Chapter 6 Genes and Dna Chapter Review Answers

Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. Dna was known to exist a long polymer composed of only four types of subunits, which resemble 1 some other chemically. Early on in the 1950s, Deoxyribonucleic acid was first examined past x-ray diffraction analysis, a technique for determining the iii-dimensional atomic structure of a molecule (discussed in Chapter 8). The early on x-ray diffraction results indicated that DNA was composed of 2 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 information encoding go apparent. In this section we examine the structure of the DNA molecule and explain in general terms how information technology is able to store hereditary data.

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 equally a Dna concatenation, or a Dna strand. Hydrogen bonds between the base of operations portions of the nucleotides agree the 2 bondage together (Figure 4-3). As we saw in Affiliate 2 (Panel 2-6, pp. 120-121), nucleotides are equanimous of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the carbohydrate is deoxyribose attached to a single phosphate grouping (hence the proper name dna), and the base may exist either adenine (A), cytosine (C), guanine (G), or thymine (T). The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus course a "backbone" of alternating saccharide-phosphate-sugar-phosphate (run across Figure 4-3). Because simply the base differs in each of the four types of subunits, each polynucleotide concatenation in DNA is coordinating to a necklace (the backbone) strung with four types of chaplet (the iv bases A, C, G, and T). These aforementioned symbols (A, C, G, and T) are likewise commonly used to denote the four different nucleotides—that is, the bases with their attached sugar and phosphate groups.

Figure four-3

Deoxyribonucleic acid and its building blocks. Dna is fabricated of four types of nucleotides, which are linked covalently into a polynucleotide chain (a Deoxyribonucleic acid strand) with a carbohydrate-phosphate courage from which the bases (A, C, Yard, and T) extend. A Deoxyribonucleic acid molecule is composed of two (more...)

The way in which the nucleotide subunits are lined together gives a Deoxyribonucleic acid strand a chemic polarity. If nosotros think of each sugar as a cake with a protruding knob (the five′ phosphate) on one side and a hole (the 3′ hydroxyl) on the other (see Figure four-iii), each completed chain, formed by interlocking knobs with holes, will take all of its subunits lined upwardly in the same orientation. Moreover, the two 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 Deoxyribonucleic acid concatenation is indicated by referring to i cease every bit the 3′ finish and the other every bit the 5′ end.

The three-dimensional structure of Dna—the double helix—arises from the chemic and structural features of its 2 polynucleotide bondage. 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 exterior (see 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 G with C (Figure four-four). This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each base pair is of similar width, thus property the saccharide-phosphate backbones an equal altitude apart along the Deoxyribonucleic acid molecule. To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones current of air around each other to class a double helix, with i consummate plow every ten base pairs (Figure iv-5).

Figure four-4

Complementary base pairs in the Dna double helix. The shapes and chemical structure of the bases allow hydrogen bonds to form efficiently only betwixt A and T and between G and C, where atoms that are able to form hydrogen bonds (run across Panel two-3, pp. 114–115) (more than...)

Effigy 4-5

The DNA double helix. (A) A space-filling model of ane.five turns of the DNA double helix. Each turn of Dna is made up of ten.iv nucleotide pairs and the middle-to-center distance between next nucleotide pairs is 3.4 nm. The coiling of the ii strands around (more than...)

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, simply if the polarity of one strand is oriented opposite to that of the other strand (see Figures 4-three and 4-4). 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 DNA Provides a Mechanism for Heredity

Genes behave biological information that must be copied accurately for transmission to the next generation each time a prison cell divides to form two daughter cells. Ii central biological questions arise from these requirements: how tin can the information for specifying an organism be carried in chemical grade, 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 department, 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—A, C, T, or G—can be considered as a alphabetic character in a four-letter alphabet that spells out biological messages in the chemic structure of the DNA. Equally we saw in Chapter i, organisms differ from i some other considering their respective Dna molecules have different nucleotide sequences and, consequently, carry different biological letters. Just how is the nucleotide alphabet used to make messages, and what do they spell out?

