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.

THE STRUCTURE OF NUCLEIC ACID CHAINS

THE STRUCTURE OF NUCLEIC ACID CHAINS

Nucleotides are joined together in DNA and RNA by phosphate ester bonds between the phosphate component of one nucleotide and the sugar component of the next nucleotide. An ester bond is a bond which occurs between a Carbon atom and an Oxygen atom.

More and more nucleotides can be added on by the same process of forming ester bonds until an immense chain is formed. But no matter how long a polynucleotide chain is, one end of the nucleic acid molecule always has a free -OH group on the sugar at the Carbon known as C3' (called the 3' end) and the other end of the molecule always has a phosphoric acid group at C5' (the 5' end). The Carbons get this name from a counting system illustrated in the next diagram.

Beginning from the "right-hand" side of the sugar, count the Carbons....1', 2', 3' (where the phosphate group of the next nucleotide in a series can be linked via a chemical bond), 4', 5' (where the phosphate group of the previous nucleotide is linked via a chemical bond).

This "counting system" allows the strand of nucleic acid to be oriented: the 5' end of the molecule always ends with a phosphate and the 3' end of the strand always ends with a sugar. You may be wondering why we don't just call the 5' end the "top" of the DNA or RNA molecule and the 3' end the "bottom" of the molecule. But in order to name something the "top", we're assuming that that end of the molecule is "up".

But how can you assume this in a cell?

YOU CAN'T! Remember that cells don't have specific orientations and that the nucleic acid within the cells is tightly wrapped and coiled around special proteins in the nucleus. So the terms "top" and "bottom" or "left" and "right" are pretty useless in this situation. Any nucleotides in between the 3' and 5' nucleotides would be involved in phosphodiester bonds. These nucleotides on the ends of each strand have a "free" end which is not involved in such a bond.

Click here to see one interpretation of the general structure of the nucleic acid DNA. The phosphate groups joined to sugar groups form what is known as the "backbone" of a nucleic acid molecule. The bases are attached to the sugars at a different point than the phosphate groups to form this generalized structure. The shape of DNA (a double-stranded molecule) is often referred to as a "double helix" or "twisted ladder" with the sugar-phosphate backbone forming the sides of the ladder and the nitrogen bases forming the rungs of the ladder in the middle.

The sequence of nucleotides in a chain is described by starting at the 5' end and identifying the bases in the order that they are linked together. Rather than write the full name of each nucleotide or each base, however, it's quicker and easier to use the simple one-letter abbreviations of the bases: A for Adenine, G for guanine, C for cytosine, T for thymine (and U for uracil in RNA). So, to describe a sequence of DNA, you might write something like -T-A-G-G-C-T-.

In the coming sections of this tutorial and in lecture, you will learn that the exact structure of a protein depends on the sequence in which the individual amino acids are connected. The same is true with nucleic acids; the exact structure of a nucleic acid molecule depends on the sequence in which individual nucleotides are connected. We'll return to this concept later in the tutorial.