Translation

By: Nathaniel Craig G. de Guzman

 Introduction
Take a moment to look at your hand. The bone,
skin, and muscle you see are made up of cells,
each of which contains many millions of
proteins. In fact, proteins are key cellular
components of every organism on Earth! They
not only give structure to cells and tissues, but
also perform a wide range of specialized tasks,
such as catalyzing reactions and transmitting
signals. How are these millions of proteins
produced inside of a cell?
The instructions for building proteins are
encoded by a cell’s DNA in the form of
genes. As described in the central dogma of
molecular biology, information from
a gene can be used to build a protein in a
process with two steps. The first
is transcription, in which the DNA
sequence of a gene acts as a template for
synthesis of an RNA transcript.
In eukaryotes, this transcript goes
through additional processing steps to
become a mature messenger RNA
(mRNA). In translation, the nucleotide
sequence of the mRNA is "decoded" to
build a polypeptide (a protein or protein
subunit) with a specific sequence of
amino acids.
First, let’s examine how the
sequence of an mRNA encodes
the sequence of a protein.
Next, we'll discuss ribosomes, the
sites of translation, and tRNAs,
special RNA molecules that bring
amino acids to the ribosome.
Finally, we'll walk step-by-step
through the three stages of
translation: initiation, elongation,
and termination.
The central dogma of molecular
biology states that information flows
from DNA (genes) to mRNA
through the process of
transcription, and then to proteins
through the process of translation.
RNA to Protein - Translation
The sequence encoded in the RNA
molecule is decoded and converted into an
amino acid sequence in a process
called translation.
In our analogy, this part is equivalent to
building the dresser out of wood according
to the written notes you brought home from
the library.
You can remember this word by
remembering that translate means 'to
convert things from one language to
another.
In our analogy, we are translating
between written words and a physical
object. In the cell, we are translating
between a nucleotide sequence and an
amino acid sequence.
The genetic code connects mRNA
codons to amino acid sequences
Genetic code table. Each three-letter
sequence of mRNA nucleotides
corresponds to a specific amino acid, or
to a stop codon. UGA, UAA, and UAG
are stop codons. AUG is the codon for
methionine, and is also the start codon.
As we saw above, an mRNA transcript is
the end product of transcription.
Translation involves “reading” the
information encoded by the mRNA in
order to build a protein. Actually, an
mRNA doesn’t always encode a whole
protein. Instead, it encodes a
polypeptide – a chain of amino acids –
that may fold into its own protein or
form part of a larger, multi-part protein.
In an mRNA, the instructions for
building a polypeptide take the form of a
series of nucleotide triplets (that is,
groups of three nucleotides) called
codons. There are 61 different codons
that specify 20 amino acids (with some
amino acids specified by multiple
codons).
Three additional “stop” codons, UAA,
UAG, and UGA, indicate when a
polypeptide is complete. One codon,
AUG, both specifies the amino acid
methionine and serves as a “start” signal.
This collection of codon-amino acid
relationships is called the genetic code,
as it allows a nucleotide sequence to be
“decoded” into a chain of amino acids.
During translation, the codons of an
mRNA transcript are read sequentially
(from 5' to 3') by special RNA molecules
called tRNAs, which are discussed
further in the next section. Each tRNA
recognizes just one or a few codons and
delivers the corresponding amino acid,
which is added to the C-terminus
(carboxyl group end) of the growing
polypeptide.
In this way, a chain of amino acids is built one
at a time, and the sequence of amino acids in
the chain mirrors the sequence of codons
found in the mRNA. Once a stop codon is
reached, the polypeptide is complete.
Each mRNA contains a series of codons
(nucleotide triplets) that each specifies an
amino acid. The correspondence between
mRNA codons and amino acids is called the
genetic code.
5' AUG - Methionine ACG - Threonine
GAG - Glutamate CUU - Leucine CGG -
Arginine AGC - Serine UAG - Stop 3‘
Overview of translation
How is an mRNA actually translated into
a polypeptide? Two molecular factors
that play key roles in translation are
tRNAs and ribosomes. Transfer RNAs
(tRNAs) are adapters that relate mRNA
sequences to amino acids, while
ribosomes are large structures that
house the translation process and
catalyze some of its steps.
Transfer RNAs (tRNAs)
Transfer RNAs (tRNAs) are RNA
molecules that carry amino acids to the
ribosome, where they can be added to a
growing polypeptide. One part of each
tRNA has a sequence of three
nucleotides called an anticodon, which
can bind to specific mRNA codons.
Ribosomes
Ribosomes are large structures made of
ribosomal RNA and proteins arranged
into two subunits (a large one and a
small one). The ribosome not only
provides a space in which tRNAs can
bind to an mRNA template, but also
catalyzes the addition of each tRNA's
amino acid to the growing polypeptide
chain.
Ribosomes are composed of a small and
large subunit and have three sites where
tRNAs can bind to an mRNA (the A, P,
and E sites). Each tRNA carries a specific
amino acid and binds to an mRNA
codon that is complementary to its
anticodon.
Translation occurs in three stages
Translation occurs in three stages:
initiation, elongation, and
termination. Here, we'll quickly
summarize what takes place during each
stage.
 Initiation
In the initiation stage, an mRNA
associates with the ribosomal subunits
and a tRNA that carries the first amino
acid of the polypeptide. Together, these
molecules form a structure called the
initiation complex. In most organisms,
the first amino acid of a polypeptide is
methionine, specified by the start codon
AUG.
 Elongation
In the elongation stage, the mRNA is
read one codon at a time, and the amino
acid corresponding to each codon is
attached to a growing polypeptide chain.
Elongation is a cycle, repeating as each
amino acid is added to the chain, and it
occurs in three steps:
1) Codon recognition
The next codon to be read is exposed in
the A site of the ribosome, where it can
bind to an incoming tRNA with a
matching anticodon (that is, a tRNA
bearing the correct amino acid).
2) Peptide bond formation
The ribosome forms a peptide bond between
the new amino acid (attached to the tRNA in
the A site) and the last amino acid in the
existing chain (attached to the tRNA in the
P site). This step transfers the polypeptide to
the tRNA in the A site.
3) Translocation
The ribosome moves one codon down on
the mRNA, shifting the polypeptide-
bearing tRNA from the A site to the P
site. At the same time, the empty tRNA
in the P site moves to the E site, where it
exits the ribosome. Translocation
exposes the next codon of the mRNA in
the A site, and the cycle begins again.
 Termination
In the termination stage, the finished
polypeptide is released from the ribosome.
Termination takes place when a stop codon
(UAG, UAA, or UGA) enters the A site. A
protein called a release factor binds to the
stop codon and breaks the bond between the
polypeptide the tRNA that holds it. The
finished polypeptide can then exit the
ribosome through a tunnel in the large
subunit.
What happens after translation?
Once a polypeptide has been translated,
it has to fold into its correct three-
dimensional shape in order to function.
Some polypeptides must also be
chemically modified, cut into smaller
pieces, combined with other
polypeptides, or shipped to particular
organelles in order to function.
 Translation_HIGH
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GOD BLESS!!!