Page 59 - Demo 1

Chromosome Abnormalities

The process of mitosis produces idencal daughter cells by arranging
chromosomes into two equal groups. When the process occurs normally,
chromosomes aach to spindles and begin to move to the middle of each
daughter cell. If chromosomes fail to aach to these spindles, however, a
daughter cell might have an extra copy of a chromosome aer the cell divides,
or it might be missing one.
Sciensts refer to the condion whereby cells have an incorrect chromosome
number as aneuploidy. Down syndrome, which is characterized by specific
facial features and higher suscepbility to certain diseases like Alzheimer's and
leukemia, is one disorder caused by the presence of an extra chromosome.

Genetic Disorders

Cells grown in the laboratory undergo a specific number of divisions, referred
to as the Hayflick limit, and then cease dividing. Cells from human embryos
have a limit of about 50 divisions – enough to produce an adult and to replace
cells during a lifeme. Adult cells can divide only about 10-30 mes, however,
embryonic stem cells have unlimited capacity to proliferate.

The maximum number of divisions in human cells is under genec control, and
many genec disorders that affect control of cell division are associated with
accelerated aging. One such disorder is progeria, in which 7- or 8-year-old
affected children look like they are 70 or 80 years old. Affected individuals
usually die of coronary heart disease in their teenage years.

Another genec disorder related to premature aging is called Werner
syndrome. In this case, the disease process starts between the ages of 15 and
20 years, and affected individuals die of age-related problems by 45-50 years
old.
Both progeria and Werner syndrome are related to defects in DNA repair,
switching cells from growth to maintenance mode, thus stopping divisions far
short of the Hayflick limit.

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