The Central Dogma: DNA → RNA → Protein

The Central Dogma of Molecular Biology describes how genetic information flows inside a cell.
It explains how instructions written in DNA are converted into RNA, and then used to build proteins, the molecules that perform most cellular functions.

This page covers:

• What transcription is
• What translation is
• Types of RNA
• How information flows
• Why this process is essential for life
• What parts (if any) can be seen under a light microscope

1. Overview of the Central Dogma

DNA → RNA → Protein

This happens in two major steps:

1. Transcription (DNA → RNA)
   Occurs in the nucleus of eukaryotes (yeast, onion, human).
   DNA is read by the enzyme RNA polymerase, producing messenger RNA (mRNA).

2. Translation (RNA → Protein)
   Occurs in the cytoplasm on ribosomes.
   The ribosome reads the mRNA and assembles amino acids into a protein.

This flow of information is universal across life.

2. Step 1: Transcription (DNA → RNA)

Transcription is the process of making an RNA copy of a section of DNA (a gene).

2.1. Key enzyme

• RNA Polymerase — binds to a gene, reads DNA, and synthesizes RNA.

2.2. Steps of transcription

1. Initiation
   RNA polymerase binds the promoter, a regulatory DNA region.

2. Elongation
   The enzyme moves along DNA, forming an RNA strand complementary to the DNA template.

3. Termination
   RNA polymerase stops and releases the RNA molecule.

2.3. Where it happens

• Eukaryotes (plants, yeasts, humans): inside the nucleus
• Prokaryotes (bacteria): directly in the cytoplasm

3. Step 2: Translation (RNA → Protein)

Translation is the process of converting mRNA information into a chain of amino acids (a protein).

3.1. Where translation occurs

• On ribosomes, found in the cytoplasm or bound to the rough ER.

3.2. Key players

mRNA (Messenger RNA)

Carries the instructions copied from DNA.

tRNA (Transfer RNA)

• Brings amino acids to the ribosome.
• Each tRNA recognizes a specific 3-letter mRNA sequence (a codon).

rRNA (Ribosomal RNA)

• Forms the core of the ribosome.
• Catalyzes peptide bond formation.

3.3. Steps of translation

1. Initiation
   Ribosome binds mRNA at the start codon (AUG).

2. Elongation
   tRNAs bring amino acids in the correct order; the ribosome links them together.

3. Termination
   When a stop codon is reached, the protein is released.

3.4. Optical visibility

Ribosomes, mRNA, and proteins cannot be seen individually with a light microscope.

4. Types of RNA

Cells use several kinds of RNA, each with a specific purpose:

4.1. mRNA (Messenger RNA)

Carries genetic instructions.

4.2. tRNA (Transfer RNA)

Delivers amino acids to ribosomes.

4.3. rRNA (Ribosomal RNA)

Structural and catalytic component of ribosomes.

4.4. snRNA / snoRNA

Used in RNA processing (splicing, modification).

4.5. miRNA / siRNA

Regulate gene expression (epigenetic control).

4.6. lncRNA

Long non-coding RNAs with regulatory roles.

5. The Genetic Code

The genetic code uses codons — groups of three nucleotides on mRNA.

Examples:

• AUG → Methionine (start)
• UGA → Stop
• GGU → Glycine

Features:

• Universal (same in yeast, plants, humans)
• Redundant (multiple codons for the same amino acid)
• Non-overlapping

6. Why the Central Dogma Matters

It explains how cells:

• build proteins
• respond to signals
• repair themselves
• differentiate into specialized types
• age
• reprogram (OSK, CRISPR)

Most biotechnology — vaccines, gene editing, RNA therapies — directly manipulates this flow.

For example:

• mRNA vaccines provide mRNA so your cells make a protein.
• CRISPR modifies DNA so transcription produces different RNA/proteins.
• OSK reprogramming changes gene expression to reverse epigenetic age.

8. Quick Summary

• Transcription: DNA → RNA
• Translation: RNA → Protein
• Ribosomes read mRNA and assemble amino acids.
• Proteins perform almost every function in the cell.
• This process is universal to life and essential for understanding biology, aging, disease, and biotechnology.