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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. Fourth edition. New York: Garland Science; 2002.

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Biologists in the 1940s had difficulty in agree DNA as the genetic material because of the evident simplicity that its chemistry. DNA was recognized to be a long polymer created of just four varieties of subunits, i m sorry resemble one one more chemically. At an early stage in the 1950s, DNA was an initial examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a molecule (discussed in chapter 8). The at an early stage x-ray diffraction results indicated that DNA was created of 2 strands the the polymer wound right into a helix. The observation that DNA to be double-stranded to be of vital significance and noted one the the major clues that led to the Watson-Crick structure of DNA. Only when this design was proposed go DNA\"s potential for replication and also information encoding come to be apparent. In this ar we study the framework of the DNA molecule and also explain in general terms exactly how it is may be to save hereditary information.

A DNA Molecule consists of two Complementary chains of Nucleotides

A DNA molecule is composed of two lengthy polynucleotide chains written of four types of nucleotide subunits. Every of this chains is known as a DNA chain, or a DNA strand. Hydrogen bonds in between the base sections of the nucleotides organize the two chains together (Figure 4-3). As we saw in chapter 2 (Panel 2-6, pp. 120-121), nucleotides room composed that a five-carbon street to which space attached one or much more phosphate groups and also a nitrogen-containing base. In the case of the nucleotides in DNA, the street is deoxyribose attached come a solitary phosphate team (hence the name deoxyribonucleic acid), and the base might be either adenine (A), cytosine (C), guanine (G), or thymine (T). The nucleotides space covalently linked together in a chain with the sugars and also phosphates, i beg your pardon thus form a “backbone” of alternate sugar-phosphate-sugar-phosphate (see number 4-3). Due to the fact that only the base differs in every of the four varieties of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung v four species of beads (the 4 bases A, C, G, and also T). These same symbols (A, C, G, and T) are additionally commonly provided to denote the four different nucleotides—that is, the bases v their attached sugar and phosphate groups.


Figure 4-3

DNA and its structure blocks. DNA is make of four types of nucleotides, i beg your pardon are connected covalently into a polynucleotide chain (a DNA strand) through a sugar-phosphate backbone from which the bases (A, C, G, and also T) extend. A DNA molecule is created of two (more...)

The way in i m sorry the nucleotide subunits space lined together offers a DNA strand a chemistry polarity. If we think of every sugar together a block v a protruding knob (the 5′ phosphate) on one side and a feet (the 3′ hydroxyl) ~ above the various other (see number 4-3), every completed chain, formed by interlocking knobs through holes, will have every one of its subunits inside wall up in the very same orientation. Moreover, the two ends that the chain will be quickly distinguishable, as one has actually a feet (the 3′ hydroxyl) and also the various other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated by referring to one finish as the 3′ end and the other as the 5′ end.

The three-dimensional framework of DNA—the twin helix—arises indigenous the chemical and also structural functions of its 2 polynucleotide chains. Because these 2 chains are hosted together through hydrogen bonding in between the bases ~ above the various strands, all the bases are on the inside of the twin helix, and also the sugar-phosphate backbones room on the external (see figure 4-3). In every case, a bulkier two-ring basic (a purine; view Panel 2-6, pp. 120–121) is paired with a single-ring base (a pyrimidine); A constantly pairs v T, and G with C (Figure 4-4). This security base-pairing enables the basic pairs come be pack in the energetically many favorable setup in the inner of the twin helix. In this arrangement, each base pair is of comparable width, for this reason holding the sugar-phosphate backbones an equal street apart along the DNA molecule. To maximize the performance of base-pair packing, the two sugar-phosphate backbones wind around each various other to kind a dual helix, v one complete turn every ten base pairs (Figure 4-5).


Figure 4-4

Complementary base pairs in the DNA twin helix. The shapes and also chemical framework of the bases allow hydrogen binding to form efficiently only in between A and T and between G and C, wherein atoms that room able to type hydrogen bond (see dashboard 2-3, pp. 114–115) (more...)


Figure 4-5

The DNA dual helix. (A) A space-filling version of 1.5 turns of the DNA double helix. Each revolve of DNA is comprised of 10.4 nucleotide pairs and also the center-to-center distance between nearby nucleotide bag is 3.4 nm. The coiling that the 2 strands roughly (more...)

The members of each base pair can fit together within the twin helix just if the two strands the the helix are antiparallel—that is, just if the polarity the one strand is oriented the opposite to the of the various other strand (see numbers 4-3 and 4-4). A an effect of this base-pairing needs is the each strand the a DNA molecule has a succession of nucleotides the is specifically complementary come the nucleotide sequence of its partner strand.

The framework of DNA provides a device for Heredity

Genes carry organic information that must be copied accurately for transmission to the following generation every time a cell divides to type two daughter cells. Two central biological concerns arise from these requirements: how can the details for point out an biology be brought in chemical form, and how is that accurately copied? The discovery of the structure of the DNA double helix to be a landmark in twentieth-century biology because it immediately argued answers come both questions, thereby solving at the molecule level the trouble of heredity. We talk about briefly the answers come these questions in this section, and we shall examine them in much more detail in succeeding chapters.

