definition of transcription protein synthesis

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Protein Synthesis

Protein synthesis, as the name implies, is the process by which every cell produces specific proteins in its ribosome. In this process, polypeptide chains are formed from varying amounts of 20 different amino acids. It is one of the fundamental biological processes in both prokaryotes and eukaryotes. This is a vital process, as the proteins formed take part in every major cellular activities, ranging from catalysis to forming various structural elements of the cell.

In 1958, Francis Crick proposed a theory called central dogma to describe the flow of genetic information from DNA to RNA to protein. According to this framework, protein is formed from RNA via translation , which in turn, is formed from DNA through transcription.

DNA → RNA → Protein

i. DNA → RNA (Transcription)

ii. RNA → Protein (Translation)

Where and When does Protein Synthesis Take Place

In both prokaryotes and eukaryotes, protein synthesis occurs in the ribosome. That’s why the ribosome is called the ‘protein factory’ of the cell.

However, in eukaryotes , the ribosomes remain scattered in the cytoplasm and are also attached to the Endoplasmic reticulum (RER). So, generally, it is said that, in eukaryotes, the process occurs in the cytoplasm and RER.

On the other hand, in prokaryotes , the ribosomes are scattered throughout the cell cytoplasm. So, commonly, it is said that, in prokaryotes, it takes place in the cytoplasm.

Process: How does it Work

The process of protein synthesis occurs in two steps: transcription and translation. In the first step, DNA is used as a template to make a messenger RNA molecule (mRNA). The mRNA thus formed, exits the nucleus through a nuclear pore and travels to the ribosome for the next step, translation. Upon reaching the ribosome, the genetic code in mRNA is read and used for polypeptide synthesis.

Below is a flowchart of the overall process:

Now, let us discuss these two steps of protein synthesis in detail:

1) Transcription: The First Step of Protein Synthesis

In this process, a single-stranded mRNA molecule is transcribed from a double-stranded DNA molecule. The mRNA thus formed is used as a template for the next step, translation.

The three steps of transcription are: initiation, elongation, and termination.

i) Initiation

The process of transcription begins when the enzyme RNA polymerase binds to a region of a gene called the promoter sequence with the assistance of certain transcription factors. Due to this binding, the double-stranded DNA starts to unwind at the promoter region, forming a transcription bubble. Among the two strands of DNA, one that is used as a template to produce mRNA is called the template, noncoding, or antisense strand. On the other hand, the other one is called the coding or sense strand.

ii) Elongation

After the opening of DNA, the attached RNA polymerase moves along the template strand of the DNA, creating complementary base pairing of that strand to form mRNA. As a result of this, an mRNA transcript containing a copy of the coding strand of DNA is formed. The only exception is, in the mRNA, the nitrogenous base thymine gets replaced by uracil. The sugar-phosphate backbone forms through RNA polymerase.

iii) Termination

Once the mRNA strand is complete, the hydrogen bonds between the RNA-DNA helix break. As a result, the mRNA detaches from the DNA and undergoes further processing.

Post Transcriptional Modification: mRNA Processing

The mRNA formed at the end of the transcription process is called pre-mRNA, as it is not fully ready prepared to enter translation. So, before leaving the nucleus, it needs to undergo some modifications or processing to transform into a mature mRNA.  Following these modifications a single gene can produce more than one protein.

a. Splicing

The pre-mRNA is comprised of introns and exons. Introns are the regions that do not code for the protein, whereas exons are the regions that code for the protein.In splicing, noncoding regions or introns of the mRNA get removed under the influence of ribonucleoproteins.

Here, the mRNA gets edited, that is, its some of the nucleotides get changed. For instance, a human protein called Apolipoprotein B (APOB), which helps in lipid transportation in the blood, comes in two different forms due to this editing. One form is smaller than the other because an earlier stop signal gets added in mRNA due to editing.

c. 5’ Capping

In this process, a methylated cap is added to the 5′ end or ‘head’ of the mRNA, replacing the triphosphate group.  This cap helps with mRNA recognition by the ribosome during translation, and also protects the mRNA from breaking down.

d. Polyadenylation

At the opposite end of the RNA transcript, that is, to the 3′ end of the RNA chain 30-500 adenines are added, forming the poly A tail. It signals the end of mRNA, and is involved in exporting mRNA from the nucleus.

