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The salivary gland chromosomes in the dipteran larvae are useful in gene mapping because -
A
They have endoreduplicated chromosomes
B
These are fused
C
These are easy to stain
D
These are much longer in size
Detailed Solution
The salivary gland cells of dipteran larvae (such as Drosophila and Chironomus) contain giant chromosomes called polytene chromosomes.
They are formed by endoreduplication: the chromosomes replicate repeatedly without any separation or cell division, so each chromosome comes to have about 1000 or more chromatids lying side by side.
Because the chromatids are aligned exactly, they show a characteristic pattern of dark bands and light interbands, and the bands correspond to particular genes.
Changes such as deletions, duplications and inversions can be seen directly in the band pattern, so the genes can be located (mapped) cytologically.
The great length and ease of staining are consequences of this polyteny; the basic reason is endoreduplication.
So they are useful in gene mapping because they have endoreduplicated chromosomes.
They are formed by endoreduplication: the chromosomes replicate repeatedly without any separation or cell division, so each chromosome comes to have about 1000 or more chromatids lying side by side.
Because the chromatids are aligned exactly, they show a characteristic pattern of dark bands and light interbands, and the bands correspond to particular genes.
Changes such as deletions, duplications and inversions can be seen directly in the band pattern, so the genes can be located (mapped) cytologically.
The great length and ease of staining are consequences of this polyteny; the basic reason is endoreduplication.
So they are useful in gene mapping because they have endoreduplicated chromosomes.