After transcription occurs, the mature mRNA needs to find a ribosome. Ribosomes are the
site of translation, the step of protein synthesis where the mRNA code translates into an actual
protein. The ribosome is made of two subunits, the large subunit and the small subunit. Both
parts are made of a mixture of protein and ribosomal RNA. The small subunit has a binding
side for the mRNA strand. The large subunit has three binding sites for tRNA molecules,
but only 2 tRNA molecules can bind at the same time to the ribosome. The binding sites
are called the A which stands for aminoacyl site, P or peptidyl site, and E or exit site.
The ribosomes move along the RNA to build protein with the help of the tRNA. More than
one ribosome can translate one mRNA molecule at the same time. This is even visible from
an electron microscope and these structures are called polysomes. Clusters of ribosomes
on a strand of mRNA in the process of translating. Each ribosome moves down the length of the
mRNA until the polypeptide is completed. tRNA molecules have the job of bringing the correct
amino acid to the ribosome for the section of mRNA that is being translated. The anticodon
of the tRNA molecule will complement the codon of the mRNA molecule, ensuring the correct
amino acid is added. To attach an amino acid to the tRNA molecule, a tRNA activating enzyme
is used. The binding process occurs in two steps: The enzyme binds ATP to the amino acid
to form an amino acid–AMP complex linked by a high energy bond and two phosphates are
released. Then, the amino acid is coupled to tRNA and the AMP is released – the tRNA
molecule is now “charged” and ready for use. The stored energy of this charged molecule
will be used to provide the energy to form peptide bonds at the ribosome during translation.
Now we’re ready for translation, the production of a polypeptide. To begin translation, the
small subunit attaches the mRNA and the start codon of the mRNA, AUG, must be complemented
by the anticodon UAC of the tRNA molecule carrying methionine. The large subunit of
the ribosome attaches to the small subunit so that the tRNA molecule is in the P site.
The next tRNA molecule attaches to the complementary codon at the A site and a peptide bond is
formed between the amino acids. Now the cyclical steps for elongation of the polypeptide will
occur. The ribosome translocates 3 bases at a time, moving the tRNA molecule in the P
site to the E site, and the tRNA molecule in the A site to the P site. The tRNA molecule
in the E site will be ejected and a new tRNA molecule will bind to the A site, creating
a new peptide bond in the process. Then the cycle repeats, with translocation of the ribosome
and tRNA molecules until the full length of the polypeptide is made. Termination occurs
when a stop codon is reached and the final polypeptide is released. the sequence and
number of amino acids in the final polypeptide that is produced is called the primary structure.
Hydrogen bonds form between carboxyl groups and amino groups creating spiraling alpha
helixes and beta pleated sheets. This is called the secondary structure. The tertiary structure
is the overall 3-dimensional shape of the protein that is formed by further folding
because of interactions between R-groups. Hydrophilic regions will orient toward the
outside and hydrophobic regions toward the inside. Additionally, R-groups of the amino
acid cysteine can form a covalent bond with the R-group of another cysteine forming a
disulphide bridge. Some proteins form a quaternary structure, where several polypeptide chains
work together to form the fully functional protein. Hemoglobin is made of four polypeptide
chains, two alpha chains and two beta chains. protein synthesis in prokaryotes can occur
immediately after transcription because there is no nuclear membrane separating the DNA
from ribosomes. In eukaryotes, the mRNA has to travel outside the nucleus and find a ribosome.
Ribosomes can be free floating in the cytosol, or bound to the endoplasmic reticulum. Free
ribosomes make proteins that are used in the cytoplasm, mitochondria and chloroplasts.
Bound ribosomes make proteins that are used in the ER, Golgi apparatus, lysosomes, the
plasma membrane or for outside the cell. Thanks for watching this episode of Teacher’s Pet,
don’t forget to like and subscribe and follow me on twitter @SciencePet.