This page has been archived and is no longer updated. The genetic information stored in DNA is a living archive of instructions that cells use to accomplish the functions of life.
Inside each cell, catalysts seek out the appropriate information from this archive and use it to build new proteins — proteins that make up the structures of the cell, run the biochemical reactions in the cell, and are sometimes manufactured for export.
Although all of the cells that make up a multicellular organism contain identical genetic information, functionally different cells within the organism use different sets of catalysts to express only specific portions of these instructions to accomplish the functions of life. When a cell divides, it creates one copy of its genetic information — in the form of DNA molecules — for each of the two resulting daughter cells.
The accuracy of these copies determines the health and inherited features of the nascent cells, so it is essential that the process of DNA replication be as accurate as possible Figure 1.
Figure 1: DNA replication of the leading and lagging strand The helicase unzips the double-stranded DNA for replication, making a forked structure. This enzyme can work only in the 5' to 3' direction, so it replicates the leading strand continuously. Lagging-strand replication is discontinuous, with short Okazaki fragments being formed and later linked together. Molecular biology: Prime-time progress.
Nature , All rights reserved. Figure Detail. 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. 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. In biology a or genetics course, some classes may want you to take an mRNA sequence and figure out what sequence of tRNAs, and hence amino acids, it codes for.
Find the first place in the mRNA sequence where the start codon, defined as a sequence of three nucleotide genetic code, begins. So all proteins start with the amino acid methionine, known as the N-formylmethionine in bacteria.
Translate each letter of the mRNA codon into an amino acid using an amino acid table, found online or in coursework books. Remember that a tRNA essentially acts as an adapter in translation.
Letters A are always complementary to Us, and Cs are complementary to Gs. Since each codon has three bases, you'll move down the mRNA transcript three bases at a time. Write down the name of each amino acid relative to the three-letter sequence.
Notice more than one mRNA codon can code for the same amino acid. That's because the third base of the tRNA doesn't have to bond as tightly to its opposite number in the mRNA transcript as do the first two bases. The third codon position is called the wobble base-pair.
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