DNA

 

A Lab Experiment
 
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.