DNA (deoxyribonucleic acid) and RNA (Ribonucleic acid) are categorised as
nucleic acids.
DNA
DNA is the blueprints of life. In every living cell there is some form of DNA, which tells the cell how to act, what to make and when to die. DNA codes for proteins and therefore enzymes. By ordering the cell to make more or less of a specific enzyme, DNA can control the structure and function of the cell.
All hereditary information is passed down through DNA. This is why you may look like your mother and father or be predisposed to a genetic disorder.
The basics (Genes)
As we discussed before DNA make up Genes. Genes are sections of DNA, which code for proteins and contain hereditary information.
All the Genes in an organisms cell makes up its Genome. All organisms, from Bacteria to humans have a genome.
So in a silk worm
The Genome contains Genes, Genes code for polypeptide chains. The polypeptide forms the silk used by the silk worm.
The basics
(DNA configuration)
DNA is made up of nucleotides. These are the basic building blocks of DNA.
The Nucleotide is made up of 3 parts:
A phosphate group,
A five carbon sugar
One of the 4 nitrogenous bases
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
Uracil (U) (in RNA only)
Base
These can be divided into 2 categories
Purines
Purines have a double ring structure
Adenine and Guanine are Purines
Pyrimidines
Pyrimidines have a single ring structure
Thymine and Cytosine are Pyrimidines
As you can see Pyrimidines (such as T or C) link up with Purines (A or G).
Double helix
As you know the base pairing rule in DNA is A-T and C-G.
This means that there is the exact same amount of C and G nucleotides, and A and T nucleotides.
So by simply knowing the bases of one strand we can easily work out the other strands sequence.
The two end of a polynucleotide are refereed to as 3’ (prime) to a 5’ (prime) ends. The 5’ end has a free phosphate group and the 3’ has a free hydroxyl group.
The strands run anti-parallel meaning one goes 3’ to 5’ while the other goes 5’ to 3’.
Packaging
The average DNA molecule in a human cell is 6.5 x 10^7 base pairs long, this equates to a single cell holding 1.8 meters of DNA. So how does the cell fit 1.8 meters to DNA into a 6µm nucleus?
By super coiling DNA.
Observe the diagram to the right. Notice how the DNA is continuoulsy coiled and compressed.
It then continues to wrap around large circular protein molecules called Histones. This forms a nucleosome.
Wrapping DNA around histones serves to efficiently package the DNA and protect it fromm enzymatic degradation.
The entire DNA and protein complex is called the chromatin.
