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Which ORF will be translated

Which ORF will be translated


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I've a question about ORF and translation into protein. Say I have an RNA transcript that contains two ORFs, one in phase 1 ; one in phase2 ; as:

ORF number 1 in reading frame 1 on the direct strand extends from base 529 to base 759.

ORF number 2 in reading frame 3 on the direct strand extends from base 849 to base 1223.

So ORF number 1 comes earlier in the transcript, but ORF number 3 is longer…

Is it always the first ORF that is translated ?

Edit >

  • It's in an eukaryoytic organism and same gene (it's the same transcript)

  • I use ORF finder (http://www.bioinformatics.org/sms2/orf_find.html) on my transcript sequence and it gives me two potential ORFs (ORF 1 and ORF 2 within two different frames)

Thanks


If both of the ORFs that the web tool predicted start with an AUG initiation codon, then the textbook answer is that the 5' cap on eukaryotic mRNAs is the first feature that is recognized by the translational apparatus, and the ribosome scans along until it finds the first AUG (it is a simplified description). However, if there are several AUGs near each other then the one that matches the best to the Kozak PWM (position weight matrix) will be preferred. The Kozak sequence can be approximated by: CCATGG which is also recognized by the restriction enzyme NcoI. Highly expressed genes tend to match most closely to the Kozak consensus.


The most common case like this is a prokaryotic mRNA, where polycistronic genes show up regularly. In that case the ribosomes are free to attach to the RNA and both genes are usually co-translated.

The proportion of translation between the two genes does vary depending on the affinity of the ribosome to the mRNA, which is partially determined by the ribosome binding site sequence.

If there is a lot of transcription activity on the mRNA, many ribosomes at once can translate any single ORF, as you can see in the micrograph above.


Translation

Genes contain the instructions a cell needs to make proteins. Making proteins from DNA requires a 2-step process:

  1. Transcription: the process of copying the gene&rsquos DNA into RNA.
  2. Translation: the process of using RNA to synthesize protein.

Taken together, these two steps make up the &ldquocentral dogma&rdquo of biology:

Figure (PageIndex<1>). (CC BY-NC-SA)

Figure (PageIndex<2>). (CC BY-NC-SA)

Transcription and processing of the newly made mRNA occurs in the nucleus of the cell.
Once a mature mRNA transcript is made it is transported to the cytoplasm for translation into protein.

Figure (PageIndex<3>). (CC BY-NC-SA)

Important Players in Translation

messenger RNA(mRNA): RNA copy of DNA that contains the instructions to make a protein.

transfer RNA (tRNA): RNA molecule responsible for delivering amino acids to the ribosome.

Amino acid: The basic building block of a protein. There are 20 different amino acids, each differs in its R group.

Figure (PageIndex<4>). (CC BY-NC-SA)

Protein: A chain of amino acids, also known as a polypeptide.

Ribosome: The organelle on which mRNA is translated into protein. Consists of a large (60S) and small subunit (40S) which are made up of ribosomal RNA (rRNA) and protein.

The Genetic Code: Translating RNA into Protein

How does the nucleotide sequence of RNA specify the specific order of amino acids in a protein? The answer lies in what is known as the genetic code.

Consider RNA and Protein as different languages:

RNA consists of four different "letters" - A, U, G and C.

Protein consists of 20 "letters" - the 20 amino acids

How can RNA code for protein?

If each RNA base codes for just 1 amino acid, RNA could code for only 4 amino acids (not enough to include all 20 amino acids).

If two RNA basescode for 1 amino acid, RNA could code for 16 amino acids (still not enough to include all 20 amino acids).

If three RNA bases code for 1 amino acid, RNA could code for 64 amino acids (more than enough to include all 20 amino acids).

Thus, the genetic code is a triplet code in which three nucletides in RNA specify one amino acid in protein.

Sets of three nucleotides that code for a specific amino acid are known as codons. These codons are recognized by, and basepair with, the tRNA molecule with the complementary anticodon. tRNA molecules act as translators because they are able to read the nucleic acid words (mRNA codons) and interpret them as protein words (amino acids). There is at least one tRNA for each of the 20 amino acids (some amino acids bind to 2 or 3 different tRNAs, so cells may contain as many as 32 different tRNAs).

Figure (PageIndex<5>). (CC BY-NC-SA)

The codon "AUG" is the start signal for translation which places the amino acid, methionine (Met) at the beginning of each protein. Three codons, UAA, UAG, and UGA, act as signals to terminate translation. They are called STOP codons.

