Human artificial chromosomes (HACs) could be useful for gene therapy by allowing large gene fragments to be stably introduced and expressed in target cells without disrupting existing genes. HACs are small, artificial microchromosomes that can carry exogenous DNA and act as entirely new chromosomes in human cells. They were first constructed in 1997 by adding alpha-satellite DNA to telomeric and genomic DNA. There are two accepted methods for creating HAC vectors - altering a natural chromosome or de novo construction of a novel chromosome, though the latter has proven more difficult. HACs are preferable to other transgenic methods like yeast/bacterial artificial chromosomes due to their separation from the original genome, which provides greater stability and prevents insert

1. Suitable vector for cloning

3. 1. It is the potential application of artificial
chromosomes is in gene therapy of human.
2. For gene therapy we need to have gene sized
human DNA fragments including their
promoters and all the control elements.
3. This DNA would have to introduced into the
target cells efficiently and would have to be
stably maintained inside the nucleus from
generation to generation.
4. This DNA would have to expressed properly
without disrupt the functions of resident
genes.

4.  A human artificial chromosome (HAC) is
a microchromosome that can act as a
new chromosome in a population of
human cells. That is, instead of 46
chromosomes, the cell could have 47 with
the 47th being very small, roughly 6-
10megabases (Mb) in size instead of 50-250
Mb for natural chromosomes, and able to
carry new genes introduced by human
researchers.

5.  HACs were first constructed de novo in 1997
by adding alpha-satellite DNA to telomeric
and genomic DNA in humanHT1080 cells. This
resulted in an entirely
new microchromosome that contained DNA
of interest, as well as elements allowing it to
be structurally and mitotically stable, such
as telomeric and centromeric sequences

6.  There are currently two accepted models for the
creation of human artificial chromosome vectors. The
first is to create a small minichromosome by altering
a natural human chromosome. This is accomplished
by truncating the natural chromosome, followed by
the introduction of unique genetic material via
the Cre-Lox system of recombination. The second
method involves the literal creation of a novel
chromosome de novo. Progress regarding de novo HAC
formation has been limited, as many large genomic
fragments will not successfully integrate into de
novo vectors. Another factor limitingde novo vector
formation is limited knowledge of what elements are
required for construction, specifically centromeric
sequences

9.  Alternative methods of creating transgenic, such
as utilizing yeast artificial
chromosomes and bacterial artificial
chromosomes, lead to unpredictable problems.
The genetic material introduced by
these vectors not only leads to different
expression levels, but the inserts also disrupt the
original genome. HACs differ in this regard, as
they are entirely separate chromosomes. This
separation from existing genetic material
assumes that no insertional mutants would
arise. This stability and accuracy makes HACs
preferable to other methods such as viral
vectors, YACs and BACs.