II. The polynucleotide structure of nucleic acids.

III. The Watson-Crick model of DNA.

IV. DNA in chromosomes.

V. Replication of DNA.

Terms.

A. In the 1860's, Miescher isolated an acidic substance from cell nuclei that he called nucleic acid. Subsequently it was shown that there are two general types of nucleic acid in biological systems. The functions were unknown.

B. Griffith (1928) proved that a component of killed, encapsulated pneumococci, when injected into mice along with a different strain of living, nonencapulated pneumococci, could transform the living strain into encapsulated forms. It was concluded that some substance that specified encapsulation in the killed bacteria had been incorporated into the living bacteria and had changed the heredity of the living bacteria. The nature of the substance was unknown.

C. Avery, MacLeod, and McCarty (1944) proved that the transforming substance was DNA.

D. Hershey and Chase (1952) further supported the role of DNA in heredity by showing that when certain bacteriophages infect the bacterium Escherichia coli, only DNA is transferred from the phage to the bacterial cell.

A. There are two types of nucleic acids in biological systems.
1. DNA (deoxyribonucleic acid) is found in eukaryotes only in the nuclei and in very small amounts in mitochondria. Except for certain viruses, DNA functions universally as the primary carrier of genetic information.

2. RNA (ribonucleic acid) is found both in nuclei and cytoplasm. RNA functions in the tranfer of genetic information from the DNA of the nucleus to the cytoplasm, where the information is used to direct protein synthesis.

B. A nucleotide consists of an organic base (either a purine or pyrimidine) to which is connected a sugar (either ribose or deoxyribose) and a phosphate group. The phosphate groups are strongly acidic, thus the name nucleic acid. A nucleotide can be represented as B–S–P (base–sugar–phosphate).
1. The general definition of a base is any substance that binds protons and therefore can neutralize acids. Chemicals that are basic can also be described as alkaline. Purines and pyrimidines have basic amine groups and therefore can be described as organic bases.

2. Sugars are organic molecules consisting of a carbon chain of 3 to 7 carbon atoms, to most of which are attached hydroxyl (–OH) groups and –H atoms. In a typical sugar (monosaccharide), each C atom has one –OH and one –H, except the end C atoms, which vary. Polysaccharides consist of multiple sugar units bound together covalently. Examples of large polysaccharides built of glucose subunits are glycogen (found in the liver), cellulose, and starch.

3. A phosphate group can be represented by an atom of P to which are bound one O by means of a double bond and three hydroxyl groups (–OH). This structure would actually be phosphoric acid, and one or more of the hydroxyl groups would likely be ionized, producing protons (H⁺ ions) and leaving negatively charged oxygens. In this form, the structure is referred to as phosphate. The hydroxyl groups can also form covalent bonds with other hydroxyl groups by splitting out water. This is what happens in the formation of nucleotides and polynucleotide chains.

C. The purine bases are adenine (A) and guanine (G), found in both DNA and RNA. The pyrimidine bases are thymine (T), found in DNA, uracil (U), found in RNA, and cytosine (C), found in both DNA and RNA. T and U are equivalent in terms of genetic information. Each type of nucleic acid thus has four types of nucleotide building blocks.

D. The nucleotides of DNA have the sugar deoxyribose; the nucleotides of RNA have ribose.

E. Nucleotides may occur in any order in a polynucleotide chain. Thus such a DNA chain can be represented as CTGAAGCCTTGAG..., where each nucleotide is represented by its base. Since nucleotides are not symmetrical, they have "direction." E.g. a nucleotide could be represented as -A>, and a polynucleotide chain as -C>-T>-G>-A>... This directionality is usually represented by 3' and 5', corresponding to structural aspects of the sugar. It is conventional to write the structure in the order 5'–CTGAAGCCTTGAG–3', but the ends of a chain should be labeled if there is any doubt.

A. The hydrogen bonds form between the bases A and T and between G and C.

B. This base pairing creates a ladder-like structure. Because the bases are hydrophobic, succeeding pairs attract each other, causing the structure to twist into a right-handed double helix.

C. The strands are described as complementary because the sequence on one determines what the sequence on the other must be. E.g. the sequence AAGTGCA on one pairs only with TTCACGT on the other.

D. The two strands are antiparallel, i.e. they run in opposite directions. Thus, 5'–AAGTGCA–3' pairs with 3'–TTCACGT–5'.

E. Genetic information is stored in the sequence of bases, or base pairs (although the second base contains no additional information). DNA can thus be viewed as a linear quaternary code, much like a computer code (which is binary). A sequence of 10 nucleotides can have 4¹⁰ forms, or 1,048,576, if all possible sequences are considered.

F. RNA is constructed similar to DNA, except for U in place of T and ribose in place of deoxyribose. Two complementary strands of RNA can form a double helix. However, RNA isolated from cells is nearly all single stranded because only one of the strands is made. An RNA strand that is complementary to a DNA strand can form a double helix with it. RNA can also carry genetic information but is not the primary repository except in a few viruses.

A. The DNA in a chromosome is a single molecule that runs from end to end. Since it is many times greater than the diameter of a nucleus, it must be folded extensively.

B. The length of the 23 molecules of DNA in a human haploid genome, laid end to end, is about a meter.

C. The number of nucleotide pairs in a haploid complement of mammalian chromosomes = ca. 3 billion.

D. Histones are very basic (alkaline) proteins. A group of 8 histone molecules forms a core around which DNA wraps. Such a structure is called a nucleosome. During chromosomal condensation, the chain of nucleosomes forms a coil, which coils again to form a supercoil, etc. This compact structure is what is observed at metaphase. During interphase, the DNA is in very extended (uncoiled) form so that genes can function.

A. This process is semiconservative, in that each of the original strands remains intact through subsequent replications. However, it is paired with a newly synthesized complementary strand each time.

B. The process of joining together the nucleotides in the growing strand is catalyzed by an enzyme, DNA polymerase. Only nucleotides that are correctly paired are added to the growing strand. However, if a mistake is made, cells have machinery for detecting errors and correcting them.

C. DNA replication must be initiated at a primer, which is a short piece of DNA or RNA that is complementary to a segment of the strand to be copied. DNA polymerase can then add additional nucleotides to the primer, but only to the 3' end. In DNA replication in cells, the primer is RNA.

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