i. Vacuoles. Vacuoles are best described as small storage areas for water,
electrolytes, and food particles.
j. Cell Membrane. (Animal cells do not have cell walls; they have cell
membranes only. Plant cells have both.) A cell membrane or unit membrane is quite
complex, having the capacity to selectively absorb certain constituents from the cell's
surrounding environment and release waste materials and other unwanted products into
the environment. The precise ultrastructure of a cellular membrane is still under
considerable debate. Its structure varies from one membrane to another, making it
difficult to describe a typical unit membrane. It is generally accepted that a unit
membrane consists of an inner and outer layer of protein with two layers of lipid in
THE STRUCTURE OF DEOXYRIBONUCLEIC ACID AND RIBONUCLEIC ACID
Prior to our discussion of protein synthesis, we will first discuss the structure of
molecules responsible for producing proteins.
a. Deoxyribonucleic Acid. Deoxyribonucleic acid (DNA), like other large
organic molecules, is composed of a relatively simple building block compound, called a
nucleotide. A nucleotide includes three parts: a phosphate group, a five carbon sugar
called deoxyribose, and an organic nitrogencontaining base. There are four different
types of nucleotides that occur in DNA, the type of each depending upon which of four
nitrogenous bases are included in its structure. The four nitrogenous bases are
adenine, guanine, thymine, and cytosine. Adenine and guanine are purine bases,
characterized by a doublering structure. Cytosine and thymine are pyrimidines, which
have only a single ring. A typical DNA molecule is doublestranded and helical and has
been simply described as appearing like a ladder twisted into a form of a helix (spiral).
The sides of the ladder are composed of alternating molecules of phosphate and sugar
while the rungs of the ladder are each composed of two nitrogenous bases. Since the
width of the DNA molecule is the same, a purine base is always attached to a
pyrimidine in forming a rung of the ladder like structure. More specifically, adenine is
always bonded to thymine, cytosine to guanine.
b. Ribonucleic Acid. Ribonucleic acid (RNA), like DNA, is composed of a basic
unit called a nucleotide. The RNA nucleotide is similar to the nucleotide of DNA in that
it contains a phosphate group, a ribose sugar, and an organic nitrogencontaining base.
The four nitrogenous bases include adenine, guanine, cytosine, and uracil.
c. Differences. Three main differences between DNA and RNA are:
Deoxyribonucleic acid is doublestranded while RNA has one strand.
Deoxyribonucleic acid contains deoxyribose; RNA contains ribose.
(3) Deoxyribonucleic acid contains thymine as one of its four nitrogenous
bases while uracil has replaced thymine in RNA.