DNA, mRNA, tRNA; It's not really making sense yet what each one actually does to make complicated proteins. This level finally brings it all together and wraps up the first Unit.

Wow, so you've made it this far. Are you confused? Don't worry, you're supposed to be, at this point. We will show you what happens after mRNA goes to the cell factory. Quick review: DNA holds data for making complex proteins. It transcribes the data over to mRNA, which matures and leaves the nucleus. The mRNA travels to the cell assembly line to make a protein.

Part 1: mRNA's Part

We've talked about how mRNA matures after being transcribed from DNA. We haven't yet talked about how these A's, U's, C's, and G's code for proteins.

Proteins are actually several polypeptides clumped in specific configurations. Polypeptides are made of amino acids. You've probably heard that amino acids are the "building blocks of life." Proteins are an important part of life, and proteins are made of amino acids.

The correct amino acid sequence for proteins is encoded in DNA. When a particular protein is needed, that portion is transcribed onto mRNA and taken out of the nucleus aboard the mRNA. Special organelles called ribosomes read mRNA and help to assemble the correct amino acid sequence, making a polypeptide and eventually a protein. mRNA stores its data in 3-letter "words". For example, the 3-letter block AGU codes for the amino acid Serine to be inserted into the next position in the chain. These "words" are called codons. Some codons don't just code for amino acids in the sequence. They tell the cell when to start insertion of proteins and when to stop. The start codon is AUG. This codon instructs the cell to begin adding amino acids into the chain as well as adding the amino acid Methionine. The Methionine may or may not be needed. If not, it will be removed when the protein is complete. The stop codons are UGA, UAA, and UAG. These stop the insertion of proteins into the polypeptide.

But how does the cell read and interpret it? It is not an intelligent being that can read and process complicated information as we can. Read on!

Part 2: tRNA's Part

tRNA has a flat portion that consists of 3 nucleotides. But wait -- the "words" that coded for amino acids in mRNA are also 3 nucleotides! Is there a connection?

Definitely. Each codon in mRNA will connect to a specific type of tRNA. The correct tRNA strand is determined by this flat part of 3 nucleotides. The tRNA will come over and bond with the mRNA codons at this flat part. This flat part is the anticodon. Just like in DNA and mRNA, a UAG codon docks with an AUC anticodon and so forth. Remember the tRNA structure. Part of it sticks out and just hangs there. This is the aminoacyl attachment site. Each little strand of tRNA has a slightly different coding, and most of them differ in this site. This site attaches amino acids to the tRNA strand.

So: A specific tRNA bonds with a specific codon, carrying with it a specific amino acid. See where this is going? The coding on mRNA determines which acid is added.

Part 3: It all comes together

You probably don't realize it, but you already know all the components of protein synthesis. All that's left is assembling it.

1. The needed gene on DNA unravels and unzips.
2. RNA polymerase transcribes this DNA code onto mRNA.
3. mRNA matures with a head and a tail and leaves the nucleus.
4. mRNA travels to the ribosomes.
5. A ribosome hitches onto the beginning part of the mRNA.
6. A tRNA molecule with the right anticodon, carrying an amino acid, recognizes the codon AUG with its UAC anticodon. It docks with this start codon on the mRNA by sliding into the first receptacle in the ribosome.
7. The amino acid is joined with the assistance of other enzymes to the growing chain.
8. The ribosome moves to the next codon on mRNA and the tRNA floats away to be recycled.

See how easy that is? Watch this animation if it is still confusing. Everything that we've talked about so far -- DNA, mRNA, RNA polymerase, tRNA, have their results come together to form a protein. That is how the codes in DNA are expressed as proteins we need. And yet, this complicated process is only one of many, many, complex processes involving DNA.

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