The differences between Eukaryotes and （真核生物和之间的区别）.pdf

PHAR 2811 Dale’s lecture 5 page 1 The differences between Eukaryotes and Prokaryotes Eukaryotic Replication. Replication is intimately linked to cell division in all organisms; both prokaryote and eukaryote. Cell division in eukaryotes is carried out in the context of the cell cycle. Unlike prokaryotes which can double under optimal conditions in as little as 20 min the eukaryotic cell cycle takes some 18 to 24 h to complete. The cell cycle consists of a number of phases: G1, S, G2 and M. • G1 = growth and preparation of the chromosomes for replication • S = synthesis of DNA (and centrosomes) • G2 = preparation for M • M = mitosis G1, S and G2 are collectively referred to as interphase. If cells are no longer dividing, such as with terminally differentiated cells, they are considered to be in G0. As the cell passes through each phase of the cell cycle certain checks must be satisfied. At critical stages the level of special cytoplasmic proteins known as cyclins rise then subside once the cell has passed through the phase. There are also a group of enzymes known as cyclin dependent kinases (cdks) which phosphorylate protein targets involved in the control of the cell cycle. The catch is that, although the cdks are present at fairly steady concentrations in the cell, independent of the cell cycle phase, they only become activated when they bind the appropriate cyclin. Certain key indicators are used at each checkpoint. A check for the completion of S phase (replication) is the presence of Okazaki fragments. If replication is complete there should be no Okazaki fragments. DNA damage is checked for at G1 before the cell enters S. Spindles are checked before the cell actually divides (M phase) PHAR 2811 Dale’s lecture 5 page 2 The similarities between prokaryotic and eukaryotic replication: