Lesson 3
RNA stands for RiboNucleicAcid. It sounds familiar to DNA, except it has ribose as a sugar as opposed to deoxyribose and thymine is not part of RNA. Instead a similair base called uracil (U) is used in thymine's place. RNA is coded just like DNA. It is created by copying one of the complementary strands of the DNA. This process is almost like our previous one of DNA replication, except that after the DNA has generated the complementary RNA strand, the RNA breaks off and the two DNA strands zip back together to become that funny looking double helix again.
Just to make things a little more complex, lets add that there are three types of RNA. Hurrah! There's ribosomal RNA (rRNA), this type of RNA along with a bunch of proteins make up the ribosomes (ribosomes are organelles in a cell that generate proteins, but you already knew that). Then there is messenger RNA (mRNA), this kind of RNA is the carrier of the instructions written in the DNA to the ribosomes. Finally there is the last RNA, the transfer RNA (tRNA). tRNA brings amino acids to the ribosomes, the amino acids attach to a chain of amino acids.
The answer to this question involved a little bit of mathematically educated guessing. First, we ask ourselves, does one base code for an amino acid? The obvious answer to this is no. It can't be the case because 4 to the 1st power is 4, meaning that there should only be four amino acids found in proteins as opposed to twenty. Okay, how about two? No way. 4 to 2nd is 16, and that is short of four of our twenty amino acids. Okay fine, how about three? That would give us 64 (4 cubed). More then enough for our twenty amino acids. So scientists chose this number, but why didn't they choose four or five, or ten? The answer is simply that they guessed, or made an assumption, and for now it has not been proven incorrect. Actually, it has essentially been proven empirically since scientists found over a number of years that each three bases account for an amino acid in protein, and furthermore, some different combinations of the bases yield the same amino acid (this is also referred to as degeneracy of the code), so the sixty-four slots are filled, and by now we know what each combination of three bases (also called a codon or a triplet) stands for.
The process called transcription is the first of two parts in the synthesis of proteins. The DNA double strands unzip. mRNA is generated that attaches to one of the strands and then breaks off. This is the part called transcription. It's called transcription because the DNA transcribes its instructions over to the mRNA when the mRNA is forming. What I mean by forming is bases that are just floating around are finding their partners on one of the unzipped strands of DNA. So lets have a semi-visual example of this:
Remember our good old imaginary gene, well here it is again!
A-T
T-A
C-G
C-G
T-A
A-T
G-C
C-G
A-T
Now it unzips.
A T
T A
C G
C G
T A
A T
G C
C G
A T
Now lets forget about the left strand, and focus on what's happening to the right strand. There it is all by itself when over a period of time mRNA begins forming along it.
A-T
A
G
C-G
U-A
T
G-C
G
A-T
Now it's complete.
A-T
U-A
C-G
C-G
U-A
A-T
G-C
C-G
A-T
Then the mRNA unzips and flys off.
The next step (which is far more interesting in my opinion) is translation. This is the part where the actual protein is created. After the mRNA has been created, it zooms out of the nuclear membrane. It carries the DNA instructions from there to the ribosomes in the cytoplasm. Each codon in the mRNA is what specifies which tRNA will come bringing with it the correct amino acid. Then at the ribosome, the mRNA is read and the amino acids brought by tRNA are attached to each other. Then at the end a new protein has been constructed. Hurray!