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Introduction. The process of translation in biology is the decoding an mRNA message into a polypeptide product. Put another way, a message written in the chemical language of nucleotides is "translated" into the chemical language of amino acids.
Translation is perhaps the most energy-intensive job a cell must do, beginning with the attachment of amino acids to their tRNAs. The basic amino-acylation reaction is the same for all amino acids. A specific aminoacyl-tRNA synthase attaches each tRNA to ( charges ) an appropriate amino acid.
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This process of protein synthesis occurs in two stages: Transcription – DNA is transcribed and an mRNA molecule is produced. Translation – mRNA (messenger RNA) is translated and an amino acid sequence is produced.
- Overview
- Introduction
- The genetic code
- Overview of translation
- Transfer RNAs (tRNAs)
- Ribosomes
- Steps of translation
- Getting started: Initiation
- Extending the chain: Elongation
- Finishing up: Termination
How the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide (protein).
•Step 1: transcription! Here, the DNA sequence of a gene is "rewritten" in the form of RNA. In eukaryotes like you and me, the RNA is processed (and often has a few bits snipped out of it) to make the final product, called a messenger RNA or mRNA.
•Step 2: translation! In this stage, the mRNA is "decoded" to build a protein (or a chunk/subunit of a protein) that contains a specific series of amino acids.
[What exactly is an "amino acid"?]
Take a moment to look at your hands. The bone, skin, and muscle you see are made up of cells. And each of those cells contains many millions of proteins1 . As a matter of fact, proteins are key molecular "building blocks" for every organism on Earth!
How are these proteins made in a cell? For starters, the instructions for making proteins are "written" in a cell’s DNA in the form of genes. If that idea is new to you, you may want to check out the section on DNA to RNA to protein (central dogma) before getting into the nitty-gritty of building proteins.
Basically, a gene is used to build a protein in a two-step process:
•Step 1: transcription! Here, the DNA sequence of a gene is "rewritten" in the form of RNA. In eukaryotes like you and me, the RNA is processed (and often has a few bits snipped out of it) to make the final product, called a messenger RNA or mRNA.
•Step 2: translation! In this stage, the mRNA is "decoded" to build a protein (or a chunk/subunit of a protein) that contains a specific series of amino acids.
[What exactly is an "amino acid"?]
During translation, a cell “reads” the information in a messenger RNA (mRNA) and uses it to build a protein. Actually, to be a little more techical, an mRNA doesn’t always encode—provide instructions for—a whole protein. Instead, what we can confidently say is that it always encodes a polypeptide, or chain of amino acids.
[Wait, what is the difference?]
In an mRNA, the instructions for building a polypeptide are RNA nucleotides (As, Us, Cs, and Gs) read in groups of three. These groups of three are called codons.
There are 61 codons for amino acids, and each of them is "read" to specify a certain amino acid out of the 20 commonly found in proteins. One codon, AUG, specifies the amino acid methionine and also acts as a start codon to signal the start of protein construction.
How is an mRNA "read" to make a polypeptide? Two types of molecules with key roles in translation are tRNAs and ribosomes.
Transfer RNAs, or tRNAs, are molecular "bridges" that connect mRNA codons to the amino acids they encode. One end of each tRNA has a sequence of three nucleotides called an anticodon, which can bind to specific mRNA codons. The other end of the tRNA carries the amino acid specified by the codons.
There are many different types of tRNAs. Each type reads one or a few codons and brings the right amino acid matching those codons.
Ribosomes are the structures where polypeptides (proteins) are built. They are made up of protein and RNA (ribosomal RNA, or rRNA). Each ribosome has two subunits, a large one and a small one, which come together around an mRNA—kind of like the two halves of a hamburger bun coming together around the patty.
The ribosome provides a set of handy slots where tRNAs can find their matching codons on the mRNA template and deliver their amino acids. These slots are called the A, P, and E sites. Not only that, but the ribosome also acts as an enzyme, catalyzing the chemical reaction that links amino acids together to make a chain.
Your cells are making new proteins every second of the day. And each of those proteins must contain the right set of amino acids, linked together in just the right order. That may sound like a challenging task, but luckily, your cells (along with those of other animals, plants, and bacteria) are up to the job.
To see how cells make proteins, let's divide translation into three stages: initiation (starting off), elongation (adding on to the protein chain), and termination (finishing up).
In initiation, the ribosome assembles around the mRNA to be read and the first tRNA (carrying the amino acid methionine, which matches the start codon, AUG). This setup, called the initiation complex, is needed in order for translation to get started.
Elongation is the stage where the amino acid chain gets longer. In elongation, the mRNA is read one codon at a time, and the amino acid matching each codon is added to a growing protein chain.
Each time a new codon is exposed:
•A matching tRNA binds to the codon
•The existing amino acid chain (polypeptide) is linked onto the amino acid of the tRNA via a chemical reaction
•The mRNA is shifted one codon over in the ribosome, exposing a new codon for reading
During elongation, tRNAs move through the A, P, and E sites of the ribosome, as shown above. This process repeats many times as new codons are read and new amino acids are added to the chain.
Termination is the stage in which the finished polypeptide chain is released. It begins when a stop codon (UAG, UAA, or UGA) enters the ribosome, triggering a series of events that separate the chain from its tRNA and allow it to drift out of the ribosome.
After termination, the polypeptide may still need to fold into the right 3D shape, undergo processing (such as the removal of amino acids), get shipped to the right place in the cell, or combine with other polypeptides before it can do its job as a functional protein.
The major steps of translation. (1) Translation begins when a ribosome (gray) docks on a start codon (red) of an mRNA molecule in the cytoplasm. (2) Next, tRNA...
Jul 31, 2022 · The process of translation is similar in prokaryotes and eukaryotes. Here we’ll explore how translation occurs in E. coli, a representative prokaryote, and specify any differences between prokaryotic and eukaryotic translation.
Translation: The big picture. Translation involves “decoding” a messenger RNA (mRNA) and using its information to build a polypeptide, or chain of amino acids. For most purposes, a polypeptide is basically just a protein (with the technical difference being that some large proteins are made up of several polypeptide chains). The genetic code.
