You're right that organisms vary greatly in the
number of chromosomes they have. Bacteria all (or
at least almost all) have only one chromosome,
fruit flies have four pairs, and a plant called
the Adder's Tongue Fern(Ophioglossum reticulatum)
has over 1,000 chromosome pairs! The number of
chromosomes is not what makes an organism
genetically unique, though.
individuals) are unique because of the content of
the DNA that makes up the chromosomes, not the
number of chromosomes.
As you may already
know, chromosomes are made of tightly packed DNA,
and DNA is made of incredibly long strands of
chemicals called nucleotides. There are four
nucleotides, often called "bases" and abbreviated
as A, T, G, and C. DNA is unwound and loose most
of the time, and it usually only gets tightly
packed into a chromosome when a cell divides.
(During cell division,the tightly packed
chromosome is easier for the cell to move around
than along, unwound strand of DNA would
The order of the bases along the
strand of DNA, called the "sequence," is the DNA
content I mentioned that makes an organism or a
species unique. If a one-cell human embryo lost 11
chromosome pairs so only 12 pairs remained, it
would not be genetically any closer to a cherry
tomato than you are right now. That's because the
sequences of its remaining DNA will not have
changed, and they don't match the DNA sequences in
cherry tomatoes. The change would simply be that a
lot of the embryo's DNA sequences were lost(11
chromosome's worth). If you replaced those 11
chromosomes with 11different chromosomes with
different DNA sequences, it would not develop into
a normal person, even though it had 23 pairs of
So it's the DNA
sequences, not the chromosome numbers, that make
all the difference. Overlap in chromosome numbers
do definitely occur. Potatoes and Chimpanzees, for
example, both have 24 pairs of chromosomes. Humans
are not the only animal with 23 pairs, either--the
Chinese subspecies of Muntiacusmuntjac, a small
kind of deer, also has 23 pairs of chromosomes.
Are you ready for something really weird, though?
A different subspecies of this same deer has only
Don't let anyone convince you
that the number of chromosomes affects the
complexity of the organism, either. It's true that
simple bacteria have only one chromosome, but look
at these numbers: domestic cats have 19 pairs of
chromosomes, and Geometrid Moths have 112 pairs.
Moths are definitely not 6times more complex than
cats, even though 6 x 19 equals about 112! And
don't forget about the ferns with over 1,000
I hope that was helpful and
interesting. Keep coming up with great questions!
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