DNA encodes information through the order, or sequence, that the nucleotides follow me each strand. Every base—A, C, T, or G—can be considered as a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the DNA. As we witnessed in thing 1, organisms different from one another because their particular DNA molecule have different nucleotide assignment and, consequently, lug different biological messages. But how is the nucleotide alphabet used to do messages, and also what carry out they assignment out?

As disputed above, the was well-known well prior to the structure of DNA was figured out that genes contain the accuse for producing proteins. The DNA messages must therefore somehow encode proteins (Figure 4-6). This relationship automatically makes the difficulty easier come understand, due to the fact that of the chemical character that proteins. As debated in thing 3, the properties of a protein, which are responsible because that its biological function, are figured out by that is three-dimensional structure, and also its framework is established in turn by the linear sequence that the amino mountain of which that is composed. The direct sequence the nucleotides in a gene must therefore somehow spell the end the straight sequence that amino acids in a protein. The specific correspondence between the four-letter nucleotide alphabet that DNA and the twenty-letter amino acid alphabet that proteins—the genetic code—is not apparent from the DNA structure, and also it took over a decade after the discovery of the double helix before it was operated out. In thing 6 we describe this password in detail in the course of elaborating the process, recognized as gene expression, through which a cell converts the nucleotide sequence of a gene right into the amino acid sequence that a protein.


The complete set of details in an organism\"s DNA is dubbed its genome, and also it carries the information for all the proteins the organism will ever synthesize. (The ax genome is additionally used to describe the DNA the carries this information.) The lot of information contained in genomes is staggering: because that example, a usual human cell contains 2 meters of DNA. Created out in the four-letter nucleotide alphabet, the nucleotide succession of a very little human gene rectal a quarter of a web page of message (Figure 4-7), while the complete sequence that nucleotides in the human genome would fill an ext than a thousand publications the dimension of this one. In addition to other an essential information, that carries the instructions for about 30,000 distinct proteins.

Figure 4-7

The nucleotide succession of the human β-globin gene. This gene carries the details for the amino acid sequence of among the two types of subunits that the hemoglobin molecule, i beg your pardon carries oxygen in the blood. A various gene, the α-globin (more...)

At each cell division, the cell need to copy the genome to pass it to both daughter cells. The discovery of the framework of DNA likewise revealed the principle that provides this copy possible: because each strand the DNA contains a succession of nucleotides the is exactly complementary to the nucleotide succession of its partner strand, each strand have the right to act as a template, or mold, because that the synthetic of a new complementary strand. In various other words, if we designate the 2 DNA strands as S and S′, strand S deserve to serve as a layout for do a brand-new strand S′, when strand S′ can serve as a theme for make a new strand S (Figure 4-8). Thus, the hereditary information in DNA deserve to be accurately copied by the beautifully simple process in i m sorry strand S separates native strand S′, and also each be separated strand then serves as a theme for the manufacturing of a new complementary companion strand that is identical to its former partner.

Figure 4-8

DNA together a layout for its own duplication. Together the nucleotide A efficiently pairs just with T, and also G with C, each strand the DNA have the right to specify the succession of nucleotides in its security strand. In this way, double-helical DNA have the right to be copied precisely. (more...)

The capacity of every strand of a DNA molecule come act as a design template for developing a complementary strand enables a cell to copy, or replicate, that genes before passing castle on to its descendants. In the following chapter we explain the elegant machine the cell supplies to do this huge task.

In Eucaryotes, DNA Is enclosed in a cabinet Nucleus

Nearly every the DNA in a eucaryotic cell is sequestered in a nucleus, which occupies about 10% that the full cell volume. This compartment is delimited by a atom envelope developed by 2 concentric lipid double layer membranes that room punctured in ~ intervals by huge nuclear pores, which transfer molecules in between the nucleus and also the cytosol. The atom envelope is directly connected to the extensive membranes that the endoplasmic reticulum. It is mechanically sustained by two networks of intermediate filaments: one, dubbed the nuclear lamina, forms a slim sheetlike meshwork within the nucleus, simply beneath the inner nuclear membrane; the other surrounds the external nuclear membrane and also is much less regularly organized (Figure 4-9).

Figure 4-9

A cross-sectional watch of a typical cell nucleus. The nuclear envelope consists of 2 membranes, the outer one being continuous with the absorbent reticulum membrane (see also Figure 12-9). The an are inside the endoplasmic reticulum (the ER lumen) (more...)

The atom envelope permits the countless proteins the act top top DNA come be concentrated where lock are necessary in the cell, and, together we watch in subsequent chapters, it additionally keeps nuclear and cytosolic enzyme separate, a function that is vital for the proper functioning of eucaryotic cells. Compartmentalization, of i beg your pardon the nucleus is an example, is vital principle the biology; it offer to develop an setting in i m sorry biochemical reactions are helped with by the high concentration of both substrates and also the enzymes that act top top them.


Genetic details is carried in the linear sequence the nucleotides in DNA. Every molecule that DNA is a dual helix created from 2 complementary strands that nucleotides hosted together by hydrogen bonds between G-C and A-T basic pairs. Duplication that the genetic information wake up by the use of one DNA strand together a theme for formation of a security strand. The genetic information save on computer in an organism\"s DNA has the accuse for all the protein the organism will ever before synthesize. In eucaryotes, DNA is contained in the cell nucleus.

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