Question

Provide a detailed description of the cell cycle and how its regulation results in control of cell proliferation and growth. Include a description of the cell cycle control system (include when, why, and how it can be paused), of apoptosis and its role in regulation, and how extracellular signals regulate cell number and fate.

Answer 1

The term cell was proposed by Howard and pelc.it is the cyclic represention of cell division.cell division occurred ina recurrent manner with a freedom to quit only one point in g1 phase.cell cycle is a progressive event no chance to retrogression.The cell cycle is controlled by checkpoints at different stages. These detect if a cell contains damaged DNA and ensure those cells do not replicate. This cell cycle is also closely regulated by cyclins which control the cell progression by activating cyclin-dependent kinase enzymes.The cell cycle is the process a cell will go through to replicate all of its material and divide itself from one cell into two identical cells. While this is commonly known as Mitosis, in fact Mitosis is just one stage of the cell cycle. In this article, we will look at the different stages of the cell cycle and what happens in each stage. We will also consider the regulation of the cell cycle, and look at some examples of when this goes wrong.division of a cell to produce two daughter cells is fundamental to most forms of life.he ‘life cycle’ of a dividing eukaryotic non-embryonic cell starts with the cell triggered to enter the cell cycle and ends with the equal partitioning of the genetic material and cleavage of the cell during cytokinesis. The whole process is called the cell cycle and consists of four main phases.

Entry to the cycle is made in Gap 1 (G1) phase and this is followed in sequence by a DNA synthesis (S) phase, Gap 2 (G2) phase, and Mitosis (M). After mitosis (M) some cells enter the G1 phase of a new cell cycle whilst others may diverge at the start of G1 into a phase called Gap O (zero). Phases G1, S and G2 are often grouped and called ‘interphase’.

Cells in G-0 (zero) are quiescent and not dividing (hence zero), this may be permanent or temporary.
Mitosis (M phase) had been observed and described in some detail by the start of the 20th century, but it was not until about 50 years later that it was discovered that DNA synthesis took place as a separate process ahead of mitosis. Between mitosis (M) in a previous cell cycle and DNA synthesis (S) there was a time gap. This was designated Gap 1 (G1). The time gap between DNA synthesis (S) and mitosis (M) was designated Gap 2 (G2).

After cell division the daughter cells follow ONE of several pathways

that can divide again immediately enter the G1 phase of a new cell cycle.

other cells enter G-0 phase. Some of them are quiescent for a time but then re-enter G1 phase.

some specialist cells, for example nerve cells, do not divide again.

other cells can be triggered by activities such as wound repair to enter G1 phase of the cell cycle to divide as required.Mitosis is visually very dramatic but it only occupies about 5% of the total cell cycle time. Within the ‘big picture’ of the life cycle of a dividing cell, interphase (phases G1, S, and G2) account for the other 95% of cell cycle time. Research evidence shows interphase (once called resting phase!) operates in a beautifully ordered systematic way. It has also been shown to be more complicated than previously thought.

The time taken for a eukaryotic cell to divide varies widely with cell type and environment. Yeast cells take from 1.5 to 3 hours, intestinal epithelial cells about 12 hours and cells in culture about 22 hours. In different organisms and at different developmental times the details of the cell cycle vary. Embryonic cells in many organisms run a cycle that is shorter than similar cells in the adult. Cells of yeast and mammal show differences in cycle detail but the general mechanism of the cell cycle has been highly conserved over the years.During the cell cycle cytoplasmic chemistry influences to a large extent the activities of the whole cell. At all other times we think in terms of the cell nucleus determining cytoplasmic activity.

important in plants and animals are extracellular signaling molecules that function within an organism to control metabolic processes within cells, the growth of tissues, the synthesis and secretion of proteins, and the composition of intracellular and extracellular fluids.  effects of extracellular signals can also be amplified by enzymatic cascades. At the initiation of the signal, a single ligand binds to a single receptor. However, activation of a receptor-linked enzyme can activate many copies of a component of the signaling cascade, which amplifies the signal.