Transcription built a strand of mRNA, but our goal is to make a protein. This next step – translation – is how the mRNA is decoded into a protein.

Not only is this the way all genes are expressed in humans, it is the way genes are expressed in all organisms on earth. That means genes from plants can be transferred into animals, fungus genes into bacteria, and bacterial genes into plants…and they still make proteins. This is the basis for genetic engineering and gene therapy, both of which have applications to medicine.

Why this is important…Since proteins are at the heart of most molecular processes, from membrane transport to enzyme reactions, understanding how proteins are formed is crucial. It also helps us understand the root of genetic disorders, which we can now think of as protein-coding disorders.

Foundations

Proteins are produced at cell structures called ribosomes. They work like enzymes, facilitating the pairing of mRNA with another type of RNA – transfer RNA (tRNA) – which carries amino acids to the ribosome. Each tRNA fits one segment of the mRNA to place amino acids in the correct order. You can probably guess how mRNA and tRNA are correctly matched – complimentary base pairing!

Key terms

Ribosome – cell structure that facilitates the decoding of mRNA into a protein. It is made of two large protein subunits which include some ribosomal RNA.

Nucleolus – section of the nucleus where ribosomes are manufactured

Codon – segment of the messenger RNA consisting of 3 nucleotides, which codes for a specific amino acid

Transfer RNA (tRNA) – molecule that carries amino acids into a ribosome

Anticodon – segment of the tRNA consisting of 3 nucleotides, which is complimentary to a specific codon

Details

Before we get to the process, let’s have a brief introduction to each of the major players:

Messenger RNA carries the “recipe” for making a protein. This nucleic acid recipe gets translated into a protein in the ribosome. This happens 3 nucleotides at a time. Each set of 3 nucleotides – a codon – codes for a specific amino acid.

Transfer RNA (tRNA) has a set of three bases that are complimentary to the codon. This is called the anticodon. The other end holds a specific amino acid which was attached by special enzymes (Figure 9-7).

Ribosomes are manufactured in the nucleolus. This is a small, round section of the nucleus that stains very darkly. Ribosomes are large structures made of proteins and ribosomal RNA (rRNA). Once assembled, they make their way into the cell’s cytoplasm or rough ER.

Two depictions of transfer RNA. One is a molecular model while the other is simplified. Both show the key elements - anticodon and amino acid that it carries.

FIGURE 9-7. Transfer RNA. Key elements are the anticodon and the amino acid it carries.
("Transfer RNA" by National Human Genome Research Institute is in the Public Domain)

VIDEO 9b: Translation

After watching this video, you should be able to:

describe when and where translation happens
explain how translation relates to gene expression
describe the process of translation
describe the relationship between codons and anticodons

Translation starts by bringing all the parts together at the start codon (initiation phase). The mRNA meets up with both parts of the ribosome plus the first tRNA. It is important that the ribosome starts “reading” mRNA at the correct location. If it starts at the wrong place, the entire protein would be made incorrectly. For this reason, translation always starts at the same codon – AUG – the start codon. This brings in the first tRNA, which has the complimentary anticodon – UAC – and carries the amino acid methionine (or Met). This is how protein construction begins.

On a related note, scientists have worked out the code for the rest of this process – which codon codes for which amino acid. This is shown in Figure 9-8, which may look humble, but is an impressive piece of work.

Codon chart. Three-nucleotide codons are translated into amino acids that they code for.

FIGURE 9-8. Codon chart. These 3-nucleotide sequences on the mRNA each code for specific amino acids, which are brought in by the transfer RNA. ("Codon chart" by OpenStax College is licensed under CC BY-SA 3.0)

The protein chain grows by adding amino acids (elongation phase). Each ribosome has two slots for tRNA molecules.

With one slot filled, another tRNA complimentary to the next codon enters the second slot.
Positioned next to one another, the amino acids are perfectly aligned to form a bond. The molecule GTP (a chemical cousin of ATP) supplies the energy needed to shift the protein strand and bind it to the new amino acid carried by the tRNA (Figure 9-9). Now the “used” tRNA leaves.
The ribosome shifts down the mRNA strand. Another tRNA, carrying an amino acid, enters the ribosome. Another bond is formed and the “used” tRNA leaves.
This process repeats until an entire polypeptide is built, which is usually several hundred amino acids long.

