Dna is Composed of Repeating Structural Units Called
Biologists in the 1940s had difficulty in accepting Dna every bit the genetic material because of the apparent simplicity of its chemistry. Deoxyribonucleic acid was known to exist a long polymer composed of just iv types of subunits, which resemble one another chemically. Early on in the 1950s, Dna was start examined past x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a molecule (discussed in Chapter viii). The early on x-ray diffraction results indicated that DNA was composed of ii strands of the polymer wound into a helix. The observation that DNA was double-stranded was of crucial significance and provided ane of the major clues that led to the Watson-Crick structure of Dna. Only when this model was proposed did DNA’s potential for replication and data encoding get credible. In this section we examine the structure of the Deoxyribonucleic acid molecule and explain in general terms how it is able to store hereditary information.
A DNA Molecule Consists of Ii Complementary Chains of Nucleotides
A Deoxyribonucleic acid molecule consists of two long polynucleotide chains composed of iv types of nucleotide subunits. Each of these chains is known as a
Deoxyribonucleic acid chain, or a
betwixt the base portions of the nucleotides hold the two bondage together (Figure 4-three). As we saw in Chapter 2 (Panel 2-6, pp. 120-121), nucleotides are composed of a 5-carbon sugar to which are fastened one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acrid), and the base of operations may exist either
adenine (A), cytosine (C), guanine (G),
thymine (T). The nucleotides are covalently linked together in a concatenation through the sugars and phosphates, which thus form a “courage” of alternating carbohydrate-phosphate-sugar-phosphate (see Figure iv-3). Considering merely the base differs in each of the four types of subunits, each polynucleotide chain in Dna is analogous to a necklace (the backbone) strung with 4 types of chaplet (the four bases A, C, Thousand, and T). These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides—that is, the bases with their fastened saccharide and phosphate groups.
The style in which the nucleotide subunits are lined together gives a Deoxyribonucleic acid strand a chemic polarity. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on i side and a hole (the 3′ hydroxyl) on the other (see Figure four-iii), each completed chain, formed by interlocking knobs with holes, volition have all of its subunits lined up in the same orientation. Moreover, the 2 ends of the concatenation will be easily distinguishable, every bit one has a pigsty (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA concatenation is indicated by referring to one finish every bit the
and the other every bit the
The 3-dimensional structure of Deoxyribonucleic acid—the double helix—arises from the chemical and structural features of its ii polynucleotide chains. Because these two chains are held together by hydrogen bonding betwixt the bases on the different strands, all the bases are on the inside of the double helix, and the saccharide-phosphate backbones are on the outside (meet Figure four-3). In each case, a bulkier 2-band base (a purine; meet Console ii-6, pp. 120–121) is paired with a single-ring base (a pyrimidine); A always pairs with T, and G with C (Figure 4-4). This
enables the base of operations pairs to be packed in the energetically most favorable system in the interior of the double helix. In this organization, each base pair is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the Deoxyribonucleic acid molecule. To maximize the efficiency of base-pair packing, the 2 sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs (Figure 4-5).
The members of each base pair can fit together inside the double helix simply if the 2 strands of the helix are antiparallel—that is, only if the polarity of one strand is oriented opposite to that of the other strand (see Figures 4-three and four-4). A issue of these base of operations-pairing requirements is that each strand of a Deoxyribonucleic acid molecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand.
The Structure of Dna Provides a Machinery for Heredity
Genes acquit biological information that must be copied accurately for manual to the adjacent generation each time a cell divides to form ii daughter cells. Two central biological questions arise from these requirements: how can the information for specifying an organism be carried in chemical form, and how is it accurately copied? The discovery of the structure of the Deoxyribonucleic acid double helix was a landmark in twentieth-century biological science because it immediately suggested answers to both questions, thereby resolving at the molecular level the trouble of heredity. Nosotros discuss briefly the answers to these questions in this department, and we shall examine them in more item in subsequent chapters.
Dna encodes information through the order, or sequence, of the nucleotides along each strand. Each base—A, C, T, or 1000—tin be considered equally a letter in a four-alphabetic character alphabet that spells out biological messages in the chemical structure of the DNA. As nosotros saw in Chapter ane, organisms differ from one another because their respective DNA molecules have different nucleotide sequences and, consequently, carry different biological messages. But how is the nucleotide alphabet used to make messages, and what do they spell out?
