Types of RNA

There are four types of RNA produced within the nucleus by transcription: (1) precursor messenger RNA (pre-mRNA), which is altered within the nucleus to form mRNA; (2) messenger RNA (mRNA), which contains the code for the synthesis of specific proteins; (3) transfer RNA (tRNA), which is needed for decoding the genetic message contained in mRNA; and (4) ribosomal RNA (rRNA), which forms part of the structure of ribosomes. The DNA that codes for rRNA synthesis is located in the part of the nucleus called the nucleolus. The DNA that codes for pre-mRNA and tRNA synthesis is located elsewhere in the nucleus.

In bacteria, where the molecular biology of the gene is best understood, a gene that codes for one type of protein produces an mRNA molecule that begins to direct protein synthesis as soon as it is transcribed. This is not the case in higher organisms, including humans. In higher cells, a pre-mRNA is produced that must be modified within the nucleus before it can enter the cytoplasm as mRNA and direct protein synthesis.

Precursor mRNA is much larger than the mRNA it forms. Surprisingly, this large size of pre-mRNA is not due to excess bases at the ends of the molecule that must be trimmed; rather, the excess bases are located within the pre-mRNA. The genetic code for a particular protein, in other words, is split up by stretches of base pairs that do not contribute to the code. These regions of noncoding DNA within a gene are called introns; the coding regions are known as exons. Consequently, pre-mRNA must be cut and spliced to make mRNA (fig. 3.19). This cutting and splicing can be quite extensive—a single gene may contain up to 50 introns, which must be removed from the pre-mRNA in order to convert it to mRNA.

Introns are cut out of the pre-mRNA, and the ends of the exons spliced, by macromolecules called snRNPs (pronounced "snurps"), producing the functional mRNA that leaves the nucleus and enters the cytoplasm. SnRNPs stands for small nuclear ribonucleoproteins. These are small, ribosome-like aggregates of RNA and protein that form a body called a spliceosome that splices the exons together.

Test Yourself Before You Continue

1. Describe the appearance and composition of chromatin and the structure of nucleosomes. Comment on the significance of histone proteins.

2. Explain how RNA is produced within the nucleus according to the information contained in DNA.

3. Explain how precursor mRNA is modified to produce mRNA.

Fox: Human Physiology, 3. Cell Structure and Text © The McGraw-Hill

Eighth Edition Genetic Control Companies, 2003

Cell Structure and Genetic Control

Cell Structure and Genetic Control

■ Figure 3.18 RNA synthesis (transcription). Notice that only one of the two DNA strands is used to form a single-stranded molecule of RNA.

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