Lecture 3 what Genes are and What they do

Lecture 3what Genes are and What they do Part II Three Chapters

How the molecule of heredity carries, replicates, and recombines information Chapter 6 DNA

How investigators pinpointed DNA as the genetic material The elegant Watson-Crick model of DNA structure How DNA structure provides for the storage of genetic information How DNA structure gives rise to the semiconservative model of molecular replication How DNA structure promotes the recombination of genetic information

The chemical composition of DNA Fig. 6.2

Chemical characterization localizes DNA in the chromosomes 1869 – Friedrich Meischer extracted a weakly acidic, phosphorous rich material from nuclei of human white blood cells which he named nuclein

Are genes composed of DNA or protein? DNA Only four different subunits make up DNA Chromosomes contain less DNA than protein by weight Protein 20 different subunits – greater potential variety of combinations Chromosomes contain more protein than DNA by weight

Bacterial transformation implicates DNA as the substance of genes 1928 – Frederick Griffith – experiments with smooth (S), virulent strain Streptococcus pneumoniae, and rough (R), nonvirulent strain Bacterial transformation demonstrates transfer of genetic material 1944 – Oswald Avery, Colin MacLeod, and MacIyn McCarty – determined that DNA is the transformation material

Griffith experiment Fig. 6.3

Griffith experiment Fig. 6.3 b

Avery, MacLeod, McCarty experiment

Hershey and Chase experiments 1952 – Alfred Hershey and Martha Chase provide convincing evidence that DNA is genetic material Waring blender experiment using T2 bacteriophage and bacteria Radioactive labels 32P for DNA and 35S for protein

Hershey and Chase Waring blender experiment Fig. 6.5 a,b

Hershey and Chase Waring blender experiment Fig. 6.5 c

The Watson-Crick Model: DNA is a double helix 1951 – James Watson learns about x-ray diffraction pattern projected by DNA Knowledge of the chemical structure of nucleotides (deoxyribose sugar, phosphate, and nitrogenous base) Erwin Chargaff’s experiments demonstrate that ratio of A and T are 1:1, and G and C are 1:1 1953 – James Watson and Francis crick propose their double helix model of DNA structure

X-ray diffraction patterns produced by DNA fibers – Rosalind Franklin and Maurice Wilkins Fig. 6.6

Chargaff’s ratios

Complementary base pairing by formation of hydrogen bonds explain Chargaff’s ratios Fig. 6.8

DNA is double helix Strands are antiparallele with a sugar-phosphate backbone on outside and pairs of bases in the middle Two strands wrap around each other every 30 Angstroms, once every 10 base pairs Two chains are held together by hydrogen bonds between A-T and G-C base pairs Fig. 6.9

Stucturally, purines (A and G )pair best with pyrimadines (T and C) Thus, A pairs with T and G pairs with C, also explaining Chargaff’s ratios Fig. 6.9 d

Double helix may assume alternative forms Fig. 6.10

Some DNA molecules are circular instead of linear • 1. Prokaryotes • 2. Mitochondria • 3. Chloroplasts • 4. Viruses • Some viruses carry single-stranded DNA • 1. bacteriophages • Some viruses carry RNA • 1. e.g., AIDS

Four requirements for DNA to be genetic material Must carry information Cracking the genetic code Must replicate DNA replication Must allow for information to change Mutation Must govern the expression of the phenotype Gene function

Some viruses use RNA as the repository of genetic information Fig. 6.13

DNA replication: Copying genetic information for transmission to the next generation Complementary base pairing produces semiconservative replication Double helix unwinds Each strand acts as template Complementary base pairing ensures that T signals addition of A on new strand, and G signals addition of C Two daughter helices produced after replication

Fig. 6.14

Fig. 6.15

Meselson-Stahl experiments confirm semiconservative replication Fig. 6.16

The mechanism of DNA replication Arthur Kornbuerg, a nobel prize winner and other biochemists deduced steps of replication Initiation Proteins bind to DNA and open up double helix Prepare DNA for complementary base pairing Elongation Proteins connect the correct sequences of nucleotides into a continuous new strand of DNA

Enzymes involved in replication Pol III – produces new stands of complementary DNA Pol I – fills in gaps between newly synthesized Okazaki segments DNA helicase – unwinds double helix Single-stranded binding proteins – keep helix open Primase – creates RNA primers to initiate synthesis Ligase – welds together Okazaki fragments

Replication is bidirectional Replication forks move in opposite directions In linear chromosomes, telomeres ensure the maintenance and accurate replication of chromosome ends In circular chromosomes, such as E. coli, there is only one origin of replication. In circular chromosomes, unwinding and replication causes supercoiling, which may impede replication Topoisomerase – enzyme that relaxes supercoils by nicking strands

The bidirectional replication of a circular chromosome Fig. 6.18

Fig. 6.18

Cells must ensure accuracy of genetic information Redunancy Basis for repair of errors that occur during replication or during storage Enzymes repair chemical damage to DNA Errors during replication are rare

Recombination reshuffles the information content of DNA During recombination, DNA molecules break and rejoin Meselson and Weigle - Experimental evidence from viral DNA and radioactive isotopes Coinfected E. coli with light and heavy strains of virus after allowing time for recombination Separated on a CsCl density gradient

Meselson and Weigle demonstrate recombination occurs by breakage and rejoining of DNA Fig. 6.19

Heteroduplexes mark the spot of recombination Products of recombination are always in exact register; not a single base pair is lost or gained Two strands do not break and rejoin at the same location; often they are hundreds of base pairs apart Region between break points is called heteroduplex

Heteroduplex region Fig .6.20

In heterozygotes, mismatches within heteroduplexes must be repaired Gene conversion – a deviation from expected 2:2 segregation of alleles due to mismatch repair. Studied most extensively in yeast where tetrad analysis makes possible to follow products of meiosis

Gene conversion in yeastMismatch leads to 3:1 ratio of a:A. Ratio of B:b and C:c which lie outside of heteroduplex are both 2:2, as expected. Fig. 6.20 c

Double stranded break model of meiotic recombination Homologs physically break, exchange parts, and rejoin. Breakage and repair create reciprocal products of recombination Recombination events can occur anywhere along the DNA molecule Precision in the exchange prevents mutations from occurring during the process Gene conversion can give rise to unequal yield of two different alleles. 50% of gene conversions are associated with crossing over of adjacent chromosomal regions, and 50% of gene conversions are not associate with crossing over

Double stranded break formationspoI protein breaks one chromatid on both strands Fig. 6.22 step 1

Resection5’ end on each side of break are degraded to produce two 3’ single stranded tails Fig. 6.22 step 2

Lecture 3 what Genes are and What they do

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