what is the end result of 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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Ribosomes, Transcription, and Translation

The genetic information stored in DNA is a living archive of instructions that cells use to accomplish the functions of life. Inside each cell, catalysts seek out the appropriate information from this archive and use it to build new proteins — proteins that make up the structures of the cell, run the biochemical reactions in the cell, and are sometimes manufactured for export. Although all of the cells that make up a multicellular organism contain identical genetic information, functionally different cells within the organism use different sets of catalysts to express only specific portions of these instructions to accomplish the functions of life.

How Is Genetic Information Passed on in Dividing Cells?

When a cell divides, it creates one copy of its genetic information — in the form of DNA molecules — for each of the two resulting daughter cells. The accuracy of these copies determines the health and inherited features of the nascent cells, so it is essential that the process of DNA replication be as accurate as possible (Figure 1).

One factor that helps ensure precise replication is the double-helical structure of DNA itself. In particular, the two strands of the DNA double helix are made up of combinations of molecules called nucleotides . DNA is constructed from just four different nucleotides — adenine (A), thymine (T), cytosine (C), and guanine (G) — each of which is named for the nitrogenous base it contains. Moreover, the nucleotides that form one strand of the DNA double helix always bond with the nucleotides in the other strand according to a pattern known as complementary base-pairing — specifically, A always pairs with T, and C always pairs with G (Figure 2). Thus, during cell division, the paired strands unravel and each strand serves as the template for synthesis of a new complementary strand.

What Are the Initial Steps in Accessing Genetic Information?

RNA molecules differ from DNA molecules in several important ways: They are single stranded rather than double stranded; their sugar component is a ribose rather than a deoxyribose; and they include uracil (U) nucleotides rather than thymine (T) nucleotides (Figure 4). Also, because they are single strands, RNA molecules don't form helices; rather, they fold into complex structures that are stabilized by internal complementary base-pairing.

mRNA is the most variable class of RNA, and there are literally thousands of different mRNA molecules present in a cell at any given time. Some mRNA molecules are abundant, numbering in the hundreds or thousands, as is often true of transcripts encoding structural proteins. Other mRNAs are quite rare, with perhaps only a single copy present, as is sometimes the case for transcripts that encode signaling proteins. mRNAs also vary in how long-lived they are. In eukaryotes, transcripts for structural proteins may remain intact for over ten hours, whereas transcripts for signaling proteins may be degraded in less than ten minutes.

Cells can be characterized by the spectrum of mRNA molecules present within them; this spectrum is called the transcriptome . Whereas each cell in a multicellular organism carries the same DNA or genome, its transcriptome varies widely according to cell type and function. For instance, the insulin-producing cells of the pancreas contain transcripts for insulin, but bone cells do not. Even though bone cells carry the gene for insulin, this gene is not transcribed. Therefore, the transcriptome functions as a kind of catalog of all of the genes that are being expressed in a cell at a particular point in time.

What Is the Function of Ribosomes?

Ribosomes are complexes of rRNA molecules and proteins, and they can be observed in electron micrographs of cells. Sometimes, ribosomes are visible as clusters, called polyribosomes. In eukaryotes (but not in prokaryotes), some of the ribosomes are attached to internal membranes, where they synthesize the proteins that will later reside in those membranes, or are destined for secretion (Figure 6). Although only a few rRNA molecules are present in each ribosome, these molecules make up about half of the ribosomal mass. The remaining mass consists of a number of proteins — nearly 60 in prokaryotic cells and over 80 in eukaryotic cells.

Within the ribosome, the rRNA molecules direct the catalytic steps of protein synthesis — the stitching together of amino acids to make a protein molecule. In fact, rRNA is sometimes called a ribozyme or catalytic RNA to reflect this function.

How Does the Whole Process Result in New Proteins?

After the transcription of DNA to mRNA is complete, translation — or the reading of these mRNAs to make proteins — begins. Recall that mRNA molecules are single stranded, and the order of their bases — A, U, C, and G — is complementary to that in specific portions of the cell's DNA. Each mRNA dictates the order in which amino acids should be added to a growing protein as it is synthesized. In fact, every amino acid is represented by a three-nucleotide sequence or codon along the mRNA molecule. For example, AGC is the mRNA codon for the amino acid serine, and UAA is a signal to stop translating a protein — also called the stop codon (Figure 7).

Molecules of tRNA are responsible for matching amino acids with the appropriate codons in mRNA. Each tRNA molecule has two distinct ends, one of which binds to a specific amino acid, and the other which binds to the corresponding mRNA codon. During translation , these tRNAs carry amino acids to the ribosome and join with their complementary codons. Then, the assembled amino acids are joined together as the ribosome, with its resident rRNAs, moves along the mRNA molecule in a ratchet-like motion. The resulting protein chains can be hundreds of amino acids in length, and synthesizing these molecules requires a huge amount of chemical energy (Figure 8).

In prokaryotic cells, transcription (DNA to mRNA) and translation (mRNA to protein) are so closely linked that translation usually begins before transcription is complete. In eukaryotic cells, however, the two processes are separated in both space and time: mRNAs are synthesized in the nucleus, and proteins are later made in the cytoplasm.

