Transcription and Translation

By the end of this lesson, you will be able to describe the flow of genetic information (central dogma), explain transcription (DNA → RNA) and translation (RNA → protein), decode codons, and identify the roles of mRNA, tRNA, and rRNA.

The Central Dogma

$$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$$

Information flows one way under normal circumstances. DNA is the master blueprint; mRNA is the working copy; protein is the product.

Transcription (DNA → mRNA)

Location: nucleus (eukaryotes)

Key enzyme: RNA polymerase

Process:

1. RNA polymerase binds to the promoter region upstream of the gene
2. Double helix unwinds; RNA polymerase reads the template strand (3' → 5')
3. Assembles complementary RNA nucleotides (5' → 3' direction); uses U (uracil) instead of T
4. At the terminator sequence, RNA polymerase detaches; pre-mRNA is released

mRNA processing in eukaryotes (before leaving nucleus):

• 5' cap and poly-A tail added → protect mRNA from degradation, aid translation initiation
• Introns (non-coding sequences) are spliced out by spliceosomes
• Exons (coding sequences) are joined → mature mRNA

The Genetic Code

Codon: a three-base sequence of mRNA that specifies one amino acid.

64 possible codons → 20 amino acids → the code is degenerate (multiple codons for the same amino acid).

Start codon: AUG (codes for methionine — the first amino acid in all proteins).

Stop codons: UAA, UAG, UGA → signal end of translation; no amino acid added.

Translation (mRNA → Protein)

Location: ribosome (cytoplasm or rough ER)

Key molecules:

• mRNA: carries codons (the message)
• tRNA: has anticodon complementary to mRNA codon; carries the specific amino acid
• rRNA: structural and catalytic component of ribosomes

Process

1. Initiation: small ribosomal subunit binds mRNA at start codon (AUG); initiator tRNA (Met) binds; large subunit joins