|
|
||||||||||||||||||||||||||||||||||||||||||||
|
Main | genes | DNA | RNA | DNA/RNA/Proteins DNA - RNA - PROTEINS According to what has been called the central dogma of molecular genetics, the function of DNA is to store information and pass it on to RNA, while the function of RNA is to read, decode and use the information received from DNA to make proteins.
Three fundamental processes
take place in the transfer and use of genetic information:
The terms important for this process are:
Now let's review what amino acids are. Amino acids are the building blocks of proteins. Everything your DNA codes for is protein, so your DNA codes for amino acids. There are only 20 amino acids total, but each one has a generalized structure.
Each of the 20 different amino acids shares the amino group, the carboxyl group, the Hydrogen atom, and the central Carbon atom. The only group which differentiates them is the "R" group. R is simply a symbol for the side group. There is the specialized apparatus for making proteins called the ribosome. There are many ribosomes in the cytoplasm of a cell, and all the ribosomes are made of a small subunit and a large subunit. These two subunits open up like a "pac-man" allowing the mRNA message to slide through. Once the mRNA message is in place and protein synthesis is ready to begin, the two subunits close again so that the mRNA is now in between the two subunits. The next player on the list is the tRNA (transfer RNA molecule). This molecule is responsible for bringing in the proper amino acids. Remember, the mRNA is now held within the two subunits of the ribosome and is relatively immobile. The amino acids are floating free in the cytoplasm. So how can we bring the amino acids down to the mRNA?This problem is solved by the action of tRNA. The tRNA molecule acts as a "taxi" whose job is to read the code from the mRNA and bring the corresponding amino acid into place. What do I mean by "corresponding" amino acid? Every tRNA molecule has its own set of three bases which is called an anticodon. This anticodon is complementary to mRNA codons. The other "end" of the tRNA molecule has an "acceptor" site where the tRNA's specific amino acid will bind. Even though there are only 20 amino acids that exist, there are
actually 64 possible tRNA molecules: 4 X 4 X 4 = 64 possible combinations There are four choices of bases for the first space (A, U, G, or C), the same four choices for the second space (you can repeat the same bases), and the same four bases as a choice for the third spot. So, 4 x 4 x 4 is 64! 61 of the tRNAs code for specific amino acids and 3 code for chain termination as a result of pairing up with "stop codons", signaling the end of the mRNA message. The table shows which codons code for which amino acids:
After looking at this chart, something should strike you...why does each amino acid have more than one codon? Isn't one codon sufficient for each amino acid? In theory, yes, this would be correct. But cellular processes do not occur in a perfect world! What if the coding sequence in a particular codon should be GUA, but, due to a mutation, the coding sequence became GUC? What would happen? Check the chart to find out! HOW CAN ONLY 20 AMINO ACIDS CREATE THE PRACTICALLY INFINITE NUMBER OF PROTEINS PRESENT IN THE BODY? It seems impossible, doesn't it? The key to all the variety is that the 20 amino acids can be linked in different combinations and in different numbers. For example, alanine-valine-tryptophan........serine is a different protein than valine-serine-tryptophan........alanine because the sequence is different, even though the same amino acids are represented. Similarly, a protein made of 200 amino acids is quite different than a protein that is 2000 amino acids. The reason for this is because a protein's function is directly related to its shape (which is related to its amino acid sequence). Thus, if you change a protein's amino acid sequence, then you change its shape; and if you change the protein's shape, you change its function! So, the key to remember here is that the FUNCTION OF THE PROTEIN IS DIRECTLY RELATED TO THE SEQUENCE OF AMINO ACIDS ! To go one step further, the sequence of amino acids is related to the code on the mRNA molecule, which is determined by the code on the DNA molecule itself! This is how DNA eventually codes for proteins!! Now you know WHY it's so important that the DNA code stays intact (no mutations) because if you change the DNA, you change the mRNA, you change the amino acids coded for, and thus, you change the protein! The problem is if you change the protein, it usually renders the protein biologically inactive (in other words, it won't work properly!). As the term "anticodon" on tRNA implies, it is complementary to the codon on mRNA. The codon is ALSO a set of three bases, but because the codon is found on the mRNA molecule, it is called something different. So, let's review this…
You might be saying to yourself, "Isn't this just a case of the same thing being called a different name depending on where it is?" YES, YOU ARE CORRECT! Try to compare yourself to this example: You may be called by your first name here at school, by a nick-name by someone you know well, and Mr. or Ms. on a job interview. So, you are still the same person, you're just called a different name depending on where you are! Protein synthesis occurs in three stages: Initiation, Elongation and Termination.
Web Author: Tug Sezen |
Replication is the process by which a replica,
or identical copy, of DNA is made. Replication occurs every
time a cell divides so that information can be preserved and
handed down to offspring. This is similar to making a copy of
a file onto a disk so you can take that file to a different
computer.
Transcription is the process by which the
genetic messages contained in DNA are "read" or transcribed.
The product of transcription, known as messenger RNA (mRNA),
leaves the cell nucleus and carries the message to the sites
of protein synthesis. This tutorial explains later why this
step is necessary in organisms with a nucleus!
Translation is the process by which the
genetic messages carried by mRNA are decoded and used to build
proteins. 