As discussed above, information technology was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. The Deoxyribonucleic acid messages must therefore somehow encode proteins (Figure 4-6). This human relationship immediately makes the problem easier to understand, because of the chemical character of proteins. As 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 construction is determined in plow by 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 between the four-letter nucleotide alphabet of Dna and the 20-letter amino acid alphabet of proteins—the genetic lawmaking—is non obvious from the Deoxyribonucleic acid structure, and information technology took over a decade subsequently the discovery of the double helix earlier information technology was worked out. In Chapter vi we describe this code in detail in the course of elaborating the process, known as gene expression, through which a cell translates the nucleotide sequence of a cistron into the amino acrid sequence of a protein.

Effigy 4-6

The relationship between genetic information carried in Deoxyribonucleic acid and proteins.

The complete prepare of information in an organism'southward DNA is called its genome, and it carries the information for all the proteins the organism will ever synthesize. (The term genome is also used to depict the DNA that carries this information.) The corporeality of information contained in genomes is staggering: for example, a typical human cell contains ii meters of DNA. Written out in the four-letter nucleotide alphabet, the nucleotide sequence of a very small homo gene occupies a quarter of a folio of text (Figure 4-seven), while the consummate sequence of nucleotides in the human genome would fill more than than a thousand books the size of this ane. In add-on to other disquisitional information, it carries the instructions for about 30,000 distinct proteins.

Figure 4-7

The nucleotide sequence of the human β-globin gene. This gene carries the information for the amino acid sequence of one of the ii types of subunits of the hemoglobin molecule, which carries oxygen in the blood. A unlike gene, the α-globin (more...)

At each cell partition, the jail cell must copy its genome to pass information technology to both daughter cells. The discovery of the structure of Dna also 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 nosotros designate the two DNA strands as S and S′, strand Due south tin serve as a template for making a new strand S′, while strand Southward′ can serve as a template for making a new strand S (Effigy 4-viii). Thus, the genetic information in Deoxyribonucleic acid can be accurately copied by the beautifully simple procedure in which strand S separates from strand S′, and each separated strand and so serves as a template for the product of a new complementary partner strand that is identical to its sometime partner.

Figure 4-8

DNA every bit a template for its own duplication. As the nucleotide A successfully pairs but with T, and G with C, each strand of Dna can specify the sequence of nucleotides in its complementary strand. In this way, double-helical Dna can be copied precisely. (more...)

The ability of each strand of a Dna molecule to deed as a template for producing a complementary strand enables a jail cell to copy, or replicate, its genes before passing them on to its descendants. In the next affiliate nosotros describe the elegant machinery the jail cell uses to perform this enormous chore.

In Eucaryotes, DNA Is Enclosed in a Cell Nucleus

Almost 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 nuclear envelope formed by ii concentric lipid bilayer membranes that are punctured at intervals by large nuclear pores, which transport molecules between the nucleus and the cytosol. The nuclear envelope is straight connected to the all-encompassing membranes of the endoplasmic reticulum. It is mechanically supported past two networks of intermediate filaments: one, chosen the nuclear lamina, forms a sparse sheetlike meshwork within the nucleus, but beneath the inner nuclear membrane; the other surrounds the outer nuclear membrane and is less regularly organized (Figure 4-9).

Figure 4-9

A cross-sectional view of a typical cell nucleus. The nuclear envelope consists of ii membranes, the outer one being continuous with the endoplasmic reticulum membrane (run into likewise Figure 12-9). The space within the endoplasmic reticulum (the ER lumen) (more...)

The nuclear envelope allows the many proteins that human action on Dna to be concentrated where they are needed in the cell, and, as we meet in subsequent capacity, it besides keeps nuclear and cytosolic enzymes separate, a characteristic that is crucial for the proper performance of eucaryotic cells. Compartmentalization, of which the nucleus is an instance, is an important principle of biology; it serves to constitute an surround in which biochemical reactions are facilitated by the loftier concentration of both substrates and the enzymes that deed on them.

Summary

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 by hydrogen bonds between M-C and A-T base pairs. Duplication of the genetic data occurs by the apply of 1 Deoxyribonucleic acid strand as a template for formation of a complementary strand. The genetic information stored in an organism's DNA contains the instructions for all the proteins the organism volition ever synthesize. In eucaryotes, Deoxyribonucleic acid is contained in the cell nucleus.

davidsonkillaimpon.blogspot.com

Source: https://www.ncbi.nlm.nih.gov/books/NBK26821/

Share this post