DNA and RNA Structure and Synthesis

This post will discuss DNA and RNA structure and synthesis which is the next topic in our Veterinary Biochemistry unit.


The Function and Characteristics of DNA

The function of DNA is to provide a template for replication and to maintain genetic identity. DNA has the ability to store genetic information, and to transfer a faithful copy of this information to daughter cells. It also has physical and chemical stability so that information can be stored for a long time. There is also the potential for small changes, which can be inherited, without the loss of parental information. A few important characteristics are listed below:

• DNA is double stranded
• DNA strands are anti parallel
• Guanine-Cytosine pairs have 3 hydrogen bonds
• Adenosine - Thymine pairs have 2 hydrogen bonds
• One strand is the complement of the other
• The double helix has major and minor grooves.

RNA (Ribonucleic acid)

There are three kinds of RNA:

• Ribosomal RNA: which make up the structure of ribosomes (small granular structures where protein synthesis takes place).
• Messenger RNA (mRNA): they record information from DNA and carry it to ribosomes.
• Transfer RNA (tRNA): they deliver amino acids to protein chains that are assembled at the ribosomes.

Differences between DNA and RNA:

• The sugar in RNA is ribose, in DNA it is deoxyribose
• RNA has the base uracil instead of thymine which is found in DNA
• RNA molecules are shorter than DNA molecules
• RNA is single stranded while DNA is double stranded
• DNA has only one function - storing genetic information.   

How DNA is Packaged Inside a Nucleus

There are two forms of eukaryotic DNA. Chromatin, which is the uncoiled form of DNA and contains over 50% protein. Chromosomes are coiled DNA and protein that form during the early stages of cell division. 

Uncoiled DNA is very long so in order for it to fit inside the nucleus of a cell it needs to be coiled. First, DNA begins to wind around proteins known as histones forming a chain of bead like nucleosomes. These nucleosomes then form a tightly coiled structure which is stabilised by histone H1. This coiled structure is packed further by arranging in loops which attach to a protein scaffold. The scaffold coils into a spiral to form a chromosome. 

The Organisation of DNA
source: http://commons.wikimedia.org/wiki/File%3AChromatin_Structures.png
please see http://creativecommons.org/licenses/by-sa/3.0/ if you'd like to use this diagram.

Please note that the original copy of the diagram above is quite large. It would be best if you follow the link in the caption to see the full sized image.

DNA Synthesis

Below is a very good video which describes the process of DNA replication.

  
The main enzymes involved are:

•  Helicase (unwinds double helix)
• Single strand binding proteins (stabilise separate strands)
•  Primase (adds a short RNA primer to DNA strand)
• DNA Polymerase (binds nucleotides to form new strands)
• Exonuclease (removes RNA primer and inserts correct base)
• DNA ligase (joins Okazaki fragments and seals other nicks in sugar-phosphate back bone)

And that's it! If you have any questions please feel free to ask.