An Organism’s Body is Constructed Using the Information Contained in

A DNA Molecule Consists of Ii Complementary Chains of Nucleotides

A Dna molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Each of these chains is known as a
Dna concatenation, or a
DNA strand.
Hydrogen bonds
between the base of operations portions of the nucleotides concur the ii chains together (Figure 4-3). As we saw in Affiliate 2 (Panel 2-six, pp. 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more than phosphate groups and a nitrogen-containing base. In the case of the nucleotides in Dna, the sugar is deoxyribose attached to a single phosphate grouping (hence the name deoxyribonucleic acid), and the base may be either
adenine (A), cytosine (C), guanine (G),
or
thymine (T). The nucleotides are covalently linked together in a concatenation through the sugars and phosphates, which thus class a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure iv-three). Because only the base of operations differs in each of the four types of subunits, each polynucleotide concatenation in DNA is coordinating to a necklace (the courage) strung with 4 types of beads (the four bases A, C, G, and T). These same symbols (A, C, Yard, and T) are too ordinarily used to denote the iv unlike nucleotides—that is, the bases with their fastened sugar and phosphate groups.

The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. If we think of each sugar as a cake with a protruding knob (the 5′ phosphate) on one side and a pigsty (the 3′ hydroxyl) on the other (see Figure iv-3), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. Moreover, the 2 ends of the chain volition be easily distinguishable, as one has a pigsty (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated past referring to one stop equally the
3′
stop
and the other equally the
v′
end.

The three-dimensional structure of DNA—the double helix—arises from the chemical and structural features of its two polynucleotide chains. Because these 2 chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the within of the double helix, and the saccharide-phosphate backbones are on the outside (run into Figure 4-3). In each case, a bulkier two-ring base of operations (a purine; come across Console 2-6, pp. 120–121) is paired with a single-band base (a pyrimidine); A always pairs with T, and Thou with C (Figure four-4). This
complementary base of operations-pairing
enables the base pairs to be packed in the energetically nearly favorable arrangement in the interior of the double helix. In this arrangement, each base pair is of similar width, thus holding the sugar-phosphate backbones an equal distance apart forth the Deoxyribonucleic acid molecule. To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with ane complete turn every 10 base of operations pairs (Figure 4-5).

The members of each base of operations pair tin can fit together inside the double helix simply if the two 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 4-3 and 4-four). A result of these base-pairing requirements is that each strand of a Dna molecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand.

The Structure of Deoxyribonucleic acid Provides a Mechanism for Heredity

Genes carry biological information that must be copied accurately for transmission to the next generation each time a jail cell divides to form two girl cells. Two central biological questions ascend from these requirements: how can the information for specifying an organism be carried in chemical course, and how is it accurately copied? The discovery of the structure of the Deoxyribonucleic acid double helix was a landmark in twentieth-century biology because it immediately suggested answers to both questions, thereby resolving at the molecular level the problem of heredity. We discuss briefly the answers to these questions in this section, and we shall examine them in more particular in subsequent chapters.

DNA encodes information through the order, or sequence, of the nucleotides along each strand. Each base of operations—A, C, T, or G—can be considered equally a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the Deoxyribonucleic acid. As we saw in Affiliate ane, organisms differ from one another because their respective DNA molecules have dissimilar nucleotide sequences and, consequently, acquit different biological messages. But how is the nucleotide alphabet used to brand messages, and what do they spell out?

As discussed above, it was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. The Dna letters must therefore somehow encode proteins (Figure 4-six). This relationship immediately makes the problem easier to empathize, because of the chemical grapheme of proteins. As discussed in Chapter iii, the properties of a protein, which are responsible for its biological function, are determined past its iii-dimensional structure, and its construction 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 exact correspondence betwixt the 4-letter nucleotide alphabet of DNA and the twenty-letter amino acid alphabet of proteins—the genetic code—is not obvious from the Dna structure, and it took over a decade later the discovery of the double helix before it was worked out. In Chapter half dozen we describe this code in detail in the form of elaborating the process, known equally
gene expression, through which a prison cell translates the nucleotide sequence of a factor into the amino acid sequence of a protein.

The consummate fix of information in an organism’s Dna is chosen its genome, and it carries the data for all the proteins the organism volition ever synthesize. (The term genome is also used to describe the DNA that carries this information.) The corporeality of data contained in genomes is staggering: for example, a typical human prison cell contains 2 meters of Deoxyribonucleic acid. Written out in the iv-alphabetic character nucleotide alphabet, the nucleotide sequence of a very small human gene occupies a quarter of a page of text (Figure four-7), while the complete sequence of nucleotides in the human genome would fill more than than a thousand books the size of this i. In addition to other disquisitional information, information technology carries the instructions for about 30,000 distinct proteins.

At each cell division, the cell must copy its genome to pass it to both girl cells. The discovery of the structure of DNA besides revealed the principle that makes this copying possible: because each strand of DNA contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand, each strand can deed as a template, or mold, for the synthesis of a new complementary strand. In other words, if nosotros designate the two DNA strands as Due south and S′, strand S can serve as a template for making a new strand S′, while strand South′ tin serve every bit a template for making a new strand Due south (Figure four-8). Thus, the genetic information in DNA can be accurately copied by the beautifully simple procedure in which strand S separates from strand S′, and each separated strand then serves equally a template for the production of a new complementary partner strand that is identical to its former partner.

The ability of each strand of a DNA molecule to human action as a template for producing a complementary strand enables a prison cell to copy, or
replicate, its genes before passing them on to its descendants. In the next chapter we depict the elegant machinery the cell uses to perform this enormous task.

In Eucaryotes, DNA Is Enclosed in a Cell Nucleus

Nearly all the Dna in a eucaryotic cell is sequestered in a nucleus, which occupies nigh ten% of the total cell volume. This compartment is delimited past a
nuclear envelope
formed by two concentric lipid bilayer membranes that are punctured at intervals past large nuclear pores, which transport molecules between the nucleus and the cytosol. The nuclear envelope is direct continued 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, just beneath the inner nuclear membrane; the other surrounds the outer nuclear membrane and is less regularly organized (Figure four-9).

The nuclear envelope allows the many proteins that act on Deoxyribonucleic acid to be full-bodied where they are needed in the cell, and, as nosotros see in subsequent chapters, it also keeps nuclear and cytosolic enzymes split, a characteristic that is crucial for the proper operation of eucaryotic cells. Compartmentalization, of which the nucleus is an case, is an important principle of biology; it serves to establish an surroundings in which biochemical reactions are facilitated by the loftier concentration of both substrates and the enzymes that human action on them.

Summary

Genetic data 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 by hydrogen bonds between G-C and A-T base of operations pairs. Duplication of the genetic information occurs by the use of ane Dna strand as a template for germination of a complementary strand. The genetic information stored in an organism’southward Dna contains the instructions for all the proteins the organism will ever synthesize. In eucaryotes, DNA is contained in the cell nucleus.