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.