2) Translation: The Second Step of Protein Synthesis

Translation, the next major step of protein synthesis is the process in which the genetic code in mRNA is read to make the amino acids, which are linked together in a particular order based on the genetic code, forming protein.

Similar to transcription, translation also occurs in three stages: initiation, elongation, and termination.

After the mature mRNA leaves the nucleus, it travels to a ribosome. The 5′ methylated cap of the mRNA, containing the strat codon binds to the small ribosomal subunit of the ribosome consisting rRNA. Next, a tRNA containing anticodons complementary to the start codon on the mRNA attaches to the ribosome.  These mRNA, ribosome, and tRNA together form an initiation complex.The ribosome reads the sequence of codons in mRNA, and tRNA bring amino acids to the ribosome in the proper sequence.

Once the initiation complex is formed, the large ribosomal subunit of ribosome binds to this complex, releasing initiation factors (IFs). The large subunit of the ribosome has three sites for tRNA binding; A site, P site, and E site. The A (amino acid) site is the region, where the complementary anticodons of aminoacyl-tRNA (tRNA with amino acid) pairs up with the mRNA codon. This ensures that correct amino acid is added to the growing polypeptide chain at the P (polypeptide) site. Once this transfer is complete, the tRNA leaves the ribosome at the E (exit) site and returns to the cytoplasm to bind another amino acid. The whole process gets repeated continuously and the polypeptide chain gets elongated. The rRNA binds the newly formed amino acids via peptide bond, forming the polypeptide chains.

The 3′ poly A tail of the mRNA holds a stop codon that signals to end the elongation stage. A specialized protein called release factor gets attached to the tail o mRNA, causing the entire initiation complex along with the polypeptide chain to break down. As a result, all the components are released.

What Happens Next

After translation, the newly formed polypeptide chain undergoes either of the two post-translational modifications discussed below:

• Proteolysis : Here, the proteins get cleaved, that is, their N-terminal, C-terminal, or the internal amino-acid residues are removed from the polypeptide by the action of proteases.
• Protein folding : In this stage, the nascent proteins get folded to achieve the secondary and tertiary structures.

After these modifications, the protein may bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates, forming lipoproteins or glycoproteins, respectively. Many proteins travel to the Golgi apparatus where they are modified according to their role in cell.

Why is Protein Synthesis Important

As we can see, this complex process of protein synthesis leads to the formation of proteins that plays several crucial roles in cells, including formation of structural components of cell, like cell membrane , cell repair, producing hormones, enzymes, and many more.

Why is it Different in Prokaryotes and Eukaryotes

The speed of protein synthesis is different in prokaryotes and eukaryotes. In prokaryotes, the process is faster, as the whole process takes place in the cytoplasm. On the other hand, in eukaryotes it is slower, as the pre- mRNA is first synthesized in the nucleus and after splicing, the mature mRNA comes to the cytoplasm for translation.

Ans.   mRNA carries the coding sequences for protein synthesis from DNA to ribosome. tRNA decodes a specific codon of mRNA and transfers a specific amino acid to the ribosome.

Ans. Three types of RNAs are involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Ans. Deoxyribonucleic acid (DNA) provides the master code for protein synthesis.

Ans . The codon AUG, coding for methionine starts protein synthesis.

Ans . The two organelles that are involved in protein synthesis are: nucleus and ribosome.

Ans . Well defined reading frames are critical in protein synthesis, because without a well-defined reading frame, the peptide made from a given sequence could be completely different.

Ans . Yes, protein synthesis requires energy.

Ans . Protein synthesis is the process of producing a functional protein molecule based on the information in the genes. On the contrary, DNA replication produces a replica of an existing DNA molecule.

• Protein Synthesis – Flexbooks.ck12.org
• Translation: DNA to mRNA to Protein – Nature.com
• What is protein synthesis – Proteinsynthesis.org
• Translation: Making Protein Synthesis Possible – Thoughtco.com
• Protein Synthesis in the Cell and the Central Dogma – Study.com

Article was last reviewed on Friday, February 17, 2023

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Protein synthesis

Protein synthesis n., plural: protein syntheses Definition: the creation of protein.

