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 (GC).
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
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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 mRNA. 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.
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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
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Translation is the process where the amino acid sequence is derived from mRNA. 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 mRNA. 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 subunits 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
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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.