Question

In studies of frameshift mutations, Crick, Barnett, Brenner, and Watts-Tobin found that either three nucleotide insertions or deletions restored the correct reading frame. (a) Assuming the code is a triplet, what effect would the addition or loss of six nucleotides have on the reading frame? (b) If the code were a sextuplet (consisting of six nucleotides), would the reading frame be restored by the addition or loss of three, six, or nine nucleotides?

Short answer

In a sextuplet code, is the reading frame restored by adding or losing 3, 6, or 9 nucleotides? Answer: In a triplet code, adding or losing six nucleotides will have no effect on the reading frame since they represent complete codons. In a sextuplet code, the reading frame will only be restored by adding or losing six nucleotides.

Step-by-step solution

Part (a) - Effect of adding or losing six nucleotides in a triplet code

In the case of a triplet code, each codon consists of three nucleotides. Now, let's analyze the addition or loss of six nucleotides. 1. Addition of six nucleotides: - In this scenario, we add six nucleotides, which represent two extra codons. This means that their effect on the reading frame would be neutral since they create complete codons and the original reading frame is maintained. 2. Loss of six nucleotides: - In this scenario, we remove six nucleotides. This means two complete codons are removed from the sequence. Again, their effect on the reading frame would be neutral since by removing two complete codons, the original reading frame remains unchanged. So, adding or losing six nucleotides will have no effect on the reading frame in a triplet code.

Part (b) - Restoration of the reading frame by adding or losing 3, 6, or 9 nucleotides in a sextuplet code

In the case of a sextuplet code, each codon consists of six nucleotides. Now, let's analyze the restoration of the reading frame by adding or losing three, six, or nine nucleotides. 1. Addition or loss of three nucleotides: - In this scenario, we are either adding or losing three nucleotides, which would disrupt the reading frame since each codon in the sextuplet code must contain six nucleotides. So, adding or losing three nucleotides will not restore the reading frame. 2. Addition or loss of six nucleotides: - In this scenario, we are either adding or losing six nucleotides, which represent a complete codon in the sextuplet code. By either adding or losing a complete codon, the reading frame is unaffected. Thus, the reading frame is restored by adding or losing six nucleotides. 3. Addition or loss of nine nucleotides: - In this scenario, we are either adding or losing nine nucleotides. However, this represents one and a half codons in the sextuplet code, which results in a frameshift mutation and disruption of the reading frame. So, adding or losing nine nucleotides will not restore the reading frame. Therefore, the reading frame in a sextuplet code will only be restored by the addition or loss of six nucleotides.

Key concepts

Frameshift Mutation

A frameshift mutation occurs when a number of nucleotides that is not a multiple of three is inserted or deleted from a DNA sequence. This disrupts the triplet reading framework that our cells use to decode DNA into proteins.

For instance, if one or two nucleotides are inserted or deleted, the grouping of three-nucleotide codons is altered, which can change every codon downstream from the mutation point. This can result in the production of defective proteins or even stop the protein from being made altogether.

Frameshift mutations frequently result in significant changes to the amino acid sequence of a protein, potentially leading to loss of function or harmful gain of function. It’s a stark reminder that the genetic code is highly regulated and even small changes can have vast molecular consequences.

Reading Frame

The reading frame is a way of dividing the sequence of nucleotides into a set of consecutive, non-overlapping triplets, or codons. Each codon represents a specific amino acid or a stop signal during protein synthesis.

There are three possible reading frames in any sequence (each shifted by one nucleotide), which means that the same sequence can be read in different ways depending on where you start. Only one of these frames will correctly encode a functional protein. Starting at the wrong base can lead to non-functional proteins, underscoring the importance of maintaining the correct reading frame.

Mutations that insert or delete nucleotides can shift the reading frame. This is why the concept of frameshift mutations is highly relevant in genetic and protein studies.

Triplet Codon

A triplet codon consists of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule. Each triplet codon specifies a particular amino acid that will be added to the growing protein chain during protein synthesis.

The "triplet" nature of codons is central to the operation of the genetic code, as it provides 64 (4^3) possible codons, which is more than enough to specify the 20 standard amino acids and stop signals.

This coding system is universal among most organisms, which emphasizes its evolutionary importance. It is like a language that cells use to communicate instructions for building proteins, making it crucial to understand how slight changes, such as frameshift mutations, can disrupt this delicate communication.

Sextuplet Code

The concept of a sextuplet code is a theoretical concept where each codon would consist of six nucleotides instead of three. This would greatly expand the potential combination of codons (4^6), offering a larger variety set without actually existing in nature.

If the genetic code operated as a sextuplet, the even distribution of nucleotide insertions or deletions matters. To avoid disturbing the reading frame, any addition or deletion must encompass complete sextuplets.

In such a framework, adding or losing six nucleotides (an entire sextuplet code) would keep the reading frame intact, just as adding or subtracting three nucleotides works with the natural triplet code. Understanding theoretical models like the sextuplet code helps underscore the precision required for effective protein synthesis.