ARS Elements, Centromeres, and Telomeres
Survival of a chromosome requires three basic
properties–replication, proper segregation upon DNA replication and cell
division, and replication and protection of the ends of the chromosome.
Multiple origins of replication exist in the chromosomes of cells. These
origins are called autonomously replicating sequences, ARS, because they can be
cloned into DNA that will replicate on its own in other cells. Such DNA,
however, does not properly partition itself into daughter cells because it
lacks the necessary signals for segregation. Frequently the daughters fail to
receive a copy of the DNA replicating under ARS control.
Classical cell biology has identified the portion
of the chromosome that is responsible for segregation of the chromosomes into
daughter cells. This is the centromere. As cells divide, the centromeres are
pulled into the two daughter cells by microtubules. It has been possible to identify
a centromere by seeking a DNA segment from a chromosome that confers the
property of more correct segregation on a DNA element containing an ARS
element.
A third necessary part to a normal chromosome is
the telomere. Telomeres also have been identified by classical biology as being
special. First, most chromosomes of eukaryotic cells are linear. This poses a problem
in DNA replication as the normal DNA polymerase cannot elongate to the ends of
both strands because it replicates only in a 5’ to 3’ direction. The end of one
strand can’t be reached. Something else must extend the portion of the strand
that cannot be completely repli - cated. Secondly, since chromosome breakage
occasionally occurs, and has dire consequences to cells, they have evolved a
way to try to rescue broken chromosomes by fancy recombination processes. The
normal ends of chromosomes are inert in these rescue processes by virtue of
special markers called telomeres. These telomeres have been identified by their
properties of permitting the existence of linear artificial chro-mosomes that
contain ARS elements and centromeres. Interestingly, telomeres are repeated
sequences of five to ten bases, largely of C’s and G’s. A special enzyme adds
these sequences onto single-stranded DNA possessing the same telomeric
sequence. These unusual enzymes must first recognize the sequence to which they
will make additions, and then they add nucleotides, one at a time, to generate
the correct telomeric structure. They do this making use of an internal RNA
molecule that provides the sequence information needed for the additions.
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