Biology 6. Genetics 6.1. Molecular Foundations of Heredity 6.1.3. From Gene to Protein 6.1.3.1. Protein Biosynthesis in Prokaryotes and Eukaryotes

6.1.3.1. Protein Biosynthesis in Prokaryotes and Eukaryotes

Table of Contents

Overview: One Principle, Two Variants

All cells use the same genetic code and the same basic logic:

DNA is transcribed into RNA (covered in detail elsewhere),
RNA (specifically mRNA) is translated into protein by ribosomes.

In this chapter the focus is on how this translation process is carried out in prokaryotes versus eukaryotes, and how their cellular organization leads to important differences.

You already know what DNA, RNA, genes, codons, and the genetic code are from other chapters; here we look at:

how ribosomes “read” mRNA,
how translation is initiated, elongated, and terminated,
and what is similar or different in bacteria/archaea versus eukaryotic cells.

We will first outline the general steps of translation, then highlight prokaryote–eukaryote differences step by step.

The Players in Translation

Translation is a biochemical “assembly line” with specific roles:

mRNA (messenger RNA) – carries the coding sequence, a series of codons.
Ribosome – RNA–protein complex that catalyzes peptide bond formation.

Has a small subunit (decoding the mRNA–tRNA pairing),
and a large subunit (forming peptide bonds).

tRNA (transfer RNA) – adaptor molecules:

One end: anticodon (3 bases complementary to an mRNA codon),
Other end: specific amino acid.

Aminoacyl‑tRNA synthetases – enzymes that attach the right amino acid to the right tRNA.
Initiation, elongation, and release factors – accessory proteins that control the steps.

The basic cycle is the same in all cells:

Ribosome binds mRNA.
tRNAs bring amino acids, matching codons.
Peptide bonds join amino acids into a growing polypeptide chain.
At a stop codon, the chain is released and the ribosome dissociates.

The Ribosome and Its Sites

Ribosomes in all domains of life have similar functional regions:

A site (aminoacyl site) – entry of the new aminoacyl‑tRNA.
P site (peptidyl site) – holds the tRNA with the growing polypeptide chain.
E site (exit site) – where the now empty tRNA leaves the ribosome.

The core catalytic activity—forming peptide bonds—is carried out by ribosomal RNA (rRNA), making the ribosome a ribozyme, not just a protein machine.

Key difference in size:

Prokaryotic ribosome: 70S

Small subunit: 30S
Large subunit: 50S

Eukaryotic ribosome: 80S

Small subunit: 40S
Large subunit: 60S

(“S” = Svedberg unit; it reflects how particles sediment, not a simple sum.)

General Stages of Translation

Even though many details differ, translation everywhere can be divided into three main stages:

Initiation – assembling the ribosome at the correct start codon.
Elongation – repeated addition of amino acids to the growing chain.
Termination – releasing the completed polypeptide at a stop codon.

We’ll describe the overall sequence first in a domain‑neutral way, then focus on what is unique in prokaryotes and eukaryotes.

1. Initiation: Finding the Start and Assembling the Complex

Core idea

The cell must:

distinguish the start codon from other identical triplets down‑stream,
place a special initiator tRNA in the P site,
and assemble both ribosomal subunits at the right position.

In both prokaryotes and eukaryotes:

The usual start codon is AUG, coding for methionine (Met).
A special initiator tRNA (not used later in elongation) delivers the first Met.
After initiation, the ribosome has:

Start codon in its P site,
Initiator tRNA bound to that codon,
A site empty and ready to receive the second aminoacyl‑tRNA.

How these conditions are achieved differs significantly between prokaryotes and eukaryotes.

2. Elongation: Repeating a Three‑Step Cycle

Once properly initiated, translation proceeds by repeating a well‑ordered cycle:

Codon recognition / tRNA entry

An aminoacyl‑tRNA, escorted by an elongation factor, enters the A site.
If anticodon and codon match, the tRNA is allowed to stay.

Peptide bond formation

The growing peptide chain is transferred from the tRNA in the P site to the amino acid on the tRNA in the A site.
This reaction is catalyzed by the rRNA of the large subunit (peptidyl transferase activity).

Translocation

The ribosome moves one codon (3 nucleotides) along the mRNA.
The tRNA that carried the growing chain shifts from A → P site.
The now empty tRNA in the P site moves to the E site and exits.
The A site is free for the next aminoacyl‑tRNA.

This cycle proceeds codon by codon until the ribosome reaches a stop codon.

Elongation is highly accurate because:

Correct tRNA–codon pairing is checked by the small subunit,
Mismatched pairs are typically rejected before peptide bond formation.

