Mendel proposed that the genetic code is transmitted to offspring in discrete, independent units that we now call genes. Recall from Chapters 2 and 4 that each gene is a sequence of nucleotide bases in the deoxyribonucleic acid (DNA) molecule.

Figure 37-1 shows a detailed view of human DNA. Beginning at the left of the diagram, you can see a fully condensed chromosome unfold toward the right, where a single double-helix strand of DNA is visible. As the genetic codes of a DNA molecule’s genes are being actively transcribed in a cell’s nucleus, the DNA is in the threadlike form called chromatin. Chromatin, as you can see in Figure 37-1, is actually a thread of DNA wound around little spools made of proteins called histones. The chromatin is thus organized into little “thread on spool” subunits called nucleosomes.

During cell division, each replicated strand of chromatin coils on itself to form a compact chromosome (see Figure 37-1). Each DNA molecule can be called either a chromatin strand or a chromosome, depending on what form it is in. Throughout this chapter we will use the term chromosome for DNA, regardless of its actual form, and the term gene for each distinct encoding segment within a DNA molecule.

Each gene in a chromosome contains a genetic code that the cell transcribes to a ribonucleic acid (RNA) molecule (see Box 5-2 on p. 120). Some RNA molecules do not code for polypeptides but have a functional role—for example, ribosomal RNA (rRNA) and transfer RNA (tRNA). A transcribed messenger RNA (mRNA) molecule, however, associates with a ribosome in which the code is translated to form a specific polypeptide molecule. By way of slight differences in editing of mRNA, one mRNA may perhaps actually produce several specific polypeptides. And the polypeptides may be complete tertiary proteins by themselves—or they may combine with any of several other polypeptides to form several different specific large quaternary proteins (see Figure 2-28 on p. 54).

Many of the protein molecules formed from the polypeptides encoded by genes are enzymes, functional proteins that help regulate the various metabolic pathways of the body by catalyzing specific chemical reactions. Because enzymes and other functional proteins such as hemoglobin regulate the biochemistry of the body, they regulate the entire structure and function of the body. Some proteins, such as collagen and keratin, are important structural components of the body—and thus determine important structural characteristics of various body parts.

As you can see, genes determine the structure and function of the human body by producing a set of specific structural proteins, along with many functional proteins and RNA molecules.

The entire collection of genetic material in each typical cell of the human body is called the genome (JEE-nome). The structure of the human genome is summarized in Figure 37-2. The typical human genome includes 46 individual nuclear chromosomes and one mitochondrial chromosome. In 2003, the Human Genome Project—a publicly funded, worldwide collaboration to map all the genes in the human genome—was completed. This landmark event coincided exactly with the fiftieth anniversary of the discovery of DNA.

We also know that less than 2% of the DNA carries protein-coding genes. A bit more of the DNA carries code for functional RNAs, such as rRNA, tRNA, and ribozymes (see Figure 5-2 on p. 115). Most of the rest of the DNA is often called “junk” code that is either not used or is edited out of mRNA before it is used to make proteins. Some of this noncoding DNA may actually be made up of broken bits of genes that are no longer functional—remnants of our evolutionary past. Termed pseudogenes, these bits of formerly functional genes are like genetic fossils that have begun to reveal an interesting history of our genetic past.

The current draft of the human genome shows us that most coding genes tend to lie in clusters rich in C (cytosine) and G (guanine), separated by long stretches of noncoding DNA rich in A (adenine) and T (thymine). Chromosome 1 has the most genes, with nearly 3000 genes, and the Y chromosome has the fewest, with just over 200 genes. Hundreds of the newly discovered genes in the human genome seem to be bacterial in origin, perhaps inserted there by bacteria in our distant ancestors.

An important thing to remember about genes is that they are not each just one sequence that codes for one protein. Besides possibly coding for a nonprotein such as RNA, each gene is made up of several separated exons that join together before protein synthesis (see Figure 5-4 on p. 116). In some cases, different combinations of some of the same exons can make up genes for different products. Recall also that some proteins are quaternary proteins made up of polypeptides that may be made from different genes. So the definition of a gene is less straightforward than it first appears!

Although we now have the essential picture of the details of the human genome, much work still lies ahead in the field of genomics (jeh-NOM-iks), the analysis of the genome’s code. Besides filling in the remaining details of the rough draft, we have much work to do in discovering all the possible mutations that might exist (see the discussion later in this chapter) and all the proteins encoded by the genes that make up the human genome (Box 37-1).

Information obtained about the human genome can be expressed in a variety of ways. As you can see in Figure 37-2, an ideogram (ID-ee-oh-gram), or simple cartoon of a chromosome, is often used in genomics to show the overall physical structure of a chromosome. The constriction in the ideogram shows the relative position of the chromosome’s centromere. The shorter segment of the chromosome is called the p-arm and the longer segment is called the q-arm.

The bands in an ideogram of a chromosome show staining landmarks and help identify the regions of the chromosome. Sometimes physical maps of genes will show exact positions of individual genes on the p-arm and q-arm of a chromosome. A more detailed representation of a gene would show the actual sequence of nucleotide bases, abbreviated a, c, g, and t for adenine, cytosine, guanine, and thymine, as shown in Figure 37-2.