RNA Molecules

RNA (ribonucleic acid) molecules differ from DNA molecules in several ways. RNA molecules are single-

stranded, and their nucleotides contain ribose rather than deoxyribose sugar. Like DNA, RNA nucleotides each contain one of four organic bases, but whereas adenine, cytosine, and guanine nucleotides occur in both DNA and RNA, thymine nucleotides are found only in DNA. In place of thymine nucleotides, RNA molecules contain uracil (U) nucleotides (fig. 4.21 and Appendix D, A Closer Look at Cellular Respiration, p. 1038).

The first step in the delivery of information from the nucleus to the cytoplasm is the synthesis of a type of RNA called messenger RNA (mRNA). In messenger RNA synthesis, RNA nucleotides form complementary base pairs with a section of a strand of DNA that encodes a particular protein. However, just as the words in a sentence must be read in the correct order to make sense, the base sequence of a strand of DNA must be "read" in the correct direction. Furthermore, only one of the two antiparallel strands of DNA contains the genetic message. An enzyme called RNA polymerase determines the correct DNA strand and the right direction for RNA synthesis (fig. 4.22a).

In mRNA synthesis, RNA polymerase binds to a promoter, which is a DNA base sequence that begins a gene. As a result of RNA polymerase binding, a section of the double-stranded DNA molecule unwinds and pulls apart, exposing a portion of the gene. RNA poly-merase then moves along the strand, exposing other portions of the gene. At the same time, a molecule of mRNA forms as RNA nucleotides complementary to those along the DNA strand are strung together. For example, if the sequence of DNA bases is A, C, A, A, T, G, C, G, T, A, the complementary bases in the developing mRNA molecule will be U, G, U, U, A, C, G, C, A, U, as figure 4.22a shows. (The other strand of DNA is not used in this process, but it is important in DNA replication, discussed later in the chapter.)

RNA polymerase continues to move along the DNA strand, exposing portions of the gene, until it reaches a special DNA base sequence (termination signal) that signals the end of the gene. At this point, the RNA poly-merase releases the newly formed mRNA molecule and leaves the DNA. The DNA then rewinds and assumes its previous double helix structure. This process of copying DNA information into the structure of an mRNA molecule is called transcription (tranz-krip'-shun).

Messenger RNA molecules can be hundreds or even thousands of nucleotides long. They exit the nucleus through the nuclear pores and enter the cytoplasm (fig. 4.22b). There they associate with ribosomes and act as patterns, or templates, for synthesizing proteins. Protein synthesis is called translation (tranz-lay'shun) (fig. 4.22b).

Because an amino acid corresponds to a sequence of three nucleotides in a DNA molecule, the same amino acid is represented in the transcribed messenger RNA by the complementary set of three nucleotides. Such a triplet of nucleotides in a messenger RNA molecule is

Chromosome

Thymine Adenine

Chromosome

Thymine Adenine

Rna Molecule Base

Nitrogenous base

Sugar

Phosphate

Nitrogenous base

Sugar

Phosphate

Segment of DNA molecule

Nucleotide strand

Segment of DNA molecule

Nucleotide strand

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment