DNA can direct the activities of the cell only by means of another type of nucleic acid—RNA (ribonucleic acid). Like DNA, RNA consists of long chains of nucleotides joined together by sugar-phosphate bonds. Nucleotides in RNA, however, differ from those in DNA (fig. 2.32) in three ways: (1) a ribonucleotide contains the sugar ribose (instead of deoxyribose), (2) the base uracil is found in place of thymine, and (3) RNA is composed of a single polynucleotide strand (it is not double-stranded like DNA).
There are three types of RNA molecules that function in the cytoplasm of cells: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). All three types are made within the cell nucleus by using information contained in DNA as a guide. The functions of RNA are described in chapter 3.
DNA nucleotides contain RNA nucleotides contain
■ Figure 2.32 Differences between the nucleotides and sugars in DNA and RNA. DNA has deoxyribose and thymine; RNA has ribose and uracil. The other three bases are the same in DNA and RNA.
Test Yourself Before You Continue
1. What are nucleotides, and of what are they composed?
2. Describe the structure of DNA and explain the law of complementary base pairing.
3. List the types of RNA, and explain how the structure of RNA differs from the structure of DNA.
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