© 2012 Pearson Education, Inc The Genetic Material Must Exhibit Four Characteristics

© 2012 Pearson Education, Inc. Section 10.1 For a molecule to serve as the genetic material, it must be able to –replicate –store information –express information –allow variation by mutation

© 2012 Pearson Education, Inc. Section 10.1 The central dogma of molecular genetics is that DNA makes RNA (transcription), which makes proteins (translation) (Figure 10.1)

© 2012 Pearson Education, Inc. Figure 10.1

© 2012 Pearson Education, Inc Until 1944, Observations Favored Protein as the Genetic Material

© 2012 Pearson Education, Inc. Section 10.2 The genetic material is physically transmitted from parent to offspring Proteins and nucleic acids were the major candidates for the genetic material In the 1940s, many geneticists favored proteins

© 2012 Pearson Education, Inc. Section 10.2 Proteins are more diverse and abundant in cells They were the subject of the most active areas of genetic research, with more known about proteins than nucleic acid chemistry DNA was thought to be too simple to be the genetic material, with only four types of nucleotides as compared to the 20 different amino acids of proteins

© 2012 Pearson Education, Inc Evidence Favoring DNA as the Genetic Material Was First Obtained during the Study of Bacteria and Bacteriophages

© 2012 Pearson Education, Inc. Section 10.3 Griffith (1927) showed that avirulent strains of Diplococcus pneumoniae could be transformed to virulence (Figure 10.2) He speculated that the transforming principle could be part of the polysaccharide capsule or a compound required for capsule synthesis

© 2012 Pearson Education, Inc. Figure 10.2

© 2012 Pearson Education, Inc. Section 10.3 Avery, MacLeod, and McCarty demonstrated that the transforming principle was DNA and not protein (Figure 10.3) In their 1944 publication they stated, "The evidence presented supports the belief that a nucleic acid of the deoxyribose type is the fundamental unit of the transforming principle of Pneumococcus Type III."

© 2012 Pearson Education, Inc. Figure 10.3

© 2012 Pearson Education, Inc. Section 10.3 Hershey and Chase (1952), using Escherichia coli and an infecting virus (bacteriophage T2), demonstrated that DNA, and not protein, is the genetic material Using radioisotope 32 P and 35 S, Hershey and Chase demonstrated that DNA enters the bacterial cell during infection and directs viral reproduction (Figures 10.4 and 10.5)

© 2012 Pearson Education, Inc. Figure 10.4

© 2012 Pearson Education, Inc. Figure 10.5

© 2012 Pearson Education, Inc. Section 10.3 Transfection, the process of infection by viral DNA into bacterial cells, proved conclusively that the viral DNA alone contains all the necessary information for production of mature viruses

© 2012 Pearson Education, Inc Indirect and Direct Evidence Supports the Concept that DNA Is the Genetic Material in Eukaryotes

© 2012 Pearson Education, Inc. Section 10.4 Protein is abundant in the cytoplasm and protein is not Mitochondria and chloroplast perform genetic function, and DNA is also part of these organelles DNA is found only where the primary genetic function occurs This provides indirect evidence for DNA as the genetic material

© 2012 Pearson Education, Inc. Section 10.4 UV light is capable of inducing mutations in the genetic material and is most mutagenic at a wavelength of 260 nm DNA and RNA absorb UV light most strongly at 260 nm, but protein absorbs most strongly at 280 nm, a wavelength at which no significant mutagenic effects are observed Again, this provides indirect evidence for DNA as the genetic material

© 2012 Pearson Education, Inc. Section 10.4 The strongest direct evidence for DNA as the genetic material comes from recombinant DNA technology Segments of eukaryotic DNA corresponding to specific genes are isolated and spliced into the bacterial DNA This complex can be inserted into a bacterial cell and monitored

© 2012 Pearson Education, Inc. Section 10.4 The presence of the eukaryotic gene product in bacteria containing the eukaryotic gene provides direct evidence that this DNA is present and functional in the bacterial cell This has been shown to occur in countless instances (e.g., insulin production by bacteria)

© 2012 Pearson Education, Inc RNA Serves as the Genetic Material in Some Viruses

© 2012 Pearson Education, Inc. Section 10.5 Some viruses have an RNA core rather than a DNA core Experiments with tobacco mosaic virus (1956) demonstrated that RNA serves as the genetic material for these viruses

© 2012 Pearson Education, Inc. Section 10.5 Replication of the viral RNA is dependent on RNA replicase

© 2012 Pearson Education, Inc. Section 10.5 Retroviruses replicate in an unusual way RNA serves as a template for synthesis of a complementary DNA by the RNA- dependent DNA polymerase called reverse transcriptase This DNA can be incorporated into the host cell genome; when transcribed, copies of the original retroviral RNA chromosomes are also produced

© 2012 Pearson Education, Inc Knowledge of Nucleic Acid Chemistry Is Essential to the Understanding of DNA Structure

