Nucleic Acids And Heredity

NUCLEIC ACIDS AND HEREDITY

How is genetic information passed on from generation to generation, or just cell to cell? How can a "bunch of letters" determine what proteins are made in the cell and direct the cell's activities?

A mechanism must exist for copying DNA in a fool-proof manner. If the information is to be used, mechanisms must exist for decoding the information held in the sequence of "letters" and for carrying out the instructions coded in that sequence.

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:

	1		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.
	2		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!
	3		Translation is the process by which the genetic messages carried by mRNA are decoded and used to build proteins.

The important point to remember is that the processes outlined above do not necessarily have to take place in that exact order all the time. For instance, if you eat a bagel with cream cheese, and the specialized cells of your pancreas need to secrete a digestive enzyme, then the one gene for that enzyme will be transcribed from DNA to mRNA and then to the protein (the digestive enzyme) which will be released into the digestive tract to do its work. Does the cell need to replicate (copy) its DNA for this? NO!! The only reason a cell has for replicating its DNA is if it's going to divide (make new cells). If the cell simply wants to make a protein for day-to-day functions, then DNA replication is not necessary.