Table of Contents

Protein synthesis is the process of creating protein molecules. In biological systems, it involves amino acid synthesis, transcription, translation, and post-translational events. In amino acid synthesis , there is a set of biochemical processes that produce amino acids from carbon sources like glucose .

Not all amino acids are produced by the body; other amino acids are obtained from the diet . Within the cells, proteins are generated involving transcription and translation processes. In brief, transcription is the process by which the mRNA template is transcribed from DNA.

The template is used for the succeeding step, translation. In translation, the amino acids are linked together in a particular order based on the genetic code. After translation, the newly formed protein undergoes further processing, such as proteolysis, post-translational modification, and protein folding.

Proteins are made up of amino acids that are arrainged in orderly fashion. Discover how the cell organizes protein synthesis with the help of the RNAs. You’re more than welcome to join us in our Forum discussion: What does mRNA do in protein synthesis?

Protein Synthesis Definition

Protein synthesis is the creation of proteins. In biological systems, it is carried out inside the cell. In prokaryotes , it occurs in the cytoplasm . In eukaryotes , it initially occurs in the nucleus to create a transcript ( mRNA ) of the coding region of the DNA . The transcript leaves the nucleus and reaches the ribosomes for translation into a protein molecule with a specific sequence of amino acids .

Protein synthesis is the creation of proteins by cells that uses DNA , RNA , and various enzymes . It generally includes transcription , translation , and post-translational events, such as protein folding, modifications, and proteolysis.

The term protein came from Late Greek prōteios , prōtos , meaning “first”. The word synthesis came from Greek sunthesis , from suntithenai , meaning “to put together”. Variant : protein biosynthesis.

Forum Question: Where does protein synthesis take place?    Best Answer!

Prokaryotic vs. Eukaryotic Protein Synthesis

Proteins are a major type of biomolecule that all living things require to thrive. Both prokaryotes and eukaryotes produce various proteins for multifarious processes and functions. Some proteins are used for structural purposes while others act as catalysts for biochemical reactions.

Prokaryotic and eukaryotic protein syntheses have distinct differences. For instance, protein synthesis in prokaryotes occurs in the cytoplasm. In eukaryotes, the first step (transcription) occurs in the nucleus. When the transcript (mRNA) is formed, it proceeds to the cytoplasm where ribosomes are located.

Here, the mRNA is translated into an amino acid chain. In the table below, differences between prokaryotic and eukaryotic protein syntheses are shown.

Genetic Code

In biology, a codon refers to the trinucleotides that specify for a particular amino acid. For example, Guanine-Cytosine-Cytosine (GCC) codes for the amino acid alanine .

The Guanine-Uracil-Uracil (GUU) codes for valine. Uracil-Adenine-Adenine (UAA) is a stop codon. The codon of the mRNA complements the trinucleotide (called anticodon) in the tRNA.

What is the Genetic Code? “The genetic code is the system that combines different components of protein synthesis, like DNA, mRNA, tRNA…” More FAQ answered by our biology expert in the Forum: What does mRNA do in protein synthesis? Come join us now!

mRNA, tRNA, and rRNA

mRNA , tRNA , and rRNA are the three major types of RNA involved in protein synthesis. The mRNA (or messenger RNA) carries the code for making a protein. In eukaryotes, it is formed inside the nucleus and consists 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 (or transfer RNA), as the name implies, transfers the specific amino acid to the ribosome to be added to the growing chain of amino acid. It consists of two major sites: (1) anticodon arm and (2) acceptor stem . The anticodon arm contains the anticodon that complementary base pairs with the codon of the mRNA. The acceptor stem is the site where a specific amino acid is attached (in this case, the tRNA with amino acid is called aminoacyl-tRNA ). A peptidyl-tRNA is the tRNA that holds the growing polypeptide chain.

Unlike the first two, rRNA (or ribosomal RNA) does not carry genetic information. Rather, it serves as one of the components of the ribosome. The ribosome is a cytoplasmic structure in cells of prokaryotes and eukaryotes that are known for serving as a site of protein synthesis. The ribosomes can be used to determine a prokaryote from a eukaryote.

