Cell Cycle

3 phases of cell cycle

Resting phase ( G0 Phase)

1. Inter Phase (Divided in to 2 sub phases).
1. G1 Phase
2. S Phase
3. G2 Phase
2. M Phase or Mitosis Phase

G0 Phase: In G0 Phase the cell exist in a quiescent state. G0 phase is viewed as either an extended G1 phase where the cell is neither dividing nor preparing to divide and or as a distinct quiescent stage which occurs outside of the cell cycle.

G0 is sometimes referred to as a "post-mitotic" state since cells in G0 are in a non-dividing phase outside of the cell cycle;

The nerve cells after reaching maturity (i.e., when they are terminally differentiated) become post-mitotic (enter G0 phase) but continue to perform their main functions for the rest of the life.

Inter Phase: The majority of eukaryotic cells spend most of their time in interphase.

In this phase cell performs the majority of its purposes including preparation for cell division. In preparation for cell division it increases its size and number of organelles, and makes a copy of its DNA. Interphase is also considered to be the 'living' phase of the cell, in which the cell obtains nutrients, grows, reads its DNA, and conducts other "normal" cell functions.

G1 Phase (Gap 1):  In this phase the cell grows and functions normally. During this time, a lot of protein synthesis occurs and the cell grows (to about double its original size) - more organelles are produced, increasing the volume of the cytoplasm. If the cell is not to divide again, it will remain in this phase.

S Phase (Synthesis Phase): In this phase the cell duplicates its DNA (via semi-conservative replication)

 G2 Phase (Gap 2):  It is the shortest subphase during interphase in which the cell undergoes a period of rapid growth to prepare for mitosis. It follows the S phase, and occurs during a period of often four to five hours (for human cells). In this phase the nucleus is well defined, bound by a nuclear envelope and contains at least one nucleolus. Although chromosomes have been replicated they cannot yet be distinguished individually because they are still in the form of loosely packed chromatin fibers.

The G2 phase prepares the cell for mitosis (M phase) which is initiated by Prophase

 M Phase (Mitosis): Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei.[1] It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle.

Criteria for Brain Death

For pediatir patients of less than 5 years

The precondition and the Physical examination are similar to adults difference lies in the duration of observation

For 7 days to 2 months children period of observation is 2 examinatin and EEG's searated by atleast 48 hours

For 2 months to 1 year children  examinatin and EEG's searated by atleast 24 hours. 

A repeat examination and EEG are not necessary if a concomittant radionuclide flow study shows no visualization of cerebral arteries

For child of > 1 year - When an irreversible cause exists laboratory testing is not required but an observation period of atleast 12 hours is recommended

Brain Death

Brain Death is defined as "irreversible sessation of all fuctions of entire brain"

Criteria for brain death

Certain preconditions to be excludede prior to proceeding for the tests of brain death

    1. Rule out depressent drug use

    2. Rule out Hypothermia ( Temp should be > 35 Degree Cetigrade)

    3. Rule out Neuromuscular blocking drugs

    4. Rule out metabolic and endocrine disturbances

On physical examination certain criteria should be met to declare brain stem death

    1.  Coma and Apnea must coexist

    2. Absence of Brainstem function should be defined by

        a. Mid position of pupil

        b. Absebce of eye movements induced by oculocephalic and caloric tests

        c. Absence of movements of bulbar musculature including Gag and Cough reflex

       d. Respiratory movements absent with patient  on respirator