Now that we’ve described DNA and RNA, it’s time to take a look at the process of protein synthesis. The synthesis of proteins takes two steps: transcription and translation. Transcription takes the information encoded in DNA and encodes it into mRNA, which heads out of the cell’s nucleus and into the cytoplasm. During translation, the mRNA works with a ribosome and tRNA to synthesize proteins.
The first step in transcription is the partial unwinding of the DNA molecule so that the portion of DNA that codes for the needed protein can be transcribed. Once the DNA molecule is unwound at the correct location, an enzyme called RNA polymerase helps line up nucleotides to create a complementary strand of mRNA. Since mRNA is a single-stranded molecule, only one of the two strands of DNA is used as a template for the new RNA strand.
The new strand of RNA is made according to the rules of base pairing:
- DNA cytosine pairs with RNA guanine
- DNA guanine pairs with RNA cytosine
- DNA thymine pairs with RNA adenine
- DNA adenine pairs with RNA uracil
For example, the mRNA complement to the DNA sequence TTGCAC is AACGUG. The SAT II Biology frequently asks about the sequence of mRNA that will be produced from a given sequence of DNA. For these questions, don’t forget that RNA uses uracil in place of thymine.
After transcription, the new RNA strand is released and the two unzipped DNA strands bind together again to form the double helix. Because the DNA template remains unchanged after transcription, it is possible to transcribe another identical molecule of RNA immediately after the first one is complete. A single gene on a DNA strand can produce enough RNA to make thousands of copies of the same protein in a very short time.
In translation, mRNA is sent to the cytoplasm, where it bonds with ribosomes, the sites of protein synthesis. Ribosomes have three important binding sites: one for mRNA and two for tRNA. The two tRNA sites are labeled the A site and P site.
Once the mRNA is in place, tRNA molecules, each associated with specific amino acids, bind to the ribosome in a sequence defined by the mRNA code. tRNA molecules can perform this function because of their special structure. tRNA is made up of many nucleotides that bend into the shape of a cloverleaf. At its tail end, tRNA has an acceptor stem that attaches to a specific amino acid. At its head, tRNA has three nucleotides that make up an anticodon.
An anticodon pairs complementary nitrogenous bases with mRNA. For example if mRNA has a codon AUC, it will pair with tRNA’s anticodon sequence UAG. tRNA molecules with the same anticodon sequence will always carry the same amino acids, ensuring the consistency of the proteins coded for in DNA.
The Process of Translation
Translation begins with the binding of the mRNA chain to the ribosome. The first codon, which is always the start codon methionine, fills the P site and the second codon fills the A site. The tRNA molecule whose anticodon is complementary to the mRNA forms a temporary base pair with the mRNA in the A site. A peptide bond is formed between the amino acid attached to the tRNA in the A site and the methionine in the P site.
The ribosome now slides down the mRNA, so that the tRNA in the A site moves over to the P site, and a new codon fills the A site. (One way to remember this is that the A site brings new amino acids to the growing polypeptide at the P site.) The appropriate tRNA carrying the appropriate amino acid pairs bases with this new codon in the A site. A peptide bond is formed between the two adjacent amino acids held by tRNA molecules, forming the first two links of a chain.
The ribosome slides again. The tRNA that was in the P site is let go into the cytoplasm, where it will eventually bind with another amino acid. Another tRNA comes to bind with the new codon in the A site, and a peptide bond is formed between the new amino acid to the growing peptide chain.
The process continues until one of the three stop codons enters the A site. At that point, the protein chain connected to the tRNA in the P site is released. Translation is complete.
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Protein synthesis is the process whereby biological cells generate new proteins; it is balanced by the loss of cellular proteins via degradation or export. Translation, the assembly of amino acids by ribosomes, is an essential part of the biosynthetic pathway, along with generation of messenger RNA (mRNA), aminoacylation of transfer RNA (tRNA), co-translational transport, and post-translational modification. Protein biosynthesis is strictly regulated at multiple steps. They are principally during transcription (phenomena of RNA synthesis from DNA template) and translation (phenomena of amino acid assembly from RNA).
The cistron DNA is transcribed into the first of a series of RNA intermediates. The last version is used as a template in synthesis of a polypeptide chain. Protein will often be synthesized directly from genes by translatingmRNA. However, when a protein must be available on short notice or in large quantities, a protein precursor is produced. A proprotein is an inactive protein containing one or more inhibitory peptides that can be activated when the inhibitory sequence is removed by proteolysis during posttranslational modification. A preprotein is a form that contains a signal sequence (an N-terminal signal peptide) that specifies its insertion into or through membranes, i.e., targets them for secretion. The signal peptide is cleaved off in the endoplasmic reticulum.Preproproteins have both sequences (inhibitory and signal) still present.
