Protein Synthesis


     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.