Genomics 101: DNA, Genes, and the Central Dogma
The foundational biology behind every bioinformatics pipeline - what a genome actually is, and how DNA becomes protein.
Before any FASTQ file, alignment, or variant call makes sense, it helps to be solid on the underlying biology. This is the 10-minute version of what a genome is and how information flows out of it.
What is a genome?
A genome is the complete set of DNA instructions in an organism - in humans, roughly 3.1 billion base pairs packaged into 23 pairs of chromosomes inside almost every cell. DNA is built from four chemical bases:
- A - Adenine
- T - Thymine
- C - Cytosine
- G - Guanine
These bases pair up in a strict, predictable way - A with T, and C with G - which is what lets DNA form its double helix and be reliably copied.
The Central Dogma
The "Central Dogma" of molecular biology, first described by Francis Crick in 1958, describes how genetic information flows in a cell:
- Transcription - an enzyme (RNA polymerase) reads a gene in the DNA and produces a matching single-stranded copy called messenger RNA (mRNA). RNA uses Uracil (U) instead of Thymine.
- Translation - the mRNA leaves the nucleus and is read by a ribosome three bases (a codon) at a time. Each codon specifies one amino acid, and the ribosome strings amino acids together into a protein - the molecule that actually does the work in a cell.
A gene is simply the stretch of DNA that contains the instructions for one (or more) protein, plus the regulatory sequences that control when and how much of it gets made. Humans have roughly 20,000 protein-coding genes, but they make up only about 1-2% of the genome - the rest is regulatory sequence, structural DNA, and regions whose function is still being mapped.
Why this matters for bioinformatics
Almost everything downstream connects back to this flow:
- Variant interpretation asks: does this DNA change affect transcription, splicing, or the resulting protein?
- RNA-seq measures which genes are being transcribed, and how much, in a given sample.
- Clinical genomics (my day job) is largely about tracing a change all the way from a DNA letter to a functional consequence a clinician can act on.
The Central Dogma: Transcription and Translation - Bozeman Science