Question

part 2: the following mutation is found in the somatic cell of an organism
original dna: atg - cgg - tta
mutated: atg - cgg - ttt
after transcription, what would be the correct mrna strand of the original and mutated sequence?
○ original: tta, mutated: ttt
○ original: aaa, mutated: aau
○ original: aat, mutated: aaa
○ original: aau, mutated: aaa

Explanation:

Step1: Recall Transcription Rules

During transcription, DNA is used to make mRNA. The base - pairing rules for transcription are: DNA's adenine (A) pairs with mRNA's uracil (U), DNA's thymine (T) pairs with mRNA's adenine (A), DNA's cytosine (C) pairs with mRNA's guanine (G), and DNA's guanine (G) pairs with mRNA's cytosine (C). Also, we need to transcribe the non - coding strand (the strand that is complementary to the coding strand, but for transcription, we use the DNA template strand. However, in the given problem, we can look at the relevant part of the DNA sequence for transcription. The original DNA has a sequence segment: for the part that is being transcribed (the last three bases, since the start is ATG - CGG - [TTA or TTT]), the DNA bases for transcription (the template strand, but here we can consider the complementary of the DNA coding strand? Wait, actually, the DNA sequence given: Original DNA: ATG - CGG - TTA, Mutated: ATG - CGG - TTT. We need to transcribe the last three bases (TTA for original, TTT for mutated) into mRNA.

Step2: Transcribe Original DNA (TTA) to mRNA

For the original DNA sequence's last three bases TTA:

• T (DNA) pairs with A (mRNA)
• T (DNA) pairs with A (mRNA)
• A (DNA) pairs with U (mRNA)

So the mRNA sequence for original (the part corresponding to TTA) is AAU? Wait, no, wait. Wait, maybe I got the template strand wrong. Wait, the DNA has a coding strand and a template strand. The coding strand has the same sequence as the mRNA (except T is replaced with U). Wait, maybe that's a better way: the coding strand of DNA has the same sequence as the mRNA, with T replaced by U. Let's check:

Original DNA (coding strand) segment: TTA. So mRNA would be UUA? No, that's not matching the options. Wait, maybe the problem is looking at the complementary of the DNA strand. Wait, the DNA sequence: ATG - CGG - TTA. Let's consider the DNA strand that is transcribed. The RNA polymerase reads the DNA template strand (which is complementary to the coding strand). So the template strand for the last three bases (TTA) of the original DNA: the template strand would be AAT (since coding strand is TTA, template is AAT, because A - T, T - A, T - A? Wait, no, A pairs with T, T pairs with A, C pairs with G, G pairs with C. So coding strand: T T A, template strand: A A T. Then mRNA is transcribed from template strand: A (template) pairs with U (mRNA), A (template) pairs with U (mRNA), T (template) pairs with A (mRNA). So mRNA would be UUA? No, that's not in the options. Wait, maybe the problem is considering the DNA sequence as the template strand. Let's try that. If the DNA sequence (template strand) is TTA (original), then mRNA is AAU (because T - A, T - A, A - U). For the mutated DNA (template strand TTT), mRNA is AAA (T - A, T - A, T - A). Wait, let's check the options. The option is Original: AAU, Mutated: AAA. Let's verify:

Original DNA (template strand) TTA:

• T (DNA) → A (mRNA)
• T (DNA) → A (mRNA)
• A (DNA) → U (mRNA)

So mRNA: AAU.

Mutated DNA (template strand) TTT:

• T (DNA) → A (mRNA)
• T (DNA) → A (mRNA)
• T (DNA) → A (mRNA)

So mRNA: AAA.

Yes, that matches the option "Original: AAU, Mutated: AAA".

Answer:

Original: AAU, Mutated: AAA (the option with "Original: AAU, Mutated: AAA")