Rough ER Protein Synthesis

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll learn what the rough ER is and what it's job is in the cell. We'll also look at the steps of protein synthesis and how it relates to the rough ER.

What Is the Rough ER?

Picture a factory making school supplies. They might make pencils, calculators, or folders. The factory gets the instructions for how to make each item, and then creates them as needed. Next, the factory ships out the materials to different places, such as stores, schools, or individual buyers.

Although the cells that make up our bodies are microscopic, they too need factories to produce goods. In the cell, these goods are called proteins, and they are needed for all body structure and function.

The factory of the cell is an organelle called the rough endoplasmic reticulum, or rough ER. This structure works with another cell part called ribosomes, which are also protein factories but much smaller. The rough ER is named so because it is dotted with tiny ribosomes, giving it a rough, or nubbed appearance under the microscope, kind of like a climbing wall.

Together, they make proteins and ship them to all other parts of the cell in a process called protein synthesis.

What Is Protein Synthesis?

There are two main steps in protein synthesis, transcription and translation.

Step 1: Transcription

Lets go back to our school supply factory. Let's say the workers need to start making calculators. First, they'll need instructions. Transcription in a cell is the process of getting instructions on how to make a protein.

Transcription starts in the nucleus, which is the brain of the cell. It contains DNA which contain all the instructions we need to make proteins. These instructions are needed by the ribosomes outside the nucleus. DNA is too important to be moved around the cell, it needs to stay there as a sort of master copy. So the cell makes another copy of it in a process called transcription.

The copy is called mRNA, or messenger ribonucleic acid. Once made, the mRNA can be taken outside the nucleus to the ribosome to get started on the next step, translation.

Transcription of mRNA

Step 2: Translation

Once the school supply factory receives instructions on how to make a calculator, they would need to assemble the parts. In a cell, assembly of parts to make a protein is called translation.

Ribosomes are tiny protein factories that read the mRNA, assemble the necessary parts, and make protein. As the ribosomes read the mRNA, another molecule called tRNA, or transfer ribonucleic acid, brings the components needed to build the protein, called amino acids. The ribosome reads the mRNA, then the tRNA gets the correct amino acid and brings it back to the ribosome. Then the ribosome puts all the amino acids together into a protein, just like the factory workers use plastic and metal bits to make a calculator.

Some ribosomes make the protein on their own, called 'free ribosomes,' and others travel to the rough ER and create proteins that will be shipped to other destinations inside the cell.

Translation at the Rough ER

Rough ER inside an animal cell
Rough ER

But how does the ribosome know which proteins should be sent to the rough ER for further modification? Well, it's sort of like if the school supply factory put the parts together for the calculator, but it still needs programming. So they put it in an envelope with the address of a programming company. The postman comes, recognizes the address, and delivers it to the programming company.

Well, as the amino acids are being put together in the ribosome, they create a pattern, called a signal sequence, like the address on an envelope. Another complex in the cell, called a signal recognition particle, or SRP, recognizes the pattern in the new protein, sort of like a postman sees the address label on the envelope. The postman of the cell, the SRP, reads the address and then directs the ribosome and new protein to the rough ER.

There are lots of different places in the cell a newly forming protein might need to go, and each one has a different signal sequence. If there is a problem with a signal sequence in a protein, it is possible that it would not be shipped to the direct location.

SRP directs proteins with the correct signal sequence to the rough ER
rough ER signal sequence

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