Question

atg tct act
which code represents the anticodon trna used during protein synthesis?
tac aga tga
gct cac gac
uac aga uga
aug ucu acu

Explanation:

Step1: Recall tRNA anticodon rules

tRNA anticodons are complementary to mRNA codons, and mRNA is complementary to DNA (with U replacing T in RNA). The DNA sequence here is ATG TCT ACT. First, find the mRNA codon: DNA to mRNA (T→U, A→U, G→C, C→G, etc.). Wait, actually, the DNA template (coding strand? Wait, no: during transcription, DNA (template strand) is used to make mRNA. But the given DNA is probably the coding strand? Wait, no, the tRNA anticodon pairs with mRNA. So first, the DNA sequence (let's assume it's the DNA coding strand, so mRNA is same as DNA but T→U. Wait, no: DNA template strand is complementary to coding strand. Wait, maybe simpler: the DNA sequence here is ATG TCT ACT. The mRNA codon would be UAC AGA UGA? No, wait, DNA to mRNA: A→U, T→A, G→C, C→G. Wait, no: transcription: DNA template strand (complementary to coding strand) is used. But maybe the given DNA is the DNA that is being transcribed, so mRNA is made by replacing T with U. Wait, no, the correct process: DNA (template) has bases, mRNA is synthesized with U instead of T, complementary to DNA. Then tRNA anticodon is complementary to mRNA. Wait, let's take the DNA sequence: ATG TCT ACT. Let's find the mRNA codon: for each DNA base, A→U, T→A, G→C, C→G? Wait, no, transcription: DNA template (e.g., if DNA is 3'-TAC AGA TGA-5', then mRNA is 5'-AUG UCU ACU-3'). Wait, maybe the given DNA is the DNA that is the non-template (coding) strand, so the template strand is TAC AGA TGA, so mRNA is AUG UCU ACU. Then tRNA anticodon is complementary to mRNA: AUG→UAC, UCU→AGA, ACU→UGA? Wait, no: tRNA anticodon is 3 bases, complementary to mRNA codon. mRNA codon: AUG (from DNA ATG, since T→U), UCU (from TCT, T→U), ACU (from ACT, T→U). So tRNA anticodon for AUG is UAC, for UCU is AGA, for ACU is UGA? Wait, no: complementary base pairing: A-U, U-A, G-C, C-G. So mRNA codon AUG: tRNA anticodon is UAC (A-U, U-A, G-C? Wait, A (mRNA) pairs with U (tRNA), U (mRNA) pairs with A (tRNA), G (mRNA) pairs with C (tRNA)? Wait, no: mRNA codon is 5'-AUG-3', tRNA anticodon is 3'-UAC-5', so when paired, it's A-U, U-A, G-C. So yes, A (mRNA) - U (tRNA), U (mRNA) - A (tRNA), G (mRNA) - C (tRNA)? Wait, no, G (mRNA) pairs with C (tRNA), C (mRNA) pairs with G (tRNA). Wait, let's do each triplet:

DNA: ATG (A-T-G) → mRNA codon: A→U, T→A, G→C? No, wait, transcription: DNA template strand is complementary to mRNA. If DNA coding strand is ATG TCT ACT, then template strand is TAC AGA TGA (complementary). Then mRNA is synthesized from template strand: T→A, A→U, C→G, G→C, A→U, T→A, G→C, A→U. Wait, template strand TAC AGA TGA (3'-TAC AGA TGA-5') → mRNA (5'-AUG UCU ACU-3'). Then tRNA anticodon is complementary to mRNA: AUG (mRNA) → tRNA anticodon UAC (A-U, U-A, G-C? Wait, A (mRNA) - U (tRNA), U (mRNA) - A (tRNA), G (mRNA) - C (tRNA)? No, G (mRNA) pairs with C (tRNA), so AUG (mRNA) is 5'-AUG-3', tRNA anticodon is 3'-UAC-5' (so written as UAC, since anticodon is usually written 3' to 5' but sometimes 5' to 3'? Wait, maybe the options: let's check the options. The options are TAC AGA TGA, GCT CAC GAC, UAC AGA UGA, AUG UCU ACU. Wait, tRNA is RNA, so it has U, not T. So tRNA anticodon should have U. Wait, the first option has T, so it's DNA, not tRNA. Second option: GCT... no. Third option: UAC AGA UGA. Let's check: mRNA codon is AUG UCU ACU (from DNA ATG TCT ACT, T→U). Then tRNA anticodon for AUG is UAC, for UCU is AGA, for ACU is UGA. So tRNA anticodon is UAC AGA UGA. Let's check the options: third option is UAC AGA UGA. Wait, but let's re-examine. Wait, maybe I messed up. Let's do step by step:

1. DNA sequence: ATG TCT ACT (let's consider this as the DNA that is being transcribed, so the mRNA is made by replacing T with U: AUG UCU ACU (since A→A? No, wait, transcription: DNA template strand is complementary to coding strand. If the given DNA is the coding strand (5'-ATG TCT ACT-3'), then the template strand is 3'-TAC AGA TGA-5', so mRNA is 5'-AUG UCU ACU-3' (since transcription uses template strand, so mRNA is complementary to template: TAC→AUG, AGA→UCU, TGA→ACU). Then tRNA anticodon is complementary to mRNA: AUG (mRNA) → UAC (tRNA), UCU (mRNA) → AGA (tRNA), ACU (mRNA) → UGA (tRNA). So tRNA anticodon is UAC AGA UGA, which is the third option.

Step2: Eliminate other options

- First option: TAC AGA TGA (has T, tRNA has U, so DNA, not tRNA)
- Second option: GCT CAC GAC (doesn't match complementarity)
- Fourth option: AUG UCU ACU (this is mRNA, not tRNA anticodon)
So the correct tRNA anticodon is UAC AGA UGA.

Answer:

UAC AGA UGA (the third option, assuming the options are labeled as: first: TAC AGA TGA, second: GCT CAC GAC, third: UAC AGA UGA, fourth: AUG UCU ACU. So the answer is the third option: UAC AGA UGA.