DNA Replication and Protein Synthesis

Every time one of your cells divides, it has to copy about 3 billion letters of DNA instruction almost perfectly. Your body does this billions of times over your life, and on average it makes fewer than 1 mistake per 1 billion letters copied. That's like retyping the entire Harry Potter series word for word 1,500 times and only making a single typo.

- The four DNA letters and how they always pair up the same way - How a cell copies its entire DNA before dividing - How cells read DNA instructions to build proteins - Why a copying mistake can sometimes matter and sometimes not

DNA is built from four chemical letters: A (adenine), T (thymine), C (cytosine), and G (guanine). These letters always pair the same way: A always pairs with T, and C always pairs with G. This strict pairing rule is what makes copying possible — if you know one side of the ladder, you already know exactly what the other side must be.

Before a cell divides, an enzyme called DNA helicase unzips the double-stranded DNA ladder right down the middle, separating the two strands. Then a second enzyme, DNA polymerase, moves along each single strand and builds a brand-new matching strand by adding the correct partner letter to each exposed letter. When it's done, there are two identical DNA ladders where there used to be one — each new ladder made of one original strand and one brand-new strand.

DNA stays safely inside the nucleus, so the cell makes a working copy called messenger RNA (mRNA) through a process called transcription. The mRNA carries the instructions out to a cell structure called a ribosome. There, during translation, the ribosome reads the mRNA three letters at a time (each three-letter group is called a codon) and matches each codon to a specific building block called an amino acid. Chain enough amino acids together in the right order and you get a protein — the molecules that build muscle, carry oxygen in blood, and fight off germs.

Sometimes DNA polymerase's proofreading misses an error, and a wrong letter slips through. This is called a mutation. Many mutations don't change anything important, because the genetic code has some built-in backup — several different codons can code for the same amino acid. But occasionally a mutation does change a protein's shape enough to change how it works, which is one reason siblings can look different even though they share the same parents' DNA instructions.