FREE ESSAY ON PROTEIN SYNTHESIS

PROTEIN SYNTHESIS

Protein Synthesis
The Expression of a Gene
The process of Protein Synthesis involves many parts of the cell. Unlike other similar
productions, this process is very complex and precise and therefore must be done in
proper sequence to work effectively. The slightest error during this process could cause
the action to experience difficulty or even fail. For example, in the production of
starch, glucose molecules are combined to be stored and eventually utilized as usable
chemical energy. The cell can break down the starch with little difficulty as if each
molecule was identical, even though there is a wide variety of molecules. This is a
different case in Protein Synthesis. In Protein Synthesis, there are twenty different
amino acids and if one is out of place than is will effect the specificity of the
protein. In a healthy person, the protein hemoglobin can be found in red blood cells,
hemoglobin is helps with the transfer of respiratory gases from the blood to the tissues
of the body. With an illness called sickle-cell anemia, the red blood cells are changed
from a round, disk shape to a floppy looking sickle shape. These cells therefore cannot
pass through small blood vessels due to their divergent shape. The actual cause of this
mutation is a gene disorder, where the sixth codon of the protein glutamaric acid is
changed with valine. This small change in the genetic code can cause severe defects in
the effected such as blood clots, severe disorders and even death. All this can result
from a misinterpretation in one codon in a chain of hundreds! Protein synthesis acts in
this way, that is if there is only the most minuscule mistake it can have monstrous
effects. 
THE BASICS OF DNA AND GENES
Protein synthesis first begins in a gene. A gene is a section of chromosome compound of
deoxyribonucleic acid or DNA. Each DNA strand is composed of phosphate, the five-carbon
sugar deoxyribose and nitrogenous bases or nucleotides. There are four types of
nitrogenous bases in DNA. They are (A)denine, (G)uanine, (T)hymine, (C)ytosine and they
must be paired very specifically. Only Adenine with Thymine (A-T) and Guanine with
Cytosine (G-C).
To form a polynucleotide DNA, many nucleotides are linked together with 3`-5`
phosphodiester linkages. In a complete molecule of DNA two of these polynucleotide
strands are linked together by nitrogenous bases at 90 degrees to the sugar-phosphate
spine (FIG. 1). The nitrogenous bases are held together with weak hydrogen bonds. One
polynitrogenous chain runs in a 3'-5' direction, the 3' being the top hydroxyl and the 5'
being the bottom phosphate attached to the carbon five of the sugar. The other string
runs the opposite. The two strands of the structure cannot be identical but they are
complimentary. There is no restrictions on the placement and sequence of the nucleotides,
which becomes important in storage of information.
TRANSCRIPTION: The Synthesis of RNA
Genetic information would be rendered useless if the stored information did not have a
way of reaching the desired focal area. Since protein synthesis occurs in the cytoplasm
and the DNA must remain in the nucleus, a way of transporting the code is essential. This
comes in the form of messenger ribonucleic acid or m-RNA. Since the information on the
DNA must stay the same on the m-RNA, the two have to be very similar. There are three
major differences between RNA and DNA. RNA is only a single strand. The five carbon sugar
of RNA is ribose opposed to deoxyribose and in RNA the pyrimidine uracil (U) replaces
DNA's pyrimidine thymine (T). Since RNA is produced from DNA, the nucleotides of RNA can
hold the same information as the nucleotides of DNA because the code for amino acids is
centered around the RNA structure.
The process in which m-RNA is synthesized is called transcription. This process is
similar to DNA replication in the way that for transcription to occur, the double helix
DNA must be unwound as in DNA replication (FIG 2). The major difference between
transcription and replication is that in transcription only one of the strands is used as
a template and only one m-RNA strand is produced. Transcription can be broken up into
three parts in order to be understood. These steps are: i)initiation, ii)elongation and
iii)termination. Initiation of transcription is how the transcription begins. The enzyme
responsible for m-RNA synthesis is called RNA polymerase 2. The RNA polymerase knows
where to begin transcription because it is coded into the DNA.
Elongation of transcription represents how the process happens. This occurs the same way
as DNA replication, with the nucleotides being added one at a time in the 5'-3' direction
as the m-RNA strand uses the DNA strand as a template. Notice that uracil replaces
thymine. 
