Module 3: Genomes

• The complete set of DNA molecules possessed by an organism is referred to as a genome.
• The human genome comprises 3200 Mb DNA split into 24 linear DNA molecules, the shortest being 48 Mb and the longest being 250 Mb, each one in a different chromosome.
• In a normal diploid cell there are 46 chromosomes, two sets of Chromosomes 1-22 and either XX or XY, amounting to 6400 Mb DNA.
• The E. coli genome comprises 4.64 Mb DNA contained in a single DNA molecule, which is circular.
• Replication of the E. coli genome begins at an origin of replication, always the same position on the genome, with two replication forks moving bidirectionally.
• Replication of human DNA also begins at origins of replication, with many on each chromosomal DNA molecule, each replication fork copying about 150 kb of DNA.
• DnaA proteins bind close to the origin of replication, causing the DNA to become wound around these proteins, this forces the base pairs to break at the origin of replication, which is A-T rich and easier to come apart.
• Dna B helicase, a helicase, forms the prepriming complex by attaching to the origin, this complex is formed by attachment of DnaB proteins to the origin.
• The primosome is formed by attachment of two primase enzymes, these make the RNA primers that initiate replication of the two leading strands.
• The events at the replication fork begin with the primosome.
• Replication of human DNA also begins at origins of replication, there are many on each chromosomal DNA molecule, each replication fork copies about 150 kb of DNA.
• DNA strand separation followed by specific interaction of the Ter C6 base with the Tus lock domain results in replication fork arrest.
• Primase is making primers on leading and lagging strands.
• Primase/DNA pol alpha is making primers on leading and lagging strands.
• DNA pol delta (two copies) is synthesizing DNA.
• Replication forks become trapped at the correct place.
• Replication of the E. coli genome always meets at the replication forks.
• Helicase breaks base pairs.
• Helicase (DNA B) breaks base pairs.
• Tus proteins allow the replication fork to pass in one direction but not the other, creating a Permissive face and Non-permissive face.
• Replication forks just merge, don’t need exact control like in a circular DNA.
• DNA pol I and DNA ligase remove primers and join Okazaki fragments.
• The E. coli genome comprises 4.64 Mb DNA contained in a single DNA molecule, which is circular.
• Single Strand Binding proteins (SSBs) protect the bare single strands.
• There are terminator sequences in the E. coli genome, each one is the binding site for a Tus protein.
• The ‘proliferating cell nuclear antigen’ (PCNA) sliding clamp holds DNA Pol delta tightly onto the DNA.
• The gamma complex (clamp loader) attaches and detaches Pol III from the lagging strand.
• FEN1 and DNA ligase remove primers and join Okazaki fragments.
• At the replication fork in E. coli DNA topoisomerase unwinds the strands.
• Beta complex (sliding clamp) holds Pol III onto template allowing it to slide.
• DNA pol III (two copies) is synthesizing DNA.
• At the replication fork in humans, DNA topoisomerase unwinds the strands.
• Higher order packaging of DNA includes chromatin, histones, and 30-nm fibres.
• The lecture is taught by Ray O’Keefe, the lecturer for BIOL10221 Molecular Biology.
• Euchromatin and Heterochromatin are different states of DNA packaging that allow proper expression of genes.
• Centromeres and telomeres are unique features of chromosomes that allow their proper maintenance and segregation during cell division.
• The human genome comprises 3200 Mb DNA split into 24 linear DNA molecules, the shortest being 48 Mb and the longest being 250 Mb, each one in a different chromosome.
• In a normal diploid cell, there are 46 chromosomes (Two sets of Chromosomes 1-22 and either XX or XY), contributing to 6400 Mb DNA.
• Human chromosome 8 is about 3.5 µm in length, containing a single DNA molecule of 145 Mb.
• DNA extracted from the nucleus is called Chromatin, which is a DNA-protein complex with the proteins spaced at regular intervals along the DNA.