Drosophila Polytene Chromosomes

Duplications and Deletions



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Drosophila Polytene Chromosomes

Chromosomes can undergo several types of changes which fall into two classes. The first type of change involves changes in chromosome number and is referred to as aneuploidy and euploidy. How these type of changes can occur and their subsequent effect on phenotype will be discussed in the next section. The chromosomal changes that we will discuss now alter the linear order of the chromosome and occur because of deletions, duplications, inversions, translocations and insertions of chromosomal DNA.

The analysis of these types of changes to a large part has been performed in genetic stocks of the fruit fly, Drosophila melanogaster. The chromosomes of this species that are of particular interest, are those found in the salivary glands of larvae. These tissues grow not by cell division but by enlargement. During this enlargement the chromosomes also undergo replication. But this replication is different than in other tissues because:

  1. the homologous chromosomes remain synapsed, that is paired throughout the duplication; and
  2. the chromosomes undergo about nine rounds of replication to produce 1000 double-stranded DNA molecules.
The banding patterns of these chromosomes are distinct from metaphase chromosomes and have been associated with specific genes. Metaphase chromosomes also have distinct bands, but these are thought to be associated with genetically inactive heterochromatin DNA. The term used to describe these chromosomes and similar chromosomes in other diptera species is polytene chromosomes and these chromosomes have primarily been usedd to:

  1. to locate genes; and
  2. to analyze structural changes in chromosomes

Copyright © 1997. Phillip McClean