bases.html: 16_UN298NitrogenBases.jpg
Adenine and guanine are purines, nitrogenous bases with two organic rings. Cytosine and thymine are pyrimidines, which have a single ring.

chargaff.html: 16_erwin_chargaff.jpg

chargaff_rules.shtml:

* Plant DNA is an apparent exception to Chargaff's rules, however, plant DNA is often heavily methylated. As long as the amount of 5-methyl-cytosine is included in the total amount of cytosine, the rules are obeyed.

¦ Some bacteriophage DNA's (e.g. PhiX174) do not adhere to Chargaff's rules - these phages have single-stranded genomes and are therefore not constrained by the requirements of a double-stranded structure.

Adapted from http://www.mun.ca/biochem/courses/3107/Topics/DNA_history.html.

chromosome.html: 16_21bChromatinPacking-U.jpg
Interactions between nucleosomes cause the 10-nm fiber to coil into a 30-nm fiber, also seen during interphase. The 30-nm fiber forms looped domains that attach to a scaffold of nonhistone proteins. The looped domains coil further during mitosis to form a 700-nm chromatid.

double_helix.html: 16_07DNADoubleHelix.jpg

Two strands of sugar-phosphate backbones on the outside are held together by hydrogen bonds between the nitrogenous bases.		The nitrogenous bases are paired in the inside of the double helix.		The 2 strands are antiparallel: they run in opposite 5' to 3' directions.

elong.html: 16_13AddingNucelotides_L.jpg
Elongation.
DNA polymerase III catalyzes the addition of a nucleotide to the 3' end of a growing DNA strand by the hydrolysis of a nucleoside triphosphate.

excision.html: 16_18DNAExcisionRepair-U.jpg
Nucleotide excision repair of DNA damage. A thymine dimer, a type of damage often caused by ultraviolet radiation. A nuclease enzyme cuts out (excises) the damaged nucleotides. DNA polymerase replaces it with a normal DNA segment. Ligase completes the process by closing the remaining break in the sugar–phosphate backbone.

fragments.html: 16_16bLaggingStrand_4-U.jpg
On the lagging strand, synthesis must occur away from the replication fork, in the 5' to 3' direction. DNA polymerase III elongates the lagging strand in short Okazaki fragments.

init.html: 16_12EukDNARepOriginsA.jpg
Initiation. In eukaryotes, DNA replication begins at many origins of replication along each chromosome. The DNA unwinds at replication forks, forming multiple replication bubbles.

lagging.html: 16_15LaggingStrandA.jpg
Elongation along the lagging strand. continue

leading.html: 16_15-LeadingStrand-U.jpg
DNA polymerase III elongates DNA strands only in the 5' to 3' direction. On the leading strand (where the 3' end of the new strand porceeds into the fork), DNA synthesis can proceed continuously.

nucleosome.html: 16_21aChromatinPacking-U.jpg
DNA and positively-charged histone molecules form "beads on a string," the 10-nm fiber comprising the interphase chromatin. A nucleosome has 8 histone molecules with the amino end (tail) of each projecting outward.
A different histone, H1, acts as a spacer between nucleosomes.

pauling-alpha_helix.html: 16_pauling-alpha_helix.jpg

pauling.html: 16_pauling-triple_helix.jpg

repair.html: 16_17DNAExcisionRepair.jpg

replication.html: 16_12EukDNARepOriginsA.jpg

replication_bubbles.html: 16_12EukDNARepOriginsB.jpg
In this micrograph, three replication bubbles are visible along the DNA of a cultured Chinese hamster cell.

replication_fork.html: 16_13ReplicationProteins-U.jpg
At the replication fork, a helicase unwinds the double helix.
A short RNA primer is added by primase in the 5' to 3' direction, using the parental DNA as a template and following the base-pairing rules.

repmodel.html: 16_10AltDNARepModelsB.jpg
In the semiconservative model of DNA replication, each of the two new daughter molecules has one parental strand, and one new strand.

short.html: 16_19DNAShortening-A.jpg
Shortening of the ends of linear DNA molecules. DNA polymerase III cannot complete the 5' end on the new lagging strand. Repeated rounds of replication produce shorter DNA molecules.

shorter.html: 16_19DNAShortening-U.jpg
After the first round of replication, the new lagging strand is shorter than its template. After a second round, both the leading and lagging strands are shorter than the original parental DNA.

telomerase.html: 16_telomerase.jpg
An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells, to prevent shorter chromosomes being passed to future generations.

telomere.html: 16_19Telomeres_LP.jpg
Eukaryotes have noncoding terminal nucleotide sequences called telomeres at the ends of their linear DNA.
The telomeres do not prevent DNA shortening, but can postpone the erosion of genes near the ends of DNA molecules

term.html: 16_16bLaggingStrand_6-U.jpg
Termination. DNA polymerase I removes the RNA primer. DNA ligase joins the Okazaki fragments along the lagging strand.

transformation.html: 16_02GriffithExperiment.jpg
 EXPERIMENT   Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic.

 RESULTS   When Griffith mixed heat-killed pathogenic bacteria with living bacteria of the nonpathogenic strain, some of the living cells became pathogenic.

 CONCLUSION   Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells.

xray.html: 16_06XrayDiffraction.jpg
X-ray diffraction "photo 51" of DNA made by Rosalind Franklin, who was working with Wilkins, provided clues for Watson and Crick (Nobel 1962) to discover the structire of DNA. _Vid_Johnson4e/VideoQuiz/DNADarkLady_ISDN.mov