Unlike DNA, RNA does not permanently store genetic information in cells. This difference may exist because transcription need not be as accurate as DNA replication (see Table 5-1, p. Second, unlike the DNA polymerases involved in DNA replication, RNA polymerases can start an RNA chain without a primer. First, and most obvious, RNA polymerase catalyzes the linkage of ribonucleotides, not deoxyribonucleotides.
TRANSCRIBE DNA TO MRNA SERIES
Molecules of RNA polymerase are visible as a series of dots along the DNA (more.)Īlthough RNA polymerase catalyzes essentially the same chemical reaction as DNA polymerase, there are some important differences between the two enzymes. The micrograph shows many molecules of RNA polymerase simultaneously transcribing each of two adjacent genes. Transcription of two genes as observed under the electron microscope. The substrates are nucleoside triphosphates (ATP, CTP, UTP, and GTP) as for DNA replication, a hydrolysis of high-energy bonds provides the energy needed to drive the reaction forward (see Figure 5-4). In this way, the growing RNA chain is extended by one nucleotide at a time in the 5′-to-3′ direction ( Figure 6-8). The RNA polymerase moves stepwise along the DNA, unwinding the DNA helix just ahead of the active site for polymerization to expose a new region of the template strand for complementary base-pairing. Like the DNA polymerase that catalyzes DNA replication (discussed in Chapter 5), RNA polymerases catalyze the formation of the phosphodiester bonds that link the nucleotides together to form a linear chain. The enzymes that perform transcription are called RNA polymerases. A DNA molecule in a human chromosome can be up to 250 million nucleotide-pairs long in contrast, most RNAs are no more than a few thousand nucleotides long, and many are considerably shorter. In addition, because they are copied from only a limited region of the DNA, RNA molecules are much shorter than DNA molecules. Thus, the RNA molecules produced by transcription are released from the DNA template as single strands. Instead, just behind the region where the ribonucleotides are being added, the RNA chain is displaced and the DNA helix re-forms. Unlike a newly formed DNA strand, the RNA strand does not remain hydrogen-bonded to the DNA template strand. Transcription, however, differs from DNA replication in several crucial ways. It is not uncommon, however, to find other types of base pairs in RNA: for example, G pairing with U occasionally.ĭNA transcription produces a single-stranded RNA molecule that is complementary to one strand of DNA. Since U, like T, can base-pair by hydrogen-bonding with A ( Figure 6-5), the complementary base-pairing properties described for DNA in Chapters 4 and 5 apply also to RNA (in RNA, G pairs with C, and A pairs with U).
It differs from DNA chemically in two respects: (1) the nucleotides in RNA are ribonucleotides-that is, they contain the sugar ribose (hence the name ribo nucleic acid) rather than deoxyribose (2) although, like DNA, RNA contains the bases adenine (A), guanine ( G), and cytosine (C), it contains the base uracil (U) instead of the thymine (T) in DNA. Like DNA, RNA is a linear polymer made of four different types of nucleotide subunits linked together by phosphodiester bonds ( Figure 6-4). The information in RNA, although copied into another chemical form, is still written in essentially the same language as it is in DNA-the language of a nucleotide sequence. The first step a cell takes in reading out a needed part of its genetic instructions is to copy a particular portion of its DNA nucleotide sequence-a gene-into an RNA nucleotide sequence. Portions of DNA Sequence Are Transcribed into RNA
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