During translocation in protein synthesis, the ribosome shifts exactly three nucleotides along the messenger RNA strand, moving transfer RNAs (tRNAs) from one internal site to the next so a new amino acid can be added to the growing protein chain. This is one step in a repeating cycle that builds proteins one amino acid at a time. The term “translocation” also applies to other biological processes, including chromosome rearrangements and sugar transport in plants, each involving a different kind of movement.
Ribosomal Translocation During Protein Synthesis
The ribosome has three binding slots for tRNA molecules, labeled A (aminoacyl), P (peptidyl), and E (exit). During translation, an incoming tRNA carrying an amino acid enters the A site, and its amino acid is linked to the growing protein chain held by the tRNA in the P site. At that point, the A-site tRNA now holds the entire chain, the P-site tRNA is empty, and the A site needs to be cleared so the next tRNA can arrive. That clearing is translocation.
During translocation, the tRNA that was in the A site moves to the P site, and the empty tRNA that was in the P site moves to the E site, where it detaches from the ribosome entirely. The mRNA slides along with the tRNAs so the next three-letter codon is now exposed in the open A site. One round of translocation advances the ribosome by one codon, or three nucleotides, along the mRNA.
How the Movement Happens
Translocation is not a single snap. It proceeds through intermediate stages. First, the acceptor ends of the tRNAs (the tips that carry amino acids) shift on the large ribosomal subunit while their other ends stay put on the small subunit. This creates what biochemists call hybrid states: the A-site tRNA sits partly in A and partly in P, while the P-site tRNA sits partly in P and partly in E. The small subunit then rotates, and a protein called elongation factor G (EF-G) binds and hydrolyzes one molecule of GTP, the cell’s energy currency. That energy input drives the tRNAs the rest of the way into their new positions. EF-G essentially acts as a molecular motor, coupling the chemical energy of GTP to the physical movement of the mRNA-tRNA assembly.
Once GTP is fully broken down and a phosphate group is released, the small subunit rotates back to its original orientation, and the tRNAs settle into their final classical positions: the former A-site tRNA is now squarely in the P site, and the former P-site tRNA is fully in the E site. The A site is empty and ready for the next aminoacyl-tRNA to arrive, restarting the cycle.
Protein Translocation Across Membranes
Translocation also describes the movement of a newly made protein through a membrane. Many proteins need to enter the endoplasmic reticulum (ER), a compartment inside the cell, to be processed and shipped elsewhere. These proteins contain a short signal sequence at their leading end that flags them for transport.
As the signal sequence emerges from the ribosome, a particle in the cytoplasm called the signal recognition particle (SRP) grabs it and temporarily slows protein synthesis. This pause gives the ribosome time to dock at the ER membrane. Once docked, SRP releases, translation resumes, and the growing protein feeds through a channel in the membrane called the Sec61 translocon. This channel is narrow enough that only unfolded proteins can pass through, and it binds tightly to the ribosome so that translation and translocation happen in lockstep. The protein emerges on the other side of the membrane, where it folds into its functional shape.
Chromosomal Translocation in Genetics
In genetics, translocation refers to a chunk of one chromosome breaking off and attaching to a different chromosome. There are two main types. In a reciprocal translocation, two non-matching chromosomes swap segments with each other. In a Robertsonian translocation, two chromosomes that have their important genetic material concentrated at one end fuse near their centers, effectively merging into one.
A person who carries a balanced translocation has all the normal genetic material, just rearranged. They typically have no symptoms. The risk shows up in reproduction: when their cells divide to form eggs or sperm, the rearranged chromosomes may sort unevenly. This can produce embryos with extra or missing genetic material (an unbalanced translocation), which can lead to developmental delays, intellectual disability, birth defects, or miscarriage.
One well-known example is the Philadelphia chromosome, found in most cases of chronic myeloid leukemia. Segments of chromosomes 9 and 22 swap places, fusing two genes called BCR and ABL. The resulting hybrid gene produces a protein with abnormally high activity that drives uncontrolled growth of blood cells.
Translocation in Plants
In plant biology, translocation is the long-distance transport of sugars and other nutrients through the phloem, a network of tube-shaped cells running from leaves to roots and growing tips. Mature leaves produce sugars through photosynthesis and are called sources. Roots, flowers, and developing fruits consume those sugars and are called sinks.
The driving force is osmotic pressure. Companion cells in the leaf actively pump sugars into the phloem’s sieve tube elements, narrow living cells that have lost their nuclei and most internal structures to serve as efficient pipes. As sugar concentration rises inside these tubes, water is pulled in by osmosis, building pressure. At the sink end, sugars are unloaded and consumed, water exits, and pressure drops. The pressure difference between source and sink pushes a steady stream of sugar-laden sap from one end to the other. Companion cells power this process by maintaining steep concentration gradients, using energy generated largely by their own chloroplasts.
Comparing the Three Types
- Ribosomal translocation: tRNAs and mRNA shift through the ribosome by one codon, powered by GTP hydrolysis, to continue building a protein.
- Protein translocation: a new protein threads through a membrane channel to reach the ER interior, guided by a signal sequence and the Sec61 translocon.
- Chromosomal translocation: a segment of DNA breaks from one chromosome and joins another, sometimes with no health effect and sometimes causing disease.
- Phloem translocation: sugars flow from leaves to roots and growing tissues through pressure-driven bulk movement in the phloem.
If you encountered this term in a molecular biology or biochemistry course, the ribosomal version is almost certainly what your textbook means. It is the step in translation where physical movement of the mRNA and tRNAs sets the stage for each new amino acid to be added, making it essential for every protein your cells produce.