are the smallest unit of life other than viruses,
which technically aren’t alive. Often they are the
building blocks for larger, multicellular organisms.
In animals, each cell contains a nucleus. Nuclei (more
than one nucleus) are the “brains” of the cell. Cells
also contain other organelles, or parts, such as mitochondria
(the “stomachs”) and ribosomes (the “factory” of the
cell which makes chemicals the cell needs to live).
Inside the nucleus is all the genetic information
the cell needs to exists and to reproduce.
Muscle cell mitochondria
most types of cells, genetic information is organized
into structures called chromosomes. Under electron
microscopes, chromosomes have the shape of the letter
X. Sometimes, though, they look like the letter Y
(as in human males). The general rule is that the
more chromosomes a species has, the more developed
it is. Fruit flies, for example, only have four chromosomes
while humans have 46. Chromosomes work in pairs; therefore,
each human cell has 23 pairs of chromosomes. In each
pair, one comes from the male parent and one from
the female. Each chromosome contains hundreds, sometimes
thousands, of smaller pieces of information called
determine the unique characteristics or traits an
organism will have. Animals with the gene for shaggy
fur will have shaggy fur, and humans with the gene
for blue eyes will have blue eyes. Each human being
has thousands of traits determined by his/her genes.
Chemically speaking, each gene is the blueprint
for a specific type of protein in the body. Proteins
are important because they do so many functions
in cells and in the human body. Proteins are made
of smaller units called amino acids.
are 20 types of amino acids. Different combinations
and numbers of these amino acids make different
types of proteins. That means that one protein may
only have four amino acids while another has 20;
or two proteins may both have 20 amino acids but
they’re in a different order in each protein. Each
amino acid is made up of three nucleotide bases
found in DNA.
DNA Double Helix
is shaped like a coiled ladder called a double helix.
DNA, or deoxyribonucleic acid, is made up of four
nitrogen bases called nucleotides, and a “backbone”
made of deoxyribose sugar. The four bases are adenine
(A), guanine (G), cytosine (C), and thymine (T). These
four bases bond to make the rungs of the spiral ladder,
but there are rules: T always bonds with A (and visa
versa) and C always bonds with G (and vice versa).
T will never bond with C or G, nor will A. When three
of these bases are “read” in order, they form an mRNA
codon through transcription. Like their name suggests,
codons are a special code to make an amino acid. There
are 64 possible codons. Since there are only 20 amino
acids, many codons code for the same amino acids.
Some codons are also used to mark the beginning and
end of a protein. The codon that codes for the amino
acid methionine also functions as an initiator codon
or beginner of a protein sequence. The codons at the
end of the sequence are called terminators.
Clockwise from upper
left: thymidine, guanidine, cytidine, and uridine
permission Florida State University