As discussed above, it was known well before the construction of DNA was determined that genes contain the instructions for producing proteins. The Dna messages must therefore somehow encode proteins (Figure 4-6). This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. Equally discussed in Chapter 3, the properties of a protein, which are responsible for its biological function, are determined by its three-dimensional structure, and its structure is determined in turn past the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a gene must therefore somehow spell out the linear sequence of amino acids in a protein. The verbal correspondence between the four-letter nucleotide alphabet of Dna and the twenty-letter amino acrid alphabet of proteins—the genetic code—is non obvious from the Dna structure, and it took over a decade afterwards the discovery of the double helix before information technology was worked out. In Affiliate half-dozen we describe this code in detail in the course of elaborating the process, known as
factor expression, through which a cell translates the nucleotide sequence of a gene into the amino acrid sequence of a protein.
The complete gear up of information in an organism’s Dna is called its genome, and information technology carries the information for all the proteins the organism will ever synthesize. (The term genome is as well used to describe the DNA that carries this information.) The amount of information contained in genomes is staggering: for example, a typical human cell contains two meters of Deoxyribonucleic acid. Written out in the iv-letter nucleotide alphabet, the nucleotide sequence of a very small human gene occupies a quarter of a page of text (Figure 4-vii), while the complete sequence of nucleotides in the homo genome would fill up more than a thousand books the size of this one. In addition to other critical data, it carries the instructions for about 30,000 singled-out proteins.
At each cell division, the cell must copy its genome to laissez passer it to both girl cells. The discovery of the structure of DNA also revealed the principle that makes this copying possible: considering each strand of Deoxyribonucleic acid contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand, each strand can act as a template, or mold, for the synthesis of a new complementary strand. In other words, if nosotros designate the two Dna strands as S and S′, strand S tin can serve as a template for making a new strand South′, while strand Southward′ tin can serve as a template for making a new strand Southward (Figure 4-8). Thus, the genetic information in DNA tin be accurately copied by the beautifully simple process in which strand S separates from strand S′, and each separated strand then serves as a template for the production of a new complementary partner strand that is identical to its sometime partner.
The power of each strand of a DNA molecule to human action as a template for producing a complementary strand enables a cell to copy, or
replicate, its genes before passing them on to its descendants. In the next affiliate we depict the elegant machinery the cell uses to perform this enormous task.
In Eucaryotes, DNA Is Enclosed in a Prison cell Nucleus
Nearly all the Deoxyribonucleic acid in a eucaryotic cell is sequestered in a nucleus, which occupies about 10% of the total cell volume. This compartment is delimited by a
formed by two concentric lipid bilayer membranes that are punctured at intervals past large nuclear pores, which transport molecules betwixt the nucleus and the cytosol. The nuclear envelope is directly connected to the extensive membranes of the endoplasmic reticulum. It is mechanically supported by two networks of intermediate filaments: one, called the
nuclear lamina, forms a thin sheetlike meshwork within the nucleus, but beneath the inner nuclear membrane; the other surrounds the outer nuclear membrane and is less regularly organized (Effigy 4-nine).
The nuclear envelope allows the many proteins that act on DNA to exist concentrated where they are needed in the cell, and, equally nosotros encounter in subsequent chapters, information technology as well keeps nuclear and cytosolic enzymes separate, a feature that is crucial for the proper functioning of eucaryotic cells. Compartmentalization, of which the nucleus is an example, is an important principle of biology; information technology serves to plant an environment in which biochemical reactions are facilitated by the high concentration of both substrates and the enzymes that human activity on them.
Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of DNA is a double helix formed from two complementary strands of nucleotides held together past hydrogen bonds between G-C and A-T base of operations pairs. Duplication of the genetic information occurs by the use of one DNA strand as a template for germination of a complementary strand. The genetic information stored in an organism’s Deoxyribonucleic acid contains the instructions for all the proteins the organism will ever synthesize. In eucaryotes, Deoxyribonucleic acid is contained in the cell nucleus.
Dna is Composed of Repeating Structural Units Called