Prokaryotes have 70S ribosomes whereas eukaryotes have 80S ribosomes. Both types, though, are each made up of two subunits of differing sizes. The larger subunit serves as the ribozyme that catalyzes the peptide bond formation between amino acids. rRNA has three binding sites: 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.

Protein Biosynthesis Steps

Major steps of protein biosynthesis:

• Transcription
• Translation
• Post-translation

Transcription is the process by which an mRNA template , encoding the sequence of the protein in the form of a trinucleotide code, is transcribed from DNA to provide a template for translation through the help of the enzyme, RNA polymerase.

Thus, transcription is regarded as the first step of gene expression. Similar to DNA replication, the transcription proceeds in the 5′ → 3′ direction. But unlike DNA replication, transcription needs no primer to initiate the process and, instead of thymine, uracil pairs with adenine.

The steps of transcription are as follows: (1) Initiation, (2) Promoter escape, (3) Elongation, and (4) Termination.

Step 1: Initiation

The first step, initiation, is when the RNA polymerase, with the assistance of certain transcription factors, binds to the promoter of DNA. This leads to the opening (unwinding) of DNA at the promoter region, forming a transcription bubble . A transcription start site in the transcription bubble binds to the RNA polymerase, particularly to the latter’s initiating NTP and an extending NTP . A phase of abortive cycles of synthesis occurs resulting in the release of short mRNA transcripts (about 2 to 15 nucleotides).

Step 2: Promoter escape

The next step is for the RNA polymerase to escape the promoter so that it can enter into the elongation step.

Step 3: Elongation

During elongation, RNA polymerase traverses the template strand of the DNA and base pairs with the nucleotides on the template (noncoding) strand. This results in an mRNA transcript containing a copy of the coding strand of DNA, except for thymines that are replaced by uracils. The sugar-phosphate backbone forms through RNA polymerase.

Step 4: Termination

The last step is termination. During this phase, the hydrogen bonds of the RNA-DNA helix break. In eukaryotes, the mRNA transcript goes through further processing. It goes through polyadenylation , capping , and splicing .

Translation is the process in which amino acids are linked together in a specific order according to the rules specified by the genetic code. It occurs in the cytoplasm where the ribosomes are located. It consists of four phases:

• Activation (the amino acid is covalently bonded to the tRNA ),
• Initiation (the small subunit of the ribosome binds to 5′ end of mRNA with the help of initiation factors)
• Elongation (the next aminoacyl-tRNA in line binds to the ribosome along with GTP and an elongation factor)
• Termination (the A site of the ribosome faces a stop codon)

Post-translation Events

Following protein synthesis are events such as proteolysis and protein folding . Proteolysis refers to the cleavage of proteins by proteases. Through it, N-terminal, C-terminal, or the internal amino-acid residues are removed from the polypeptide.

Post-translational modification refers to the enzymatic processing of a polypeptide chain following translation and peptide bond formation. The ends and the side chains of the polypeptide may be modified in order to ensure proper cellular localization and function. Protein folding is the folding of the polypeptide chains to assume secondary and tertiary structures.

Watch this video about Protein Translation:

Has this info helped you understand the topic? Got any question? How about hearing answers directly from our community? Join us in our Forum: What does mRNA do in protein synthesis? Let’s keep it fun and simple!

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Further reading.

• Protein Synthesis. (2019). Retrieved from Elmhurst.edu website: http://chemistry.elmhurst.edu/vchembook/584proteinsyn.html
• Protein Synthesis. (2019). Retrieved from Estrellamountain.edu website: https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookPROTSYn.html
• Protein Synthesis. (2019). Retrieved from Nau.edu website: http://www2.nau.edu/lrm22/lessons/protein-synthesis/protein-synthesis.htm

© Biology Online. Content provided and moderated by Biology Online Editors

Last updated on August 25th, 2023

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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 a ntisense strand.  The sequence of bases on the opposite strand of DNA is called the 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?

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.

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 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.

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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Transcription and Translation Lesson Plan

This list of websites provide tools and resources for teaching the concepts of transcription and translation, two key steps in gene expression .