Figure (PageIndex<6>). (CC BY-NC-SA)

Translation: RNA to Protein

Figure (PageIndex<7>). (CC BY-NC-SA)

Translation Initiation: The small subunit binds to a site upstream (on the 5' side) of the start of the mRNA. It proceeds to scan the mRNA in the 5'-->3' direction until it encounters the START codon (AUG). The large subunit attaches and the initiator tRNA, which carries methionine (Met), binds to the P site on the ribosome.

Figure (PageIndex<8>). (CC BY-NC-SA)

Elongation: A tRNA bound to its amino acid (known as an aminoacyl-tRNA) that is able to base pair with the next codon on the mRNA arrives at the A site. The preceding amino acid (Met at the start of translation) is covalently linked to the incoming amino acid with a peptide bond. The initiator tRNA moves to the E site and the ribosome moves one codon downstream. This shifts the more most recent tRNA from the A site to the P site, opening up the A site for the arrival of a new aminoacyl-tRNA. Polypeptide synthesis repeats, the tRNA residing in the E site is released from the complex, the tRNAs in the P site and A site shift over and the next amino acid is added to the growing polypeptide chain. This cycle repeats until a stop codon is reached.

Figure (PageIndex<9>). (CC BY-NC-SA)

Termination: Translation ends when the ribosome reaches a STOP codon (UAA, UAG or UGA). There are no tRNA molecules with anticodons for stop codons, instead protein release factors recognize these codons when they arrive at the A site. Binding of a release protein causes the polypeptide (protein) to be released from the ribosome. The ribosome subunits dissociate (split) from each other and can be reassembled later for another round of protein synthesis.

Figure (PageIndex<10>). (CC BY-NC-SA)

/>
Translation Tutorial by Dr. Katherine Harris is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.


The Genetic Code

To summarize what we know to this point, the cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template converts nucleotide-based genetic information into a protein product. Protein sequences consist of 20 commonly occurring amino acids therefore, it can be said that the protein alphabet consists of 20 letters. Each amino acid is defined by a three-nucleotide sequence called the triplet codon . The relationship between a nucleotide codon and its corresponding amino acid is called the genetic code .

Given the different numbers of “letters” in the mRNA and protein “alphabets,” combinations of nucleotides corresponded to single amino acids. Using a three-nucleotide code means that there are a total of 64 (4 × 4 × 4) possible combinations therefore, a given amino acid is encoded by more than one nucleotide triplet ([Figure 2]).

Figure 2: This figure shows the genetic code for translating each nucleotide triplet, or codon, in mRNA into an amino acid or a termination signal in a nascent protein. (credit: modification of work by NIH)

Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called stop codons . Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also serves as the start codon to initiate translation. The reading frame for translation is set by the AUG start codon near the 5′ end of the mRNA. The genetic code is universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all life on Earth shares a common origin.


9.4 Translation

The synthesis of proteins is one of a cell’s most energy-consuming metabolic processes. In turn, proteins account for more mass than any other component of living organisms (with the exception of water), and proteins perform a wide variety of the functions of a cell. The process of translation, or protein synthesis, involves decoding an mRNA message into a polypeptide product. Amino acids are covalently strung together in lengths ranging from approximately 50 amino acids to more than 1,000.

The Protein Synthesis Machinery

In addition to the mRNA template, many other molecules contribute to the process of translation. The composition of each component may vary across species for instance, ribosomes may consist of different numbers of ribosomal RNAs ( rRNA ) and polypeptides depending on the organism. However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. Translation requires the input of an mRNA template, ribosomes, tRNAs, and various enzymatic factors (Figure 9.19).

In E. coli, there are 200,000 ribosomes present in every cell at any given time. A ribosome is a complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides. In eukaryotes, the nucleolus is completely specialized for the synthesis and assembly of rRNAs.

Ribosomes are located in the cytoplasm in prokaryotes and in the cytoplasm and endoplasmic reticulum of eukaryotes. Ribosomes are made up of a large and a small subunit that come together for translation. The small subunit is responsible for binding the mRNA template, whereas the large subunit sequentially binds tRNAs , a type of RNA molecule that brings amino acids to the growing chain of the polypeptide. Each mRNA molecule is simultaneously translated by many ribosomes, all synthesizing protein in the same direction.

