Rna Structure And Function

Like DNA, RNA is a nucleic acid made up of nucleotides. However, as shown in Figure 10-13, the structure of RNA differs from that of DNA in four basic ways. First, RNA contains the sugar ribose, not the sugar deoxyribose found in DNA. Second, RNA contains the nitrogenous base uracil instead of the nitrogenous base thymine found in DNA. Third, RNA is usually single stranded rather than double stranded like DNA. However, within a single-stranded RNA molecule, some regions fold to form short double-stranded sections. In the double-stranded regions, guanine forms base pairs with cytosine, and uracil forms base pairs with adenine. Fourth, RNA is usually much shorter in length than DNA (about the length of one gene). On the other hand, DNA is usually long, containing hundreds or thousands of genes.

Types of RNA

Cells have three major types of RNA, as shown in Figure 10-14. Each type of RNA plays a different role in protein synthesis. The first type of RNA is messenger RNA (mRNA), a single-stranded RNA molecule that carries the instructions from a gene to make a protein. In eukaryotic cells, mRNA carries the genetic "message" from DNA in the nucleus to the ribosomes in the cytosol. The second type of RNA is ribosomal RNA (rRNA), which is part of the structure of ribosomes. Ribosomes are organelles in the cell where protein synthesis occurs. Ribosomes are made of rRNAs and many proteins. Figure 10-14 shows a model of a ribosome. The third type of RNA is transfer RNA (tRNA), which transfers amino acids to the ribosome to make a protein. Although the entire tRNA is made of nucleotides linked together, only three are emphasized in Figure 10-14.

mRNA

mRNA

rRNA

(shown as part of a ribosome)

rRNA

(shown as part of a ribosome)

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