Termination of transcription represents how the process stops. Transcription is stopped
by certain sequences coded into the DNA template. These sequences are called terminators.
At the terminator sequence, RNA polymerase 2 stops or pauses, causing the transcription
to be completed and the m-RNA to be released.
DNA REPLICATION
DNA can replicate prior to mitotic division. This process is called semiconservative,
meaning that each daughter duplex contains one parental and a complimentary replicated
chain. For DNA to replicate, it must first be unwound. This is done by an enzyme called
helicase; using ATP as an energy source. The helicase helps this in process by breaking
the weak hydrogen bonds between nitrogenous bases. While unwinding, the strands can
become tangled and knotted. This problem is solved by an enzyme called gyrase which can
make transient breaks in the strand relieving tension and then rejoins the ends. DNA
replication occurs in a partially unwound are where some of the duplex region is still
present, known as the replication fork. For DNA synthesis, all four nucleotides must be
present. The existing DNA strands serve as templates which dictate the nucleotide
sequence of the new strand. Growth of the new chain only occurs in the 5'-3' direction.
The Genetic Code
DNA has the capacity to determine the sequences of specific proteins. The proteins are
composed of amino acids; of which there are twenty types. Since there are only four types
of nucleotides to blueprint, DNA uses combinations of three nucleotides to form codons
(FIG. 3). Each gene has its own amount and series of codons, depending on the protein.
There are sixty-four codons each having its own meaning. The only codon that has a double
meaning is AUG. This codon symbolizes the amino acid metheonine and also signals where
the polypeptide synthesis should start.
Translation
Translation is the process where the amino acid sequence is derived from m-RNA. To
understand translation, one must first understand transfer RNA, t-RNA (FIG. 4). The
function of t-RNA is to serve as a transporter for amino acids and an intermediate
between m-RNA codons and their corresponding amino acids. Transfer RNA have anticodons
which make them correspond to the codons of m-RNA. These t-RNA, that is with the help of
an enzyme called aminoacyl t-RNA synthetase, carry the proper amino acids to the proper
position in the m-RNA chain. When an amino acid is bonded to a t-RNA molecule, ATP
supplies the energy. When an amino acid is bonded to another amino acid by a peptide
bond, the ATP supplies the energy. The final component of the translation process is the
ribosome. Ribosome's are a cellular organelle that causes the t-RNA, the m-RNA and the
amino acid sequence to come together and form a polypeptide chain. Ribosome's are
composed of two unequal sub-units. Each sub-unit contains ribosomal RNA and ribosomal
protein. Ribosome's are attached to the m-RNA, read the codons, make sure that the proper
t-RNA is in place and then bonds the amino acids together by peptide bonds (FIG. 5).
There are three m-RNA codons that cause translation termination. There are not any
t-RNA's that correspond to these codons. Instead, they are recognized by proteins as
release factors. These release factors cause the release of the polypeptide chain from
its t-RNA and the ribosome. Then the polypeptide chain folds back up into its original
structure. With the release of the chain, the ribosome leaves the m-RNA. The ribosomal
sub-units are then ready to repeat the process for another m-RNA. See FIG 6 for complete
description.
Mutations
Mutations can occur either in body cells or reproductive (germinal) cells. Only diseases
of germinal cells can be passed through generations. Mutations can alter a single gene
point ( point mutations) or can effect and change the structure of many chromosomes (
chromosomal mutations). Mutations are not always bad because they can cause adaptation
and variation in people.
Point Mutations and Base Pair Mutations
The most common type of mutation involves a change in only a single base pair. This
change only effects a single codon of the gene. There are three types of base pair
mutations: silent, missense, and chain termination. Silent mutations involves the
repositioning of the third codon. This does not effect the amino acid sequence. Missense
mutation is where one codon is altered to code for a different amino acid (sickle cell
anemia). Chain termination mutations involve the codon being changes to a stop codon.
This causes the protein synthesis to remain incomplete and lose most of the biological
activity.
Frame shift Mutations and Mutagens
This is the addition or deletion of one or more base pair but not multiples of three.
This causes the ribosome to read the codon incorrectly causing and entirely different
amino acid sequence. Mutagens are agents that increase the frequency of mutations. X-rays
or other radiation are causes of mutagens.