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Half of your DNA comes from
your mum and half from your
dad. When the sperm and egg
combined to make you, 23
chromosomes from the egg
combined with 23 chromosomes
from the sperm to form a
full complement of human DNA
- 46 chromosomes.
Chromosomes pair up and
copy themselves every time
before cells divide. This
division happens billions of
times in your lifetime as
you grow, and to replace old
cells (like skin cells or
cells in the lining of your
mouth).
If a cell is stopped
during cell division, and
stained with Giemsa dye, the
23 pairs of human
chromosomes are visible with
a light microscope. The dye
stains regions of
chromosomes that are rich in
the base pairs adenine (A)
and thymine (T), producing
banding patterns in the
chromosomes, each one
different from the rest.
Chromosomes are best
seen during cell division
when they bunch up ready to
copy themselves.
DNA is packaged so
tightly together that even
the thinnest bands contain
over a million base pairs
and potentially hundreds of
genes.
The chromosomes can be
matched in their pairs,
arranged and numbered by
size from largest to
smallest based on the
banding patterns that you
see and the position of the
centromere. The centromere
is the central most
condensed and constricted
region of a chromosome. It
is also the part that the
spindle fibre attaches to
during cell division,
allowing the chromosomes to
separate.
A human karyotype. The
23 pairs of chromosomes are
lined up according to size,
from largest to smallest.
Can you tell whether this
DNA came from a male or
female?
Lining up the chromosomes
produces an image called a
karyotype.
Genetic diseases can
result if a person:
- has too many or too
few chromosomes
- is missing pieces of
chromosomes
- has mixed up pieces of
chromosomes.
Karyotyping is one of
many techniques that can
detect chromosomal
abnormalities by looking at
the number and structure of
chromosomes.
Cytogenetics is the study
of chromosomes using a
microscope.
Chromosome preparations
can be taken from different
types of tissue including
blood, bone marrow, amniotic
fluid, and embryonic tissue. |