3. Termination: Ending at the Right Place

Three codons do not code for amino acids but signal “stop”:

UAA, UAG, UGA

There are no tRNAs with anticodons for these codons.
Instead, release factors (proteins) recognize stop codons:

A release factor binds to the A site when a stop codon is present.
It triggers hydrolysis of the bond between the peptide and the tRNA in the P site.
The completed polypeptide chain is released.
The ribosomal subunits dissociate from the mRNA and can be reused.

The chemical principle (hydrolysis and disassembly) is similar in all cells, but the exact proteins and names of release factors differ between domains.

Protein Biosynthesis in Prokaryotes

Prokaryotic cells (bacteria and archaea) lack a nucleus and complex internal compartmentalization. This has direct consequences for translation.

Coupling of Transcription and Translation

In prokaryotes, DNA, RNA, and ribosomes all reside in the same cellular space (cytoplasm).
As soon as an mRNA begins to be transcribed from DNA, ribosomes can start translating it.
This leads to transcription–translation coupling:

RNA polymerase at the front, synthesizing the mRNA,
several ribosomes behind it, already making protein from the same mRNA.

Consequences:

Rapid protein production, important for fast growth.
Regulation of gene expression can involve both transcription and translation happening almost simultaneously.

Polycistronic mRNAs and Operons

Prokaryotic mRNAs are often polycistronic:

One mRNA molecule contains coding sequences for several proteins.
These proteins usually function in the same pathway (e.g., all enzymes of a metabolic pathway).

Such gene clusters are called operons.
Each coding region on the polycistronic mRNA has its own translation initiation signals, allowing separate ribosome entry points.

Initiation in Prokaryotes: Shine–Dalgarno Sequence

Prokaryotes have a specific way to position the ribosome correctly:

Upstream (before) the start codon AUG, prokaryotic mRNAs often contain a Shine–Dalgarno sequence.

A short, purine‑rich sequence, complementary to a region on the 16S rRNA of the 30S subunit.

Steps of initiation (simplified):

The 30S small subunit binds to the mRNA by base pairing between 16S rRNA and the Shine–Dalgarno sequence.
The initiator tRNA carrying a modified methionine (N‑formylmethionine, fMet) pairs with the AUG start codon in the P site.
Several initiation factors (e.g., IF1, IF2, IF3 in bacteria) assist in correct assembly.
The 50S large subunit joins, forming a functional 70S initiation complex.

Key differences to note:

Use of fMet‑tRNA\_fMet as initiator in bacteria.
Direct recognition of an internal start codon via the Shine–Dalgarno sequence (no scanning from the 5′ end).

Elongation in Prokaryotes

The elongation cycle in bacteria uses specific elongation factors, for example:

EF‑Tu – delivers aminoacyl‑tRNA to the A site.
EF‑Ts – regenerates EF‑Tu in its active form.
EF‑G – promotes translocation of the ribosome along the mRNA.

These factors hydrolyze GTP, which provides energy and also acts as a timing mechanism to ensure correct tRNA selection and movement.

Termination in Prokaryotes

Stop codons (UAA, UAG, UGA) are recognized by protein release factors:

RF1 and RF2 read specific stop codons.
RF3 helps complete the termination process.

After peptide release, ribosome recycling factors and initiation factors help dissociate the ribosomal subunits and release mRNA and tRNA.

Protein Biosynthesis in Eukaryotes

Eukaryotic cells have a nucleus and various membrane‑bound organelles. This changes where and how translation occurs.

Separation of Transcription and Translation

Transcription occurs in the nucleus.
The initial RNA transcript is processed to form mature mRNA:

5′ cap structure,
splicing,
poly(A) tail (covered in detail in RNA chapters).

Mature mRNA is exported through nuclear pores to the cytoplasm.
Translation takes place in:

the cytosol (free ribosomes),
or on ribosomes bound to the rough endoplasmic reticulum (RER).

Consequences:

No direct coupling between transcription and translation.
Additional levels of regulation (e.g. mRNA processing, nuclear export, localization).

Monocistronic mRNAs

Eukaryotic mRNAs are usually monocistronic:

Each mRNA generally encodes one main polypeptide.

Regulation is often exerted at the level of:

which exons are included (alternative splicing),
which mRNAs are exported and stabilized,
and how efficiently each mRNA is translated.

Initiation in Eukaryotes: Cap‑Dependent Scanning

Eukaryotic initiation is more complex and strongly dependent on the 5′ cap:

5′ cap – a modified guanine nucleotide added to the 5′ end of mRNA; recognized by the translation machinery.
Kozak sequence – an optimal context around the start codon (e.g. GCC(A/G)CCAUGG in vertebrates) that enhances recognition of the correct AUG.

Typical steps of eukaryotic initiation (simplified):

The small 40S subunit binds the initiator tRNA:

The initiator tRNA carries methionine (Met) (not formylated).
It forms a pre‑initiation complex with several eukaryotic initiation factors (eIFs).

