Key Concepts Of Section 105

Morphology and Functional Elements of Eukaryotic Chromosomes

■ During metaphase, eukaryotic chromosomes become sufficiently condensed that they can be visualized individually in the light microscope.

■ The karyotype, the set of metaphase chromosomes, is characteristic of each species. Closely related species can have dramatically different karyotypes, indicating that similar genetic information can be organized on chromosomes in different ways.

■ Banding analysis and chromosome painting, a more precise method, are used to identify the different human metaphase chromosomes and to detect translocations and deletions (see Figure 10-29).

■ In certain cells of the fruit fly Drosophila melanogaster and related insects, interphase chromosomes are reduplicated 10 times, generating polytene chromosomes that are visible in the light microscope (see Figure 10-31).

■ The highly reproducible banding patterns of polytene chromosomes make it possible to localize cloned Drosophila DNA on a Drosophila chromosome by in situ hybridization (Figure 10-30) and to visualize chromosomal deletions and rearrangements as changes in the normal pattern of bands.

■ When metaphase chromosomes decondense during interphase, certain regions, termed heterochromatin, remain much more condensed than the bulk of chromatin, called euchromatin.

■ Three types of DNA sequences are required for a long linear DNA molecule to function as a chromosome: a replication origin, called ARS in yeast; a centromere (CEN) sequence; and two telomere (TEL) sequences at the ends of the DNA (see Figure 10-32).

■ DNA fragments up to 106 base pairs long can be cloned in yeast artificial chromosome (YAC) vectors.

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