Heredity Essay, Research Paper
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)