Evolution in 1-2-3
1) Descent with modification
2) Adaptation and natural selection
DNA (Deoxyribonucleic acid) consists of a four-letter alphabet, four amino acids - adenine, thymine, cytosine and guanine - labeled A, T, C and G. The four letters create three-letter words which are called codons. There are 64 different codons in your DNA, i.e. 64 different three-letter combinations of the four letters.
Just like words, the codons have different meanings. For instance, ATG denotes "start reading" whereas TAG, TAA and TGA denote "stop reading". Most of the 64 codons represent one of the 20 amino acids which are used to build proteins. But in order to get 20 amino acids from a code consisting of 4 amino acids, the code must be translated.
First, a copy of the DNA code from the "start reading" codon to the "stop reading" codon is made. The copy is called a Messenger RNA (mRNA). The mRNA is brought to the Ribosome where the message is read, codon by codon (word for word). The Ribosome uses a Transfer RNA (tRNA) to translate the codon on the mRNA to an amino acid which shall become part of a larger protein. It is called Transfer RNA because one end of the correct tRNA attaches to the mRNA while the other end of that tRNA contains a corresponding amino acid. At the end of the process, i.e. when the Ribosome reaches the "stop reading" codon on the mRNA, a string of amino acids have been joined together by the Ribosome.
The string of amino acids which was just produced has chemical properties which makes it curl into a three dimensional structure, a large molecule called a protein. Different configurations of amino acids create different three dimensional structures. The human body may consist of as many as two million different proteins, from less than 50 amino acids long to up to 27000 amino acids long. Many proteins even go through a post-translation modification where carbonhydrates are attached to it, changing its structure further.
The three dimensional structure of a specific protein makes it fit to fill a specific function. It can be a building block, joining several different proteins to constitute an even larger structure, or it can be a single enzyme, a means of transportation or flexible as in muscles, facilitating biochemical processes in the organism.
The fact that everything in a living cell is interdependet with everything else in the cell is one of the great challenges in origin of life research. Origin of life is however not the focus of this site.
DNA is nature's two-dimensional instruction manual for building three-dimensional living things. Letters create words, words create sentences, sentences relay meaningful instructions to nano-machinery which follow the instructions and produce functional building blocs.
But nothing is perfect. Every time a process is repeated, a chance of error exists. And the processes of life are repeated billions and billions of times.
It is from this platform that the facts of evolution start.