The DNA code for the protein remains in the nucleus , but a copy, called mRNA, moves from the nucleus to the ribosomes where proteins are synthesised in the cytoplasm. The protein produced depends on the template used, and if this sequence changes a different protein will be made. Carrier molecules bring specific amino acids to add to the growing protein in the correct order.
There are only about 20 different naturally-occurring amino acids. Each protein molecule has hundreds, or even thousands, of amino acids joined together in a unique sequence. In turn, proteins account for more mass than any other macromolecule of living organisms. They perform virtually every function of a cell, serving as both functional e.
The process of translation , or protein synthesis , the second part of gene expression, involves the decoding by a ribosome of an mRNA message into a polypeptide product. A protein sequence consists of 20 commonly occurring amino acids. Each amino acid is defined within the mRNA by a triplet of nucleotides called a codon.
The relationship between an mRNA codon and its corresponding amino acid is called the genetic code. The three-nucleotide code means that there is a total of 64 possible combinations 4 3 , with four different nucleotides possible at each of the three different positions within the codon. This number is greater than the number of amino acids and a given amino acid is encoded by more than one codon Figure 1. This redundancy in the genetic code is called degeneracy.
Typically, whereas the first two positions in a codon are important for determining which amino acid will be incorporated into a growing polypeptide, the third position, called the wobble position , is less critical. In some cases, if the nucleotide in the third position is changed, the same amino acid is still incorporated.
Whereas 61 of the 64 possible triplets code for amino acids, three of the 64 codons do not code for an amino acid; they terminate protein synthesis, releasing the polypeptide from the translation machinery. These are called stop codons or nonsense codons. Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also typically serves as the start codon to initiate translation.
Each set of three nucleotides following this start codon is a codon in the mRNA message. The genetic code is nearly universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all extant life on earth shares a common origin.
However, unusual amino acids such as selenocysteine and pyrrolysine have been observed in archaea and bacteria. In the case of selenocysteine, the codon used is UGA normally a stop codon. Pyrrolysine uses a different stop codon, UAG.
Figure 1. This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a termination signal in a nascent protein. The first letter of a codon is shown vertically on the left, the second letter of a codon is shown horizontally across the top, and the third letter of a codon is shown vertically on the right.
In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation.
The composition of each component varies across taxa; for instance, ribosomes may consist of different numbers of ribosomal RNAs rRNAs and polypeptides depending on the organism.
However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. A ribosome is a complex macromolecule composed of catalytic rRNAs called ribozymes and structural rRNAs , as well as many distinct polypeptides.
Prokaryotes have 70S ribosomes, whereas eukaryotes have 80S ribosomes in the cytoplasm and rough endoplasmic reticulum, and 70S ribosomes in mitochondria and chloroplasts. Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation. The small subunit is responsible for binding the mRNA template, whereas the large subunit binds tRNAs discussed in the next subsection.
The complete structure containing an mRNA with multiple associated ribosomes is called a polyribosome or polysome. In both bacteria and archaea , before transcriptional termination occurs, each protein-encoding transcript is already being used to begin synthesis of numerous copies of the encoded polypeptide s because the processes of transcription and translation can occur concurrently, forming polyribosomes Figure 2.
This allows a prokaryotic cell to respond to an environmental signal requiring new proteins very quickly. In contrast, in eukaryotic cells, simultaneous transcription and translation is not possible. Although polyribosomes also form in eukaryotes, they cannot do so until RNA synthesis is complete and the RNA molecule has been modified and transported out of the nucleus.
Figure 2. In prokaryotes, multiple RNA polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate the mRNA transcripts into polypeptides.
In this way, a specific protein can rapidly reach a high concentration in the bacterial cell. Bacterial species typically have between 60 and 90 types.
Serving as adaptors, each tRNA type binds to a specific codon on the mRNA template and adds the corresponding amino acid to the polypeptide chain. 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.
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. At the end of the mRNA coding is a stop codon which will end the elongation stage. 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.
Amoeba Sisters. Parker, N. Microbiology [online]. Figure In Microbiology Section Protein synthesis. The process by which DNA is copied transcribed to mRNA in order transfer the information needed for protein synthesis. Cells which have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. 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.
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 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.
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