The number _____ and _____ sign are assigned to the first nucleotide of a transcriptional unit.

a. 0, positive

b. 1, negative

c. 1, positive

d. 0, no sign

I would choose "d". Please check.

To determine the correct answer, let's first understand the terms involved.

A transcriptional unit refers to a segment of DNA that is transcribed into RNA. It typically includes the coding region (the part that contains the instructions for making a protein) as well as the regulatory regions (sequences that control gene expression).

In transcription, the process of converting DNA into RNA, a specific nucleotide called the transcription start site is identified. This nucleotide marks the beginning of a transcriptional unit. It is from this point that RNA polymerase, the enzyme responsible for synthesizing RNA, starts transcribing the DNA into RNA.

Now, let's consider the options provided:

a. 0, positive: This option suggests that the number 0 and a positive sign are assigned to the first nucleotide of a transcriptional unit. However, in transcription, a basic numbering system starting from 1 is commonly used, so this option is unlikely to be correct.

b. 1, negative: This option suggests that the number 1 and a negative sign are assigned to the first nucleotide of a transcriptional unit. In transcription, the numbering convention typically starts from 1, so this option is a plausible choice.

c. 1, positive: This option suggests that the number 1 and a positive sign are assigned to the first nucleotide of a transcriptional unit. As mentioned earlier, the numbering starting from 1 is commonly used in transcription, so this option is also a plausible choice.

d. 0, no sign: This option suggests that the number 0 and no sign are assigned to the first nucleotide of a transcriptional unit. However, as mentioned before, the numbering in transcription generally starts from 1, so this option is not the most likely choice.

Based on this analysis, it is more likely that the correct answer is either option b or c. Both suggest that the first nucleotide is assigned the number 1, but differ in terms of the sign assigned. Without further context or information, it is difficult to choose definitively between the two options. Please review the material or consult a reliable source to determine the correct answer.