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DNA What is it

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					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.