Stop & Think #7: If the mRNA codon was UUC, what is the complimentary tRNA anticodon and what amino acid would it carry?

Stop & Think #8: What amino acids are coded for by the mRNA sequence: AUGUACGGG
(Hint: there should be three amino acids)

Translation. Ribosome pictured with several tRNA molecules matching up with the mRNA. As they enter, amino acids are transferred to a growing chain of amino acids, pictured as a series of colored spheres.

FIGURE 9-9. Translation. This process occurs at a ribosome which holds the messenger RNA. Transfer RNA molecules enter the ribosome carrying amino acids. If the anticodon is complimentary to the codon, the amino acid is added to the chain. ("Translation" by National Human Genome Research Institute is in the Public Domain)

The protein is done growing when the stop codon is reached (termination phase). To verify that the ribosome has reached the end of the coding part of the mRNA a stop codon signals that the work is complete. The tRNA complimentary to the stop codon does not carry an amino acid. Instead, it signals for the release of all the parts – the ribosome disassembles, mRNA floats away, and the polypeptide drifts off to be put to use. All these parts can be reused to make other proteins.

Now that you have learned about transcription and translation, try modeling the process yourself! The Genetic Science Learning Center site has tool to let you try both: Transcribe and Translate a Gene.

After the protein is made, it gets folded and sent to its destination. Folding is the result of interactions between the amino acids and their environment. For example, amino acids with hydrophobic R-groups tend to “hide” from water and move to the inside of the protein as it folds. Other R-groups attract one another and hold the protein chain in loops or coils. This results in a specific shape that allows the protein to do its job. That concept should sound fairly familiar by now.

Stop & Think #9: Relate the process of gene expression (transcription and translation) to inheritance. How does it explain how genotypes become phenotypes?

Now that you know how proteins are made, mutations will make more sense. Mutations to DNA mess up proteins. If there is a change to the genetic code, that error is copied into the mRNA and then to the protein. DNA nucleotides can be changed, added or deleted for a variety of reasons. Mutations are often due to breaks in DNA caused by a chemical or radiation. If these errors are not repaired, the proteins they code for might not work. The seriousness of the error depends on how big of a change is made to the protein. Some mutations are catastrophic and ruin the protein. Others have no effect, either because they happen outside of genes or the new code happens to code for the same amino acids.

Stop & Think #10: Transcribe then translate the following DNA sequences to find out what amino acid it codes for. Then do the same for the new sequence to see what affect the mutation has on the amino acid.
a. TTC changes to TAC
b. AAG changes to AAA

Learning objectives

Describe the process of translation including the roles of mRNA, tRNA, and ribosomes.
Transcribe and translate a DNA sequence to determine the correct protein sequence. You should be able to use the codon chart to accomplish this.
Describe the range of effects mutations have on proteins.
Relate gene expression to genotypes and phenotypes
Explain how gene expression differs in prokaryotes and eukaryotes.

Stop & Think Questions Explained

7. The complimentary tRNA anticodon would be AAG. This tRNA would carry the amino acid “Phe.” If you thought the amino acid should be “Lys” then you read the codon chart incorrectly. The codon chart gets us from the codon of the mRNA to the amino acid, it doesn’t tell us about anticodons.

8. AUGUACGGG would give the amino acid sequence Met-Tyr-Gly.

9. The genotypes that Mendel described are the genes, made of DNA. When that DNA is expressed, it codes for mRNA which codes for proteins. When proteins are made by many cells together, we can observe their effects, which we could call a phenotype. So, a phenotype is the cumulative effect of proteins made in cells.

10. TTC codes for AAG, which codes for the amino acid Lys.
The new code, TAC, codes for AUG, which codes for Met.
b. AAG transcribes to UUC, which translates to Phe.
The new code, AAA, transcribes to UUU, which also translates to Phe (this is known as a silent mutation).