Biologists in the 1940s had difficulty in accepting Dna as the genetic material because of the apparent simplicity of its chemical science. Dna was known to be a long polymer equanimous of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined past x-ray diffraction assay, a technique for determining the iii-dimensional atomic structure of a molecule (discussed in Affiliate viii). The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. The ascertainment that DNA was double-stranded was of crucial significance and provided one 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 information encoding become apparent. In this section nosotros examine the construction of the Deoxyribonucleic acid molecule and explicate in general terms how information technology is able to store hereditary information.
A DNA Molecule Consists of Two Complementary Chains of Nucleotides
A DNA molecule consists of two long polynucleotide chains equanimous 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 2 bondage together (Figure iv-3). Equally we saw in Chapter 2 (Console 2-half-dozen, pp. 120-121), nucleotides are composed of a five-carbon saccharide to which are attached 1 or more than phosphate groups and a nitrogen-containing base of operations. In the example of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name dna), and the base may be either
adenine (A), cytosine (C), guanine (G),
thymine (T). The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating carbohydrate-phosphate-sugar-phosphate (come across Effigy 4-iii). 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 backbone) strung with 4 types of beads (the 4 bases A, C, G, and T). These same symbols (A, C, M, and T) are too commonly used to denote the four different nucleotides—that is, the bases with their fastened carbohydrate and phosphate groups.
The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. If nosotros think of each sugar equally a block with a protruding knob (the 5′ phosphate) on one side and a hole (the three′ hydroxyl) on the other (run across Figure 4-3), each completed chain, formed by interlocking knobs with holes, volition have all of its subunits lined upwards in the same orientation. Moreover, the two ends of the chain will be easily distinguishable, as one has a pigsty (the iii′ hydroxyl) and the other a knob (the v′ phosphate) at its terminus. This polarity in a DNA chain is indicated past referring to one finish every bit the
and the other as the
The three-dimensional structure of Dna—the double helix—arises from the chemical and structural features of its two polynucleotide chains. Considering these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside (run across Figure 4-3). In each case, a bulkier two-ring base (a purine; see Panel 2-6, pp. 120–121) is paired with a single-ring base (a pyrimidine); A always pairs with T, and K with C (Figure 4-4). 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 like width, thus holding the sugar-phosphate backbones an equal distance apart forth the Deoxyribonucleic acid molecule. To maximize the efficiency of base of operations-pair packing, the two sugar-phosphate backbones wind effectually each other to course a double helix, with ane complete turn every 10 base pairs (Figure 4-5).
The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel—that is, only if the polarity of 1 strand is oriented opposite to that of the other strand (see Figures 4-3 and four-four). A consequence of these base-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 Deoxyribonucleic acid Provides a Mechanism for Heredity
Genes deport biological information that must be copied accurately for transmission to the next generation each time a cell divides to course two girl cells. Two key biological questions arise from these requirements: how can the data for specifying an organism exist carried in chemical grade, and how is information technology accurately copied? The discovery of the structure of the Dna 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 hash out briefly the answers to these questions in this section, and nosotros shall examine them in more detail in subsequent chapters.
DNA encodes information through the gild, or sequence, of the nucleotides forth each strand. Each base of operations—A, C, T, or Chiliad—can be considered as a letter of the alphabet in a four-letter alphabet that spells out biological messages in the chemical construction of the DNA. As we saw in Chapter one, organisms differ from one another considering their respective Deoxyribonucleic acid molecules have different nucleotide sequences and, consequently, conduct dissimilar biological messages. Merely how is the nucleotide alphabet used to make messages, and what do they spell out?
As discussed to a higher place, it was known well before the structure of DNA was determined that genes incorporate the instructions for producing proteins. The DNA messages must therefore somehow encode proteins (Figure 4-half-dozen). This relationship immediately makes the trouble easier to sympathise, because of the chemical character of proteins. As discussed in Affiliate 3, the backdrop of a poly peptide, which are responsible for its biological function, are determined past its three-dimensional structure, and its structure is adamant in turn by the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a cistron must therefore somehow spell out the linear sequence of amino acids in a protein. The exact correspondence between the iv-letter nucleotide alphabet of Dna and the twenty-letter of the alphabet amino acid alphabet of proteins—the genetic lawmaking—is not obvious from the DNA structure, and information technology took over a decade afterwards the discovery of the double helix earlier it was worked out. In Chapter half-dozen nosotros draw this code in detail in the course of elaborating the process, known as
cistron expression, through which a prison cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein.
The complete set of information in an organism’s DNA is called its genome, and it carries the data for all the proteins the organism will ever synthesize. (The term genome is likewise used to describe the Deoxyribonucleic acid that carries this information.) The corporeality of information contained in genomes is staggering: for example, a typical human being cell contains 2 meters of Dna. Written out in the four-letter nucleotide alphabet, the nucleotide sequence of a very small human cistron 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 a grand books the size of this 1. In add-on to other critical data, information technology carries the instructions for about 30,000 distinct proteins.
At each cell sectionalization, the cell must copy its genome to pass it to both daughter cells. The discovery of the structure of Dna as well 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 act as a template, or mold, for the synthesis of a new complementary strand. In other words, if we designate the two Dna strands as S and Southward′, strand Due south can serve as a template for making a new strand South′, while strand S′ tin serve equally a template for making a new strand South (Figure four-8). Thus, the genetic information in Dna can be accurately copied by the beautifully simple procedure in which strand Due south separates from strand South′, and each separated strand so serves as a template for the production of a new complementary partner strand that is identical to its quondam partner.
The ability of each strand of a DNA molecule to act equally a template for producing a complementary strand enables a cell to re-create, or
replicate, its genes before passing them on to its descendants. In the next affiliate we describe the elegant machinery the prison cell uses to perform this enormous task.
In Eucaryotes, Dna Is Enclosed in a Jail cell Nucleus
Near all the Dna in a eucaryotic cell is sequestered in a nucleus, which occupies about 10% of the full 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 send molecules between the nucleus and the cytosol. The nuclear envelope is directly connected to the all-encompassing 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 inside the nucleus, just beneath the inner nuclear membrane; the other surrounds the outer nuclear membrane and is less regularly organized (Effigy 4-ix).
The nuclear envelope allows the many proteins that act on DNA to exist concentrated where they are needed in the cell, and, as we run into in subsequent chapters, it too keeps nuclear and cytosolic enzymes split, a feature that is crucial for the proper operation of eucaryotic cells. Compartmentalization, of which the nucleus is an example, is an important principle of biology; it serves to establish an surround in which biochemical reactions are facilitated by the high concentration of both substrates and the enzymes that act on them.
Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of Dna is a double helix formed from 2 complementary strands of nucleotides held together by hydrogen bonds between G-C and A-T base pairs. Duplication of the genetic information occurs by the use of one Deoxyribonucleic acid strand as a template for germination of a complementary strand. The genetic information stored in an organism’s DNA contains the instructions for all the proteins the organism will always synthesize. In eucaryotes, Dna is contained in the cell nucleus.
What Makes Dna a Good Molecule to Store Information