47 5.7 Protein Synthesis
Created by: CK-12/Adapted by Christine Miller
The Art of Protein Synthesis
This amazing artwork (Figure 5.7.1) shows a process that takes place in the cells of all living things: the production of proteins. This process is called protein synthesis, and it actually consists of two processes — transcription and translation. In eukaryotic cells, transcription takes place in the nucleus. During transcription, DNA is used as a template to make a molecule of messenger RNA (mRNA). The molecule of mRNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. During translation, the genetic code in mRNA is read and used to make a polypeptide. These two processes are summed up by the central dogma of molecular biology: DNA → RNA → Protein.
Transcription
Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions in DNA to mRNA. During transcription, a strand of mRNA is made to complement a strand of DNA. You can see how this happens in Figure 5.7.2.
Transcription begins when the enzyme RNA polymerase binds to a region of a gene called the promoter sequence. This signals the DNA to unwind so the enzyme can “read” the bases of DNA. The two strands of DNA are named based on whether they will be used as a template for RNA or not. The strand that is used as a template is called the template strand, or can also be called the antisense strand. The sequence of bases on the opposite strand of DNA is called the non-coding or sense strand. Once the DNA has opened, and RNA polymerase has attached, the RNA polymerase moves along the DNA, adding RNA nucleotides to the growing mRNA strand. The template strand of DNA is used as to create mRNA through complementary base pairing. Once the mRNA strand is complete, and it detaches from DNA. The result is a strand of mRNA that is nearly identical to the coding strand DNA – the only difference being that DNA uses the base thymine, and the mRNA uses uracil in the place of thymine
Processing mRNA
In eukaryotes, the new mRNA is not yet ready for translation. At this stage, it is called pre-mRNA, and it must go through more processing before it leaves the nucleus as mature mRNA. The processing may include splicing, editing, and polyadenylation. These processes modify the mRNA in various ways. Such modifications allow a single gene to be used to make more than one protein.
- Splicing removes introns from mRNA, as shown in Figure 5.7.3. Introns are regions that do not code for the protein. The remaining mRNA consists only of regions called exons that do code for the protein. The ribonucleoproteins in the diagram are small proteins in the nucleus that contain RNA and are needed for the splicing process.
- Editing changes some of the nucleotides in mRNA. For example, a human protein called APOB, which helps transport lipids in the blood, has two different forms because of editing. One form is smaller than the other because editing adds an earlier stop signal in mRNA.
- 5′ Capping adds a methylated cap to the “head” of the mRNA. This cap protects the mRNA from breaking down, and helps the ribosomes know where to bind to the mRNA
- Polyadenylation adds a “tail” to the mRNA. The tail consists of a string of As (adenine bases). It signals the end of mRNA. It is also involved in exporting mRNA from the nucleus, and it protects mRNA from enzymes that might break it down.
Translation
Translation is the second part of the central dogma of molecular biology: RNA → Protein. It is the process in which the genetic code in mRNA is read to make a protein. Translation is illustrated in Figure 5.7.4. After mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins. The ribosome reads the sequence of codons in mRNA, and molecules of tRNA bring amino acids to the ribosome in the correct sequence.
Translation occurs in three stages: Initiation, Elongation and Termination.
Initiation:
After transcription in the nucleus, the mRNA exits through a nuclear pore and enters the cytoplasm. At the region on the mRNA containing the methylated cap and the start codon, the small and large subunits of the ribosome bind to the mRNA. These are then joined by a tRNA which contains the anticodons matching the start codon on the mRNA. This group of molecues (mRNA, ribosome, tRNA) is called an initiation complex.
Elongation:
tRNA keep bringing amino acids to the growing polypeptide according to complementary base pairing between the codons on the mRNA and the anticodons on the tRNA. As a tRNA moves into the ribosome, its amino acid is transferred to the growing polypeptide. Once this transfer is complete, the tRNA leaves the ribosome, the ribosome moves one codon length down the mRNA, and a new tRNA enters with its corresponding amino acid. This process repeats and the polypeptide grows.
Termination:
At the end of the mRNA coding is a stop codon which will end the elongation stage. The stop codon doesn’t call for a tRNA, but instead for a type of protein called a release factor, which will cause the entire complex (mRNA, ribosome, tRNA, and polypeptide) to break apart, releasing all of the components.