Depending on the species, 40 to 60 types of tRNA exist in the cytoplasm. Serving as adaptors, specific tRNAs bind to sequences on the mRNA template and add the corresponding amino acid to the polypeptide chain. Therefore, tRNAs are the molecules that actually “translate” the language of RNA into the language of proteins. For each tRNA to function, it must have its specific amino acid bonded to it. In the process of tRNA “charging,” each tRNA molecule is bonded to its correct amino acid.

The Genetic Code

To summarize what we know to this point, the cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template converts nucleotide-based genetic information into a protein product. Protein sequences consist of 20 commonly occurring amino acids therefore, it can be said that the protein alphabet consists of 20 letters. Each amino acid is defined by a three-nucleotide sequence called the triplet codon . The relationship between a nucleotide codon and its corresponding amino acid is called the genetic code .

Given the different numbers of “letters” in the mRNA and protein “alphabets,” combinations of nucleotides corresponded to single amino acids. Using a three-nucleotide code means that there are a total of 64 (4 × 4 × 4) possible combinations therefore, a given amino acid is encoded by more than one nucleotide triplet (Figure 9.20).

Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called stop codons . Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also serves as the start codon to initiate translation. The reading frame for translation is set by the AUG start codon near the 5' end of the mRNA. The genetic code is universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all life on Earth shares a common origin.

The Mechanism of Protein Synthesis

Just as with mRNA synthesis, protein synthesis can be divided into three phases: initiation, elongation, and termination. The process of translation is similar in prokaryotes and eukaryotes. Here we will explore how translation occurs in E. coli, a representative prokaryote, and specify any differences between prokaryotic and eukaryotic translation.

Protein synthesis begins with the formation of an initiation complex. In E. coli, this complex involves the small ribosome subunit, the mRNA template, three initiation factors, and a special initiator tRNA. The initiator tRNA interacts with the AUG start codon, and links to a special form of the amino acid methionine that is typically removed from the polypeptide after translation is complete.

In prokaryotes and eukaryotes, the basics of polypeptide elongation are the same, so we will review elongation from the perspective of E. coli. The large ribosomal subunit of E. coli consists of three compartments: the A site binds incoming charged tRNAs (tRNAs with their attached specific amino acids). The P site binds charged tRNAs carrying amino acids that have formed bonds with the growing polypeptide chain but have not yet dissociated from their corresponding tRNA. The E site releases dissociated tRNAs so they can be recharged with free amino acids. The ribosome shifts one codon at a time, catalyzing each process that occurs in the three sites. With each step, a charged tRNA enters the complex, the polypeptide becomes one amino acid longer, and an uncharged tRNA departs. The energy for each bond between amino acids is derived from GTP, a molecule similar to ATP (Figure 9.21). Amazingly, the E. coli translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200-amino acid polypeptide could be translated in just 10 seconds.

Termination of translation occurs when a stop codon (UAA, UAG, or UGA) is encountered. When the ribosome encounters the stop codon, the growing polypeptide is released and the ribosome subunits dissociate and leave the mRNA. After many ribosomes have completed translation, the mRNA is degraded so the nucleotides can be reused in another transcription reaction.

Concepts in Action

Transcribe a gene and translate it to protein using complementary pairing and the genetic code at this site.


Since codons in mRNA are read in the 5′ → 3′direction, anticodons are oriented in the 3′ → 5′ direction, as Figure 3-19 shows. Each tRNA is specific for only one amino acid and carries that amino acid attached at its free 3′ end. Amino acids are added to the tRNA by enzymes called aminoacyl-tRNA synthetases.

A codon is a trinucleotide sequence of DNA or RNA that corresponds to a specific amino acid. The genetic code describes the relationship between the sequence of DNA bases (A, C, G, and T) in a gene and the corresponding protein sequence that it encodes. The cell reads the sequence of the gene in groups of three bases.


Describe the process of translation.

Translation is a universal process in biology where a protein formed of amino acids is made by using messenger Ribonucleic acid (mRNA) to dictate the order of amino acids.

Following transcription (the conversion of DNA into mRNA) an mRNA strand leaves the nucleus and enters the cytoplasm via the nuclear pore. Here it is recognised and becomes attached to biological structures called ribosomes forming a 'polysome'. Each ribosome attached to an mRNA strand will create a seperate polypeptide.

