Biology Reference
In-Depth Information
Cytogenetics and Cochlear
Expressed Sequence Tags (ESTs) for
Identification of Genes Involved in
Hearing and Deafness
ANNE B. SKVORAK GIERSCH and CYNTHIA C. MORTON
1. Introduction
A variety of approaches is being used to study the genetics of mammalian
hearing. Most methods have the same goal: to first identify the chro-
mosomal location of genes that contribute to hearing loss, and then to dis-
cover the gene itself. This chapter addresses two methodologies that can
be employed in gene discovery in the auditory system: cytogenetics and
cochlear expressed sequence tags. Cytogenetics is the study of chromo-
somes. Structural rearrangements of chromosomes, such as deletions or
translocations, may be associated with disease. These rearrangements can
cause disruption or deletion of a particular gene(s), thus detection of a chro-
mosomal rearrangement can lend clues to where a disease gene resides.
Cochlear expressed sequence tags (ESTs) offer a glimpse at gene expres-
sion in the sensory end organ for hearing. Sequence analysis of ESTs
provides a survey of genes expressed in the cochlea and a collection of posi-
tional candidate transcripts for deafness loci.
2. Cytogenetics
Eukaryotic chromatin, composed largely of DNA, histones and nonhistone
proteins, is in a decondensed form in the nucleus throughout the majority
of the cell cycle. However, during the relatively brief mitotic stage of the
cycle, M phase, newly replicated DNA condenses into discrete units, or
chromosomes, in preparation for cell division. Cytogenetic studies are
concerned primarily with the chromosomes in mitosis, when each of the
individual chromosomes can be visualized simultaneously as a separate,
subnuclear component using the light microscope.
Tjio and Levan (1956) were the first to determine that the correct number
of chromosomes per human somatic cell is 46. Human chromosomes were
grouped according to size and centromere position, but were not individu-
ally identifiable until the 1970s, when a variety of banding techniques was
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