Mutations and Genetic Code

The Link Between Genes and DNA

The relationship between genes and DNA is best understood by studying mutations. You have already encountered mutations and their effects in Chapter 4. Large-scale changes — like deletions or rearrangements of a long DNA segment — are easy to grasp: they can cause the loss or gain of an entire gene, and therefore the loss or gain of a function. But what about tiny, single-letter changes? Those are called point mutations, and they reveal the most precise details of how the genetic code works.

The Classic Example: Sickle Cell Anemia

A point mutation is a change in a single base pair. The textbook’s classic example is in the gene for the beta globin chain of haemoglobin. A single base-pair substitution changes the amino acid glutamate to valine at one position in the protein. This one change produces the disease sickle cell anemia. It shows that even a one-letter change in DNA can have a dramatic biological effect.

Frameshift Mutations: Insertions and Deletions

To understand what happens when a base is inserted or deleted, the book gives a simple word analogy. Think of a sentence made of three-letter words — like the genetic code itself:

RAM HAS RED CAP

Now, insert one extra letter (B) between HAS and RED, and rearrange the words into three-letter groups:

RAM HAS BRE DCA P

The meaning is destroyed from the point of insertion onward. The reading frame has shifted.

If you insert two letters at the same place (say, BI):

RAM HAS BIR EDC AP

Again, the frame shifts and the message becomes nonsense.

But if you insert three letters together (say, BIG):

RAM HAS BIG RED CAP

The original sentence is preserved after the insertion. The reading frame is not disturbed.

The same exercise works for deletions. Delete the letters R, E, and D one by one from the original sentence and rearrange:

• Delete R: RAM HAS EDC AP
• Delete E: RAM HAS DCA P
• Delete D: RAM HAS CAP

Each single deletion shifts the frame; the final deletion of all three letters restores the frame.

The Key Conclusion

• Insertion or deletion of one or two bases changes the reading frame from that point onward. These are called frameshift mutations. …