Chromosomes
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Just before the cell divides, the DNA of each chromosome
replicates
(makes a copy of
itself.) Following this, the chromosomes coil up tightly which
allows them to
be sorted and moved to the new cells without tangling and breaking apart.
This sorting and moving process occurs before the cell divides and is
called
mitosis.
For a brief discussion of mitosis,
click here.
During this
period of cell division, chromosomes appear as dense, bulky objects when the cell is viewed
through a microscope. At very high magnifications, they have the
shape of a fuzzy, bulky X. Each half of the X comprises one chromatid--an exact copy of the original chromosome. The two chromatids
(often referred to as "sister chromatids") are joined together at a specific small region called the
centromere.
Using a microscope, it is possible to count and characterize the
individual chromosomes during the time they are coiled and condensed.
A photograph of the entire set of chromosomes can
be made. Then
the
images of the individual chromosome can be cut out and arranged by shape
and size in an orderly arrangement called a
karyotype
(see below).
(This is a nightmare project to the untrained eye, a simulation of which
is often assigned to 10th grade students as a learning exercise!)
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In most higher plants
and animals, including humans, chromosomes from the body cells can be matched up in pairs.
The two chromosomes of a pair are called
homologous chromosomes.
The members of most homologous pairs
of chromosomes look alike. They are the same length, their
centromeres are in the same position, they show the same pattern of light
and dark bands when stained, and they
carry genes for the same
inherited characteristics, line up on the chromosome in the same order.
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The occurrence of pairs of chromosomes in our karyotype is a result of our
sexual origins. We inherit one member of each chromosome pair from each
parent. So the 46 chromosomes in our somatic cells are actually
two sets of 23
chromosomes—a
maternal set
(from our mother) and a
paternal set
(from our father.)
A cell with two of
each kind of chromosome is called a
diploid cell
and is said to contain a
diploid, or 2n, number of chromosomes.
In humans, the
homologous pairs
are defined and numbered and
carry the genes for
the same trait in each person.
For example, human
chromosome #1 contains, along with many others, the genes for the Rh blood protein and for a
starch-digesting enzyme in the saliva. However, the corresponding
genes on the two homologous chromosomes are not necessarily identical. For
instance, some chromosomes have a gene for the protein that makes a person
Rh-positive, and some have a gene coding for a different version of this
protein (Rh-negative) at the Rh location.
Different versions of
the same gene are referred to as
alleles.
An
individual with two genes the same for a trait is said to be
homozygous
for that trait. A person with two different alleles for the same
trait is
heterozygous
for that trait.
In human males, the partners of 22 of the pairs of chromosomes look similar, but
the twenty-third pair is mismatched with two unlike chromosomes, called X and Y
(see the far right chromosome pair in the bottom row of the male karyotype
above.) In the cells of a female, both members of homologous pair
#23 are X chromosomes (far right pair of chromosomes in the bottom row in the
female karyotype.) The X and Y chromosomes are called the
sex chromosomes,
because they differ between the sexes and because they carry the genes
that determine the sex of the individual. The other 22 chromosomes
are called
autosomal
chromosmes
or simply
autosomes.
It is the Y
chromosome that is of major interest to the genealogist because, as it is
handed from father to son, virtually unchanged, it becomes a signature or
fingerprint for the surname which is passed down in the same way in many cultures.
For a chart
demonstrating how the Y chromosome is passed through a family,
click here.
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To find
out more about chromosomes and what can be learned about and from them,
have a look at
What Can Our Chromosome Tell Us at the
University of Utah's Genetic Science Learning Center. Particularly
interesting are pages describing and explaining chromosomal abnormalities
(mixed
up pieces of chromosomes?,
too many / too few chromosomes?,
46
chromosomes?,
missing pieces of chromosomes?). |
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