of the details in this unit are consequences of that single constraint.
Answer these in your own words before you open the answers.
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| Step | What happens | Where |
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A question that gives you a length and asks for another length is asking you to know which steps add material and which remove it. Only one step here adds any.