🧬 Biology introductory Lesson 3 of 6 1 min read

Protein Synthesis: Transcription and Translation

How the information in genes becomes protein, covering transcription of DNA into RNA, mRNA processing, translation at the ribosome, and the properties of the genetic code.

Reading level

What you'll learn

Describe the three stages of transcription and where each takes place.
Explain how a primary transcript is processed into mature mRNA in eukaryotes.
Trace how the ribosome, tRNA, and mRNA cooperate to build a polypeptide.
State the defining properties of the genetic code and their significance.

Transcription

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA Polymerase.

Steps

Initiation — Transcription factors bind the promoter (e.g., TATA box in eukaryotes); RNA pol II assembles into a pre-initiation complex.
Elongation — RNA pol moves 3’→5’ along the template strand, synthesizing RNA 5’→3’.
Termination — A polyadenylation signal (AAUAAA) in eukaryotes triggers cleavage and poly-A tail addition; the transcript is capped at the 5’ end.

mRNA Processing (Eukaryotes)

5’ 7-methylguanosine cap → ribosome recognition and nuclease protection
Poly-A tail (~200 A nucleotides) → nuclear export and stability
Splicing — spliceosome removes introns; exons are joined to form mature mRNA

Translation

Translation occurs at the ribosome and converts mRNA codons into an amino acid chain.

Key Players

Component	Role
mRNA	Carries codon sequence from nucleus
Ribosome (large + small subunit)	Scaffold; catalytic peptidyl transferase activity (rRNA)
tRNA	Anticodon matches codon; carries cognate amino acid
Aminoacyl-tRNA synthetases	Charge tRNA with correct amino acid
Initiation, elongation, release factors	GTP-driven cycle machinery

Ribosome Sites

A site — aminoacyl-tRNA entry
P site — peptidyl-tRNA (growing chain)
E site — exit; uncharged tRNA leaves

The Genetic Code

The genetic code is:

Triplet — 3 nucleotides = 1 codon
Degenerate — most amino acids encoded by multiple codons (wobble position)
Universal — same across virtually all life (minor exceptions in mitochondria)

Clinical Relevance

Many antibiotics exploit differences between prokaryotic (70S) and eukaryotic (80S) ribosomes: aminoglycosides and tetracyclines target the 30S subunit; macrolides and chloramphenicol target the 50S subunit.

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

Wobble and why 61 codons need only ~40 tRNAs

Crick's wobble hypothesis explains the degeneracy of the code: pairing at the third codon position is relaxed, so a single tRNA anticodon can read several synonymous codons. The modified base inosine at the wobble position can pair with U, C, or A, which is why cells manage with far fewer than 61 distinct tRNAs. This flexibility also buffers many third-position point mutations, making them synonymous (silent) and phenotypically neutral.

Folding, chaperones, and post-translational fate

The linear polypeptide is not the finished product. Signal peptides at the N-terminus direct ribosomes to the rough ER via the signal recognition particle, routing proteins into the secretory pathway. Chaperones such as the Hsp70 and chaperonin families use ATP to prevent misfolding and aggregation, while post-translational modifications, including glycosylation, phosphorylation, and proteolytic cleavage, determine activity, localization, and half-life. Failures of folding underlie diseases ranging from cystic fibrosis (misfolded CFTR degraded before reaching the membrane) to the amyloid aggregates of neurodegeneration.

Key terms

Transcription: The synthesis of an RNA copy from a DNA template, carried out by RNA polymerase.
RNA polymerase: The enzyme that reads a DNA template and builds a complementary RNA strand in the 5' to 3' direction.
mRNA: Messenger RNA; the transcript that carries the coding sequence from DNA to the ribosome.
Splicing: The removal of introns and joining of exons to produce mature mRNA.
Translation: The process by which the ribosome reads mRNA codons and assembles the corresponding chain of amino acids.
Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal.
tRNA: Transfer RNA; an adaptor molecule whose anticodon matches a codon while it carries the matching amino acid.
Genetic code: The set of rules relating three-nucleotide codons to amino acids; it is triplet, degenerate, and nearly universal.

Check your understanding

5 questions · answers reveal instantly.

1.During transcription, RNA is synthesized in which direction?
2.Which mRNA processing step removes non-coding sequences from the primary transcript?
3.What matches a codon to the correct amino acid during translation?
4.The genetic code is described as 'degenerate' because:
5.Many antibiotics work by targeting bacterial ribosomes because:

Citations & References

Links open publicly available educational and peer-reviewed sources.