DNA replication processivity

A further important function of the bending and deformation of DNA in a protein complex can be to partially melt the sequences, thereby making them accessible for recognition in transcription or DNA replication processes. 

Despite the near-perfect fidelity of genetic replication, infrequent, unrepaired mistakes in the DNA replication process lead to changes in the nucleotide sequence of DNA, producing a genetic mutation (Fig. 1-32) and changing the instructions for some cellular component. Incorrectly repaired damage to one of the DNA strands has the same effect. Mutations in the DNA handed down... 

The DNA replication process begins at origin points in the parent DNA strand which are targeted by DNA binding proteins resulting in the separation of the strands and the initiation of the replication fork complex via the helicase enzyme. The origin points tend to be A-T rich regions because of... 

Using the number for the enzymes, select the correct sequence for the DNA replication process. 

Distinguish between the roles of RNA polymerase and primase in the DNA replication process in bacteria. 

Although this reaction is in principle quite simple, it is significantly complicated by specific features of the DNA double helix. First, the two strands of the double helix run in opposite directions. Because DNA strand synthesis always proceeds in the 5 -to 3 direction, the DNA replication process must have special mechanisms to accommodate the oppositely directed strands. Second, the two strands of the double helix interact with one another in such a way that the edges of the bases on which the newly synthesized DNA is to be assembled are occupied- Thus, the two strands must be separated from each other so as to generate appropriate templates. Finally, the two strands of the double helix wrap around each other. Thus, strand separation also entails the unwinding of the double helix. This unwinding creates supercoils that must themselves be resolved as replication continues, as described in Section 28.2. We begin with a consideration of the chemistry that underlies the formation of the phosphodiester backbone of newly synthesized DNA. 

Figure 4. Schematics of the DNA replication process in the cell (see the text for explanation). DNA strands are shown in straight lines, DNA strands in the wavy lines.

An in-vitro nuclear system prepared from HeLa cells, described by Friedman and Mueller (1968), appears to continue the DNA replication process observed in vivo. The system requires intact nuclei, the four deoxyribonucleoside triphosphates, magnesium ion, ATP, and, in addition, a heat-labile cytoplasmic factor. The activity of the system was similar to the DNA synthetic activity observed in intact cells in synchronized culture (Friedman and Mueller, 1968). Cytoplasmic factors also appear to stimulate in-vitro nuclear systems prepared from normal and regenerating rat livers (De Beilis, 1969). The cytoplasmic factors are present in both normal and regenerating liver cytoplasm and stimulate nuclear DNA synthesis in both systems. The stimulation was most marked using regenerating liver factors and normal liver nuclei (De Beilis, 1969). When mouse liver nuclei are recombined with cell free cytoplasmic extracts from mouse ascitic or L-cells active in DNA synthesis there is a marked stimulation of DNA synthesis in the isolated nuclei (Thompson and McCarthy, 1968). Cytoplasmic preparations from HeLa cells also stimulated DNA synthesis in mouse liver nuclei. 

Histone phosphorylation and dephosphorylation occurs at different stages of cell division and the DNA replication processes (below) are believed to be affected by the DNA-histone binding. 

In a first approach to determine the mode of action of the cytostatically active clavines, incorporation studies were performed (Eich et al., 1984a). They revealed that the incorporation of PH]-thymidine into DNA is reduced in the presence of 1-propylfestuclavine. On the other hand, no influence has been determined for [ H]-uridine incorporation into RNA and PH]-phenylalanine incorporation into protein. The inhibition of the thymidine incorporation rate was only observed after a preincubation period with the clavine derivative for 24 h and was not detectable after a pretreatment period of 2h. The assumption was favoured that clavines interfere with DNA replication processes in L5178y cells rather than an unspecific effect on cell growth. This conclusion was based on the observation that all active clavines caused unbalanced growth , a property which they share with other cytostatic agents acting selectively by... 

Another regulator of transcriptional rate is, possibly, associated with the organization of the DNA replication processes. As mentioned in Part I, the transcriptional process is postponed in the late replicated regions. 

---