Bio 1000- DNA structure, Replication & Organization- An Introduction

As you may or may not know, DNA is what makes us who we are. From the colour of our eyes to the height that we're going to inherit, it is all determined by a microscopic structure in our cells called DNA.

WHAT IS DNA AND WHAT'S ITS STRUCTURE?

DNA stands for deoxyribonucleic acid and is found in the nucleus of the cell. It is comprised of nucleotides made up of :

1. Five carbon sugar deoxyribose
2. Phosphate group
3.  One of four nitrogenous bases 
1. Adenine (A)
2. Guanine (G)
3. Thymine (T)
4. Cytosine (C)

These nitrogenous bases are further subdivided into purines and pyrimidines:

1. Purines are nitrogenous bases built from a pair of fused rings of carbon and nitrogen atoms
• Includes Adenine and Guanine
2. Pyrimidines are nitrogenous bases built from a single carbon ring
• Includes Thymine and Cytosine

Researcher Erwin Chargaff discovered what is now known as Chargaff's Rules. This rule suggests that the number of purines equals the number of pyrimidines, that is the amount of adenine equals the amount of thymine and the amount of guanine equals the amount of cytosine.

WHAT THE HECK IS POLYPEPTIDE CHAINS?

DNA contains nucleotides that join together to form polypeptide chains. In polypeptide chains, the deoxyribose sugars are linked by phosphate groups in an alternating sugar-phosphate-sugar-phosphate pattern, creating a sugar-phosphate backbone. Each phosphate group attaches between the 3' carbon of one sugar and the 5' carbon of another sugar, creating a bridge between the sugars known as phosphodiester bond.

The polynucleotide chain has directionality, that is, the two ends of the chain are not the same. At one end, a phosphate group is bound to the 5' carbon of the deoxyribose sugar  whereas the other end, a hydroxyl is bound to the 3' carbon of the deoxyribose sugar. To simply:

1. 5' end is where the phosphate group is attached
2. 3' end is where the hydroxyl group is attached

WAIT, WHAT DNA IS A DOUBLE HELIX?

Yes, DNA does take the shape of a double helix. In the double helix, the two sugar-phosphate backbones are separated from each other by a regular distance. The bases then extend to fill the space by which the bonding of a purine and a pyrimidine fills the central space perfectly. This led to what is referred to as complementary base pairing, a purine must bond with a pyrimidine to fill the central space. Complementary base pairing exists as:

1. Adenine binds with Thymine
2. Guanine binds with Cytosine

These base pairs are stabilized by hydrogen bonds:

1. 2 hydrogen bonds between A-T
2. 3 hydrogen bonds between G-C

However, complementary base pairing can only exist if the DNA strands are antiparallel, that is, if they run in opposite directions. One strand runs from 3' to 5' and the other strand runs from 5' to 3'.

 

 

Note: The distance between each pair bases are 0.34 nm and each full twist of DNA double helix is 3.4 nm