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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.

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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

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Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.

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The Cell: A Molecular Approach. 2nd edition.

Chapter 7 protein synthesis, processing, and regulation.

Transcription and RNA processing are followed by translation, the synthesis of proteins as directed by mRNA templates. Proteins are the active players in most cell processes, implementing the myriad tasks that are directed by the information encoded in genomic DNA. Protein synthesis is thus the final stage of gene expression. However, the translation of mRNA is only the first step in the formation of a functional protein. The polypeptide chain must then fold into the appropriate three-dimensional conformation and, frequently, undergo various processing steps before being converted to its active form. These processing steps, particularly in eukaryotes, are intimately related to the sorting and transport of different proteins to their appropriate destinations within the cell.

Although the expression of most genes is regulated primarily at the level of transcription (see Chapter 6), gene expression can also be controlled at the level of translation, and this control is an important element of gene regulation in both prokaryotic and eukaryotic cells. Of even broader significance, however, are the mechanisms that control the activities of proteins within cells. Once synthesized, most proteins can be regulated in response to extracellular signals by either covalent modifications or by association with other molecules. In addition, the levels of proteins within cells can be controlled by differential rates of protein degradation. These multiple controls of both the amounts and activities of intracellular proteins ultimately regulate all aspects of cell behavior.

• Translation of mRNA
• Protein Folding and Processing
• Regulation of Protein Function
• Protein Degradation
• References and Further Reading
• Cite this Page Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Chapter 7, Protein Synthesis, Processing, and Regulation.

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Translation: Making Protein Synthesis Possible

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Protein synthesis is accomplished through a process called translation. After DNA is transcribed into a messenger RNA (mRNA) molecule during transcription, the mRNA must be translated to produce a protein. In the translation process in protein synthesis, mRNA along with transfer RNA (tRNA) and ribosomes work together to produce proteins.

Stages of Translation in Protein Synthesis

• Initiation:  Ribosomal subunits bind to mRNA.
• Elongation:  The  ribosome  moves along the mRNA molecule linking  amino acids  and forming a polypeptide chain.
• Termination:  The ribosome reaches a stop codon, which terminates protein synthesis and releases the ribosome.

Transfer RNA

Transfer RNA  plays a huge role in the translation process in protein synthesis. Its job is to translate the message within the nucleotide sequence of mRNA to a specific  amino acid  sequence. These sequences are joined together to form a protein . Transfer RNA  is shaped like a clover leaf with three loops. It contains an amino acid attachment site on one end and a special section in the middle loop called the anticodon site. The anticodon recognizes a specific area on a mRNA called a  codon .

Messenger RNA Modifications

The translation process in protein synthesis occurs in the  cytoplasm . After leaving the  nucleus , mRNA must undergo several modifications before being translated. Sections of the mRNA that do not code for amino acids, called introns, are removed. A poly-A tail, consisting of several adenine bases, is added to one end of the mRNA, while a guanosine triphosphate cap is added to the other end. These modifications remove unneeded sections and protect the ends of the mRNA molecule. Once all modifications are complete, mRNA is ready for translation.

Translation

Mariana Ruiz Villarreal/Wikimedia Commons 

Once messenger RNA has been modified and is ready for translation, it binds to a specific site on a ribosome . Ribosomes consist of two parts, a large subunit and a small subunit. They contain a binding site for mRNA and two binding sites for transfer RNA (tRNA) located in the large ribosomal subunit.

During the translation process in protein synthesis, a small ribosomal subunit attaches to a mRNA molecule. At the same time, an initiator tRNA molecule recognizes and binds to a specific  codon sequence  on the same mRNA molecule. A large ribosomal subunit then joins the newly formed complex. The initiator tRNA resides in one binding site of the ribosome called the  P  site, leaving the second binding site, the  A  site, open. When a new tRNA molecule recognizes the next codon sequence on the mRNA, it attaches to the open  A  site. A peptide bond forms connecting the  amino acid  of the tRNA in the  P  site to the amino acid of the tRNA in the  A  binding site.

As the ribosome moves along the mRNA molecule, the tRNA in the  P  site is released and the tRNA in the  A  site is translocated to the  P  site. The  A  binding site becomes vacant again until another tRNA that recognizes the new mRNA codon takes the open position. This pattern continues as molecules of tRNA are released from the complex, new tRNA molecules attach, and the  amino acid  chain grows.

Termination

The ribosome will translate the mRNA molecule until it reaches a termination codon on the mRNA. When this happens, the growing  protein  called a polypeptide chain is released from the tRNA molecule and the ribosome splits back into large and small subunits.

The newly formed polypeptide chain undergoes several modifications before becoming a fully functioning protein. Proteins have a  variety of functions . Some will be used in the  cell membrane , while others will remain in the  cytoplasm  or be transported out of the  cell . Many copies of a protein can be made from one mRNA molecule. This is because several  ribosomes  can translate the same mRNA molecule at the same time. These clusters of ribosomes that translate a single mRNA sequence are called polyribosomes or polysomes.

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