Origin of Selfish DNA and Junk DNA

It is thought that most of the repetitive and non-coding DNA found in the chromosomes of eukaryotes is useless to the organism concerned. Such useless DNA is sometimes referred to as junk DNA. Where did it come from? It is thought that many centromere Structure found on a chromosome and used to build and organize microtubules during mitosis centromere sequence (CEN) A recognition sequence found at the centromere and needed for attachment of the spindle fibers

DNA fingerprint Individually unique pattern due to multiple bands of DNA produced using restriction enzymes, separated by electrophoresis and usually visualized by Southern blotting heterochromatin Highly condensed form of chromatin that is genetically inert junk DNA Defective selfish DNA that is of no use to the host cell it inhabits and which can no longer move or express its genes mini-satellite Another term for a VNTR (variable number tandem repeats) VNTR See variable number tandem repeats variable number tandem repeats (VNTR) Cluster of tandemly repeated sequences in the DNA, whose number of repeats differs from one individual to another

Origin of Selfish DNA and Junk DNA 85

DNA that consists of very large numbers of tandem repeats may well have a base composition different from that of the genome as a whole. If so, the satellite DNA will have a different buoyant density from the rest of the DNA, as this property depends on the base composition. DNA may be fractionated according to density by ultracentrifugation in a gradient of the heavy metal salt, cesium chloride (CsCl). Each fraction of DNA forms a band at the position corresponding to its own density. If the %GC varies by 5 percent or more, separate bands are obtained. When mouse DNA is run on a CsCl density gradient, two DNA bands are seen (Fig. 4.09). One contains 92 percent of the DNA with a density of 1.701 gm/cm3 and the smaller, satellite band contains 8 percent of the DNA with a density of 1.690 gm/cm3. Satellite DNA was originally defined by this density separation. However, in cases where the average satellite DNA base composition is close to that of the genome as a whole, the satellite DNA cannot be physically separated using a density gradient.

FIGURE 4.09 Density Gradient Centrifugation and Satellite Bands

A cesium chloride gradient will reveal two (or more) bands of fragmented DNA if these differ in density. In this case, the lighter DNA contains sequences that are primarily satellite DNA.

Much of our genome consists of the defunct remains of viruses and transposable elements.

Selfish DNA, of no benefit to the host cell, accumulates in the genomes of slow-growing, multi-cellular organisms.

repetitive sequences and other non-coding DNA, including even some introns, may have originated from viral DNA that was inserted into the chromosome of the eukary-otic host cell. Retroviruses, in particular, have played a large role in generating such insertions. In addition, transposable elements (mobile DNA; see Ch. 15) are probably responsible for generating a significant fraction of the repetitive DNA.

The development of repetitive DNA would involve two processes. The original sequences of either viral DNA or transposable elements must have been intact and functional to insert in the first place. Both types of element might replicate and duplicated copies would then be inserted at more locations in the chromosomes of the infected cell. Thus, the numbers of these parasitic sequences would increase. In addition, many of these sequences would mutate, both by base changes and deletions. The result would be a family of related sequences, most of which are no longer functional (Fig. 4.10). Transposable elements that are solely concerned with their own survival and replication, rather than benefiting the host cell in any way are referred to as selfish DNA.

selfish DNA A sequence of DNA that manages to replicate but which is of no use to the host cell it inhabits

A Chromosome

Insertion

â– Mobile genetic element

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