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.
Each nucleotide has an affinity for its partner: A pairs with T, and C pairs with G. In most multicellular organisms, every cell carries the same DNA, but this genetic information is used in varying ways by different types of cells. In other words, what a cell "does" within an organism dictates which of its genes are expressed. Nerve cells, for example, synthesize an abundance of chemicals called neurotransmitters, which they use to send messages to other cells, whereas muscle cells load themselves with the protein-based filaments necessary for muscle contractions.
Transcription is the first step in decoding a cell's 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.
Messenger RNA mRNA molecules carry the coding sequences for protein synthesis and are called transcripts; ribosomal RNA rRNA molecules form the core of a cell's ribosomes the structures in which protein synthesis takes place ; and transfer RNA tRNA molecules carry amino acids to the ribosomes during protein synthesis.
Other types of RNA also exist but are not as well understood, although they appear to play regulatory roles in gene expression and also be involved in protection against invading viruses. 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.
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. The coding region of each mRNA molecule is composed of codons , which are three nucleotides , representing a particular amino acid of the polypeptide chain. Each codon in the mRNA molecule is read by the anticodon of the tRNA to bring the specific amino acid to the ribosome.
The secondary structure of tRNA is a clover-leaf shape. It is composed of four loop structures known as D-loop, anticodon loop, variable loop, and T-loop. Anticodon loop is composed of a specific anticodon that scans the complement codon in the mRNA molecule. The amino acid is loaded into the CCA tail at the end of the acceptor stem. Some anticodons form base pairs with several codons by the wobble base pairing.
The secondary structure of a tRNA molecule is shown in figure 2. The ribosome is the protein-synthesizing organelle in the cell, translating the coding sequence on an mRNA molecule into a polypeptide chain. The synthesis of rRNA occurs at the nucleolus. Both rRNA molecules combine with ribosomal proteins to form a small subunit and a large subunit. The large subunit of rRNA serves as the ribozyme that catalyzes the peptide bond formation. During translation, small subunit and large subunit come together to form the ribosome.
The mRNA molecule is sandwiched between the small and large subunit. Each ribosome is composed of three binding sites for the binding of tRNA molecules. Figure 3. In eukaryotes, synthesis, cutting, and assembly of rRNA into ribosomes takes place in the nucleolus region of the nucleus, but these activities occur in the cytoplasm of prokaryotes.
Neither of these types of RNA carries instructions to direct the synthesis of a polypeptide, but they play other important roles in protein synthesis. Ribosomes are composed of rRNA and protein. The rRNA ensures the proper alignment of the mRNA, tRNA, and the ribosomes; the rRNA of the ribosome also has an enzymatic activity peptidyl transferase and catalyzes the formation of the peptide bonds between two aligned amino acids during protein synthesis.
Although rRNA had long been thought to serve primarily a structural role, its catalytic role within the ribosome was proven in Because of the importance of this work, Steitz shared the Nobel Prize in Chemistry with other scientists who made significant contributions to the understanding of ribosome structure.
It carries the correct amino acid to the site of protein synthesis in the ribosome. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to be inserted in the polypeptide chain being synthesized Figure 4. Any mutations in the tRNA or rRNA can result in global problems for the cell because both are necessary for proper protein synthesis Table 1.
Figure 4. A tRNA molecule is a single-stranded molecule that exhibits significant intracellular base pairing, giving it its characteristic three-dimensional shape. Thus, RNA clearly does have the additional capacity to serve as genetic information. Although RNA is typically single stranded within cells, there is significant diversity in viruses.
Rhinoviruses, which cause the common cold; influenza viruses; and the Ebola virus are single-stranded RNA viruses. Rotaviruses, which cause severe gastroenteritis in children and other immunocompromised individuals, are examples of double-stranded RNA viruses. Because double-stranded RNA is uncommon in eukaryotic cells, its presence serves as an indicator of viral infection.
A nucleic acid is purified from a mixture. The molecules are relatively small, contain uracil, and most are covalently bound to an amino acid.
0コメント