In protein synthesis, a succession of tRNA molecules charged with appropriate amino acids are brought together with an mRNA molecule and matched up by base-pairing through the anti-codons of the tRNA with successive codons of the mRNA. The amino acids are then linked together to extend the growing protein chain, and the tRNAs, no longer carrying amino acids, are released. This whole complex of processes is carried out by the ribosome, formed of two main chains of RNA, called ribosomal RNA (rRNA), and more than 50 different proteins. The ribosome latches onto the end of an mRNA molecule and moves along it, capturing loaded tRNA molecules and joining together their amino acids to form a new protein chain.
Protein biosynthesis, although very similar, is different for prokaryotes and eukaryotes.
Main article: Transcription (genetics)
In transcription an mRNA chain is generated, with one strand of the DNA double helix in the genome as a template. This strand is called the template strand. Transcription can be divided into 3 stages: initiation, elongation, and termination, each regulated by a large number of proteins such as transcription factors and coactivators that ensure that the correct gene is transcribed.
Transcription occurs in the cell nucleus, where the DNA is held and is never able to leave. The DNA structure of the cell is made up of two helixes made up of sugar and phosphate held together by hydrogen bonds between the bases of opposite strands. The sugar and the phosphate in each strand are joined together by stronger phosphodiester covalent bonds. The DNA is "unzipped" (disruption of hydrogen bonds between different single strands) by the enzyme helicase, leaving the single nucleotide chain open to be copied. RNA polymerase reads the DNA strand from the 3-prime (3') end to the 5-prime (5') end, while it synthesizes a single strand of messenger RNA in the 5'-to-3' direction. The general RNA structure is very similar to the DNA structure, but in RNA the nucleotide uracil takes the place that thymine occupies in DNA. The single strand of mRNA leaves the nucleus through nuclear pores, and migrates into the cytoplasm.
The first product of transcription differs in prokaryotic cells from that of eukaryotic cells, as in prokaryotic cells the product is mRNA, which needs no post-transcriptional modification, whereas, in eukaryotic cells, the first product is called primary transcript, that needs post-transcriptional modification (capping with 7-methyl-guanosine, tailing with a poly A tail) to give hnRNA (heterogeneous nuclear RNA). hnRNA then undergoes splicing of introns (noncoding parts of the gene) via spliceosomes to produce the final mRNA.
Main article: Translation (biology)
Phenomena of amino acid assembly from RNA. The synthesis of proteins from RNA is known as translation. In eukaryotes, translation occurs in the cytoplasm, where the ribosomes are located. Ribosomes are made of a small and large subunit that surround the mRNA. In translation, messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the trinucleotide genetic code. This uses an mRNA sequence as a template to guide the synthesis of a chain of amino acids that form a protein. Translation proceeds in four phases: activation, initiation, elongation, and termination (all describing the growth of the amino acid chain, or polypeptide that is the product of translation).
In activation, the correct amino acid (AA) is joined to the correct transfer RNA (tRNA). While this is not, in the technical sense, a step in translation, it is required for translation to proceed. The AA is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond. When the tRNA has an amino acid linked to it, it is termed "charged". Initiation involves the small subunit of the ribosome binding to 5' end of mRNA with the help of initiation factors (IF), other proteins that assist the process. Elongation occurs when the next aminoacyl-tRNA (charged tRNA) in line binds to the ribosome along with GTP and an elongation factor. Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). When this happens, no tRNA can recognize it, but releasing factor can recognize nonsense codons and causes the release of the polypeptide chain. The capacity of disabling or inhibiting translation in protein biosynthesis is used by some antibiotics such as anisomycin, cycloheximide, chloramphenicol, tetracycline, streptomycin, erythromycin, puromycin, etc.
Events during or following protein translation
Main articles: Proteolysis, Posttranslational modification, and Protein folding
Events that occur during or following biosynthesis include proteolysis, post-translational modification and protein folding. Proteolysis may remove N-terminal, C-terminal or internal amino-acid residues or peptides from the polypeptide. The termini and side-chains of the polypeptide may be subjected to post-translational modification. These modifications may be required for correct cellular localisation or the natural function of the protein. During and after synthesis, polypeptide chains often fold to assume, so called, native secondary and tertiary structures. This is known as protein folding and is typically required for the natural function of the protein.
- ^Kafri M, Metzl-Raz E, Jona G, Barkai N. 2016. The Cost of Protein Production. Cell Rep 14:22–31. https://dx.doi.org/10.1016/j.celrep.2015.12.015
- ^ abAlberts, Bruce (2002). Molecular biology of the cell. New York: Garland Science. p. 760. ISBN 0-8153-3218-1.
- ^Alberts, Bruce. Molecular Biology of the Cell, 5e. New York: Garland Science, 2008.