Correct answer = B. The two DNA strands are complementary to each other, with A base-paired with T and G base-paired with C. So, for example, the 20 A on the first strand would be paired with 20 T on the second strand, the 25 G on the first strand would be paired with 25 C on the second strand, and so forth. When these are all added together, the correct numbers of each base are indicated in choice B. Notice that, in the correct answer, A = T and G = C.
0.4. List the order in which the following enzymes participate in prokaryotic replication.
A. Ligase
B. Polymerase I (3′→5′ exonuclease activity)
C. Polymerase I (5′→3′ exonuclease activity)
D. Polymerase I (5′→3′ polymerase activity)
E. Polymerase III
F. Primase
Correct answer: F, E, C, D, B, A. Primase makes the RNA primer; polymerase (pol) III extends the primer with DNA (and proofreads); pol I removes the primer with its 5′→3′ exonuclease activity, fills in the gap with its 5′→3′ polymerase activity, and removes errors with its 3′→5′ exonuclease activity; and ligase makes the 5′→3′-phosphodiester bond that links the DNA made by pols I and III.
0.5. Dideoxynucleotides lack a 3′-hydroxyl group. Why would incorporation of a dideoxynucleotide into DNA stop replication?
The lack of the 3′-OH group prevents formation of the 3′-hydroxyl → 5′phosphate bond that links one nucleotide to the next in DNA.
RNA Structure, Synthesis, and Processing 31
For additional ancillary materials related to this chapter, please visit thePoint.
I. OVERVIEW
The genetic master plan of an organism is contained in the sequence of deoxyribonucleotides in its DNA. However, it is through ribonucleic acid (RNA), the “working copies” of DNA, that the master plan is expressed (Fig. 31.1). The copying process, during which a DNA strand serves as a template for the synthesis of RNA, is called transcription. Transcription produces messenger RNA (mRNA), which are translated into sequences of amino acids (proteins), and ribosomal RNA (rRNA), transfer RNA (tRNA), and additional RNA molecules that perform specialized structural, catalytic, and regulatory functions and are not translated. That is, they are noncoding RNA (ncRNA). Therefore, the final product of gene expression can be RNA or protein, depending upon the gene. [Note: Only ~2% of the genome encodes proteins.] A central feature of transcription is that it is highly selective. For example, many transcripts are made of some regions of the DNA. In other regions, few or no transcripts are made. This selectivity is due, at least in part, to signals embedded in the nucleotide sequence of the DNA. These signals instruct the RNA polymerase where to start, how often to start, and where to stop transcription. Several regulatory proteins are also involved in this selection process. The biochemical differentiation of an organism’s tissues is ultimately a result of the selectivity of the transcription process. [Note: This selectivity of transcription is in contrast to the “all-or-none” nature of genomic replication.] Another important feature of transcription is that many RNA transcripts that initially are faithful copies of one of the two DNA strands may undergo various modifications, such as terminal additions, base modifications, trimming, and internal segment removal, which convert the inactive primary transcript into a functional molecule. The transcriptome is the complete set of RNA transcripts expressed by a genome.
II. RNA STRUCTURE