Watch this video “Protein Synthesis (Updated) with the Amoeba Sisters” to see this process in action:
Protein Synthesis (Updated), Amoeba Sisters, 2018.
What Happens Next?
After a polypeptide chain is synthesized, it may undergo additional processes. For example, it may assume a folded shape due to interactions between its amino acids. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travel to the Golgi apparatus within the cytoplasm to be modified for the specific job they will do.7 Summary
5.7 Summary
- Protein synthesis is the process in which cells make proteins. It occurs in two stages: transcription and translation.
- Transcription is the transfer of genetic instructions in DNA to mRNA in the nucleus. It includes three steps: initiation, elongation, and termination. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm.
- Translation occurs at the ribosome, which consists of rRNA and proteins. In translation, the instructions in mRNA are read, and tRNA brings the correct sequence of amino acids to the ribosome. Then, rRNA helps bonds form between the amino acids, producing a polypeptide chain.
- After a polypeptide chain is synthesized, it may undergo additional processing to form the finished protein.
5.7 Review Questions
- Relate protein synthesis and its two major phases to the central dogma of molecular biology.
- Explain how mRNA is processed before it leaves the nucleus.
- What additional processes might a polypeptide chain undergo after it is synthesized?
- Where does transcription take place in eukaryotes?
- Where does translation take place?
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5.7 Explore More
Protein Synthesis, Teacher’s Pet, 2014.
Attributions
Figure 5.7.1
How proteins are made by Nicolle Rager, National Science Foundation on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.7.2
Transcription by National Human Genome Research Institute, (reworked and vectorized by Sulai) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.7.3
Pre mRNA processing by Christine Miller is used under a CC BY-NC-SA 4.0 (https://creativecommons.org/licenses/by-nc-sa/4.0/) license.
Figure 5.7.4
Translation by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
References
Amoeba Sisters. (2018, January 18) Protein synthesis (Updated). YouTube. https://www.youtube.com/watch?v=oefAI2x2CQM&feature=youtu.be
Parker, N., Schneegurt, M., Thi Tu, A-H., Lister, P., Forster, B.M. (2016, November 1). Microbiology [online]. Figure 11.15 Translation in bacteria begins with the formation of the initiation complex. In Microbiology (Section 11-4). OpenStax. https://openstax.org/books/microbiology/pages/11-4-protein-synthesis-translation
Teacher’s Pet. (2014, December 7). Protein synthesis. YouTube. https://www.youtube.com/watch?v=2zAGAmTkZNY&feature=youtu.be
The part of each hemisphere of the cerebrum that is involved in functions such as touch, reading, and arithmetic.
The process of creating protein molecules.
The process by which DNA is copied (transcribed) to mRNA in order transfer the information needed for protein synthesis.
The process in which mRNA along with transfer RNA (tRNA) and ribosomes work together to produce polypeptides.
Cells which have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles.
A central organelle containing hereditary material.
Deoxyribonucleic acid - the molecule carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.
A large family of RNA molecules that convey genetic information from DNA to the ribosome, where they specify the amino acid sequence of the protein products of gene expression.
A large complex of RNA and protein which acts as the site of RNA translation, building proteins from amino acids using messenger RNA as a template.
The jellylike material that makes up much of a cell inside the cell membrane, and, in eukaryotic cells, surrounds the nucleus. The organelles of eukaryotic cells, such as mitochondria, the endoplasmic reticulum, and (in green plants) chloroplasts, are contained in the cytoplasm.
A nucleic acid of which many different kinds are now known, including messenger RNA, transfer RNA and ribosomal RNA.
A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.
The addition of a poly(A) tail to a messenger RNA. The poly(A) tail consists of multiple adenosine monophosphates.
A sequence of 3 DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis.
A small RNA molecule that participates in protein synthesis. Each tRNA molecule has two important areas: an anticodon and a region for attaching a specific amino acid.
Amino acids are organic compounds that combine to form proteins.
A substance that is insoluble in water. Examples include fats, oils and cholesterol. Lipids are made from monomers such as glycerol and fatty acids.
A biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1. Complex carbohydrates are polymers made from monomers of simple carbohydrates, also termed monosaccharides.
A membrane-bound organelle found in eukaryotic cells made up of a series of flattened stacked pouches with the purpose of collecting and dispatching protein and lipid products received from the endoplasmic reticulum (ER). Also referred to as the Golgi complex or the Golgi body.
