There is no denying the fact
that protein is a vital factor for getting and balancing the body with the significant
rudiments of food in a systematic way. Codon is the code – needed to convert
mRNA into protein language.Each Codon coherent with 3 nitrogen bases
codes for one amino acid. This is the genetic code. The genetic
code is universal. 64 possible combinations– it is noted that some do not code
for an amino acid but provide instructions for making a protein (UAA is a STOP
codon indicating that the protein chain ends at that point).
All metabolic reactions are
catalyzed by proteins in the form of enzymes, including energy releasing and
energy capturing reactions.
Proteins offer structure to
cells and organisms, such as the cytoskeleton.
DNA has the stored information
needed to determine the sequence of amino acids in proteins.
It is essential to understand
how proteins are synthesized in order to fully understand how they work.
The building of proteins is
called protein synthesis.
The assembly of proteins occurs
outside the nucleus in the ribosomes.
The ribosome will translate the mRNA molecule until
it reaches a termination codon on the mRNA. When this happens, the growing protein
called a polypeptide chain is released from the tRNA molecule and the ribosome
splits back into large and small subunits.
The newly formed polypeptide chain undergoes several
modifications before becoming a fully functioning protein. Proteins have a variety of functions .
Some will be used in the cell membrane ,
while others will remain in the cytoplasm
or be transported out of the cell .
Many copies of a protein can be made from one mRNA molecule. This is because
can translate the same mRNA molecule at the same time. These clusters of
ribosomes that translate a single mRNA sequence are called polyribosome or
DNA and RNA
DNA sends instructions for
building proteins to the ribosomes in the form of messenger RNA (mRNA).
RNA is a nucleic acid. RNA is made up of a chain of nucleotides (phosphate
+ sugar + nitrogen base).
RNA - consists of only a single
strand of nucleotide (Remember DNA is two strands). RNA is half a ladder
The sugar in RNA differs from
the DNA sugar. The sugar in RNA is ribose.
The bases found in RNA differ as
well. DNA is made up of adenine, guanine, thymine and cytosine.
Adenine, guanine and cytosine also are found in RNA. But instead of
thymine RNA contains the base uracil.
In RNA, cytosine bonds to guanine
and adenine bonds to uracil.
Types of RNA
RNA is the “workers” for protein
synthesis. DNA provides the workers with the instructions for making the
proteins and the workers build the proteins amino acid by amino acid (Remember
amino acid + amino acid + amino acid = Protein).
3Types of RNA that makes proteins (Workers in the protein assembly line)
mRNA – RNA messenger – brings
instructions from the DNA in the nucleus to the cells factory floor, the
cytoplasm. Once the mRNA is on the factory floor (cytoplasm) it moves to
the assembly line, a ribosome.
The ribosome is made up of
ribosomal RNA (rRNA); it binds the mRNA and uses the instructions to assemble
the amino acids in the correct order.
The transfer RNA (tRNA) is the
supplier – tRNA delivers the amino acids to the ribosome to be assembled into a
The process by which the DNA
message is copied into a strand of mRNA is called transcription. This
mRNA is then used for the construction of a protein molecule.
Transcription takes places in the NUCLEUS because DNA cannot leave.
The DNA double helix starts to uncoil.
Once the two strands of DNA
separate from one another only one strand participates in the synthesis of a
complementary mRNA strand.
The mRNA strand is synthesized
“made” with the help of an enzyme called RNA polymerase.
After mRNA synthesis is
complete, the two strands of DNA recouple and the molecules of DNA recoil to
assume its double helix.
The proceeded mRNA leaves the
nucleus and enters the cytoplasm.
mRNA carries the instructions
that direct the assembly of a specific protein to a designated area on the
ribosome. The instructions are carried in a sequence of three nitrogen
bases called a codon.
Codon is the code – needed to
convert mRNA into protein language. Each codon (3 nitrogen bases)
codes for one amino acid. This is the genetic code. The genetic
code is universal.
64 possible combinations– Note some do not code for an amino acid, but provide
instructions for making a protein (UAA is a STOP codon indicating that the
protein chain ends at that point).
AUG is a START codon as well as the codon for methionine.
Note that more than one codon can code for the same amino acid.
Once the message has reached the
ribosome, the protein is ready to be assembled. The process of building
the protein from the mRNA instructions is called translation. The
transfer RNA (tRNA) and the ribosomal RNA (rRNA) are involved in translation.
In the cytoplasm, a ribosome attaches to the strand of mRNA like a clothes pin
clamped to a close line.
tRNA is responsible for carrying the amino acid acids (the building blocks of
proteins) to the ribosome so they can be linked in a specific order that makes
up a single protein. Each tRNA attaches to only one type of amino acid
(correct translation of mRNA depends on the joining of each mRNA codon with the
correct tRNA molecule).
How does this happen? One end of the tRNA carries a three-base sequence called
an anticodon, which matches up with a particular codon on the mRNA. They
are complementary to each other.
The Translation Process
The tRNA carries the first amino acid to the to the mRNA strand (see Figure
The anticodon forms as base pair
with codon mRNA (see figure 11.9B). This places the amino acid in the
correct position for forming a peptide bond with the next amino acid (Remember
peptide bonds bond amino acids together in proteins).
The ribosome slides down the
mRNA chain top the next codon and a new tRNAmolecule bring another
molecule (Figure 11.9C).
The amino acids bond, the first
tRNA releases its amino acid and detaches from the mRNA (Figure 11.9D).
The tRNA molecule is now free to pick up and deliver another molecule of its
specific amino acid to a ribosome. Again the ribosome slides down to the
next codon; a new tRNA molecule arrives and its amino acid bonds with the
previous one. rRNA helps bond the amino acids together to form the
When the STOP codon is reached
on the mRNA strand translation ends and amino acid strand is released from the
ribosome (Figure 11E)
Amino acid chains become
proteins when they are released from the ribosome.
The Central Dogma
Crick and Watson had a model for
the flow of information in cells. This model incorporates DNA, RNA, and proteins.
DNA may replicate itself
DNA may be transcribed to RNA
RNA may be translated into proteins
RNA and DNA are NOT made from protein
DNA is NOT made from RNA
Called the Central Dogma of Molecular Biology – DNA to RNA to protein The
genetic code between DNA and protein is in the sequence of nucleotides in DNA
DNA is transcribed into mRNA
mRNA is translated into a polypeptide with the help of tRNA, tRNA with a
specific sequence that matches mRNA, carries the specific amino acid to
the ribosome to help form a polypeptide
Ribosomes are the site of protein synthesis.
Transcription involves making a strand of RNA complementary to DNA.
RNA is single stranded-only one strand of DNA is transcribed.
RNA produced is messenger RNA (mRNA) because it carries the genetic message
from DNA to the site of protein production.
DNA does not leave the nucleus, this
information is used to code for a protein made up of amino acids
if there are 4 letters in DNA language, and there are 20 amino acids that make
Amino acids are coded for by groups of 3 nucleotides, called CODONS.
There are 4x4x4 = 64 codons.
With these and only 20 amino
acids, there are extra codons.
Each amino acid is coded by more
than 1 codon.
NO codon codes for more than 1 amino
Four codons have special functions in the genetic code
one start AUG codes for methionine
3 stop codons you do not have to
memorize these. These determine the beginning and end of the polypeptide
In view of the above,