Question

Given below are certain functional activities of specific structures in the bodies of living organisms. Name the structure responding for the same. (i) Initiate cell - division in animal cells.
(ii) structure that are present in specific sequences of nucleotides on a chromosome. (iii) The complex in which the DNA strand binds around a core of eight histone molecules.

Answer 1

The cell division cycle

In prokaryotes, DNA synthesis can take place uninterrupted between cell divisions, and new cycles of DNA synthesis can begin before previous cycles have finished. In contrast, eukaryotes duplicate their DNA exactly once during a discrete period between cell divisions. This period is called the S (for synthetic) phase. It is preceded by a period called G1(meaning “first gap”) and followed by a period called G2, during which nuclear DNA synthesis does not occur.

The four periods G1, S, G2, and M (for mitosis) make up the cell division cycle. The cell cycle characteristically lasts between 10 and 20 hours in rapidly proliferating adult cells, but it can be arrested for weeks or months in quiescent cells or for a lifetime in neurons of the brain. Prolonged arrest of this type usually occurs during the G1 phase and is sometimes referred to as G0. In contrast, some embryonic cells, such as those of fruit flies (vinegar flies), can complete entire cycles and divide in only 11 minutes. In these exceptional cases, G1 and G2 are undetectable, and mitosis alternates with DNA synthesis. In addition, the duration of the S phase varies dramatically. The fruit fly embryo takes only four minutes to replicate its DNA, compared with several hours in adult cells of the same species.

Controlled proliferation

Several studies have identified the transition from the G1 to the S phase as a crucial control point of the cell cycle. Stimuli are known to cause resting cells to proliferate by inducing them to leave G1 and begin DNA synthesis. These stimuli, called growth factors, are naturally occurring proteins specific to certain groups of cells in the body. They include nerve growth factor, epidermal growth factor, and platelet-derived growth factor. Such factors may have important roles in the healing of wounds as well as in the maintenance and growth of normal tissues. Many growth factors are known to act on the external membrane of the cell, by interacting with specialized protein receptormolecules. These respond by triggering further cellular changes, including an increase in calciumlevels that makes the cell interior more alkaline and the addition of phosphate groups to the amino acidtyrosine in proteins. The complex response of cells to growth factors is of fundamental importance to the control of cell proliferation

The cell division cycle

In prokaryotes, DNA synthesis can take place uninterrupted between cell divisions, and new cycles of DNA synthesis can begin before previous cycles have finished. In contrast, eukaryotes duplicate their DNA exactly once during a discrete period between cell divisions. This period is called the S (for synthetic) phase. It is preceded by a period called G1(meaning “first gap”) and followed by a period called G2, during which nuclear DNA synthesis does not occur.

The four periods G1, S, G2, and M (for mitosis) make up the cell division cycle. The cell cycle characteristically lasts between 10 and 20 hours in rapidly proliferating adult cells, but it can be arrested for weeks or months in quiescent cells or for a lifetime in neurons of the brain. Prolonged arrest of this type usually occurs during the G1 phase and is sometimes referred to as G0. In contrast, some embryonic cells, such as those of fruit flies (vinegar flies), can complete entire cycles and divide in only 11 minutes. In these exceptional cases, G1 and G2 are undetectable, and mitosis alternates with DNA synthesis. In addition, the duration of the S phase varies dramatically. The fruit fly embryo takes only four minutes to replicate its DNA, compared with several hours in adult cells of the same species.

Controlled proliferation

Several studies have identified the transition from the G1 to the S phase as a crucial control point of the cell cycle. Stimuli are known to cause resting cells to proliferate by inducing them to leave G1 and begin DNA synthesis. These stimuli, called growth factors, are naturally occurring proteins specific to certain groups of cells in the body. They include nerve growth factor, epidermal growth factor, and platelet-derived growth factor. Such factors may have important roles in the healing of wounds as well as in the maintenance and growth of normal tissues. Many growth factors are known to act on the external membrane of the cell, by interacting with specialized protein receptormolecules. These respond by triggering further cellular changes, including an increase in calciumlevels that makes the cell interior more alkaline and the addition of phosphate groups to the amino acidtyrosine in proteins. The complex response of cells to growth factors is of fundamental importance to the control of cell proliferation