Definitions

Transcription is the process of making an RNA copy of a gene sequence. This copy, called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes. Here is a more complete definition of transcription:  Transcription Translation is the process of translating the sequence of a messenger RNA (mRNA) molecule to a sequence of amino acids during protein synthesis. The genetic code describes the relationship between the sequence of base pairs in a gene and the corresponding amino acid sequence that it encodes. In the cell cytoplasm, the ribosome reads the sequence of the mRNA in groups of three bases to assemble the protein. Here is a more complete definition of translation:  Translation

Teachers' Domain: Cell Transcription and Translation

Teachers' Domain is a free educational resource produced by WGBH with funding from the NSF, which houses thousands of media resources, support materials, and tools for classroom lessons.One of these resources focuses on the topics of transcription and translation.This resource is an interactive activity that starts with a general overview of the central dogma of molecular biology, and then goes into more specific details about the processes of transcription and translation.In addition to the interactive activity, the resource also includes a background narrative and discussion questions that could be used for assessment.Although the material is designated as appropriate content for grades, 9-12, it would serve as an excellent introduction to the topic for biology majors, or would be well suited for non-biology majors at the post-secondary level. See:  Teachers' Domain: Cell Transcription and Translation  

The DNA Learning Center's (DNALC) The Howard Hughes Medical Institute's DNA interactive (DNAi) The University of Utah's Genetic Science Learning Center

The DNA Learning Center's (DNALC) website, the Howard Hughes Medical Institute's DNA interactive (DNAi) website, and the University of Utah's Genetic Science Learning Center website listed below contain excellent narrated animations describing transcription and translation. These animations are useful as a lecture supplement or for students to review on their own. The DNALC animations cover central dogma, transcription (basic and advanced), mRNA splicing, RNA splicing, triplet code and translation (basic and advanced). The DNAi modules," Reading the Code" and "Copying the Code," describe the history of the process, the scientists involved in the discovery, and the basics of the process, and also include an animation and interactive game. Particularly useful to students are the interactive animations from the University of Utah that allow one to, for example,"Transcribe/Translate a Gene"or examine the effects of gene mutation as they "Test Neurofibromin Activity in a Cell."

The DNA Learning Center's (DNALC):  3-D Animation Library The Howard Hughes Medical Institute's DNA interactive: (DNAi):  Code The University of Utah's Genetic Science Learning Center:  Transcribe and Translate a Gene

The Nature Education website, Scitable, is a great study resource for students who want to learn more about, or are having difficulty understanding, transcription and translation. The site contains a searchable library, including many "overviews" of transcription, translation, and related topics. Students have access to a Genetics "Study Pack", which provides explanations, animations, and links to other resources.In addition, Scitable has an "Ask An Expert" feature that allows students to submit specific genetics-related questions. See:  Scitable

NHGRI Talking Glossary of Genetics Terms iPhone App and Website

The Talking Glossary of Genetics Terms website and iPhone app provide an easily transportable and accessible reference for your students. Many times the unfamiliar vocabulary is the major stumbling block to student comprehension. This app/site gives them a handy reference to common terms used in describing the components involved on transcription and translation. Talking Glossary of Genetics Terms Talking Glossary of Genetics Terms iPhone App

University of Buffalo Case Study Collection: Decoding the Flu

This "clicker case" was designed to develop students' ability to read and interpret information stored in DNA. Making use of personal response systems ("clickers") along with a PowerPoint presentation, students follow the story of "Jason," a student intern at the Centers for Disease Control & Prevention (CDC). While working with a CDC team in Mexico, Jason is the only person who does not get sick from a new strain of flu. It is up to Jason to use molecular data collected from different local strains of flu to identify which one may be causing the illness. Although designed for an introductory biology course for science or non-science majors, the case could be adapted for upper-level courses by including more complex problems and aspects of gene expression, such as the excision of introns." See:  Decoding the Flu

Protein Synthesis Animation from Biology-Forums.com

Translation is the process of producing proteins from the mRNA. This YouTube video shows the molecular components involved in the process. It also animates how the peptide is elongated through interaction between mRNA, ribosome, tRNA, and residues.  Protein Synthese Animation

