Question

the dna code for normal hemoglobin contains the following segment: tgc-gga-ctc-ctc which of these is the mrna code for this segment of dna? a acg-ccu-gag-gag b tcc-ggt-ctc-ctc c acg-cct-gga-gaa d ugc-gua-cuc-cuc

Explanation:

Step1: Recall Transcription Rules

During transcription, DNA's thymine (T) pairs with mRNA's adenine (A), adenine (A) pairs with uracil (U), cytosine (C) pairs with guanine (G), and guanine (G) pairs with cytosine (C). Also, note that in mRNA, T is replaced by U, but for DNA to mRNA, we use base - pairing: A - U, T - A, C - G, G - C.

Step2: Transcribe Each DNA Triplet

- For the first DNA triplet TGC:
- T (in DNA) pairs with A (in mRNA), G (in DNA) pairs with C (in mRNA), C (in DNA) pairs with G (in mRNA)? Wait, no. Wait, the correct base - pairing for DNA to mRNA transcription: DNA has bases A, T, C, G. mRNA has A, U, C, G. So the pairing is: A (DNA) - U (mRNA), T (DNA) - A (mRNA), C (DNA) - G (mRNA), G (DNA) - C (mRNA).
- So for TGC:
- T (DNA) → A (mRNA)
- G (DNA) → C (mRNA)
- C (DNA) → G (mRNA)? Wait, no, I made a mistake. Wait, the correct transcription is: DNA template strand to mRNA. The base - pairing is A - U, T - A, G - C, C - G. So for each base in DNA:
- T (DNA) is transcribed to A (mRNA)
- G (DNA) is transcribed to C (mRNA)
- C (DNA) is transcribed to G (mRNA)? No, wait, let's take the DNA sequence TGC - GGA - CTC - CTC.
- First triplet: TGC
- T (DNA) → A (mRNA)
- G (DNA) → C (mRNA)
- C (DNA) → G (mRNA)? No, that's not right. Wait, the correct base - pairing for transcription (DNA to mRNA) is: A (DNA) pairs with U (mRNA), T (DNA) pairs with A (mRNA), C (DNA) pairs with G (mRNA), G (DNA) pairs with C (mRNA).
- So T (DNA) → A (mRNA)
- G (DNA) → C (mRNA)
- C (DNA) → G (mRNA)? Wait, no, let's do it step by step.
- T (DNA) and A (mRNA) (since T in DNA pairs with A in mRNA)
- G (DNA) and C (mRNA) (G in DNA pairs with C in mRNA)
- C (DNA) and G (mRNA) (C in DNA pairs with G in mRNA)? Wait, no, I think I messed up. Wait, the correct transcription: DNA has a template strand. The coding strand (the one we are given here, since it's the DNA code for hemoglobin) is complementary to the template strand. But when transcribing, we use the template strand. But if we are given the coding strand (the non - template strand, which has the same sequence as the mRNA except T is U), wait, no. Wait, the coding strand (sense strand) has the same sequence as the mRNA, except that T is replaced by U. Wait, that's a simpler way. So if the DNA coding strand is TGC - GGA - CTC - CTC, then the mRNA will have U instead of T? No, no. Wait, no: the template strand is complementary to the coding strand. The mRNA is complementary to the template strand. So the mRNA sequence is the same as the coding strand, except that T is replaced by U? No, that's not correct. Wait, let's recall:

The DNA double helix has two strands: the template strand (used for transcription) and the coding strand (which has the same sequence as the mRNA, except T is U). Wait, no, the coding strand (also called the non - template strand) has the same sequence as the mRNA, with T instead of U. So when transcribing, the RNA polymerase reads the template strand (3' to 5') and synthesizes mRNA (5' to 3') which is complementary to the template strand. So the mRNA sequence is the same as the coding strand (the one we are given) except that T is replaced by U? No, that's a confusion. Let's take an example:

If the DNA coding strand (non - template) is 5' - ATG - 3', then the template strand is 3' - TAC - 5', and the mRNA is 5' - AUG - 3'. So the mRNA has the same sequence as the coding strand, with T replaced by U. Ah! So that's the key. So in this case, the DNA code (coding strand) is TGC - GGA - CTC - CTC. So the mRNA will have U instead of T? Wait, no, in the example above, the coding strand was ATG, mRNA was AUG (T replaced by U). Wait, yes! So the coding strand (the DNA sequence given here) has T, and the mRNA will have U? No, wait in the example, DNA coding strand ATG (A, T, G), mRNA AUG (A, U, G). So T in DNA coding strand is replaced by U in mRNA. Wait, that's correct. So let's apply this to our problem.

The DNA sequence is TGC - GGA - CTC - CTC.