The bases (individual components of mRNA) are read in non-overlapping 3s (a codon) by the ribosome. This is the 'triplet code'. Once mRNA has become attached the start codon is held in the peptidyl (P) site and the next codon in the aminoacyl (A) site. There is also an exit (E) site located on the other side of the P site. Synthesis occurs in a 5' to 3' direction of mRNA.

For amino acids to attach they are first conjugated (bonded) to a specific transfer RNA (tRNA), each tRNA has an anticodonwhich is complementaryto a codon in the mRNA. The tRNA and mRNA are matched by codon to anticodon hydrogen bonding. This causes the conjugated amino acid to be positioned so it can be bonded to the previous amino acid by a peptide bond. The start codon, thus the first amino acid in a peptide is usually methionine.

The formation of the polysome and the attachment of methionine is known as the 'initiation' step.

Following initiation, the rest of the peptide is synthesised. The mRNA and tRNA-amino acid in the A site are pushed into the P site, forcing that in the P site into the E site and a new codon moves into the A site. At the E site the tRNA now without it's amino acid dissociates from the mRNA and goes to bind another amino acid, it is recycled. This is called 'elongation' and it repeats until a stop codon is reached. Causing the peptide chain to grow by one amino acid each cycle.

Once a stop codon arrives at the A site there is no matching amino acid. Instead a release factor protein triggers the disassembly of the ribosome-mRNA complex. This step is called 'termination'. Following this the protein is complete and is released by the ribosome.


Which ORF will be translated - Biology

As discoveries are being made at a rapid rate in the fields of genetics, agriculture, and other biological sciences (biology), it is clear that there is a need for a greater understanding of genomics in the general population. In order to better understand the newest innovations in the science of genomics we must first understand the cell. The following paragraphs explain the basics of cellular biology using a small town library analogy.

A cell is very much like a town because, like a town, each cell has a purpose and components that are needed by other cells or communities. In a town you can find a library with copy machines and factories. Towns often contain some sort of manufacturing. In this analogy the manufacturing occurs in factories. The goods manufactured must be usable or they would not be produced. However, the goods produced are specific to each town. A cell is also like a town in the sense that it also has a library or a nucleus that contains important genetic information. Cells also contain "copy machines" to transcribe information and "factories" to produce products known as proteins (translation).

The library is analogous to the nucleus of the cell. Within the nucleus is the DNA. This is the genetic material that determines physical characteristics of the cell and ultimately the organism. DNA does not leave the nucleus. Instead, messenger RNA is used to retrieve the information from nucleus for the production of proteins outside the nucleus. The messenger RNA is a "copy" of the information contained in specific sequences of DNA. This copy (mRNA) is transported to a seperate region of the cell where proteins are made.

It is important to remember that the books cannot be removed from the library. Therefore, a copy must be made. This copy can only be used to make a certain amount of product at the factory before the copy wears out and the process must be repeated.

The copy machine is an analogy for the process of transcription. The copies are the messenger RNA (mRNA) that take the information within the nucleus and bring it to the site of translation. Translation usually occurs in the cytoplasm. Translation is the process of making the proteins in the cell. In our analogy "translation" is the process of turning the instructions into a product in the factory (see below). The mRNA has a given limited life-span and mRNA is typically broken down after a certain amount of time.

In most towns there is a factory. The production of goods is part of the economy of a town or community. It is essential for the factory of the town to be involved in production of necessary goods. As we have already learned, the information housed in the library contains blueprints or instructions on how the factory must produce the goods. The products of the cell factory are the essential building blocks for the communities. Factories in the town each produce a component of the total goods produced by the town. Each town produces its own unique set of products.

A more technical explaination of the factory of the cell would include that the factory represents the process of translation. Translation is the process of the nucleic acids being translated into the "amino acid" language.

The scheme as we have shown you is how information flows from DNA to mRNA to the protein level. This is the central concept of molecular biology. With this understanding, you will be better able to understand the concepts of genomics and biotechnology.

You should now recognize that the structure of a cell is similar analogous to a town. In addition, within each cell is the storage of information. You now realize that this information is DNA. Because DNA or books cannot be removed from the library a copy must be made. This copy is messenger RNA which is taken to the factories for the production of goods. There may be more than one factory in the town. However, each of these factories will produce a portion of the total product for the town or cell. For example, if the product for the town was a car, one factory would produce the tires while another would produce the interior. Once these products are put together, the overall outcome for the town is a car. Another set of books may be used to manufacture scissors. Similarly, cells "manufacture" many different proteins that are important for the cell, the whole organism, and how the orgamism functions in the environment.


Found in all living organisms.

  • Thalassemia - group of blood disorders characterized by a deficiency of haemoglobin, the blood protein that transports oxygen to the tissues.
  • Thalassemia is caused by genetically determined abnormalities in the synthesis of one or more of the polypeptide chains that make up the globin part of haemoglobin.
  • The various forms of the disorder are distinguished by different combinations of three variable
    • The particular polypeptide chain or chains that are affected
    • Whether the disorder is inherited from one parent (heterozygous) or from both parents (homozygous).
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        Biology transcription and translation

        The definition of translation. Molecular biology and biology genetics provide the following definition of translation in biology. Translation means the process of translating an mRNA (messenger RNA) sequence into amino acids. As most cells are made up of protein, DNA translation is a fundamental process for cells creation.

        Phases of translation

        Some people claim that there are four phases of translation: initiation, elongation, translocation, and termination. However, most scientists believe that there are only three steps of translation in biology. The same notions are used in the process of transcription for describing the process of making the mRNA string. The difference is that in translation the polypeptide string is created. Let have a closer look at these steps.

        At this phase the tRNA, the mRNA, and the amino acid are combined in a ribosome. The tRNA attaches to the start codon, that is the set of three nucleotides that begins the coded sequence of a gene and specifies the amino acid methionine. Methionine gets into the ribosome by attaching to the tRNA with the right anticodon. The start codon is AUG. UAC is its anticodon. If you don&rsquot know about this, you should pay attention to the rules of complementary base pairing. When the tRNA attaches to the codon AUG, the methionine is attached to tRNA.

        As peptide bonds are created, more amino acids appear. Their chain becomes longer. Thus, a polypeptide is formed. A tRNA and the amino acid enter the ribosome. If the anticodon matches the mRNA codon, the ribosome will link two amino acids together. If they don&rsquot, the wrong amino acid is rejected. Linking amino acids together, the ribosome moves them forward. The procedure is repeated when the next pair on the tRNA and the amino acid enters the ribosome.

        If the ribosome reaches one of three stop codons, it won&rsquot have a corresponding tRNA. Thus, proteins will stimulate the release of the polypeptide chain. Protein release factors recognize stop codons only when they appear at the A site. The A site is called so because it binds only to the incoming aminoacyl-tRNA (the tRNA which brings the next amino acid). The ribosome releases from the mRNA. Its subunits dissociate. Small ones will connect with the new combinations of tRNAs and methionine. A new translation will begin.

        Thus, we can make a conclusion that there are two steps of gene expression: transcription and translation. Transcription is the encoding of DNA information into RNA molecules. Translation is the encoding of information of mRNA nucleotides into a sequence of amino acids in a protein.

        As for the eukaryotes, transcription and translation are separated in time. They also take place in different places. The process of transcription of DNA into mRNA takes place in nucleus. The process of translation of mRNA into polypeptides occurs on polysomes in the cytoplasm. As bacteria don&rsquot have nucleus, the processes of translation and transcription occur simultaneously.

        In order to understand the information better, check out the following sources:

        Make sure you can answer the following questions. If not, read this information one more time or get some professional help from the best biology tutors.


        Which ORF will be translated - Biology

        Translation is the process by which ribosomes convert the information carried by messenger RNA(mRNA) to the synthesis of proteins. It can also be defined as the process in which sequence of nucleotides in mRNA is translated into the sequence of amino acids. It can also be defined as the translation of the language available in the form of mRNA into the language of proteins.

        mRNA (translation) &rarr Proteins

        Translation involves the transport of amino acids from the intercellular pool to the ribosomes where they are assembled into proteins elsewhere in the cytoplasm. Transfer of amino acids to the ribosome surface is accomplished by mRNA.

        Requirement of protein synthesis:

        Various molecules are required for the process of protein synthesis. They are:

        1)D.N.A - D.N.A is a double helical prime molecule that determines the kind of protein needed to be synthesized. The protein synthesis is initiated, guided and regulated by DNA molecule.

        2)Messenger R.N.A(mRNA)- mRNA is a single-stranded molecule that carries information from D.N.A to the cytoplasm for protein synthesis. The information stored in the form of a base sequence of mRNA is complementary to the base sequence present on template D.N.A.

