What Is Protein Synthesis – How Are Proteins Made – Transcription And Translation


Protein synthesis. So, how are proteins built? In this video we are going to go through
a very basic look at how this happens. There are two major
events that happen during protein synthesis,
transcription and translation. Transcription has three phases,
initiation, elongation and termination. The initiation phase of transcription
occurs in the nucleus of a cell, as specific enzymes
unwind DNA, which has a double helix formation,
in the region of a gene. A gene is a segment of a
DNA molecule that contains information or directions
on how to build a protein. Next, an enzyme called RNA
polymerase attaches to the DNA strand and moves to the promoter,
or start region of a gene. The hydrogen bonds between
the DNA strands are broken and there is a
bubbling out in that region. This allows the nitrogenous
bases to be exposed. In the elongation phase, free
RNA nucleotides come in a pair up with the exposed bases in
the template strand of DNA. There are four different bases in DNA,
and each of these pair up with only one type of base in the developing
strand of RNA, which are shown here. DNA bases on the left and
RNA bases on the right. Thymine with Adenine,
adenine with uracil, cytosine with guanine, and
guanine with cytosine. DNA and RNA bases differ in that RNA
contains uracil and DNA contains thymine. These base pairings are done
through hydrogen bonds. This process continues as the
RNA polymerase moves down the DNA until the entire
gene has been transcribed. Once this is completed, a new pre
messenger or mRNA has been formed. Termination occurs once the terminal
region of the gene is reached. The hydrogen bonds between the DNA and
messenger RNA are broken, the RNA polymerase is released, the DNA
returns to its double helix formation and the newly formed pre
messenger RNA strand has got its recipe for synthesizing
a new protein. Next, the pre messenger RNA is modified. The strand of pre messenger RNA has
sections called introns and exons. The intron sections are
non coding regions that are removed by structures
called spliceosomes. Enzymes then come in and splice
together the exon sections. Two other modifications
are also made to the messenger RNA. At one end of
the strand capping occurs, which increases the stability of
the strand, and at the other end a polyA tail is added, which helps
in transportation of the strand. Now, a final edited version
of messenger RNA has been created, and it leaves the
nucleus through nuclear pores, enters into the cytoplasm and
travels to a ribosome for the second major stage of
protein synthesis, translation. Translation occurs as a
complex is formed that is comprised of the large and
small subunits of a ribosome, the messenger RNA and
tRNA, or transfer RNA. The messenger RNA is read three
bases at a time, these three base sections are called codons, and they
determine the amino acid required. Transfer RNA has a binding site, referred
to as the anticodon, that contains the three bases that pair with the
bases in the messenger RNA codon. So, now the ribosome moves
along the strand of messenger RNA until it
reaches the start codon. A transfer RNA picks up the required
amino acid from a pool of free floating amino acids in the cytoplasm,
and brings it to the ribosome. Here the anticodon and codon attach,
putting the amino acid into place. Next, the ribosome again
moves down the messenger RNA strand, a transfer RNA brings
in the required amino acid, and the two amino acids
form a peptide bond. This process keeps happening, forming
a long chain of amino acids. A release factor binds to the complex
to signal the end of the process, and the two subunits of the ribsome
are separated from the messenger RNA, and the newly synthesized protein,
or polypeptide chain is released. One last note, the messenger RNA can
be read by more than one ribosome at a time. So, many copies of a protein
can be produced very quickly. And that be the basics on
the protein synthesis.

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