The Central Dogma of Molecular Biology is the fundamental principle that explains how genetic information flows inside cells. Biology is full of beautiful simplicity hidden within complex processes. One of the most fundamental frameworks in molecular biology is the central dogma: the flow of information from DNA to RNA to protein. In this post, we’ll break down the essential steps—replication, transcription, and translation—and explore how genes are structured and distributed across different organisms.
DNA Replication: Copying the Blueprint
Before any cell can divide, it must replicate its DNA. Replication follows a semi-conservative strategy, meaning each daughter DNA molecule contains one original strand and one newly synthesized strand.
- DNA synthesis always occurs in the 5′ → 3′ direction.
- The leading strand is synthesized continuously.
- The lagging strand is synthesized in small segments called Okazaki fragments.
This process ensures that every new cell inherits an identical copy of the genetic blueprint.
Transcription: From DNA to RNA
Transcription is the process of copying a DNA sequence into RNA. Here’s how it works:
- RNA polymerase binds to a transcription start site.
- It produces an RNA strand complementary to the DNA template strand.
- The resulting RNA is almost identical to the coding strand of DNA, except that RNA contains uracil (U) instead of thymine (T).
RNA molecules are less stable than DNA, but they are the active copies that carry genetic instructions for building proteins.
Translation: Building Proteins
Translation turns RNA instructions into proteins—the workhorses of the cell.
- mRNA provides the code.
- tRNAs deliver amino acids to the ribosome.
- The ribosome links amino acids into a growing polypeptide chain.
This chain folds into a functional protein, which may act as an enzyme, receptor, hormone, or structural component. Proteins carry out almost all of life’s essential functions.
What Is a Gene?
A gene is a DNA segment with defined boundaries:
- Transcriptional start site
- Coding region
- Polyadenylation site
Most genes begin with a start codon (AUG), though some use alternatives. On average, human genes are 1–2 kb long, encoding proteins of 300–600 amino acids.
- Longest gene: Dystrophin, spanning ~2.4 million base pairs. Mutations here cause Duchenne Muscular Dystrophy.
- Smallest genes: only 30–40 amino acids long.
Introns—noncoding sequences—make up much of human gene length, highlighting the complexity of eukaryotic genomes.
How Many Genes Do Organisms Have?
Gene counts vary widely across life:
- Humans: 20,000–25,000 genes (earlier estimates were ~100,000).
- E. coli: ~4,000 genes.
- Mycoplasma genitalium: just 467 genes—the bare minimum for life.
- Octopus: ~33,000 genes, more than humans! Many are N-cadherins supporting their distributed neural networks.
- Some fleas: ~31,000 genes.
Interestingly, genome size does not always correlate with complexity. Plants often have huge genomes, but not necessarily more genes.
Final Thoughts
The central dogma reminds us of biology’s elegance: DNA encodes RNA, which builds proteins, which create life. Genes vary in size and number across organisms, from the minimalist Mycoplasma to the gene-rich octopus. Yet the underlying principle is universal—a simple flow of information that sustains the diversity of life on Earth.