        3)Transfer R.N.A(tRNA)-tRNA helps in protein synthesis by picking up activated amino acids from the amino acid pool and transporting them to the ribosomes where it recognizes a specific triplet codon of mRNA. Each amino acid is carried by a specific tRNA as the lowermost segment of tRNA has three base sequences anticodon loop which are complementary to the triplet codons of mRNA.

        4)Ribosomes-These are the sites of protein synthesis and are found in the cytoplasm, They contain a number of enzymes responsible for the formation of the polypeptide chain. Each ribosome has two subunits- a larger subunit and a smaller subunit. Larger subunit has two sites:

        I) Aminoacyl site (A site) or acceptor site

        II) Peptide site (P site) or donor site

        5)Amino acids-These are the building blocks of a polypeptide chain or protein. There are 20 types of amino acids which occur in cytoplasm forming an amino acid pool. These amino acids are assembled in polypeptide chain to form a protein.

        6)Enzymes-A number of enzymes are responsible for the process of transcription. Aminoacyl-tRNA synthetase is one of them.

        source: www.biologydiscussion.com fig: Subunits of ribosomes

        Steps of translation-

        The process of translation is much more complex than that of transcription. It involves the following steps:

        1)Binding of mRNA to ribosomes:

        During transcription, DNA molecule synthesizes three types of RNAs inside the nucleus. Then, these RNAs migrate into the cytoplasm through the nuclear pore. Out of these RNAs, mRNA carries the genetic information and it is joined to the ribosomal subunits by the initiation codon 'AUG' found on its 5'end. This union forms mRNA ribosomal complex. [Many ribosomes lined up on a chain is known as poly-ribosomes.]

        2)Activation of amino acid:

        Amino acids are found in the amino acid pool in the cytoplasm in an inactive form. To form polypeptide chain, the amino acids must be activated before they are joined to the tRNA. The enzyme aminoacyl synthetase activates the amino acid in the presence of ATP and Mg ++ .

        Amino acid + Aminoacyl-Synthetase + ATP&rarr Aminoacyl-AMP enzyme complex(Activated amino acid) + Ppi

        3)Attachment of activated amino acid with tRNA:

        The activated amino acids are joined to the 3' end of the tRNA and form amino-acyl-tRNA complex.

        Activated amino acid + tRNA&rarr Aminoacyl-AMP enzyme complex + AMP + enzyme

        There are more than 20 different enzymes and 20 tRNA molecules in the cell. So a specific amino acid attaches to a specific aminoacyl-tRNA molecule to form chained tRNA. This chain of tRNA serves as an adaptor molecule for decoding the information to mRNA till it reaches the last codon. As one ribosome moves along mRNA, the initiating part of mRNA becomes free. In this site, new ribosomes get lined up to form polyribosome.

        4)Initiation of polypeptide chain:

        Each mRNA molecule has initiation codon AUGm which signals the beginning of polypeptide chain. In this process, mRNA first binds to the subunits of ribosomes. The AUG codon lies near 'P' peptidyl site of the larger subunit of the ribosome. This codon codes for amino acid methionine. This means, activated methionine bearing tRNA has anticodon UAC. The second codon on mRNA leads close to 'A' site of the ribosome. Then, the 2 nd aminoacyl-tRNA complex with anticodon bonds with the 2 nd codon of mRNA and occupies the 'A'-site of the ribosome.

        5)Elongation of polypeptide chain:

        The elongation begins with the formation of the peptide bond (-CO-NH-) between the amino acids present in 'P' and 'A' sites of the ribosomes. This is catalyzed by enzyme peptide synthetase. It causes the transfer of amino acid from 'A' site to 'P' site and formation of amino acid chain on 'A' site and releases the tRNA from P-site.

        During the elongation of the polypeptide chain, ribosomes move along mRNA till it reaches the last codon. As one ribosome moves along mRNA, the initiating point of mRNA becomes free. In this site, new ribosome gets lined up to form polyribosomes.

        6) Termination of polypeptide chain:

        When the ribosome reaches the end of mRNA strand (3' end) the synthesis of the polypeptide chain is completed. It is signaled by the termination codon UAA, UGA, and UAG. During this process:

        &bullOne polypeptide chain or protein molecule is released from tRNA.

        &bullRibosomes are set free and hence dissociates into two subunits.

        Thus, protein synthesis takes place in the above steps.

        source: www.mun.ca fig:Translation

        Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.


        Watch the video: Zázrak z Wörglu (July 2022).


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