All newly synthesized polynucleotide strands must be initiated by a specialized RNA polymerase called primase. Primase initiates polynucleotide synthesis and by creating a short RNA polynucleotide strand complementary to template DNA strand. This short stretch of RNA nucleotides is called the primer. Therefore, the two newly-synthesized strands grow in opposite directions because the template strands at each replication fork are antiparallel.
The pieces are called Okazaki fragments, and each fragment begins with its own RNA primer. Eukaryotic chromosomes have multiple origins of replication, which initiate replication almost simultaneously. Each origin of replication forms a bubble of duplicated DNA on either side of the origin of replication. However, DNA polymerase cannot catalyze the formation of a phosphodiester bond between the two segments of the new DNA strand, and it drops off.
These unattached sections of the sugar-phosphate backbone in an otherwise full-replicated DNA strand are called nicks. Later, during G 2 , the cell similarly checks its readiness to proceed to mitosis. Together, the G 1 , S, and G 2 phases make up the period known as interphase. Cells typically spend far more time in interphase than they do in mitosis. Of the four phases, G 1 is most variable in terms of duration, although it is often the longest portion of the cell cycle Figure 1.
Figure Detail. In order to move from one phase of its life cycle to the next, a cell must pass through numerous checkpoints. At each checkpoint, specialized proteins determine whether the necessary conditions exist. If so, the cell is free to enter the next phase. If not, progression through the cell cycle is halted. Errors in these checkpoints can have catastrophic consequences, including cell death or the unrestrained growth that is cancer. Each part of the cell cycle features its own unique checkpoints.
For example, during G 1 , the cell passes through a critical checkpoint that ensures environmental conditions including signals from other cells are favorable for replication. If conditions are not favorable, the cell may enter a resting state known as G 0.
Some cells remain in G 0 for the entire lifetime of the organism in which they reside. For instance, the neurons and skeletal muscle cells of mammals are typically in G 0.
Another important checkpoint takes place later in the cell cycle, just before a cell moves from G 2 to mitosis. Here, a number of proteins scrutinize the cell's DNA, making sure it is structurally intact and properly replicated. Mitosis is the nuclear division process in eukaryotic cells and ensures that each daughter cell receives the same number of chromosomes as the original parent cell. Mitosis can be divided into the following phases: prophase, metaphase, anaphase, and telophase.
Prophase: During prophase, the chromatin condenses and the chromosomes become visible. Also the nucleolus disappears, the nuclear membrane fragments, and the spindle apparatus forms and attaches to the centromeres of the chromosomes. Metaphase: During metaphase, the nuclear membrane fragmentation is complete and the duplicated chromosomes line up along the cell's equator.
Anaphase: During anaphase, diploid sets of daughter chromosomes separate and are pushed and pulled toward opposite poles of the cell.
This is accomplished by the polymerization and depolymerization of the microtubules that help to form the spindle apparatus. Telophase: During telophase, the nuclear membrane and nucleoli reform, cytokinesis is nearly complete, and the chromosomes eventually uncoil to chromatin. Usually cytokinesis occurs during telophase. During cytokinesis, the dividing cell separates into two diploid daughter cells.
In animal cells, which lack a cell wall and are surrounded only by a cytoplasmic membrane, microfilaments of actin and myosin attached to the membrane form constricting rings around the central portion of the dividing cell and eventually divide the cytoplasm into two daughter cells.
In the case of plant cells , which are surrounded by a cell wall in addition to the cytoplasmic membrane, carbohydrate-filled vesicles accumulate and fuse along the equator of the cell forming a cell plate that separates the cytoplasm into two daughter cells. Compare prokaryotic and eukaryotic DNA replication in terms of origins of replication. With the advent of modern medicine, preventative health care, and healthier lifestyles, the human life span has increased, and there is an increasing demand for people to look younger and have a better quality of life as they grow older.
In , scientists found that telomerase can reverse some age-related conditions in mice. This may have potential in regenerative medicine. Telomerase reactivation in these mice caused extension of telomeres, reduced DNA damage, reversed neurodegeneration, and improved the function of the testes, spleen, and intestines. Thus, telomere reactivation may have potential for treating age-related diseases in humans. Cancer is characterized by uncontrolled cell division of abnormal cells.
The cells accumulate mutations, proliferate uncontrollably, and can migrate to different parts of the body through a process called metastasis. Scientists have observed that cancerous cells have considerably shortened telomeres and that telomerase is active in these cells. Interestingly, only after the telomeres were shortened in the cancer cells did the telomerase become active.
If the action of telomerase in these cells can be inhibited by drugs during cancer therapy, then the cancerous cells could potentially be stopped from further division. Replication in eukaryotes starts at multiple origins of replication. The mechanism is quite similar to that in prokaryotes. A primer is required to initiate synthesis, which is then extended by DNA polymerase as it adds nucleotides one by one to the growing chain. The leading strand is synthesized continuously, whereas the lagging strand is synthesized in short stretches called Okazaki fragments.
Telomerase, an enzyme with an inbuilt RNA template, extends the ends by copying the RNA template and extending one strand of the chromosome. In this way, the ends of the chromosomes are protected.
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