The nucleus is the organelle that contains the DNA of a cell. A gene is a length of DNA that codes for the production of a specific polypeptide chain. In order for genes to be expressed, they must first direct the production of complementary RNA molecules. That process is called genetic transcription.
Most cells in the body have a single nucleus. Exceptions include skeletal muscle cells, which have two or more nuclei, and mature red blood cells, which have none. The nucleus is enclosed by two membranes—an inner membrane and an outer membrane— that together are called the nuclear envelope (fig. 3.14). The outer membrane is continuous with the endoplasmic reticulum in the cytoplasm. At various points, the inner and outer membranes are fused together by structures called nuclear pore complexes. These structures function as rivets, holding the two membranes together. Each nuclear pore complex has a central opening, the nuclear pore (fig. 3.15), surrounded by interconnected rings and columns of proteins. Small molecules may pass through the complexes by diffusion, but movement of protein and RNA through the nuclear pores is a selective, energy-requiring process.
Transport of specific proteins from the cytoplasm into the nucleus through the nuclear pores may serve a variety of functions, including regulation of gene expression by hormones (see chapter 11). Transport of RNA out of the nucleus, where it is formed, is required for gene expression. As described in this section, genes are regions of the DNA within the nucleus. Each gene contains the code for the production of a particular type of RNA called messenger RNA (mRNA). As an mRNA molecule is transported through the nuclear pore, it becomes associated with ribo-
■ Figure 3.14 The structure of a nucleus. The nucleus of a liver cell, with its nuclear envelope and nucleolus, is shown in this electron micrograph.
somes that are either free in the cytoplasm or associated with the granular endoplasmic reticulum. The mRNA then provides the code for the production of a specific type of protein.
The primary structure of the protein (its amino acid sequence) is determined by the sequence of bases in mRNA. The base sequence of mRNA has been previously determined by the sequence of bases in the region of the DNA (the gene) that codes for the mRNA. Genetic expression therefore occurs in two stages: first genetic transcription (synthesis of RNA) and then genetic translation (synthesis of protein).
Each nucleus contains one or more dark areas (see fig. 3.14). These regions, which are not surrounded by membranes, are called nucleoli. The DNA within the nucleoli contains the genes that code for the production of ribosomal RNA (rRNA).
The Human Genome Project began in 1990 as an international effort to sequence the human genome. In February of 2001, two versions were published: one sponsored by public agencies that was published in the journal Science, and one produced by a private company that was published in the journal Nature. It soon became apparent that human DNA is 99.9% similar among people; a mere 0.1% is responsible for human genetic variation. It also seems that humans have only about 30,000 to 40,000 genes (segments that code for polypeptide chains), rather than the 100,000 genes that scientists had previously believed.
62 Chapter Three
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