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The Role of Ribosomes and Peptide Bonds in Genetic Translation

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Instructor: April Koch

April teaches high school science and holds a master's degree in education.

Ribosomes live on the endoplasmic reticulum surrounding the nucleus and are key in the process of polypeptide assembly. This is essential for genetic translation because amino acid chains are linked together by peptide bonds. Explore the structure of ribosomes and how peptide bonds are formed. Updated: 08/18/2021

Review of Polypeptide Assembly

So far, we've had a lot of practice with the process of translation. It's the second step in the central dogma, which involves converting the genetic code in mRNA into a chain of amino acids. During translation, tRNA molecules first match up with the amino acids that fit their attachment sites. Then, they attach to the mRNA strand by matching their anticodons to the mRNA codons. We know that amino acids are assembled in the correct order because of good codon recognition. But, how do we begin polypeptide assembly? Do the amino acids just magically bond together? How do we make sure we get the tRNA, the mRNA, and the amino acids all in the same place at the same time?

You may recall a tiny structure in the cell called a ribosome. Ribosomes are the little 'dots' that live on the rough endoplasmic reticulum surrounding the nucleus of a cell. They are very important organelles that make polypeptide assembly possible. In this lesson, we're going to focus on the ribosome and the role it plays in helping to begin genetic translation.

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  • 0:18 Review of Polypeptide Assembly
  • 1:09 Ribosomes and Peptide Bonds
  • 3:19 Ribosome Structure
  • 6:05 Peptide Bond Formation
  • 6:59 Lesson Summary
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Ribosomes and Peptide Bonds

Translation is easier to understand when you think of it like following a recipe for your favorite dish. The mRNA strand is like the recipe because it contains all the instructions for making a product. mRNA is the type of RNA that encodes the genetic information found in DNA. When mRNA leaves the nucleus, it first goes looking for a tiny structure called a ribosome. This will be the actual site of translation.

You might recall that a ribosome is a cell organelle that helps in assembling chains of amino acids. It's made of ribosomal RNA, or rRNA, and protein. Ribosomes are located on the rough endoplasmic reticulum, or RER, that surrounds a cell's nucleus. In fact, ribosomes are the reason that the RER is called 'rough;' they give the endoplasmic reticulum a rough appearance when viewed under a microscope.

Anyway, you can think of the ribosome as sort of the 'skillet' of translation; it's the place where all the action happens. The ribosome serves as a central hub where all of the ingredients are combined by the cell's machinery. So, if the ribosome is the skillet, and mRNA is the recipe, then what exactly are the ingredients?

Proteins are made up of many of the twenty available amino acids
Twenty Different Amino Acids

The ingredients for our protein product are going to be the amino acids. You may recall that amino acids are the organic molecules that serve as the monomers for proteins. There are 20 different amino acids to choose from, and their exact combinations are unique to every protein, so it's crucial that we put the amino acids in the correct order.

When two amino acids are joined together by a chemical bond, we call it a peptide bond. A peptide bond is a covalent bond between two amino acids. We know that proteins are made from long chains of amino acids. So, if you have an amino acid chain, then you also have lots of peptide bonds. For this reason, we often use the word polypeptide to describe an amino acid chain. A polypeptide is a chain of amino acids linked together by peptide bonds. 'Poly' means 'many,' just like in the word 'polymer;' and 'peptide' refers to a peptide bond, so a molecule with many peptide bonds is called a polypeptide. If a polypeptide is the final product that we get from translation, then the peptide bonds are like the mixing of the ingredients in our skillet.

Ribosome Structure

We've got a lot of new terms and molecules floating around here. So, let's look at how everything fits together. We'll start with the cell. Inside the cell is the nucleus, and surrounding it is the endoplasmic reticulum. We can see dots on the rough ER, which are the ribosomes. They like to sit close to the nucleus because they're waiting for the mRNAs to come out. The mRNA strands result from transcription, which happens inside the nucleus. So, once transcription is done, mRNA comes out, and sitting there waiting are all the ribosomes. The mRNAs link up with the ribosomes in order to start translation.

When mRNA leaves the nucleus, it seeks out a ribosome
mRNA Seeks Out Ribosomes

Okay, so now we've got an mRNA who has found itself a ribosome. The ribosome is a special protein that is built to accept the mRNA strand. It actually has two different parts, with one part roughly twice as big as the other. The small part is called the small ribosomal subunit, and the mRNA strand sits on top of it. The large part is called the large ribosomal subunit. It sits on top of the mRNA strand and has special spots for the tRNA molecules to come in.

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