n., plural: ranslations
Definition: a biological process wherein the genetic code from a strand of mRNA is “translated” into amino acids
Table of Contents
Translation, in general, is the conversion of something into another form, such as a word from one language to another. But what is translation in biology? In biology, translation is a step in protein biosynthesis where a genetic code from a strand of mRNA is decoded to produce a particular sequence of amino acids.
In both prokaryotes and eukaryotes, it takes place on the ribosomes. In eukaryotes, though, it occurs on the ribosomes that are attached to the surface of the endoplasmic reticulum (ER) so that the newly-formed protein after translation would undergo further maturation inside the ER and then be labeled in the Golgi apparatus for transport within or outside the cell. The steps of translation are basically the same for both prokaryotes and eukaryotes. These steps are initiation, translation elongation, and translation termination.
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Translation is the process of translating (converting) the transcript (genetic information) from the mRNA into a specific sequence of amino acids in the growing chain of a polypeptide. The three general steps of translation are initiation, translation elongation, and translation termination.
- During initiation, the ribosome binds to the mRNA and then the tRNA attaches to the start codon of the transcript.
- This is followed by translation elongation wherein a specific order of amino acids is brought to the ribosomal site by the tRNAs according to the sequence of codons in the mRNA transcript. The amino acids are joined in a chain by a peptide bond.
- The stop codon in the transcript signifies that the termination phase is reached. Eventually, translation stops, and the newly formed protein undergoes maturation (e.g. protein folding or post-translation modifications).
Etymology: The term translation comes from the Latin trānslātiōn, meaning “a transferring”, equivalent to trānslāt(us) + -iōn.
In the central dogma of molecular biology, the genetic information is schematically shown to flow from DNA to DNA (by replication) and from DNA to mRNA (transcription) to protein (translation). In the latter, the genetic code from the mRNA is read as a trinucleotide codon, i.e. a set of three adjacent nucleotides.
DNA Replication vs. Translation
DNA replication is the process of producing an exact copy of a polynucleotide strand such as DNA. The flow of genetic information will be from DNA to DNA by complementary base pairing in terms of adenine-thymine (AT) and guanine-cytosine (GC) base pairing. The enzymes involved in the process are DNA polymerases. Conversely, the flow of genetic information in translation is from mRNA to protein via complementary base pairing with tRNA in terms of adenine-uracil (AU) and guanine-cytosine (GC) base pairing. The enzymes involved in translation are ribozymes.
DNA replication is a preparatory step to cell division (mitosis or meiosis). Translation, following transcription, is a step in protein synthesis. Thus, the product of DNA replication is a copy of the DNA whereas the product of translation is a polypeptide chain or protein.
In prokaryotes, DNA replication occurs in the cytoplasm, whereas in eukaryotes it occurs in the nucleus. Translation, in contrast, occurs on the ribosomes of prokaryotes and eukaryotes.
Transcription vs. Translation
Protein biosynthesis is the biological process of creating protein molecules. The first step is amino acid synthesis. Amino acids may be produced from carbon sources, e.g. glucose. Nevertheless, not all amino acids need to be synthesized. Some of them can be obtained from dietary sources.
After amino acid synthesis, transcription is the next step. However, in genetic expression, it is taken as the first step. A segment of the DNA is copied into an mRNA template. Unlike DNA replication, transcription does not need a primer. Rather, it occurs when a gene is turned on. Furthermore, complementary base pairing occurs in terms of AU and CG base pairing.
The template produced from transcription is also called mRNA transcript since the genetic code is transcribed into mRNA. This transcript is decoded in the next step, translation. At this stage, the mRNA transcript is translated into amino acids in a specific sequence in a polypeptide chain.
After translation, the newly-created polypeptide further undergoes protein maturation, e.g. post-translational modifications and protein folding. See the table below for the comparison between transcription and translation.
Table 1: Differences between transcription and translation
|The process of creating a copy of DNA into mRNA
|The process of translating mRNA transcript to amino acid
(2) Promoter escape
|In prokaryotes, in the cytoplasm
In eukaryotes, in the nucleus
|In prokaryotes and eukaryotes, in the cytoplasm where ribosomes are located
|Polypeptide or protein
|The first step of gene expression
|The second step of gene expression
Peptide or protein synthesis
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mRNA, tRNA, and rRNA
Three RNAs are involved in biological translations. They are mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).
- mRNA is the RNA produced from transcription. It is comprised of a 5′ cap, 5’UTR region, coding region, 3’UTR region, and poly(A) tail. The copy of a DNA segment for gene expression is located in its coding region. It begins with a start codon at 5’end and a stop codon at the 3′ end.
- tRNA is the RNA that transfers the specific amino acid to the ribosome to be added to the growing chain of amino acids. It has two major sites: one is the anticodon arm containing the anticodon and the other is the acceptor stem, which is the site for the amino acid. tRNA with amino acid is called aminoacyl-tRNA. The –COOH of the amino acid attaches to the 3′-OH of the adenine in the CCA tail by a covalent bond. Another type of tRNA is the peptidyl tRNA, which is a tRNA carrying the growing peptide chain.