- First triplet: TGC
- T (DNA) → U? No, wait in the example, DNA coding strand ATG, mRNA AUG. So T (DNA) → U (mRNA)? Wait, no, in the example, A (DNA) → A (mRNA), T (DNA) → U (mRNA), G (DNA) → G (mRNA). Wait, that's the case. Wait, no, the template strand is complementary to the coding strand. The template strand for ATG (coding strand) is TAC (template strand). Then mRNA is AUG (complementary to template strand TAC: A - U, U - A, G - C? No, template strand TAC (3' - TAC - 5'), mRNA is 5' - AUG - 3', which is complementary: T (template) - A (mRNA), A (template) - U (mRNA), C (template) - G (mRNA). So the mRNA is complementary to the template strand, and the template strand is complementary to the coding strand. So the mRNA is the same as the coding strand, with T replaced by U.

So let's re - do the base - pairing correctly:

For each base in the DNA coding strand:

- T (DNA) → U (mRNA)? No, in the example, DNA coding strand T was in the middle: ATG, mRNA AUG. So T (DNA) → U (mRNA). Wait, yes. So:

DNA coding strand: T G C - G G A - C T C - C T C

mRNA: U? No, wait no, in the example, the coding strand was ATG (A, T, G), mRNA was AUG (A, U, G). So T (DNA) → U (mRNA). So let's apply:

First triplet: TGC

- T → U? No, that contradicts the earlier example. Wait, no, I think I mixed up template and coding strand. Let's start over.

Transcription process:

1. DNA has two strands: template (used for transcription) and coding (non - template).
2. The template strand is read by RNA polymerase in the 3' to 5' direction.
3. The mRNA is synthesized in the 5' to 3' direction, complementary to the template strand.
4. The coding strand (non - template) has the same sequence as the mRNA, except that T is replaced by U.

So if the DNA sequence given is the coding strand (since it's the "DNA code for normal hemoglobin", which is the coding strand), then the mRNA will have the same sequence as the coding strand, with T replaced by U.

Wait, but in our options, option A is ACG - CCU - GAG - GAG, option B is TCC - GGT - CTC - CTC, option C is ACG - CCT - GGA - GAA, option D is UGC - GUA - CUC - CUC.

Wait, maybe my initial assumption about the coding strand is wrong. Maybe the given DNA sequence is the template strand. Let's try that.

If the DNA sequence is the template strand (3' to 5' direction, but we can consider it as 5' to 3' for base - pairing):

Template strand: TGC - GGA - CTC - CTC (5' to 3')

Then mRNA is complementary to the template strand:

- T (template) pairs with A (mRNA)
- G (template) pairs with C (mRNA)
- C (template) pairs with G (mRNA)
- Wait, no, base - pairing for DNA to mRNA: A (DNA) - U (mRNA), T (DNA) - A (mRNA), G (DNA) - C (mRNA), C (DNA) - G (mRNA)

So let's take each triplet:

First triplet: TGC

- T (DNA) → A (mRNA)
- G (DNA) → C (mRNA)
- C (DNA) → G (mRNA)? No, that gives A - C - G, which is ACG.

Second triplet: GGA

- G (DNA) → C (mRNA)
- G (DNA) → C (mRNA)
- A (DNA) → U (mRNA)? Wait, no: G (DNA) pairs with C (mRNA), G (DNA) pairs with C (mRNA), A (DNA) pairs with U (mRNA). So that would be CCU.

Third triplet: CTC

- C (DNA) → G (mRNA)
- T (DNA) → A (mRNA)
- C (DNA) → G (mRNA)? No, C (DNA) pairs with G (mRNA), T (DNA) pairs with A (mRNA), C (DNA) pairs with G (mRNA)? Wait, no, C (DNA) - G (mRNA), T (DNA) - A (mRNA), C (DNA) - G (mRNA) would be GAG.

Fourth triplet: CTC

- C (DNA) → G (mRNA)
- T (DNA) → A (mRNA)
- C (DNA) → G (mRNA) → GAG.

So putting it together: ACG - CCU - GAG - GAG, which is option A.

Let's verify:

Template strand DNA: TGC (T, G, C)

mRNA: A (complementary to T), C (complementary to G), G (complementary to C)? Wait, no, T (DNA) pairs with A (mRNA), G (DNA) pairs with C (mRNA), C (DNA) pairs with G (mRNA). So T - A, G - C, C - G. So TGC (DNA) → ACG (mRNA)? Wait, no, T (DNA) is paired with A (mRNA), G (DNA) with C (mRNA), C (DNA) with G (mRNA). So T (DNA) → A (mRNA), G (DNA) → C (mRNA), C (DNA) → G (mRNA). So TGC (DNA) → ACG (mRNA).

Second triplet: GGA (DNA)

G (DNA) → C (mRNA), G (DNA) → C (mRNA), A (DNA) → U (mRNA). So GGA (DNA) → CCU (mRNA).

Third triplet: CTC (DNA)

C (DNA) → G (mRNA), T (DNA) → A (mRNA), C (DNA) → G (mRNA). So CTC (DNA) → GAG (mRNA).

Fourth triplet: CTC (DNA) → GAG (mRNA).

So the mRNA sequence is ACG - CCU - GAG - GAG, which is option A.

Answer:

A. ACG - CCU - GAG - GAG