The Central Dogma Animation by RIKEN Omics Science Center

The 'Central Dogma' of molecular biology is that 'DNA makes RNA makes protein'. This anime shows how molecular machines transcribe the genes in the DNA of every cell into portable RNA messages, how those messenger RNA are modified and exported from the nucleus, and finally how the RNA code is read to build proteins.  Animation: The Central Dogma

A Prezi of this information can be found at:  NHGRI Teacher Resouces-Central Dogma

Contributing Team of Educators:

Kari D. Loomis, Ph.D., Mars Hill College Luisel Ricks, Ph.D., Howard University Mark Bolt, Ph.D., University of Pikeville Cathy Dobbs, Ph.D., Joliet Junior College Changhui Yan, Ph.D., North Dakota State University Solomon Adekunle, Ph.D., Southern University

Last updated: February 13, 2014

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Translation: DNA to mRNA to Protein

The genes in DNA encode protein molecules, which are the "workhorses" of the cell , carrying out all the functions necessary for life. For example, enzymes, including those that metabolize nutrients and synthesize new cellular constituents, as well as DNA polymerases and other enzymes that make copies of DNA during cell division , are all proteins.

In the simplest sense, expressing a gene means manufacturing its corresponding protein, and this multilayered process has two major steps. In the first step, the information in DNA is transferred to a messenger RNA ( mRNA ) molecule by way of a process called transcription . During transcription , the DNA of a gene serves as a template for complementary base-pairing , and an enzyme called RNA polymerase II catalyzes the formation of a pre-mRNA molecule, which is then processed to form mature mRNA (Figure 1). The resulting mRNA is a single-stranded copy of the gene, which next must be translated into a protein molecule.

Where Translation Occurs

Within all cells, the translation machinery resides within a specialized organelle called the ribosome . In eukaryotes, mature mRNA molecules must leave the nucleus and travel to the cytoplasm , where the ribosomes are located. On the other hand, in prokaryotic organisms, ribosomes can attach to mRNA while it is still being transcribed. In this situation, translation begins at the 5' end of the mRNA while the 3' end is still attached to DNA.

In all types of cells, the ribosome is composed of two subunits: the large (50S) subunit and the small (30S) subunit (S, for svedberg unit, is a measure of sedimentation velocity and, therefore, mass). Each subunit exists separately in the cytoplasm, but the two join together on the mRNA molecule. The ribosomal subunits contain proteins and specialized RNA molecules—specifically, ribosomal RNA ( rRNA ) and transfer RNA ( tRNA ) . The tRNA molecules are adaptor molecules—they have one end that can read the triplet code in the mRNA through complementary base-pairing, and another end that attaches to a specific amino acid (Chapeville et al. , 1962; Grunberger et al. , 1969). The idea that tRNA was an adaptor molecule was first proposed by Francis Crick, co-discoverer of DNA structure, who did much of the key work in deciphering the genetic code (Crick, 1958).

Within the ribosome, the mRNA and aminoacyl-tRNA complexes are held together closely, which facilitates base-pairing. The rRNA catalyzes the attachment of each new amino acid to the growing chain.

The Beginning of mRNA Is Not Translated

Interestingly, not all regions of an mRNA molecule correspond to particular amino acids. In particular, there is an area near the 5' end of the molecule that is known as the untranslated region (UTR) or leader sequence. This portion of mRNA is located between the first nucleotide that is transcribed and the start codon (AUG) of the coding region, and it does not affect the sequence of amino acids in a protein (Figure 3).

So, what is the purpose of the UTR? It turns out that the leader sequence is important because it contains a ribosome-binding site. In bacteria , this site is known as the Shine-Dalgarno box (AGGAGG), after scientists John Shine and Lynn Dalgarno, who first characterized it. A similar site in vertebrates was characterized by Marilyn Kozak and is thus known as the Kozak box. In bacterial mRNA, the 5' UTR is normally short; in human mRNA, the median length of the 5' UTR is about 170 nucleotides. If the leader is long, it may contain regulatory sequences, including binding sites for proteins, that can affect the stability of the mRNA or the efficiency of its translation.

Translation Begins After the Assembly of a Complex Structure

Table 1 shows the N-terminal sequences of proteins in prokaryotes and eukaryotes, based on a sample of 170 prokaryotic and 120 eukaryotic proteins (Flinta et al. , 1986). In the table, M represents methionine, A represents alanine, K represents lysine, S represents serine, and T represents threonine.

