DNA as a Genetic Template

The Watson-Crick model, proposed in 1953, elucidated two structural features of the DNA molecule that provide a basis for heredity: its double-stranded nature and the four nucleotide bases. 

This model proposed that DNA is made up of two strands of nucleotides that twist around each other to form a right-handed helix.

These strands are anti-parallel in nature - meaning the 3’ end of one strand faces the 5’ end of the other strand.

Each strand of DNA contains a sequence of nucleotides that is exactly complementary to its partner strand - enabling each strand to act as a template for its partner.

Thus, a cell can replicate its DNA before cell division by separating two strands of DNA and producing two new strands of DNA that are exactly complementary to each other. 

Additionally, the Watson-Crick model posited that base-pairing takes place between a purine - either adenine or guanine - and a pyrimidine - either cytosine or thymine. 

Adenine, guanine, cytosine, and thymine comprise the four-letter nucleotide ‘alphabet.’ 

We now know that when these nucleotide letters are strung together into a three-letter codon during translation, they can code for one of the 20 amino acids - much like letters in an alphabet can be arranged to spell out words. 

These amino acid ‘words’ are linked into sentences, forming chains that fold into different proteins. 

Different permutations of codons can result in different genes - much like how different combinations of words and sentences result in different books. 

The genome is the entirety of the information contained within an organism’s genes and encompasses all of the RNA molecules and proteins that an organism will produce over its lifetime.

Differences between genomes result in genotypically and phenotypically distinct organisms and species.