Protein Synthesis

Protein Synthesis


DNA is very long and cumbersome. The strands of DNA also code for many different
proteins. To get the code for the desired protein from
the nucleus to the ribosomes in the cytoplasm, a copy of the small necessary section of DNA
will be made. The copying process is called transcription
and it occurs in the nucleus. It’s the first step in protein synthesis–the
process of making protein. The second step is called translation. A portion of the DNA is unzipped so that the
mRNA can be made from the DNA like a template–that’s transcription. Nucleotides of RNA match with one strand of
DNA and make mRNA. RNA polymerase unzips the DNA and puts RNA
nucleotides into the right place. C and G nucleotides match up. A and T nucleotides match up, but any time
the mRNA places a compliment to an A on the DNA it places a U instead of a T on the RNA. Remember, RNA uses Uracil instead of Thymine. So, if the DNA strand looked like this, the
complimentary mRNA strand would be the compliment but each time there would normally be a Thymine
there’s a Uracil in its place. The mRNA also doesn’t copy the entire strand
of DNA. It only copies the portion of DNA that codes
for the protein it wants at the time. In fact, most of DNA doesn’t code for anything. Only about two percent of it codes for protein. So this small section of coding DNA has the
start and stop signal. After the mRNA is formed, it moves out of
the nucleus through a nuclear pore and goes into the cytoplasm. Remember, there are three different types
of RNA. The purpose of the mRNA is to get the protein
code out of the nucleus without pulling the DNA out of the nucleus. Then the mRNA will need to find a ribosome
in the cytoplasm. Ribosomes can be found on the rough ER or
floating in the cytosol. mRNA is read three bases at a time and these
three bases are called codons. Now, the ribosomes are made of protein and
RNA. And the ribosomes are the protein making machines
that read the mRNA code and add the correct amino acid using tRNA. Remember, tRNA stands for: transfer RNA and
that’s because its purpose is to transfer the right amino acid to the ribosome to build
the protein. The tRNA has an anti-codon on one end which
will match a specific codon on the mRNA and has a specific amino acid on the other end. Together these three parts will make translation
happen. Translation occurs in the cytoplasm of the
cell wherever there is a ribosome. To get translation started, mRNA attaches
to a ribosome and a start codon must be read. It’s usually AUG and I remember that school
usually starts in August and that helps me remember AUG. The first amino acid is brought in by tRNA. The anti-codon on tRNA matches up to the codon
on mRNA then the next the next tRNA molecule moves in and matches up with the mRNA codon. This time the amino acids form a peptide bond
and link together, then the first tRNA can detach and the mRNA shifts through like ticker
tape and the next tRNA molecule can come in. The protein grows until a stop codon is reached. Once the stop codon is read, the protein is
formed and ready to finish folding to become functional. And that’s the end of protein synthesis. Now we can figure out the sequence of amino
acids using the mRNA and amino acid chart like this one. Let’s use an example. Starting with the first three letters “AUG”
we can find the amino acids. The first letter is A which narrows it down
to this row. The second letter is U which narrows it down
to this column and the third letter is G which means that this is methionine which is the
starting amino acid. Next is “CCC” which is the second row, second
column and second row on the right. But we don’t even have to look at the third
letter in this case because every third letter will bring the same amino acid: proline. Next is “GGC”. Fourth row, fourth column, second row on the
right. That gives us glycine. And lastly is “UAA”. Row one, column three, row three and that
gives us STOP. There’s no amino acid here. It just means the protein will detach and
it marks the end of protein synthesis. Thanks for watching this episode of Teacher’s
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