Definition and Phases of Cell Cycle - Practice Questions & MCQ

Cell Cycle: Introduction

• All cells reproduce by dividing into daughter cells.
• Therefore, cell cycle can be defined as an orderly set of stages that take place when a cell divides and then the daughter cells also divide. 
• Hence, the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
• The cell cycle is divided into the following phases:

                            (a) Interphase
                            (b) Division phase

Utility of Cell Division:

• Cell division enables a single cell to eventually produce many cells, allowing an organism to grow and develop.
• Cell division also occurs when a repair is needed and worn-out tissues have to be replaced.
• Cell division results in the formation of gametes that fuse to form the zygote. Hence, the reproduction and restoration of the chromosome number is maintained by the cell division.
• In unicellular organisms, cell division results in the reproduction of the organism.    

Previous Works on Cell Division:

• Prevost and Dumas (1824) were the first to study cell division during the cleavage of zygote of frog. 
• Nagelli (1846) was the first to propose that new cells are formed by the division of pre-existing cells.
• Rudolf Virchow (1859) proposed "omnis cellula e cellula" and "cell lineage theory”

Cell Cycle: Interphase

• Cell spent most of its time in interphase.
• Cell performs its usual functions during this time.
• In interphase cell grows in size and prepares itself for the next division. 
• Interphase is most active phase of cell cycle. 
• The interphase last more than 95% of the duration of the cell cycle.
• It was earlier regarded as resting phase because the metabolic activities performed by the cell were not visible under a microscope.

Howard and Pelc classified interphase into three sub-stages:
1. G1 - phase or Pre DNA synthesis phase (First Gap phase):

• The cell recovers from the previous division.
• The cell increases in size, doubles its organelles (such as mitochondria and ribosomes), and accumulates materials that will be used for DNA synthesis.
• Cells are constantly performing their normal daily functions, including communicating with other cells, secreting substances, and carrying out cellular respiration.

2. S - phase (DNA synthesis phase):

• Following G1, the cell enters the S stage, when DNA synthesis or replication occurs.
• At the beginning of the S stage, each chromosome is composed of one DNA double helix.
• Following DNA replication, each chromosome is composed of two identical DNA double helix molecules.
• Each double helix is called a chromatid.
• Another way of expressing these events is to say that DNA replication has resulted in duplicated chromosomes, and the two chromatids will remain attached until they are separated during mitosis.
• If the initial amount of DNA is denoted as 2C then it increases to 4C.
• However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.

3. G2 - phase (Second Gap phase) or Post DNA synthesis phase:

• Following the S stage, G 2 is the stage from the completion of DNA replication to the onset of mitosis.
• At this stage, the proteins that will be helpful during the cell division are synthesized.
• For example, proteins that form microtubules will be synthesized.

TIP:
Go - Phase or Quiescent Stage: 

• Some cells, such as nerve and muscle cells, typically do not complete the cell cycle and are permanently arrested.
• These cells are said to have taken an exit from G1 phase and entered a Go stage. 
• Cells in this stage remain metabolically active but do not divide unless specifically required.
• Hence, from the perspective of cell division, this stage is said to be inactive.