- rRNA is the RNA component of the ribosome. The ribosome is a cytoplasmic structure in cells of prokaryotes and eukaryotes. The ribosome of prokaryotes is the 70S whereas the ribosome of eukaryotes is 80S. Both 70S and 80S are made up of a large subunit and a small subunit. The large subunit serves as a ribozyme catalyzing the peptide bond formation between two amino acids. In contrast to tRNA and mRNA that carry genetic information, rRNA does not. Nevertheless, it has three binding sites for RNA: A, P, and E sites. The A (aminoacyl) site is where aminoacyl-tRNA docks. The P (peptidyl) site is where peptidyl-tRNA binds. The E (exit) site is where the tRNA leaves the ribosome.
Codon Definition (Biology)
- Guanine-Cytosine-Cytosine (GCC) is the codon for the amino acid alanine.
- Guanine-Uracil-Uracil (GUU) codes for valine.
- Cytosine-Uracil-Adenine (CUA) codes for leucine.
- Uracil-Adenine-Adenine (UAA) is a stop codon.
Anticodon Definition (Biology)
Anticodon refers to the sequence of three adjacent nucleotides located on tRNA. It complementary-base -pairs with the codon of mRNA. For example, the anticodon for glycine is CCC (Cytosine-Cytosine-Cytosine) that binds to the codon GGG (Guanine-Guanine-Guanine) of mRNA.
Prior to initiation, a pre-translation step occurs. Called bio-activation, the amino acid binds to the corresponding tRNA by a covalent bond.
Step 1: Initiation
Translation initiation is the first major step of translation wherein the genetic code carried by mRNA is decoded to produce the specific sequence of amino acids in a polypeptide chain. The small subunit of the ribosome binds to the 5′ end of mRNA as facilitated by initiation factors (IF). The first tRNA attaches to the initiation or start codon. An initiation codon is the codon specified usually by AUG in mRNA. It is recognized by formylmethionyl-tRNA (tRNAf) in prokaryotes and by methionyl-tRNA in eukaryotes.
Step 2: Translation elongation
After initiation is transcription elongation. This is when the next aminoacyl-tRNA in line binds to the ribosome along with GTP and elongation factor (EF). The ribosome then translocates to the next mRNA codon resulting in the elongation of the amino acid chain.
Step 3: Translation termination
The last step is translation termination. This is when a peptidyl tRNA encounters a stop codon (e.g. UAA, UAG, or UGA). A stop codon does not code for any amino acid but serves as a termination signal of translation. When the termination codon is reached, the newly produced protein goes through maturation through protein folding or post-translation modifications.
Prokaryotic vs. Eukaryotic Translations
The major steps of translation in prokaryotes and eukaryotes are the same (i.e. initiation, elongation, translocation, and termination) and in both cells occurs on the ribosome. Translation in prokaryotes, though, occurs on 70S-type of ribosomes whereas translation in eukaryotes occurs on 80S-type of ribosomes.
Because prokaryotes lack membrane-bound organelles, their mRNA transcript is synthesized in the cytoplasm. In eukaryotes, mRNA is synthesized in the nucleus and then released into the cytoplasm where ribosomes are located. In eukaryotes, the growing chain of amino acids is released into the lumen of the endoplasmic reticulum via the ribosome attached to it.
Table 2: Differences between prokaryotes and eukaryotes – translations
|Translation in prokaryotes
|Translation in eukaryotes
|mRNA transcript from DNA is synthesized in the cytoplasm
|mRNA transcript from DNA is synthesized in the nucleus
|mRNA is polycistronic
|mRNA is monocistronic
|Translation occurs in 70S ribosome
|Translation occurs in 80S ribosome
|Translation initiation mechanism is cap-independent
|Translation initiation mechanism is cap-independent or cap-dependent
|First tRNA is Met-tRNAf
|First tRNA is Met-tRNA
|Initiation factors: IF1, IF2, IF3
|Initiation factors: eIF1, eIF2, eIF3, eIF4, eIF5A, eIF5B, eIF6
|Elongation factors: EF-Tu and EF-Ts
|Elongation factors: eEF1 and eEF2
|Release factors on termination: RF1, RF2, RF3
|Release factors on termination: eRF1
|Relatively faster, about 20 amino acids per second
|Relatively slower, 1 amino acid per second
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Unorthodox Ways of Translating Proteins
Within the seemingly straightforward standard steps of initiation, elongation, and termination that the translation machinery takes are the off-track nonconvential routes that it treads. Such unorthodox steps appear to help generate an even more so unique protein products, plausibly with new function.
It’s as if reading a sentence differently to discover a whole new different context.
As a matter of fact, such mechanisms are employed by certain bcteria and viruses in order to evade the host’s immune system while unraveling novel avenues for replication and pathogenesis. Here are some mechanisms that help create a set of diverse protein products that may have a potential new use or function, all from just a single mRNA:
- Frameshifting: when the ribosome shifts the reading frame as it reads the mRNA sequence, such as by +1 (one base forward) or -1 (one base backward), leading to the prodcution of protein isoforms. It helps make various proteins from the same genetic instructions by reading the genes differently. This mechanism is employed by the HIV virus to synthesize the viral enzymes reverse transcriptase and integrase.
- Readthrough (Stop Codon Suppression): when a stop codon doesn’t stop the translation. Translations still continues resulting in longer or larger protein.
- Ribosomal hopping or bypassing: when the ribosome skips a portion of the reading frame, resulting in a protein with a different amino acid sequence/ composition/intermolecular interactions.
Try to answer the quiz below to check what you have learned so far about translation.
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