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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