DNA
Deoxyribonucleic acid and ribonucleic acid are two chemical substances involved
in transmitting genetic information from parent to offspring. It was known early
into the 20th century that chromosomes, the genetic material of cells, contained
DNA. In 1944, Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty concluded
that DNA was the basic genetic component of chromosomes. Later, RNA would be
proven to regulate protein synthesis. (Miller, 139)
DNA is the genetic material found in most viruses and in all cellular organisms.
Some viruses do not have DNA, but contain RNA instead. Depending on the organism,
most DNA is found within a single chromosome like bacteria, or in several
chromosomes like most other living things. (Heath, 110) DNA can also be found
outside of chromosomes. It can be found in cell organelles such as plasmids in
bacteria, also in chloroplasts in plants, and mitochondria in plants and animals.
All DNA molecules contain a set of linked units called nucleotides. Each
nucleotide is composed of three things. The first is a sugar called deoxyribose.
Attached to one end of the sugar is a phosphate group, and at the other is one
of several nitrogenous bases. DNA contains four nitrogenous bases. The first two,
adenine and guanine, are double-ringed purine compounds. The others, cytosine
and thymine, are single-ringed pyrimidine compounds. (Miller, 141) Four types of
DNA nucleotides can be formed, depending on which nitrogenous base is involved.
The phosphate group of each nucleotide bonds with a carbon from the deoxyribose.
This forms what is called a polynucleotide chain. James D. Watson and Francis
Crick proved that most DNA consists of two polynucleotide chains that are
twisted together into a coil, forming a double helix. Watson and Crick also
discovered that in a double helix, the pairing between bases of the two chains
is highly specific. Adenine is always linked to thymine by two hydrogen bonds,
and guanine is always linked to cytosine by three hydrogen bonds. This is known
as base pairing. (Miller, 143)
The DNA of an organism provides two main functions. The first function is to
provide for protein synthesis, allowing growth and development of the organism.
The second function is to give all of it?s descendants it?s own protein-
synthesizing information by replicating itself and providing each offspring with
a copy. The information within the bases of DNA is called the genetic code. This
specifies the sequence of amino acids in a protein. (Grolier Encyclopedia, 1992)
DNA does not act directly in the process of protein synthesis because it does
not leave the nucleus, so a special ribonucleic acid is used as a messenger
(mRNA). The mRNA carries the genetic information from the DNA in the nucleus out
to the ribosomes in the cytoplasm during transcription. (Miller, 76)
This leads to the topic of replication. When DNA replicates, the two strands of
the double helix separate from one another. While the strands separate, each
nitrogenous base on each strand attracts it?s own complement, which as mentioned
earlier, attaches with hydrogen bonds. As the bases are bonded an enzyme called
DNA polymerase combines the phosphate of one nucleotide to the deoxyribose of
the opposite nucleotide.
This forms a new polynucleotide chain. The new DNA strand stays attached to the
old one through the hydrogen bonds, and together they form a new DNA double
helix molecule. (Heath, 119) (Miller, 144-145)
As mentioned before, DNA molecules are involved in a process called protein
synthesis. Without RNA, this process could not be completed. RNA is the genetic
material of some viruses. RNA molecules are like DNA. They have a long chain of
macromolecules made up of nucleotides. Each RNA nucleotide is also made up of
three basic parts. There is a sugar called ribose, and at one end of the sugar
is the phosphate group, and at the other end is one of several nitrogenous bases.
There are four main nitrogenous bases found in RNA. There are the double-ringed
purine compounds adenine and guanine, and there is the single-ringed pyrimidine
compounds of uracil and cytosine. (Miller, 146)
RNA replication is much like that of DNA?s. In RNA synthesis, the molecule being
copied is one of the two strands of a DNA molecule. So, the molecule being
created is different from the molecule being copied. This is known as
transcription. Transcription can be described as a process where information is
transferred from DNA to RNA. All of this must happen so that messenger RNA can
be created, the actual DNA cannot leave the nucleus. (Grolier Encyclopedia,
1992)
For transcription to take place, the RNA polymerase enzyme is needed first
separate the two strands of the double helix, and then create an mRNA strand,
the messenger. The newly formed mRNA will be a duplicate of one of the original
two strands. This is assured through base pairing. (Miller, 147)
When information is given from DNA to RNA, it comes coded. The origin of the
code is directly related to the way the four nitrogenous bases are arranged in
the DNA. It is important that DNA and RNA control protein synthesis. Proteins
control both the cell?s movement and it?s structure. Proteins also direct
production of lipids, carbohydrates, and nucleotides. DNA and RNA do not
actually produce these proteins, but tell the cell what to make. (Heath, 111-
113)
For a cell to build a protein according to the DNA?s request, a mRNA must first
reach a ribosome. After this has occurred, translation can begin to take place.
Chains of amino acids are constructed according to the information which has
been carried by the mRNA. The ribosomes are able to translate the mRNA?s
information into a specific protein. (Heath, 116) This process is also dependent
on another type of RNA called transfer RNA (tRNA). Cytoplasm contains all amino
acids needed for protein construction. The tRNA must bring the correct amino
acids to the mRNA so they can be aligned in the right order by the ribosomes.
(Heath, 116) For protein synthesis to begin, the two parts of a ribosome must
secure itself to a mRNA molecule. (Miller, 151)
Methods and Materials:
For the first part of the lab, colored paper clips were needed to construct two
DNA strands. Each color paper clip represented one of the four nitrogenous bases.
Black was used as adenine, white was thymine, blue was cytosine, and yellow
represented guanine. A short sequence of the human gene that controls the body?s
growth hormone was then constructed using ten paper clips. The complementary
strand of the gene was then made using ten more clips. The two model strands
were laid side by side to show how the bases would bond with each other. The
model molecule was then opened and more nucleotides were added to show what
happens during replication.
For the second part of the lab, models of DNA, mRNA, tRNA, and amino acids were
used to simulate transcription, translation, and protein synthesis. The model
molecules were cut out with scissors and placed on the table. The DNA and mRNA
molecules were put on the left side of the table, the others on the right. To
simulate transcription, the mRNA molecule was slid down the DNA strand until the
nucleotides matched. The mRNA molecule was then moved from the left side of the
table to the right, showing it?s movement from the nucleus to the cytoplasm. The
tRNA molecules were then matched up with an amino acid. Once matched up, they
were slid along the mRNA until their nucleotides matched.
Conclusions:
The most surprising discovery made was finding out that there are only four main
bases needed in a DNA and RNA molecule. Also, each of these bases will only bond
with one other base. It is important to realize how DNA greatly affects a cell?s
functions, in growth, movement, protein building, and many other duties. DNA is
not nearly complex in structure as I had thought either. Containing only it?s
three main parts of a sugar, phosphate, and of course it?s base. From these
studies it is easy to see how DNA and RNA greatly affect the life and functions
of an organism.
Bibliography:
Emmel, Thomas C. Biology Today. Chicago: Holt, Rinehart and Winston, 1991.
Foresman, Scott. Biology. Oakland, New Jersey: Scott Foresman and Company, 1988.
Hole, John W., Jr. Essentials. Dubuque, Iowa: Wm. C. Brown Company Publishers,
1983.
Mader, Sylvia S. Inquiry Into Life. New York: Wm. C. Brown Company Publishers,
1988.
McLaren, Rotundo. Heath Biology. New York: Heath Publishing, 1987.
Miller, Kenneth R. Biology. New Jersey: Prentice Hall, 1993.
Welch, Claude A. Biological Science. Boston: Houghton Mifflin Company, 1968.