DNA is condensed into chromosomes

DNA molecules are incredibly long and must be tightly packed up to fit within the nucleus of a eukaryotic cell. 

DNA molecules are wound around proteins known as histones to form a DNA-histone complex.

These complexes coil further to pack the DNA into chromosomes whereby each chromosome contains just a single molecule of DNA.

How to interpret tables that show codons for amino acids

The genetic code can be summarised into a table as shown below. The code is often shown as mRNA bases rather than DNA, so the base uracil (U) is shown instead of thymine (T).

Each amino acid is coded for by a sequence of three mRNA bases known as a codon (similar to a DNA triplet).

We can see from the table above, that many amino acids are coded for by multiple codons. For example, the amino acid phe (phenylalanine) is coded for by codons UUU and UUC. 

The genetic code

A gene is a short section of DNA that codes for a polypeptide (a protein). Each gene is located at a specific position along a chromosome known as a locus. 

The complete set of genes within a cell is known as the genome. The full range of proteins that a cell is capable of producing is known as the proteome.

The genetic code refers to the sequence of bases that code for amino acids.

Scientists found that each amino acid is coded for by a sequence of three DNA bases, known as a triplet.

The genetic code has the following features: 

1. Universal - Each DNA triplet codes for the same amino acid in all organisms (with a few minor exceptions). 
2. Non-overlapping - Each base in the DNA sequence is only read once (e.g. CGTATC is read as CGT and ATC).
3. Degenerate - Most amino acids are coded for by more than one triplet (e.g. ACA and ACG both code for cysteine).