Table 1: N-Terminal Sequences of Proteins

* Methionine was removed in all of these proteins

** Methionine was not removed from any of these proteins

Once the initiation complex is formed on the mRNA, the large ribosomal subunit binds to this complex, which causes the release of IFs (initiation factors). The large subunit of the ribosome has three sites at which tRNA molecules can bind. The A (amino acid) site is the location at which the aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that correct amino acid is added to the growing polypeptide chain. The P (polypeptide) site is the location at which the amino acid is transferred from its tRNA to the growing polypeptide chain. Finally, the E (exit) site is the location at which the "empty" tRNA sits before being released back into the cytoplasm to bind another amino acid and repeat the process. The initiator methionine tRNA is the only aminoacyl-tRNA that can bind in the P site of the ribosome, and the A site is aligned with the second mRNA codon. The ribosome is thus ready to bind the second aminoacyl-tRNA at the A site, which will be joined to the initiator methionine by the first peptide bond (Figure 5).

The Elongation Phase

Next, peptide bonds between the now-adjacent first and second amino acids are formed through a peptidyl transferase activity. For many years, it was thought that an enzyme catalyzed this step, but recent evidence indicates that the transferase activity is a catalytic function of rRNA (Pierce, 2000). After the peptide bond is formed, the ribosome shifts, or translocates, again, thus causing the tRNA to occupy the E site. The tRNA is then released to the cytoplasm to pick up another amino acid. In addition, the A site is now empty and ready to receive the tRNA for the next codon.

This process is repeated until all the codons in the mRNA have been read by tRNA molecules, and the amino acids attached to the tRNAs have been linked together in the growing polypeptide chain in the appropriate order. At this point, translation must be terminated, and the nascent protein must be released from the mRNA and ribosome.

Termination of Translation

There are three termination codons that are employed at the end of a protein-coding sequence in mRNA: UAA, UAG, and UGA. No tRNAs recognize these codons. Thus, in the place of these tRNAs, one of several proteins, called release factors, binds and facilitates release of the mRNA from the ribosome and subsequent dissociation of the ribosome.

Comparing Eukaryotic and Prokaryotic Translation

The translation process is very similar in prokaryotes and eukaryotes. Although different elongation, initiation, and termination factors are used, the genetic code is generally identical. As previously noted, in bacteria, transcription and translation take place simultaneously, and mRNAs are relatively short-lived. In eukaryotes, however, mRNAs have highly variable half-lives, are subject to modifications, and must exit the nucleus to be translated; these multiple steps offer additional opportunities to regulate levels of protein production, and thereby fine-tune gene expression.

References and Recommended Reading

Chapeville, F., et al. On the role of soluble ribonucleic acid in coding for amino acids. Proceedings of the National Academy of Sciences 48 , 1086–1092 (1962)

Crick, F. On protein synthesis. Symposia of the Society for Experimental Biology 12 , 138–163 (1958)

Flinta, C., et al . Sequence determinants of N-terminal protein processing. European Journal of Biochemistry 154 , 193–196 (1986)

Grunberger, D., et al . Codon recognition by enzymatically mischarged valine transfer ribonucleic acid. Science 166 , 1635–1637 (1969) doi:10.1126/science.166.3913.1635

Kozak, M. Point mutations close to the AUG initiator codon affect the efficiency of translation of rat preproinsulin in vivo . Nature 308 , 241–246 (1984) doi:10.1038308241a0 ( link to article )

---. Point mutations define a sequence flanking the AUG initiator codon that modulates translation by eukaryotic ribosomes. Cell 44 , 283–292 (1986)

---. An analysis of 5'-noncoding sequences from 699 vertebrate messenger RNAs. Nucleic Acids Research 15 , 8125–8148 (1987)

Pierce, B. A. Genetics: A conceptual approach (New York, Freeman, 2000)

Shine, J., & Dalgarno, L. Determinant of cistron specificity in bacterial ribosomes. Nature 254 , 34–38 (1975) doi:10.1038/254034a0 ( link to article )

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