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Translation Study Guide
This study guide is a written version of the material you have seen presented in the replication unit.
In translation, the cell uses the genetic information contained in mRNA to make the proteins that carry out the cell’s work. The cell translates the code contained in the mRNA into a new language, the language of proteins, based on amino acids. Other types of RNA, such as transfer RNA (tRNA) also assist in the protein-assembly process.
Components
In this section, you will be introduced to the components involved in the process of protein synthesis, called translation. This process requires a protein/RNA complex called the ribosome and several other components to read the mRNA and produce a new protein.
Ribosome
What is a ribosome?
- A ribosome is a molecular machine that coordinates protein assembly.
- Ribosomes are composed of several proteins with tightly coiled RNA (called ribosomal RNA or rRNA) wrapped around them.
- Ribosomes consist of two parts, the large and small subunits, which clamp around the mRNA that needs to be translated.
What is a ribosome’s role in translation?
- A ribosome brings together correctly the mRNA, which needs to be translated, and the tRNA, which assists in the translation process, to come together correctly.
- During translation, tRNA molecules carrying amino acids are positioned in the ribosome’s two docking sites.
- Afterwards, this machinery can disassemble and be reused many times.
- In the upcoming animation, you will see a ribosome assembling a polypeptide chain, a future protein.
Codons
What is a codon?
- A codon is a sequence of three nucleotides on an mRNA strand that encodes a specific amino acid.
- The mRNA sequence, in turn, is derived from the cell’s DNA.
Which codons correspond to which amino acids?
- This chart shows the correspondence between codons and amino acids.
- This code has been deciphered. By knowing a DNA sequence, it is possible to determine the sequence of amino acids in the polypeptide chain for which the DNA codes.
- Sixty-four different three nucleotide combinations (codons) can be made using the four nucleotides in mRNA (4^3 = 64 combinations).
- Three codons are called stop codons, which tell the cell to stop translating the mRNA and do not encode an amino acid.
- The code is redundant. For most amino acids, multiple codons exist, each of which could code for that amino acid.
Process Flow Chart
Animation script
In translation, the cell uses an mRNA strand that it has just transcribed from its genetic code as a template to assemble proteins. The cell has just transcribed this mRNA strand from its DNA, and it now translates the mRNA’s nucleotide sequence into a chain of amino acids. This chain, called a polypeptide, forms the basic structure of a protein.
A cellular component called a ribosome coordinates the translation process. A ribosome is a molecular machine that synthesizes proteins in the cell. It consists of two main parts, a large and small subunit.
The ribosome brings together the mRNA to be translated and a set of molecules called transfer RNAs, or tRNAs, which are floating in the cell. tRNAs are adapter molecules that coordinate between the mRNA and the polypeptide chain that the cell needs to build. Each tRNA is designed to carry a specific amino acid that it can add to a polypeptide chain.
tRNAs bring their amino acids to the mRNA in a specific order. This order is determined by the attraction between a codon, a sequence of three nucleotides on the mRNA, and a complementary nucleotide triplet on the tRNA, called an anticodon. This anticodon also specifies the particular amino acid that the tRNA carries. Only the tRNA carrying the next amino acid in the polypeptide chain has the anticodon that binds to the appropriate location on the mRNA. This system ensures that amino acids are added to the chain in the correct order.
At the beginning of translation, the ribosome and a tRNA attach to the mRNA. The tRNA is located in the ribosome’s first docking site.
Using a ratcheting mechanism, the ribosome advances the mRNA, three nucleotides at a time. The ribosome also shifts the tRNA carrying the polypeptide chain into its recently vacated docking site.
A tRNA whose anticodon is complementary to this next mRNA codon is attracted to the ribosome and the mRNA.
Once again, the polypeptide chain is transferred to the new tRNA, the empty tRNA is released, and the ribosome ratchets through the mRNA another three nucleotides, simultaneously shifting the tRNA as well. As this process continues, the polypeptide chain grows longer.
Translation continues until the ribosome encounters a stop codon in the mRNA. This nucleotide triplet signals that the polypeptide chain is complete.
The stop codon causes all the components of translation to separate. The ribosome can disassemble and be used again. The mRNA is discarded by being degraded back into its building blocks, the nucleotides. New mRNA can be synthesized via transcription when more of that protein is needed.
The cell has now successfully translated one of its genes into a polypeptide chain, the raw material of a protein. This chain will be further processed and folded into a protein. Every protein-encoding gene in the cell is transcribed and translated in this manner so that the cell can create the thousands of proteins that it needs to carry out all of its essential functions.
Translation, then, is one stage in the process in which the cell’s genetic information is used to create proteins. The cell’s DNA is first transcribed in a temporary copy (mRNA), which is then translated into the amino acid sequence of a protein.
Glossary of terms
amino acids – twenty molecules that are the building blocks of proteins. String of amino acids make up protein’s primary structure.
anticodon – a sequence of three nucleotides on a tRNA molecule that bond to a complementary sequence on an mRNA molecule. The anticodon sequence determines the amino acid that the tRNA carries.
codon – a sequence of three nucleotides on a mRNA molecule that encode a specific amino acid
complementary - matching, such as between pairs of nucleotides in a DNA molecule
DNA - the molecule that stores and encodes an organism’s genetic information. DNA is a double helix molecule made up of two twisted strands that are held together by hydrogen bonds between paired nucleotides. The two strands are chemically oriented in opposite directions.
enzyme - a type of protein that performs cellular activities
hydrogen bond - a weak bond that holds together complementary base pairs in a DNA molecule
