Our DNA is like a library - found in the nucleus of our cells - with thousands of books.
Some of these books - called genes - are extremely important, because they carry the recipes for every single protein found in the cell.
These genes are scattered among 23 pairs of chromosomes - which are like the bookcases of the library.
Chromosomes come in homologous pairs because - one comes from mom and one comes from dad.
Each chromosome of the pair carries different versions of the same genes, called alleles.
Now, on the molecular level, DNA is made up of two strands of nucleotides, so each gene is just a segment of this nucleotide sequence.
There are four types of nucleotides: adenine, guanine, thymine, and cytosine - or A, G, T, C.
Gene expression includes transcription and translation.
Transcription is where the enzyme RNA polymerase uses the gene as a template to create a molecule that can leave the nucleus.
This molecule is called messenger RNA or mRNA and it has the same nucleotide sequence as the gene, with one tweak: it has uracil nucleotides - or U - instead of thymine.
This mRNA molecule - or message - is encoded so that any 3 nucleotides equate to a specific codon which codes for an amino acid or is a stop codon which signals that the protein is complete.
In translation, specialized proteins in the cytoplasm - called ribosomes - use the mRNA template to recognize the specific codons, and match them with the corresponding amino acids that will make up the protein.
Now there are 64 different codons, and each of them codes for a single amino acid - but there are only 20 amino acids.
That’s because some amino acids are encoded by more than one nucleotide triplet.
Now - a mutation, put simply, is an alteration in the nucleotide sequence of one or more genes - but can sometimes affect large chunks of chromosomes.
These mutations can affect the chromosomes in somatic cells - meaning any cell in our body other than the gametes - or the chromosomes in gametes.
Mutations in gametes are called germline mutations, because they can be passed on to the next generation.
Now, mutations can happen spontaneously, or they can be induced by mutagens.
Mutagens include physical agents, like UV rays and chemicals, to biological agents like viruses.
Often, mutations occur during DNA replication - which happens right before a cell divides.