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


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


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.

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