Chapter 17 Nucleotides, Nucleic Acids, and Heredity

The Molecules of Heredity Each cell of our bodies contains thousands of different proteins. How do cells know which proteins to synthesize out of the extremely large number of possible amino acid sequences? From the end of the 19th century, biologists suspected that the transmission of hereditary information took place in the nucleus, more specifically in structures called chromosomes. The hereditary information was thought to reside in genes within the chromosomes. Chemical analysis of nuclei showed chromosomes are made up largely of proteins called histones and nucleic acids. By the 1940s, it became clear that deoxyribonucleic acids (DNA) carry the hereditary information. Other work in the 1940s demonstrated that each gene controls the manufacture of one protein. Thus the expression of a gene in terms of an enzyme protein led to the study of protein synthesis and its control.

Nucleic Acids There are two kinds of nucleic acids in cells: Ribonucleic acids (RNA) Located elsewhere in the nucleus and even outside the nucleus (cytoplasm) Deoxyribonucleic acids (DNA) Present in the chromosomes of the nucleic of eukaryotic cells Both RNA and DNA are polymers built from monomers called nucleotides. A nucleotide is composed of: A base, a monosaccharide, and a phosphate

Purine/Pyrimidine Bases For DNA, the bases are A, G ,C and T For RNA, the bases are A, G, C and U Figure 17.1 The five principal bases of DNA and RNA.

Nucleosides Nucleoside: A compound that consists of D-ribose or 2-deoxy-D- ribose bonded to a purine or pyrimidine base by a -N- glycosidic bond. Base sugar

Nucleotides Nucleotide: A nucleoside in which a molecule of phosphoric acid is esterified with an -OH of the monosaccharide, most commonly either at the 3’ or the 5’-OH. Anhydride Ester AMP ADP ATP

Nucleotides Deoxythymidine 3’-monophosphate (3’-dTMP),

Nucleotides Adenosine 5’- triphosphate (ATP) serves as a common currency into which energy gained from food is converted and stored.

Table 17.1 The Eight Nucleosides and Eight Nucleotides in DNA and RNA

DNA—Primary (1°) Structure For nucleic acids, primary structure is the sequence of nucleotides, beginning with the nucleotide that has the free 5’ terminus. The strand is read from the 5’end to the 3’end. Thus, the sequence AGT means that adenine (A) is the base at the 5’ terminus and thymine (T) is the base at the 3’ terminus.

Structure of DNA and RNA Figure 17.2 Schematic diagram of a nucleic acid molecule. The four bases of each nucleic acid are arranged in various specific sequences. The base sequence is read from the 5’ end to the 3’ end.

FIGURE 25. 3 Primary structure of the DNA backbone FIGURE 25.3 Primary structure of the DNA backbone. The hydrogens shown in blue cause the acidity of nucleic acids. In the body, at neutral pH, the phosphate groups carry a charge of –1 and the hydrogens are replaced by Na+ and K+.

DNA—2° Structure Secondary structure: The ordered arrangement of nucleic acid strands. The double helix model of DNA 2° structure was proposed by James Watson and Francis Crick in 1953. Double helix: A type of 2° structure of DNA in which two polynucleotide strands are coiled around each other in a screw-like fashion.

THE DNA Double Helix Figure 17.4 Three- dimensional structure of the DNA double helix.

Base Pairing Figure 17.5 A and T pair by forming two hydrogen bonds. G and C pair by forming three hydrogen bonds.  The complemetary base pairs

FIGURE 25. 7 Only one hydrogen bond is possible for TG or CA FIGURE 25.7 Only one hydrogen bond is possible for TG or CA. These combinations are not found in DNA. Compare this figure with Figure 25.5.

Superstructure of Chromosomes DNA is coiled around proteins called histones. Histones are rich in the basic amino acids Lys and Arg, whose side chains have a positive charge. The negatively-charged DNA molecules and positively- charged histones attract one another and form units called nucleosomes. Nucleosome: A core of eight histone molecules around which the DNA helix is wrapped. Nucleosomes are further condensed into chromatin. Chromatin fibers are organized into loops, and the loops into the bands that provide the superstructure of chromosomes.

Superstructure of Chromosomes Figure 17.8

Superstructure of Chromosomes Figure 17.8 cont’d

Superstructure of Chromosomes Figure 25.8 cont’d

Superstructure of Chromosomes Figure 25.8 cont’d

DNA and RNA The three differences in structure between DNA and RNA are: DNA bases are A, G, C, and T; the RNA bases are A, G, C, and U. the sugar in DNA is 2-deoxy-D-ribose; in RNA it is D-ribose. DNA is always double stranded; there are several kinds of RNA, all of which are single-stranded.

Information Transfer

Different Classes of RNA Messenger RNA( mRNA): produced in the process called transcription and they carry the genetic information from the DNA in the nucleus directly to the cytoplasm. Containing average 750 nucleotides Not-long lived Transfer RNA (tRNA): transport amino acid to the site of protein synthesis in ribosomes 74-93 nucleotides per chain Contains cytosine, guanine, adenine, uracil and amodified nucleotide called 1-methylguanosine

Different Classes of RNA Ribosomal RNA (rRNA) Ribosomes: small spherical bodies located in the cells but outside the nuclei, contain rRNA Consists of about 35% protein and 65% ribosomal RNA Small Nuclear RNA (snRNA): found in the nucleus of eukaryotic cells. 100-200 nucleotides long, neither subunit tRNA or rRNA To help with the processing of the initial mRNA transcribed from DNA into a mature form

Different Classes of RNA Micro RNA (miRNA): another type of small RNA 20-22 nucleotides long Important in the timing of an organism’s development. They inhibit translation of mRNA into protein and promote the degredation of mRNA Small Interfering RNA (siRNA): eliminate expression of an undersirable gene, such as one that causes uncontrolled cell growth or one that came from a virus Has been used to protect mouse liver from hepatitis and to help clear infected liver cells of the disease