![]() DNA hybridization (described in detail in Chapter 8) can be used to distinguish these human chromosomes by “painting” each one a different color ( Figure 4-10). Thus, each human cell contains a total of 46 chromosomes-22 pairs common to both males and females, plus two so-called sex chromosomes (X and Y in males, two Xs in females). The only nonhomologous chromosome pairs are the sex chromosomes in males, where a Y chromosome is inherited from the father and an X chromosome from the mother. The maternal and paternal chromosomes of a pair are called homologous chromosomes (homologs). With the exception of the germ cells, and a few highly specialized cell types that cannot multiply and lack DNA altogether (for example, red blood cells), each human cell contains two copies of each chromosome, one inherited from the mother and one from the father. Therefore, our discussion of chromosome structure will focus almost entirely on eucaryotic chromosomes. Even less is known about how DNA is compacted in archaea. Although often called the bacterial “ chromosome,” it does not have the same structure as eucaryotic chromosomes, and less is known about how the bacterial DNA is packaged. This DNA is associated with proteins that package and condense the DNA, but they are different from the proteins that perform these functions in eucaryotes. ![]() In addition to the proteins involved in packaging the DNA, chromosomes are also associated with many proteins required for the processes of gene expression, DNA replication, and DNA repair.īacteria carry their genes on a single DNA molecule, which is usually circular (see Figure 1-30). The complex of DNA and protein is called chromatin (from the Greek chroma, “color,” because of its staining properties). Each chromosome consists of a single, enormously long linear DNA molecule associated with proteins that fold and pack the fine DNA thread into a more compact structure. For example, the human genome-approximately 3.2 × 10 9 nucleotides-is distributed over 24 different chromosomes. In eucaryotes, the DNA in the nucleus is divided between a set of different chromosomes. Amazingly, although the DNA is very tightly folded, it is compacted in a way that allows it to easily become available to the many enzymes in the cell that replicate it, repair it, and use its genes to produce proteins.Įucaryotic DNA Is Packaged into a Set of Chromosomes This is geometrically equivalent to packing 40 km (24 miles) of extremely fine thread into a tennis ball! The complex task of packaging DNA is accomplished by specialized proteins that bind to and fold the DNA, generating a series of coils and loops that provide increasingly higher levels of organization, preventing the DNA from becoming an unmanageable tangle. Each human cell contains approximately 2 meters of DNA if stretched end-to-end yet the nucleus of a human cell, which contains the DNA, is only about 6 μm in diameter. We also confront the serious challenge of DNA packaging. In addition, we describe the specialized DNA sequences that allow a chromosome to be accurately duplicated and passed on from one generation to the next. The genomes of eucaryotes are divided up into chromosomes, and in this section we see how genes are typically arranged on each chromosome. The most important function of DNA is to carry genes, the information that specifies all the proteins that make up an organism-including information about when, in what types of cells, and in what quantity each protein is to be made.
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