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.

NUCLEOSIDES

NUCLEOSIDES

Both DNA and RNA contain nucleotides with similar components. In RNA, the sugar component is ribose, as indicated by the name "ribonucleic acid". In DNA, or deoxyribonucleic acid, the sugar component is deoxyribose. The prefix deoxy means that an oxygen atom is missing from one of the ribose Carbon atoms.

When a sugar bonds together with a Nitrogen base, you now have two of the three components of a nucleotide. This structure is known as a nucleoside.

There are FIVE Nitrogen bases that are found in DNA and RNA (although Uracil is found ONLY in RNA!). These five bases are divided into two categories based on their molecular structure. Click onto the hyperlinks and look at the structures below to see if you can tell what differentiates each category!

1 purines (Adenine and Guanine)
2 pyrimidines (Thymine, Cytosine, and Uracil)



You should notice that the purines have two ring structures while pyrimidines have only one ring structure. The way I've always remembered the difference is that the LONGER word (pyrimidine) represents the SMALLER structure (only one ring), and vice-versa!

COMPOSITION OF NUCLEIC ACIDS

COMPOSITION OF NUCLEIC ACIDS

This section of the tutorial begins to look at the composition of nucleic acids starting from the biggest subunit and going all the way down to the very chemical elements that make them up.

Nucleic acids are one of several macromolecules in the body in addition to fats, proteins and carbohydrates. So it isn't surprising that nucleic acids are built like these other macromolecules. Nucleic acids and the other macromolecules just mentioned are polymers made up of individual molecules linked together in long chains.

• Proteins are polypeptides made up of individual amino acids linked together,
• Carbohydrates are polysaccharides made up of individual monosaccharides linked together, and
• Nucleic acids are polynucleotides made up of individual nucleotides linked together.

If you go even further, a nucleotide can itself be further broken down to yield three components:

• a sugar,
• a Nitrogen (amine) base, and
• phosphoric acid.
  

As mentioned in the Introduction to this tutorial, there are two types of nucleic acids: DNA and RNA. DNA stores genetic information, and RNA allows that information to be made use of in the cell. Before we discuss the overall structure of nucleic acids as polymers, we should probably find out how their individual component parts are joined together and how DNA and RNA differ.