Wednesday, April 6, 2011

Synthesizers: They're in Your Cells

This marks two posts in a row for Biology! Don't worry...I have a few personal ones stewing away.

Pretty much everyone knows that DNA is in your cells. That's a well known, widely accepted fact.

Not as many people know that proteins do the dirty work of the body. When you look in the mirror, you are seeing the result of the work of proteins.

But this means that, somehow, the information encoded in your DNA has to be used to make proteins. How is this done?

The answer to this question is the RNA molecule. RNA is somewhat similar to DNA, but it has a few important differences. First off, uracil is used as a base instead of thymine. (Both uracil and thymine are pyrimidines, but uracil is missing a methyl group.) Then, the sugar used in its formation is ribose instead of deoxyribose. RNA also only has one strand instead of DNA's two.

Now, there are three classes of RNA. Let's take some time to examine each.

Messenger RNA (mRNA)

Messenger RNA is what carries the information from DNA to the ribosomes to create proteins. It's almost like the photo negative--every base in RNA is the complement of the base in DNA. For example, a DNA sequence of


would be transcribed as


(Remember, RNA uses uracil instead of thymine.)

Then, the strand of mRNA exits the nucleus and heads to the ribosome. Here, it meets up with...

Transfer RNA (tRNA)

Transfer RNA is a single strand of RNA that curls back on itself, with a location for an amino acid. It's typically portrayed in textbooks as a cross-ish shape, like

This isn't the actual shape of tRNA, but this particular representation is useful for several reasons. First off, it shows the "intramolecular base pairing" in the arms of the cross--G and C, U and A. It also is a "charged" RNA, meaning that it is carrying an amino acid. Finally, below the molecule, we can see the "codon," and at the bottom of the molecule itself, we have the "anticodon." Codons are groups of three bases that each have their own specific meaning. (Because there are four possible bases and three bases in a codon, there are 43=64 possible codons.)

Let's track a certain sequence of three bases through transcription (where DNA is transcribed to RNA) to translation (where mRNA is used by tRNA to create an amino acid sequence). Imagine that the sequence AGT is sitting on a DNA molecule. When transcribed to mRNA, it becomes UCA (remember, RNA becomes the complements of the bases in the DNA). This mRNA exits the nucleus and goes to the ribosomes. In the ribosome, it is paired with the tRNA carrying its anticodon, AGU. (As a matter of fact, this would result in the protein serine being added to the chain.)

Then, this general process continues for all of the codons. A series of amino acids is compiled, and finally a protein is made. Now, remember, all of this is happening in the ribosome, and an important part of the ribosome is

Ribosomal RNA (rRNA)

The ribosome is comprised of two subunits, both composed of rRNA. These two components are creatively known as the who on earth named these things large subunit (LSU) and the small subunit (SSU). When mRNA enters the ribosome for translation, it slides between the two subunits. Ribosomes have three binding sites for tRNA to enter as the mRNA is translated (known as A, P, and E). The A site binds with a charged tRNA--one that is carrying an amino acid. At the P site, another charged tRNA is waiting--and the amino acids of the P tRNA and the A tRNA bond. The tRNA that was at the P site moves to the E site, leaving its amino acid behind, and the molecule occupying the A site moves to the P site. Another tRNA molecule comes in to the A site, and the cycle continues.

That's a brief overview of the various types of RNA!

I have a confession: Over the past month, my writing on science stuff has seemed increasingly dry. I haven't liked it, and I don't know why. Has anyone else noticed this?

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