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General, Organic, and Biochemistry, 7e. Bettelheim, Brown, and March. Chapter 24. Nucleotides, Nucleic Acids, and Heredity. Introduction. each cell of our bodies contains thousands of different proteins
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General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March
Chapter 24 Nucleotides, Nucleic Acids, and Heredity
Introduction • 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
Introduction • 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) • deoxyribonucleic acids (DNA) • Both RNA and DNA are polymers built from monomers called nucleotides • A nucleotide is composed of • a base • a monosaccharide • a phosphate
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
Nucleotides • Nucleotide: a nucleoside in which a molecule of phosphoric acid is esterified with an -OH of the monosaccharide, most commonly either the 3’ or the 5’-OH
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
Structure of DNA and RNA • Primary Structure:the sequence of bases along the pentose-phosphodiester backbone of a DNA or RNA molecule • base sequence is read from the 5’ end to the 3’ end
Nucleic Acid - 1° Structure • A schematic diagram of a nucleic acid
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 molecules in which two antiparallel polynucleotide strands are coiled in a right-handed manner about the same axis
Higher Structure of DNA • DNA is coiled around proteins called histones • histones are rich in the basic amine acids Lys and Arg, whose side chains have a positive charge • the negatively-charged DNA molecules and positively-charged histones attract each other 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
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
RNA • RNA molecules are classified according to their structure and function
Genes, Exons, and Introns • Gene: a segment of DNA that carries a base sequence that directs the synthesis of a particular protein, tRNA, or mRNA • there are many genes in one DNA molecule • in bacteria the gene is continuous • in higher organisms the gene is discontinuous • Exon: a section of DNA that, when transcribed, codes for a protein or RNA • Intron: a section of DNA that does not code for anything functional
Genes, Exons, and Introns • introns are cut out of mRNA before the protein is synthesized
DNA Replication • Replication involves separation of the two original strands and synthesis of two new daughter strands using the original strands as templates • DNA double helix unwinds at a specific point called an origin of replication • polynucleotide chains are synthesized in both directions from the origin of replication; that is, DNA replication is bidirectional • at each origin of replication, there are two replication forks, points at which new polynucleotide strands are formed
DNA Replication • DNA is synthesized from its 5’ -> 3’ end (from the 3’ -> 5’ direction of the template) • the leading strand is synthesized continuously in the 5’ -> 3’ direction toward the replication fork • the lagging strand is synthesized semidiscontinuously as a series of Okazaki fragments, also in the 5’ -> 3’ direction, but away from the replication fork • Okazaki fragments of the lagging strand are joined by the enzyme DNA ligase • replication is semiconservative: each daughter strand contains one template strand and one newly synthesized strand
Replisomes • Replisomes are assemblies of “enzyme factories”
DNA Replication • Opening up the superstructure • during replication, the very condensed superstructure of chromosomes is opened by a signal transduction mechanism • one step of this mechanism involves acetylation and deacetylation of key lysine residues • acetylation removes a positive charge and thus weakens the DNA-histone interactions
DNA Replication • Relaxation of higher structures of DNA • tropoisomerases (also called gyrases) facilitate the relaxation of supercoiled DNA by introducing either single strand or double strand breaks in the DNA • once the supercoiling is relaxed by this break, the broken ends are joined and the tropoisomerase diffuses from the location of the replication fork
DNA Replication • Unwinding the DNA double helix • replication of DNA starts with unwinding of the double helix • unwinding can occur at either end or in the middle • unwinding proteins called helicases attach themselves to one DNA strand and cause separation of the double helix • the helicases catalyze the hydrolysis of ATP as the DNA strand moves through; the energy of hydrolysis promotes the movement
DNA Replication • Primer/primases • primers are short oligonucleotides of four to 15 nucleotides long • they are required to start the synthesis of both daughter strands • primases are enzymes that catalyze the synthesis of primers • primases are placed at about every 50 nucleotides in the lagging strand synthesis
DNA Replication • DNA polymerases are key enzymes in replication • once the two strands have separated at the replication fork, the nucleotides must be lined up in proper order for DNA synthesis • in the absence of DNA polymerase, alignment is slow • DNA polymerase provides the speed and specificity of alignment • along the lagging (3’ -> 5’) strand, the polymerases can synthesize only short fragments, because these enzymes only work from 5’ -> 3’ • these short fragments are called Okazaki fragments • joining the Okazaki fragments and any remaining nicks is catalyzed by DNA ligase
DNA Repair • The viability of cells depends on DNA repair enzymes that can detect, recognize, and repair mutations in DNA • Base excision repair(BER): one of the most common repair mechanisms • a specific DNA glycosylase recognizes the damaged base • it catalyzes the hydrolysis of the b-N-glycosidic bond between the incorrect base and its deoxyribose • it then flips the damaged base, completing the excision • the sugar-phosphate backbone remains intact
DNA Repair • BER (cont’d) • at the AP (apurinic or apyrimidinic) site thus created, an endonuclease catalyzes the hydrolysis of the backbone • an exonuclease liberates the sugar-phosphate unit of the damaged site • DNA polymerase inserts the correct nucleotide • DNA ligase seals the backbone to complete the repair • NER (nucleotide excision repair) removes and repairs up to 24-32 units by a similar mechanism involving a number of repair enzymes
Cloning • Clone: a genetically identical population • Cloning: a process whereby DNA is amplified by inserting it into a host and having the host replicate it along with the host’s own DNA • Polymerase chain reaction (PCR): an automated technique for amplifying DNA using a heat-stable DNA polymerase from a thermophilic bacterium
Nucleic Acids End Chapter 24
