all organisms, the coded instructions for specifying the
characteristics of the organism are carried in DNA.
The genetic code is contained in the four
nitrogenous bases of DNA; adenine, guanine, cytosine,
These bases are often indicated only
using their beginning letters A, G, C, and T. Each
individual DNA strand serves as a template or model for
the formation of other DNA molecules by replication.
codes for the formation of RNA in the nucleus of the
cell. RNA is short for another kind of nucleic acid
called ribonucleic acid. RNA is very
similar in structure to DNA except for three small
differences. These differences include the fact
that RNA is a single stranded molecule, lacks the base thymine
(T) as it is replaced by the base uracil (U), and its five
carbon sugar ribose has one more oxygen atom than the sugar in
DNA. Three different types of RNA exist,
mRNA or messenger RNA, tRNA or transfer
RNA, and rRNA or ribosomal RNA.
store and use coded information. The genetic information
stored in DNA is used to direct the synthesis of the thousands of proteins
that each cell requires. The chemical and structural properties
of DNA are the basis for how the genetic information that underlies heredity.
DNA is encoded in the sequence of nitrogenous bases which
directs the formation of proteins in the cell. How does
this process work? First, the DNA code is copied
on to the mRNA (messenger RNA) codon. A codon is a
sequence of three nitrogenous bases. This process
is called transcription. This mRNA codon is
then carried from the nucleus out to the ribosome.
Messenger RNA attaches to another kind of RNA called tRNA
(transfer RNA). Transfer RNA attaches to amino acids and
carries them to the ribosome. This assembly
of amino acids due to the code provided to RNA by the original
DNA molecule is what produces proteins for the cell.
Remember a protein is a long molecule formed from amino acid
In summary, the code of DNA
directs the synthesis of RNA, which in turn directs the making
of proteins on the ribosomes. This is
sometimes referred to as being the central dogma or idea of
biology. There are 64 possible combinations of triplets
(sequences of 3 nitrogenous bases) which code for the 20
different possible amino acids. As the DNA
of different organisms and most individuals (except for
identical twins) is different, this means the proteins
produced by different humans and other organisms exhibit
differences. It is these differences which
make us unique individuals.
work of the cell is carried out by the many different types of
molecules it assembles, mostly proteins. Protein molecules are long,
usually folded chains made from 20 different kinds of amino acids in a specific
sequence. This sequence influences the shape of the protein. The shape of the protein, in
turn, determines its function.
resemble their parents because they inherit similar genes (DNA
code for the production of proteins that form similar structures and
perform similar functions.
functions are regulated. Regulation occurs both through
changes in the activity of proteins and through the selective expression of
individual genes, as humans and other organisms have genes
which direct the expression of other genes. This regulation
allows cells to respond to their environment and to control
and coordinate cell growth and division.