YACs and the Common Mouse

The common house mouse, Mus musculus, is among the oldest and most valuable subjects for genetic study. It's an excellent genetic organism — small, prolific, and easy to keep, with a short generation time (about 3 months). It tolerates inbreeding well; so a large number of inbred strains have been developed through the years. Finally, being a mammal, the mouse is genetically and physiologically more similar to humans than are other organisms used in genetics studies, such as bacteria, yeast, corn, and fruit flies.

Powerful tools of molecular biology have enhanced the mouse's role in probing fundamental questions of heredity. New and altered genes can be added to the mouse genome by injecting DNA directly into embryos that are implanted into surrogate mothers. The resulting transgenic mice can be bred to produce offspring carrying the new genes.

Today, it is possible to introduce not just individual genes, but entire chromosomes into mouse cells. In 1983, the first artificial chromosomes, made of parts culled from yeast and protozoans, were created for studying chromosome structure and segregation. In 1987, David Burke and Maynard Olson (at Washington University, St. Louis) used yeast to create much larger artificial chromosomes called yeast artificial chromosomes or YACs. Each YAC includes the three essential elements of a chromosome: a centromere, a pair of telomeres, and an origin of replication. These elements ensure that artificial chromosomes will segregate in mitosis and meiosis, will not be degraded, and will replicate successfully. Large chunks of extra DNA from any source can be added to a YAC, and the new artificial chromosome can be inserted into a cell. Eukaryotic centromeres, telomeres, and origins of replication are similar in different organisms; so YACs function well in almost any eukaryotic cell.

In 1993, molecular geneticists successfully modified YACs so that they could be transferred to mouse cells. Previously, transgenic mice could carry only relatively small pieces of DNA, usually no more than 50,000 bp. Now, large genes as well as the surrounding DNA, which may be important in the regulation of those genes, can be added to mouse-cell nuclei. Artificial chromosomes have also been made from chromosomal components of bacteria (BACs) and mammals (MACs).

The successful construction of YACs, BACs, and MACs illustrates the fundamental nature of eukaryotic chromosomes: huge amounts of DNA complexed with proteins and possessing telomeres, centromeres, and origins of replication. In this chapter, we explore the molecular nature of chromosomes, including details of the DNA-protein complex and the structure of telomeres and centromeres; origins of replication will be discussed in Chapter 12.

Much of this chapter focuses on a storage problem: how to cram tremendous amounts of DNA into the limited confines of a cell. Even in those organisms having the smallest amounts of DNA, the length of genetic material far exceeds the length of the cell. Thus, cellular DNA must be highly folded and tightly packed, but this packing creates problems — it renders the DNA inaccessible, unable to be copied or read. Functional DNA must be capable of partly unfolding and expanding so that individual genes can undergo replication and transcription. The flexible, dynamic nature of DNA packing will be a central theme of this chapter.

We begin this chapter by considering supercoiling, an important tertiary structure of DNA found in both prokaryotic and eukaryotic cells. After a brief look at the bacterial chromosome, we examine the structure of eukary-otic chromosomes. After considering chromosome structure, we pay special attention to the working parts of a chromosome, specifically centromeres and telomeres. We also consider the types of DNA sequences present in many eukaryotic chromosomes and how DNA sequences are analyzed.

The second part of this chapter focuses on genes that move. For many years, biologists viewed genes as static entities that occupied fixed positions on chromosomes. But we now recognize that many genetic elements do not occupy fixed positions. Genes that can move have been given a variety of names, including transposons, transposable genetic elements, mobile DNA, movable genes, controlling elements, and jumping genes. We will refer to mobile DNA sequences as transposable elements, and by this term we mean any DNA sequence that is capable of moving from one place to another place within the genome.

We begin the second part of the chapter by outlining some of the general features of transposable elements and the processes by which they move from place to place. We then consider several different types of transposable elements found in prokaryotic and eukaryotic genomes. Finally, we consider the evolutionary significance of trans-posable elements.

www.whfreeman.com/pierce More information about YACs and how genes are cloned into YACs and more information about mouse genetics

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