The assembly workers. Putting together any sort

The ribosome is the cellular structure responsible for decoding your DNA.

In this lesson, we’ll learn about ribosome structure, function and location – characteristics that make it a very good genetic translator.

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Ribosomes Perform Protein Synthesis

The central dogma of biology and the types of RNA
Central Dogma Visual

We’ve reached a point in science where it’s possible to find out all the letters in your DNA sequence, and that’s pretty cool. But if you’ve ever looked at a DNA sequence, you might have noticed it isn’t exactly English. We can find out all the As, the Gs, the Ts and the Cs in a human genome, but it’s a completely different language than the one you likely learned in grade school. How can you tell which of these letters made your eyes blue and which ones gave you those big ears?Fortunately, your cells can read your genome language just fine, though it takes a few steps.

Your DNA tells your cells what proteins to make. Remember that it does this through the central dogma, where your DNA is first transcribed into RNA. DNA is a huge manual of instructions, so transcription is like writing down one page of DNA instructions at a time. There are three types of RNA made during transcription – messenger RNA (or mRNA), transfer RNA (or tRNA) and ribosomal RNA (or rRNA). In the second step of the central dogma, mRNA is translated into protein during translation. You’ll remember that proteins are built from several different amino acids in a specific order.

So, after that refresher, it’s time to introduce you to the smarty-pants of the cell who is able to speak this DNA language. This would be the ribosome, the cellular structure that performs translation, or protein synthesis. Ribosomes are both language translators and efficient assembly workers. Putting together any sort of protein is like putting together a piece of furniture that has three boxes of more than 100 parts – and the instructions are in a foreign language, with no pictures. We’ve all tried to do it, and it’s not pretty.

But for ribosomes, this is a cinch. They read an mRNA sequence, translate it and use tools to build it into a protein. This protein is then used by the cell. Specifically, ribosomes use tRNA as tools. This helps bring the correct amino acids to the building protein.

The location of free and bound ribosomes in cells
cytoplasm until they receive an mRNA message to translate. When they receive it, they waste no time in cracking the code. Ribosomes can either be free ribosomes in the cytoplasm of the cell or they may find that the mRNA they are translating has a specific destination.

In this case, some ribosomes will attach themselves to another cellular structure, and this is called the endoplasmic reticulum. Here, these are called the bound ribosomes. Proteins produced by bound ribosomes are usually destined for some kind of specific structures, or they’re meant to exit the cell through exocytosis. There are also ribosomes within the mitochondria and chloroplasts of different types of cells, but we’ll discuss that in other lessons.

Ribosome Structure

Although different organisms might have slightly different-sized ribosomes, they are very similar in the structure and all perform translation.

Specifically, translation is protein synthesis by creating a chain of amino acids linked together by polypeptide bonds.Ribosomes are composed of two different subunits, called the large subunit and the small subunit. Both subunits are composed of rRNA and protein.

These subunits usually exist separately in the cytoplasm until the small subunit recognizes and binds to a specific sequence of an mRNA message. It then recruits the large subunit to form a complete ribosome.

The structure of ribosomes
Ribosome Structure

Ribosome structure allows the mRNA message to be translated by holding the mRNA and tRNA molecules in the right place at the right time. Each tRNA carries a specific amino acid. The ribosome is responsible for matching the correct tRNA to the mRNA sequence. This is done with the help of three tRNA binding sites on the large subunit.

They are called the A, P and E sites. They are used in that order, spelling ‘ape.’ The A site stands for the ‘amino acid site,’ where a specific tRNA binds to a specific mRNA sequence.

The P site stands for ‘polypeptide site,’ where a polypeptide bond is formed between a new amino acid and the growing amino acid chain. The E site is easy; it stands for the ‘exit site,’ where the tRNA exits. We’ll learn more about the steps in translation during the translation lesson. For now, you can appreciate that this structure is such an efficient translator able to decipher our genetic instructions by having very good organizational skills.

Lesson Summary

The ribosome is the cellular structure and location of translation, or protein synthesis. It is composed of rRNA and protein.

Normally, ribosomes are in the cytoplasm as a small subunit that recognizes and binds to an mRNA transcript and a large subunit, which is recruited to the small subunit to translate the mRNA into protein. During translation, a ribosome reads and translates the mRNA sequence, matching it with a specific amino acid using tRNA as a tool. This tRNA moves through the A, P and E sites as it adds new amino acids to a growing protein. A translating ribosome can perform its function free in the cytoplasm or bound to the endoplasmic reticulum.

Some ribosomes are also located in structures called mitochondria and chloroplasts.

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