RNA

RIBONUCLEIC ACID (RNA)

Learning Objectives for this Section

RNA is a single-stranded molecule composed of building blocks called ribonucleotides (def). A ribonucleotide is composed of 3 parts: a molecule of the sugar ribose, a nitrogenous base, and a phosphate group (see Fig. 1).

a. ribose. Ribose (def) is a ringed 5-carbon sugar (see Fig. 2) similar to deoxyribose except it has a hydroxyl (OH) group) on its 2' carbon. The nitrogenous base is attached to the 1' carbon of the sugar and the phosphate group is bound to the 5' carbon. During RNA synthesis, the phosphate group of a new ribonucleotide is attached by the enzyme RNA polymerase (def) to the 3' carbon of a ribonucleotide.

b. a nitrogenous base (def). There are four nitrogenous bases found in RNA: adenine, guanine, cytosine, or uracil. Adenine and guanine are known as purine (def) bases while cytosine and uracil are known as pyrimidine bases (def) (see Fig. 3).

c. a phosphate group (see Fig. 4).

RNA differs from DNA in severaln ways. First of all, RNA is single-stranded, not double-stranded. Unlike DNA polymerases, RNA polymerases are able to join RNA nucleotides together without requiring a preexisting strand of RNA. In addition, RNA has the base uracil in place of thymine. Uracil, like thymine, can form hydrogen bond with adenine. Also, RNA and has the sugar ribose instead of deoxyribose. Finally, there are three functionally different types of RNA:

a. messenger RNA (mRNA)

Messenger RNA copies the genetic information in the DNA by complementary base pairing (def) and carries this "message" to the ribosomes where the proteins are assembled.

b. transfer RNA (tRNA)

Transfer RNAs picks up specific amino acids (def), transfers the amino acids to the ribosomes, and insert the correct amino acids in the proper place according to the mRNA message.

c. ribosomal RNA (rRNA)

Ribosomal RNA and ribosomal proteins form the ribosomal subunits (def).

d. other RNA transcripts

In recent years a variety of other RNA molecules transcribed off of DNA have been found. These RNA molecules are not translated into proteins but rather perform a wide range of direct genetic regulatory functions. Examples include antisense RNAs, microRNAs, and riboswitch RNAs.

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