Dna Structure And Function Study Guide Answer
Knowing what this genetic molecule looked like would provide much insight into how it worked because in biology, structure and function are very closely related.
- Dna Structure And Function Study Guide Answers
- Chapter 13 Dna Structure And Function Study Guide Answers
Biologists in the 1940s had difficulty in accepting as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a long composed of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a (discussed in Chapter 8). The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. The observation 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 we examine the structure of the DNA molecule and explain in general terms how it is able to store hereditary information. A DNA Molecule Consists of Two Complementary Chains of Nucleotides A consists of two long polynucleotide chains composed of four types of subunits. Each of these chains is known as a DNA chain, or a DNA strand. Hydrogen bonds between the portions of the nucleotides hold the two chains together. As we saw in Chapter 2 (, pp. 120-121), nucleotides are composed of a five-carbon to which are attached 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 ), and the base may be either adenine (A), cytosine (C), guanine , or thymine (T).
The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see ). Because only 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 four types of beads (the four bases A, C, G, 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 attached sugar and phosphate groups.
DNA and its building blocks. DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. A DNA molecule is composed of two The way in which the subunits are lined together gives a strand a chemical polarity. If we think of each as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ ) on the other (see ), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated by referring to one end as the 3′ end and the other as the 5′ end.
The three-dimensional structure of — the —arises from the chemical and structural features of its two polynucleotide chains. Because 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 -phosphate backbones are on the outside (see ). In each case, a bulkier two-ring (a; see, pp. 120–121) is paired with a single-ring base (a ); A always pairs with T, and with C. This base-pairing enables the to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA. To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs.
The DNA double helix. (A) A space-filling model of 1.5 turns of the DNA double helix. Each turn of DNA is made up of 10.4 nucleotide pairs and the center-to-center distance between adjacent nucleotide pairs is 3.4 nm. The coiling of the two strands around The members of each can fit together within the only if the two strands of the helix are —that is, only if the polarity of one strand is oriented opposite to that of the other strand (see and ). A consequence of these base-pairing requirements is that each strand of a contains a sequence of nucleotides that is exactly to the sequence of its partner strand. The Structure of DNA Provides a Mechanism for Heredity Genes carry biological information that must be copied accurately for transmission to the next generation each time a cell divides to form two 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 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, and we shall examine them in more detail in subsequent chapters. Encodes information through the order, or sequence, of the nucleotides along each strand.
Dna Structure And Function Study Guide Answers
Each —A, C, T, or —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 saw in Chapter 1, organisms differ from one another because their respective DNA molecules have different 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 structure of was that genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins.
This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. As discussed in Chapter 3, the properties of a, which are responsible for its biological function, are determined by its three-dimensional structure, and its structure is determined in turn by the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a must therefore somehow spell out the linear sequence of amino acids in a protein.
The exact correspondence between the four-letter alphabet of DNA and the twenty-letter alphabet of proteins—the —is not obvious from the DNA structure, and it took over a decade after the discovery of the before it was worked out. In Chapter 6 we describe this code in detail in the course of elaborating the process, known as gene, through which a cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein.
The relationship between genetic information carried in DNA and proteins. The complete set of information in an organism's is called its, and it carries the information for all the proteins the organism will ever synthesize. (The term is also 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 2 meters of DNA. Written out in the four-letter alphabet, the nucleotide sequence of a very small human occupies a quarter of a page of text , while the complete sequence of nucleotides in the human genome would fill more than a thousand books the size of this one. In addition to other critical information, it carries the instructions for about 30,000 distinct proteins. The nucleotide sequence of the human β-globin gene.
This gene carries the information for the amino acid sequence of one of the two types of subunits of the hemoglobin molecule, which carries oxygen in the blood. A different gene, the α-globin At each, the cell must copy its to pass it to both daughter cells.
Chapter 13 Dna Structure And Function Study Guide Answers
The discovery of the structure of also revealed the principle that makes this copying possible: because each strand of DNA contains a sequence of nucleotides that is exactly to the sequence of its partner strand, each strand can act as a, or mold, for the synthesis of a new complementary strand. In other words, if we designate the two DNA strands as S and S′, strand S can serve as a for making a new strand S′, while strand S′ can serve as a template for making a new strand S. Thus, the genetic information in DNA can 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 former partner.
DNA as a template for its own duplication. As the nucleotide A successfully pairs only 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. The ability of each strand of a to act as a for producing a strand enables a cell to copy, or replicate, its genes before passing them on to its descendants.
In the next chapter we describe the elegant machinery the cell uses to perform this enormous task. In Eucaryotes, DNA Is Enclosed in a Cell Nucleus Nearly all the in a eucaryotic cell is sequestered in a, which occupies about 10% of the total cell volume. This is delimited by a formed by two concentric membranes that are punctured at intervals by large nuclear pores, which transport molecules between the nucleus and the.
The nuclear envelope is directly connected to the extensive membranes of the. It is mechanically supported by two networks of intermediate filaments: one, called the, forms a thin sheetlike meshwork inside the nucleus, just beneath the; the other surrounds the and is less regularly organized.
A cross-sectional view of a typical cell nucleus. The nuclear envelope consists of two membranes, the outer one being continuous with the endoplasmic reticulum membrane (see also Figure 12-9). The space inside the endoplasmic reticulum (the ER lumen) The allows the many proteins that act on to be concentrated where they are needed in the cell, and, as we see in subsequent chapters, it also keeps nuclear and cytosolic enzymes separate, a feature that is crucial for the proper functioning of eucaryotic cells.
Compartmentalization, of which the is an example, is an important principle of biology; it serves to establish an environment in which biochemical reactions are facilitated by the high concentration of both substrates and the enzymes that act on them.
DNA REPLICATION: Before the lagging-strand DNA exits the replication factory, its RNA primers must be removed and the Okazaki fragments must be joined together to create a continuous DNA strand. The first step is the removal of the RNA primer. RNAse H, which recognizes RNA-DNA hybrid helices, degrades the RNA by hydrolyzing its phosphodiester bonds. Next, the sequence gap created by RNAse H is then filled in by DNA polymerase which extends the 3' end of the neighboring Okazaki fragment.