Cell division, replication

During cell division (replication), part of the double helix unwinds, temporarily separating the base pairs. The individual strands then act as templates for producing new, complementary strands. After replication, each double helix contains one old and one new strand, thereby ensuring that all genes are intact. DNA is responsible also for the synthesis of KNA. 

How is replication tied to cell division Replication is tied to cell division by several proteins including the origin recognition complex, replication activator protein, and rep-hcation licensing factors. The process is controlled by cyclins, proteins produced during the Gj and S phases that bind to cyclin-dependent kinases and activate replication. 

All organisms share basic developmental processes such as membrane synthesis, cell division, replication of nucleic acids, and ribosome-mediated protein synthesis. 

ITowever, most normal somatic cells lack telomerase. Consequently, upon every cycle of cell division when the cell replicates its DNA, about 50-nucleotide portions are lost from the end of each telomere. Thus, over time, the telomeres of somatic cells in animals become shorter and shorter, eventually leading to chromosome instability and cell death. This phenomenon has led some scientists to espouse a telomere theory of aging that implicates telomere shortening as the principal factor in cell, tissue, and even organism aging. Interestingly, cancer cells appear immortal because they continue to reproduce indefinitely. A survey of 20 different tumor types by Geron Corporation of Menlo Park, California, revealed that all contained telomerase activity. 

The consequence of ADA deficiency is accumulation of adenosine and 2 -deoxyadenosine, substances toxic to lymphocytes, important cells in the immune response. 2 -Deoxyadenosine is particularly toxic because its presence leads to accumulation of its nucleotide form, dATP, an essential substrate in DNA synthesis. Elevated levels of dATP actually block DNA replication and cell division by inhibiting synthesis of the other deoxynncleoside 5 -triphosphates (see Chapter 27). Accumulation of dATP also leads to selective depletion of cellular ATP, robbing cells of energy. Children with ADA SCID fail to develop normal immune responses and are susceptible to fatal infections, unless kept in protective isolation. 

The double helix model provides a simple explanation for cell division and reproduction. In the reproduction process, the two DNA chains unwind from each other. As this happens, a new matching chain of DNA is synthesized on each of the original ones, creating two double helices. Since the base pairs in each new double helix must match in the same way as in the original, the two new double helices must be identical to the original. Exact replication of genetic data is thereby accomplished, however complex that data may be. 

Microbial growth was discovered with replication of each cell to three daughter cells. With the growth data define the mean time for the cell divisions. Table E.10.1 shows the cell dry weight increases with culture incubation time. 

Alkylating drugp interfere with the process of cell division of malignant and normal cells. The drug binds with DNA, causing breaks and preventing DNA replication. 

Fig. 24.1 The requirement for a cloning vector (A) fragments of DNA introduced into the hacterium hy transformation do not undergo replication and gradually are diluted out of the population (B) DNA fragments introduced into plasmids are inherited hy both daughter progeny cell division.

If these results suggest a positive role for nucleolin on proliferation, they do not indicate which of its activities are responsible for it. One hypothesis is that the stimulation of ribosome biogenesis by nucleolin is indispensable for active cell division. Indeed, a direct link between protein translation and cancer is clearly emerging (Ruggero and Pandolfi, 2003). However the studies mentioned above suggest a more direct impact of nucleolin on cell division. If part of its effects could come from its capacity to repress p53 mRNA translation (Takagi et al, 2005), a more direct role of nucleolin in DNA replication can also be considered. 

When cells divide, each daughter cell must receive an accurate copy of the genetic information. DNA replication is the process in which each chromosome is duplicated before cell division. 

Gj phase (gap 2) is a period of cellular growth after DNA synthesis but preceding mitosis. Replicated DNA is checked for any errors before cell division. 

Genetic information is transmitted from parent to progeny by replication of parental DNA, a process in which two daughter DNA molecules are produced that are each identical to the parental DNA molecule. During DNA replication, the two complementary strands of parental DNA are pulled apart. Each of these parental strands is then used as a template for the synthesis of a new complementary strand (semiconservative replication). During cell division, each daughter cell receives one of the two identical DNA molecules. 

The cell cycle is a key process that recurs in a periodic manner. Early cell cycles in amphibian embryos are driven by a mitotic oscillator. This oscillator produces the repetitive activation of the cyclin-dependent kinase cdkl, also known as cdc2 [131]. Cyclin synthesis is sufficient to drive repetitive cell division cycles in amphibian embryonic cells [132]. The period of these relatively simple cell cycles is of the order of 30 min. In somatic cells the cell cycle becomes longer, with durations of up to 24 h or more, owing to the presence of checkpoints that ensure that a cell cycle phase is properly completed before the cell progresses to the next phase. The cell cycle goes successively through the phases Gl, S (DNA replication), G2, and M (mitosis) before a new cycle starts in Gl. After mitosis cells can also enter a quiescent phase GO, from which they enter Gl under mitogenic stimulation. 

---