This complex is recruited to the 5′ cap of the mRNA:

Cap‑binding proteins (part of the eIF group) recognize and bind the 5′ cap.

Scanning:

The 40S subunit with the initiator tRNA moves along the mRNA from 5′ → 3′.
It scans for an AUG start codon in a good Kozak context.

Start codon recognition:

When the correct AUG is found, base pairing occurs between the codon and the anticodon of the initiator tRNA in the P site.
Many initiation factors are released.

60S large subunit joins:

This forms the functional 80S ribosome with Met‑tRNA in the P site, ready for elongation.

Additional points:

Eukaryotes have a large set of initiation factors (eIF1, eIF2, eIF3, …), allowing fine‑tuned regulation (for example, controlling global translation rates during stress).
Some viruses and certain mRNAs can bypass cap‑dependent initiation using internal ribosome entry sites (IRES), but cap‑dependent scanning is the standard mechanism.

Elongation in Eukaryotes

The core cycle is the same, but with different named factors:

eEF1α – delivers aminoacyl‑tRNAs to the A site (analogous to bacterial EF‑Tu).
eEF1βγ – regenerates eEF1α.
eEF2 – promotes ribosome translocation (analogous to bacterial EF‑G).

GTP hydrolysis again powers and controls these steps.

Termination in Eukaryotes

A single main release factor in eukaryotes (eRF1) recognizes all three stop codons (UAA, UAG, UGA).
eRF3, a GTP‑binding protein, assists peptide release.
After termination, subunits dissociate, and translation components can be recycled.

Polyribosomes (Polysomes): Many Ribosomes on One mRNA

In both prokaryotes and eukaryotes, multiple ribosomes can translate the same mRNA molecule at once:

This structure is called a polyribosome (polysome).
It enables the cell to synthesize many protein copies rapidly from a single mRNA.
In prokaryotes, polysomes can form on an mRNA that is still being transcribed.
In eukaryotes, polysomes form on processed mRNAs in the cytoplasm or on the rough ER.

Localization of Translation and Protein Targeting

A key difference lies not just in how translation starts, but where and what happens to the newly made protein.

Prokaryotes

With few internal membranes, most translation occurs in the cytoplasm.
Some proteins may be directed to the cell membrane or secreted through signal sequences and membrane transport systems.

Eukaryotes: Cytosolic vs. RER‑Bound Ribosomes

Free ribosomes in the cytosol synthesize proteins that typically function:

in the cytosol,
in the nucleus,
in mitochondria or chloroplasts (when appropriate targeting signals are present).

Rough ER‑bound ribosomes synthesize proteins destined for:

secretion (e.g. hormones, antibodies),
insertion into cellular membranes,
lysosomes or other organelles of the endomembrane system.

Targeting to the ER often involves:

An N‑terminal signal peptide in the growing protein,
Recognized by the signal recognition particle (SRP),
Which pauses translation, docks the ribosome to the ER membrane, and resumes translation into the ER lumen or membrane.

This coupling of translation with membrane targeting is a hallmark of eukaryotic cells with complex internal compartmentalization.

Summary: Comparing Prokaryotic and Eukaryotic Protein Biosynthesis

Shared features:

Use of the same genetic code (with minor exceptions in some organelles and microorganisms).
Same essential stages: initiation → elongation → termination.
Ribosome with A, P, and E sites; tRNAs as adaptors; rRNA as catalytic core.

Main differences:

Cell organization

Prokaryotes: no nucleus, transcription and translation are coupled.
Eukaryotes: nucleus separates transcription from translation; additional RNA processing.

mRNA structure

Prokaryotes: often polycistronic, with Shine–Dalgarno sequences.
Eukaryotes: typically monocistronic, 5′ cap, 3′ poly(A) tail, Kozak context around start codon.

Ribosome size

Prokaryotes: 70S (30S + 50S).
Eukaryotes: 80S (40S + 60S).

Initiation

Prokaryotes: initiation at internal AUGs guided by Shine–Dalgarno–16S rRNA base pairing; initiator tRNA carries N‑formylmethionine (fMet).
Eukaryotes: cap‑dependent scanning from 5′ end to first suitable AUG; initiator tRNA carries Met (unmodified).

Translation regulation and targeting

Prokaryotes: rapid, tightly coupled to transcription, few intracellular compartments.
Eukaryotes: heavily regulated at multiple levels; translation occurs on free or ER‑bound ribosomes, integrating protein synthesis with complex intracellular trafficking.

These similarities and differences are central for understanding how antibiotics can selectively target bacterial translation, how viral infection takes over host translation, and how gene expression is tuned in different types of cells—all topics that build on the basic picture of protein biosynthesis you’ve now seen.