DNA multiple choice question

HI, please could someone help me with this question. I thought the answer would be A since a change in the base can change the mRNA which will change the amino acid and thus polypeptide and it is also smaller than a nucleotide however the answer is D? Thanks!

Question: What is the smallest unit of a DNA molecule that can be altered by a mutation and cause a change to the coding of a polypeptide?
A: base
B: codon
C: gene
D; nucleotide

Honestly, Im not very confident in my answer but I’ll try my best…

Here is what we know

Nitrogenous bases are fundamentally the functional units of nucleotides. There are four of them in DNA - A T G C.

Nucleotides are the fundamental units of DNA consisting of base + phosphate + sugar

Mutation is defined as alteration in Nucleotides sequence with the smallest being point mutations which involve either substitution of a base or insertion/deletion of a base.

Now, it does seem like base when I say this but you need to understand one distinction here-

The nitrogenous base is not mutated since its structure is still intact. What IS changed however, is the nucleotide sequence due to the base being either substituted or in the wrong place. Mutation can also be understood as the alteration of something’s biochemistry which honestly would help you more out here.

Apologies.. my answer was me jotting down my thought process since even I was confused upon reading your question but let me elaborate-

Let’s take an example, adenine is a purine nitrogenous base, its heterocyclic, double ringed with nitrogen at position 1,3,7,9. I just defined the structure of a nitrogenous base.

Now, a nucleotide is the unit of DNA made of sugar, Phosphate and THIS nitrogenous base.

Now let’s take an example of Sickles cell anaemia- It is caused by single base substitution (GAG to GUG). This is a mutation in the nucleotide sequence because the chemical nature of the bases uracil and adenine are completely intact.
What is changes however is the sequence of base —- keynote sequence because fundamentally the nature of the bases are very much intact. But the nucleotide sequence is now mutated resulting in the attachment of Valine instead of glutamic acid

Apologies.. my answer was me jotting down my thought process since even I was confused upon reading your question but let me elaborate-

Let’s take an example, adenine is a purine nitrogenous base, its heterocyclic, double ringed with nitrogen at position 1,3,7,9. I just defined the structure of a nitrogenous base.

Now, a nucleotide is the unit of DNA made of sugar, Phosphate and THIS nitrogenous base.

Now let’s take an example of Sickles cell anaemia- It is caused by single base substitution (GAG to GUG). This is a mutation in the nucleotide sequence because the chemical nature of the bases uracil and adenine are completely intact.
What is changes however is the sequence of base —- keynote sequence because fundamentally the nature of the bases are very much intact. But the nucleotide sequence is now mutated resulting in the attachment of Valine instead of glutamic acid

Hi, thanks for this, I understand what you mean but please could you expand on what you mean by the chemical nature of the bases being intact? Also, isn’t it actually base sequence because everything in a nucleotide stays constant except the bases which changes?

(Note- nucleotide includes the base so change in base means nucleotide changes as well)

Okay, maybe the example of substituting bases was a bit confusing so let’s use addition-

Original nucleotide sequence -
AGATCG
After mutation-
AGATCCG

Now, a nucleotide with a base of C has been attached to the mutated chain.

Chemical formula of cytosine is first chain-
C4H5N3O

Chemical formula of cytosine in mutated chain-
C4H5N3O

Just for the sake of clarity-
The two codons of the first chain are- AGA and TCG
The two codons of the mutated chain are- AGA and TCC

These will ofcourse code for different polypeptide chains,and not because of the chemical nature of the bases but a completely different base being present i.e a different nucleotide being present.