CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

 

8. DNA and RNA. The Molecular Basis of Heredity

 

8.3. RNA Structure and Function

 

Recall from chapter 3 that ribonucleic acid (RNA) is another type of nucleic acid and is important in protein production. RNA’s nucleotides are different from DNA’s nucleotides. RNA’s nucleotides contain a ribose sugar whereas the nucleotides of DNA contain a deoxyribose sugar. Ribose and deoxyribose sugars differ by one chemical functional group (figure 8.5).

 

 

FIGURE 8.5. A Comparison of DNA and RNA Nucleotides

DNA and RNA differ from each other chemically and in the nitrogenous bases that are present in each molecule. (a) The deoxyribose sugar of DNA has a circled -H. DNA also contains the nitrogenous base thymine (T). (b) The ribose sugar of RNA has a circled -OH. RNA contains the nitrogenous base uracil (U) instead of thymine (T). Both DNA and RNA have the nitrogenous bases; adenine, guanine, and cytosine.

 

Ribose has an —OH group and deoxyribose has an —H group on the second carbon.

RNA differs from DNA in other important ways. RNA contains the nitrogenous bases uracil (U), guanine (G), cytosine (C), and adenine (A). Note that the sets of nitrogenous bases in DNA and RNA are also slightly different. RNA has uracil, whereas DNA has thymine. In addition, when RNA is synthesized from DNA, it exists only as a single strand. This is different from DNA because DNA is typically double-stranded.

Cells also use DNA and RNA differently. DNA is found in the cell’s nucleus and is the original source for information to make proteins. RNA is made in the nucleus and then moves into the cytoplasm of the cell where it becomes directly involved in the process of protein assembly.

The protein-coding information in RNA comes directly from DNA. RNA is made by enzymes that read the proteincoding information in DNA. Like DNA replication, RNA synthesis also follows base-pairing rules where the RNA nitrogenous bases pair with the DNA nitrogenous bases: Guanine and cytosine still pair during RNA synthesis but RNA contains uracil, not thymine, so adenine in DNA pairs with uracil in RNA. The thymine in DNA still pairs with adenine in RNA (table 8.1).

 

TABLE 8.1. Nucleic Acid Base-Pairing Rules

 

DNA paired

with DNA

DNA paired

with RNA

RNA paired

with RNA

DNA

DNA

DNA

RNA

RNA

RNA

A pairs with T

A pairs with U

A pairs with U

T pairs with A

T pairs with A

U pairs with A

G pairs with C

G pairs with C

G pairs with C

C pairs with G

C pairs with G

C pairs with G

 

8.3. CONCEPT REVIEW

6. What role does RNA play in the cell?

7. Describe three differences in the structure of DNA and RNA.