In which of the following phase (s) of Cell cycle, both, the RNA synthesis and Protein synthesis occurs:
Ans. a (G 1). (Ref Robin's pathology 7th ed., 178; Harrison's internal medicine 18th ed. Ch. 103, 357)THE CELL CYCLE (G, A G2 and M)# During development from stem to fully differentiated, cells in the body alternately divide (mitosis) and "appear" to be resting (interphase). This sequence of activities exhibited by cells is called the cell cycle.# Interphase:- Interphase generally lasts at least 12 to 24 hours in mammalian tissue.- During this period, cell is constantly synthesizing RNA, producing protein, and growing in size.- The inactive X can be seen in the interphase nucleus as a darkly staining small mass in contact with the nuclear membrane known as the Barr body or X chromatin.- By studying molecular events in cells, scientists have determined that interphase i.e. the cell cycle can be divided into 4 phases:# Phase of cell cycle:GO (Resting phase)There are times when a cell will leave the cycle and quit dividing.This may be a temporary resting period or more permanent.An example of the latter is a cell that has reached an end stage of development and will no longer divide (e.g. neuron).G1 (Gap 1)Cells increase in size in Gap 1, crocface RNA os well as synthesizAn important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis i.e. the cell determines its readiness to commit to DNA synthesis.S(DNA synthesis)To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.Thus the genetic material is replicated and no re-replication is permitted. In-situ DNA nick end labeling can quantitative fraction of cells in synthetic (S) phase of cell cycle.G2 (Gap 2)During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins.At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.Thus then fidelity of DNA replication is assessed and errors are corrected.Mitosis or M phaseCell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells.Thus, the replicated chromosomes are separated and packaged into new nuclei by mitosis and the cytoplasm is divided between the two daughter cells by cytokinesis.Mitosis is much shorter than interphase, lasting perhaps only one to two hours.As in both G1 and G2, there is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division.# The continuously dividing human cell takes approximately 22 hours to divide.# Normally time required for different phases are as follows:- G2 takes five hours,- M phase takes one hour,- S phase takes eight hours, and- G1 phase takes eight hours.# However the time required for G, and G2 phase are characteristically variable.# Most normal cells are in G() phase and rapidly dividing cells have shorter G, phase.# It is notable that tumor cells can no longer enter G0 phase.# The cells in G2M, especially in M-phase, are most radiosensitive.# The cells in S phase are radioresistant.Also remember:a) Cell cycle nonspecific: Kill resting as well as dividing cells, e.g. nitrogen mustard, cyclophos phamide, chlorambucil, carmustine, dacarbazine, busulfan, L-asparaginase, cisplatin, procarba zine, actinomycin D.b) Cell cycle specific:Kill only actively dividing cells. Their toxicity is generally expressed in S phase. However, these drugs may show consi durable phase selectivity, e.g.-TimeG 1 :VinblastineS :Mtx, cytarabine, 6-TG, 6-MP, 5-FU, hydroxyurea, mitomycin C, doxorubicin, daunorubicin.G2: Daunorubicin, bleomycin, etoposide, topotecan.M:Vincristine, docetaxel.
G1
G2
S
All of the above
Molecular Genetics