When one thinks about the genetic makeup of a human, or indeed any organism, it is natural to focus on the protein-coding genes. After all, that is the part of the genome that controls biochemical activities of cells and the processes of growth and development. But the protein-coding genes whose function is summarized in the "Central Dogma" (DNA ↔ mRNA → polypeptide) account for only about 3% of the DNA in a human cell. The genome also contains a large array of DNA sequences that have other functions (Figure 4-1) or that perhaps have no function at all. Some sequences represent the no-longer functional copies of duplicated genes, pseudogenes, produced at an earlier time in a species' history. In other cases, the regulatory functions of regions like microRNAs have only recently been recognized. Thus, the genome must be understood as a package of informational, historical, and noncoding DNA along with regions that hold secrets that researchers continue to unravel with the tools of molecular biology.
Overview of the kinds of DNA sequences found in the human genome (after Stracham and Read, Garland Science, NCBI Bookshelf). For additional details, see Tables 4-1 and 4-2. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
In Chapter 1 we saw that the chromosomes of eukaryotes (Figure 4-2) are made up of DNA complexed with proteins to form a nucleoprotein structure. The DNA molecule in each chromosome is a single, very long double helix. If one took each of the 23 chromosomes in one haploid set of human chromosomes, removed the protein, and stretched the DNA molecules out end-to-end, they would reach about a meter in total length. On average, then, each human chromosome's DNA strand is about 4.3 cm long (100 cm/23 linkage groups) and can be composed of as many as several hundred million nucleotide base pairs. Within this molecule, some genes follow the diploid organization we have assumed to this point, with one copy of each gene per haploid genome. But many genes are actually found in multi-gene families that often have large numbers of copies, and in fact the number of copies can change over time.
A typical eukaryotic chromosome showing some of the genetic structures and activities it can carry. (Reprinted with permission from Brooker RJ: Genetics: Analysis & Principles, 3rd ed. New York: McGraw-Hill, 2008.)
The first step is to understand the kinds of sequences present in the genome and their functions, if any, for the cell or their use to researchers, which is not necessarily the same thing. We will then explore how this vast amount of DNA is packed within the tiny confines of a nucleus ...