The four nucleotides are commonly
abbreviated A, C, G and U. There |
are at least 20 types of RNA and the
number of nucleotides varies from |
under 200 to almost 10,000. RNA, like
the closely related DNA, can be |
copied and the sequence of
nucleotides can be interpreted as a code. |
One category of viruses, the
retroviruses, use RNA as their genetic |
material - this category includes
HIV-1, HIV-2 (causes of AIDS) and |
some leukemias. Several other
categories of viruses that use RNA |
instead of DNA include the viruses
responsible for Ebola, polio, the |
common
cold, influenza, measles, SARS, rabies, hepatitis C, hepatitis
E, |
and West Nile fever. |
In humans and in a great many other
organisms there are multiple types |
of RNA present, but genetic
information is coded in deoxyribonucleic |
acid (DNA). DNA is very similar to
RNA: it usually uses thymine (T) |
instead of uracil and it is typically
much longer - millions of nucleotides. |
Humans have their DNA arranged in 23
pairs of long strings known as |
chromosomes. The number of chromosomes is not much of a guide:
|
many canines, including dogs, have 78. The Red king crab
(Paralithodes |
camtschatica - also known as the Kamchatka crab; see below)
has 208 |
|
while the fruit fly
(Drosophila_melanogaster) has only eight chromosomes |
|