What kind of molecule serves as a template for translation

A set of transfer RNA tRNA molecules, each of which incorporates a particular amino acid subunit into the growing protein when it recognizes a specific group of three adjacent bases in the mRNA. DNA maintains genetic information in the nucleus. RNA takes that information into the cytoplasm, where the cell uses it to construct specific proteins, RNA synthesis is transcription; protein synthesis is translation.

RNA differs from DNA in that it is single stranded, contains Uracil instead of Thymine and ribose instead of deoxyribose, and has different functions.

The central dogma depicts RNA as a messenger between gene and protein, but does not adequately describe RNA's other function. Transcription is highly controlled and complex. In Prokaryotes, genes are expressed as required, and in multicellular organisms, specialized cell types express subsets of gene. Transcription factors recognize sequences near a gene and bind sequentially, creating a binding transcription. 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. Figure 5: An electron micrograph of a prokaryote Escherichia coli , showing DNA and ribosomes This Escherichia coli cell has been treated with chemicals and sectioned so its DNA and ribosomes are clearly visible.

The DNA appears as swirls in the center of the cell, and the ribosomes appear as dark particles at the cell periphery. Courtesy of Dr. Abraham Minsky Ribosomes are the sites in a cell in which protein synthesis takes place.

Cells have many ribosomes, and the exact number depends on how active a particular cell is in synthesizing proteins. For example, rapidly growing cells usually have a large number of ribosomes Figure 5. 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.

Eukaryotic and prokaryotic ribosomes are different from each other as a result of divergent evolution. These differences are exploited by antibiotics, which are designed to inhibit the prokaryotic ribosomes of infectious bacteria without affecting eukaryotic ribosomes, thereby not interfering with the cells of the sick host.

Figure 6: The endoplasmic reticulum of this eukaryotic cell is studded with ribosomes. Electron micrograph of a pancreatic exocrine cell section. The cytosol is filled with closely packed sheets of endoplasmic reticulum membrane studded with ribosomes. At the bottom left is a portion of the nucleus and its nuclear envelope. Image courtesy of Prof. Orci University of Geneva, Switzerland. Merging cultures in the study of membrane traffic.

Nature Cell Biology 6 , doi 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. Figure 7: The ribosome and translation A ribosome is composed of two subunits: large and small. During translation, ribosomal subunits assemble together like a sandwich on the strand of mRNA, where they proceed to attract tRNA molecules tethered to amino acids circles.

A long chain of amino acids emerges as the ribosome decodes the mRNA sequence into a polypeptide, or a new protein. 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.

Figure 8: The major steps of translation 1 Translation begins when a ribosome gray docks on a start codon red of an mRNA molecule in the cytoplasm.

A second tRNA molecule, bound to two, connected amino acids, is attached to the 4 th , 5 th , and 6 th nucleotide from the left. It no longer has amino acids bound to its terminus. In step 4, the tRNA molecule that formerly had two connected amino acids attached to its terminus, has now accumulated four amino acids total. Different colored spheres represent different amino acid types, and the four spheres are connected end-to-end in a chain.

Polypeptides are formed when the amino group of one amino acid forms an amide i. In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation. A ribosome is a complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides.

Ribosomes exist in the cytoplasm in prokaryotes and in the cytoplasm and rough endoplasmic reticulum in eukaryotes. Ribosomes are made up of two subunits. Mammalian ribosomes have a small 40S subunit and a large 60S subunit, for a total of 80S. Serving as adaptors, specific tRNAs bind to sequences on the mRNA template and add the corresponding amino acid to the polypeptide chain. Of the 64 possible mRNA codons—or triplet combinations of A, U, G, and C—three specify the termination of protein synthesis and 61 specify the addition of amino acids to the polypeptide chain.

Each tRNA anticodon can base pair with one of the mRNA codons and add an amino acid or terminate translation, according to the genetic code.