Bacteria, as well as eukaryotes, can carry transposable elements. They typically have repeats associated with their ends. They cannot replicate free of the bacterial chromosome (unless as part of a plasmid). Two types are known: Insertion (IS) sequences, which lack bacterial genes and Transposons (TN), which have bacterial genes. Both normally have transposase genes allowing movement. Transposons often also carry antibiotic resistance genes. Transposon tagging is a method for genetic analysis which involves disrupting a gene by insertion of a transposon. The gene that is mutated can be identified by selecting for the transposon (e.g., antibiotic resistance) and studying the cloned gene with inserted transposon.
Genome sizes generally increase with organismal complexity. This is quite apparent in comparing prokaryotes and eukaryotes. The correlation breaks down among the eukaryotes due to the high genome sizes in some plants and animals.
The field of molecular genetics really began with Watson and Crick’s discovery of the structure of DNA. The two had complementary backgrounds: Watson was a biologist and Crick was a physicist. The information they used included: Chargaff’s rules (A=T, G=C), knwoing the function s of the genetic material (replication, information, mutation) and model building. The model building was based in part on X-ray diffraction data collected by Rosalind Franklin.
The structure of DNA proposed by Watson and Crick explained how it could function as the genetic material. Bases project into the center of the double helix, A pairing with T and G pairing with C through hydrogen bonds. The sugar deoxyribose attaches to the bases through the 1’ carbon. Phosphate groups link the sugars through the 5’ and 3’ carbons of the sugar. The two chains run in opposite directions (are anti-parallel). Growth is at the 3’ end, where a hydroxyl group attacks the first phosphate of a tri-phosphate, forming a new phosphate bond. The major and minor grooves of the double helix are exposed, allowing proteins to recognize the bases in specific fashion.
Replication of the DNA molecule is semi-conservative This is the model originally proposed by Watson and Crick. Alternative models include conservative and dispersive replication. The classic Meselson-Stahl experiment used density-labelled DNA to prove the semi-conservative model. N-15 vs. N-14 labelled DNA was distinguished using CsCl density gradient centrifugation.
Topoisomerase enzymes help unwind the double helix by nicking it prior to replication. They also are involved in sealing nicks. Circular prokaryotic chromosomes can replicate in two different ways. A theta structure is present in the replication of the E. coli chromosome, for example. Rolling circle replication is present in the replication of the F factor, for example. Multiple origins of replication are present per chromosome eukaryotes, unlike the normal situation in prokaryotes. This is likely because of the large size of eukaryotic chromosomes, in order for replication to be completed in a reasonable length of time.