© 2012 Pearson Education, Inc. Section 10.6 Nucleotides are the building blocks of DNA They consist of –a nitrogenous base –a pentose sugar –a phosphate group

© 2012 Pearson Education, Inc. Section 10.6 There are two kinds of nitrogenous bases –Purines Adenine (A) Guanine (G) –Pyrimidines Cytosine (C) Thymine (T) Uracil (U) (Figure 10.7)

© 2012 Pearson Education, Inc. Figure 10.7

© 2012 Pearson Education, Inc. Figure 10.7a

© 2012 Pearson Education, Inc. Figure 10.7b

© 2012 Pearson Education, Inc. Section 10.6 DNA and RNA both contain A, C, and G Only DNA contains T Only RNA contains U

© 2012 Pearson Education, Inc. Section 10.6 RNA contains ribose as its sugar DNA contains deoxyribose (Figure 10.7b)

© 2012 Pearson Education, Inc. Figure 10.7

© 2012 Pearson Education, Inc. Section 10.6 A nucleoside contains the nitrogenous base and the pentose sugar A nucleotide is a nucleoside with a phosphate group added (Figure 10.8)

© 2012 Pearson Education, Inc. Figure 10.8

© 2012 Pearson Education, Inc. Section 10.6 The C-5' position is the location of the phosphate group on a nucleotide

© 2012 Pearson Education, Inc. Section 10.6 Nucleotides can have one, two, or three phosphate groups and are called NMPs, NDPs, and NTPs, respectively (Figure 10.9)

© 2012 Pearson Education, Inc. Figure 10.9

© 2012 Pearson Education, Inc. Section 10.6 Nucleotides are linked by a phosphodiester bond between the phosphate group at the C-5' position and the OH group on the C-3' position (Figure 10.10)

© 2012 Pearson Education, Inc. Figure 10.10

© 2012 Pearson Education, Inc. Figure 10.10a

© 2012 Pearson Education, Inc. Figure 10.10b

© 2012 Pearson Education, Inc The Structure of DNA Holds the Key to Understanding Its Function

© 2012 Pearson Education, Inc. Section 10.7 Chargaff (1949–1953) showed that the amount of A is proportional to T and the amount of C is proportional to G, but the percentage of C + G does not necessarily equal the percentage of A + T (Table 10.3)

© 2012 Pearson Education, Inc. Table 10.3

© 2012 Pearson Education, Inc. Section 10.7 X-ray diffraction studies by Rosalind Franklin (1950–1953) of DNA showed a 3.4 angstrom periodicity, characteristic of a helical structure (Figure 10.11)

© 2012 Pearson Education, Inc. Figure 10.11

© 2012 Pearson Education, Inc. Section 10.7 Watson and Crick (1953) proposed that DNA is a right-handed double helix in which the two strands are antiparallel and the bases are stacked on one another The two strands are connected by A-T and G-C base pairing, and there are 10 base pairs per helix turn (Figure 10.12)

© 2012 Pearson Education, Inc. Figure 10.12

© 2012 Pearson Education, Inc. Figure 10.12a

© 2012 Pearson Education, Inc. Figure 10.12b

© 2012 Pearson Education, Inc. Section 10.7 The A-T and G-C base pairing provides complementarity of the two strands and chemical stability to the helix

© 2012 Pearson Education, Inc. Section 10.7 A-T base pairs form two hydrogen bonds, and G-C base pairs form three hydrogen bonds (Figure 10.14)

© 2012 Pearson Education, Inc. Figure 10.14

© 2012 Pearson Education, Inc. Section 10.7 The arrangement of sugars and bases along the axis provides another stabilizing factor

© 2012 Pearson Education, Inc The Structure of RNA Is Chemically Similar to DNA, but Single Stranded

© 2012 Pearson Education, Inc. Section 10.9 In RNA, –the sugar ribose replaces deoxyribose of DNA and –uracil replaces thymine of DNA

© 2012 Pearson Education, Inc. Section 10.9 Most RNA is single stranded, although some RNAs form double-stranded regions as they fold into different secondary structures In addition, some viruses have a double- stranded RNA genome

© 2012 Pearson Education, Inc. Section 10.9 There are three classes of cellular RNAs that function during expression of genetic information –messenger RNA (mRNA) –ribosomal RNA (rRNA) –transfer RNA (tRNA) These all originate as complementary copies of one of the two DNA strands during transcription

© 2012 Pearson Education, Inc. Section 10.9 The characteristics of these RNAs in prokaryotic and eukaryotic cells are summarized in Table 10.4

© 2012 Pearson Education, Inc. Table 10.4

© 2012 Pearson Education, Inc. Section 10.9 rRNAs are structural components of ribosomes for protein synthesis mRNAs are the template for protein synthesis tRNAs carry amino acids for protein synthesis