Cell Cycle: Introduction, Duration, Phases of cell cycle, Regulation, Practice Problems and FAQs

Have you ever wondered how you started your life as a single cell and grew up to be an individual made up of millions of cells? Well, this was possible because cells are capable of dividing and multiplying. Remember what Rudolf Virchow had added to the cell theory? ‘Omnis cellula-e-cellula’ which means that all cells arise from pre-existing cells. Do you know how new cells arise from old ones? It is by the process of cell division. Most cell divisions in our body result in the formation of daughter cells which are exact replicas of the parent cells, except for the cell division which result in the formation of gametes. In such a division, the daughter cells receive only half the amount of genetic material from the parent cell.

Fig: Cell Division

So, now you know that cell division is an important life process that helps us to grow. But what happens once we grow up? Do our cells stop dividing? Well no, they don’t. Cell division not only helps in growth but also helps in repair and regeneration of old and worn out cells in the body. But do all our body cells require repair and regeneration at the same rate? No, they don’t. While the cells of your skin divide once in every month, the ones in your liver divide only once a year. How is this controlled? 

In fact, the entire process of cell division is a complicated event which involves growth of the cell, replication of cell DNA and the actual division of the cell, all orchestrated and coordinated in a controlled manner. This entire sequence of events is known as the cell cycle. 

The cell cycle has multiple stages and multiple checkpoints to regulate the transition from one stage to the next. These checkpoints are genetically controlled by the formation of certain proteins which decide when the cell should start dividing and when it should stop. Sounds interesting right? In this article we will learn about the different stages of cell cycle.

Table of contents:

• Definition of cell Cycle
• Duration of cell cycle
• Phases of cell cycle
• Regulation of cell cycle
• Practice problems
• FAQs

Definition of cell cycle

The series of events in a cell that involve duplication of its DNA and other cellular constituents and their distribution and division into two daughter cells is known as the cell cycle. It is an orderly sequence of events in a cell that occurs from the time of its formation till the time it completely divides into two daughter cells.

Fig: Cell Cycle

Why do cells need to divide?

Once a cell grows to a particular size, it divides to avoid getting bigger. Transport of substances in and out of the cell becomes inefficient as the cell grows larger. Thus, to keep a small size, cells divide after a point of time. 

Also, the DNA content of the cell will remain the same, even if it continues to grow larger. But with larger size, it becomes difficult for the DNA of the cell to keep up with the demands of a larger cell. Thus, the cell divides to keep the size small enough for the cellular DNA to be able to meet all its demands.

Duration of cell cycle

The period taken to complete one cell cycle, that is, from the beginning of one cell division to the beginning of the next, is known as the generation time. Duration of cell cycle varies from cell to cell. A human cell can complete a cell cycle in 24 hours while a yeast cell takes 90 minutes.

Phases of cell cycle

Cell cycle occurs in two phase:

• Interphase: A phase of cell cycle when the cell prepares itself for cell division.
• M phase: A phase of cell cycle when the actual division occurs.

Fig: Phases of cell cycle

Interphase

Introduction

Before the cell divides, it must have all the necessary requirements for producing two daughter cells. Production of necessary requirements of the cell cycle occurs at the interphase stage. Interphase stage occupies 95% of the total duration of the cell cycle. If a cell takes 24 hours to complete a cell cycle, the interphase stage takes 23 hours of the total 24 hours.

Fig: Distribution of time among phases of cell cycle

Significance

There are a few important tasks that a cell needs to accomplish at interphase stage:

• Growth in the size of cells in terms of organelles and membrane such that it can distribute enough cellular constituents to the two daughter cells.
• Creating a copy of the genetic material, i.e, DNA so that genetic material can be distributed among the daughter cells.
• Synthesis and storage of ATP to be used as an energy currency for the various stages of cell division.
• Division of the centrosome and production of two centriole pairs in case of animal cells.

Fig: Formation of copy of centriole

Stages of interphase

Interphase is divided into 3 stages:

• G1 Phase (Gap 1)
• S Phase (Synthesis phase)
• G2 Phase (Gap 2)

Fig: Phases of cell cycle

G1 Phase (Gap 1)

The duration of this phase can vary from a few minutes to several days. Cells which divide very rarely have a longer G1 phase compared to the ones which divide very frequently. The phase between mitosis and S phase of the cell cycle. The cell is metabolically active in this phase and grows larger physically. Production of nutrients and protein required for S phase are synthesised in this phase. Copies of cell organelles are also produced.

A cell in G1 phase can have three fates - 

• It can either commit itself to division and enter the S phase after a brief lag
• It can get arrested at a specific point of the G1 phase itself
• It can enter into the Go or quiescent stage. We will discuss about this stage shortly.

S Phase (Synthesis Phase)

The phase between G1 and G2 phase of the cell cycle. DNA is known as the blueprint of life as it holds all the information required to build the cell and run all the activities of the cell. Daughter cells inherit the genetic material from the parent cell and in order to distribute genetic material equally among daughter cells, a copy of existing genetic material is required. The S phase or synthesis phase involves replication of DNA and production of a copy of the genetic material of the cell. Along with DNA, synthesis of histone proteins, assembly of kinetochore units and formation of nucleosomes also occurs at this stage. S phase commits the cell to divide.

Fig: DNA replication

DNA is present in the form of condensed chromosomes in the nucleus of the cell. During S Phase, if the initial amount of DNA molecules is 2C, it doubles up to 4C by end of synthesis phase. Although the amount of DNA doubles, the number of chromosomes remain the same.

Fig: Change in DNA and Chromosomal Content in S Phase

G2 Phase (Gap 2)

Starts after the S phase of the cell cycle. During this phase the cell size increases, nucleus grows in size, and ATP production and storage continues. There is intensive synthesis of RNA and proteins, Division and multiplication of cell organelles like mitochondria, chloroplasts, etc occurs during the G2 phase. Biochemicals needed for formation of spindle fibres are actively synthesised during this phase.

G0 Phase/Gap Phase/Quiescent stage

Not all cells in our body divide and there are also cells which divide occasionally. Cells which do not divide further enter into the inactive stage called G0 phase after the G1 phase. Cells in this phase, although metabolically active, do not divide unless and until are directed to do so as per the requirement of the cell. Cells such as nerve cells, skeletal muscles, RBCs do not divide after their growth and differentiation and remain in the G0 phase forever.

Fig: G0 Phase in Cell Cycle

M Phase

It is the phase of the cell cycle that occurs after the interphase. It involves the mitotic or meiotic division of a cell. Mitotic division equally distributes the chromosomes in the two daughter cells whereas meiotic division distributes only half the number of chromosomes into the daughter cells. 

Stages of mitosis

Mitosis is divided into two stages:

• Karyokinesis
• Cytokinesis

GIF: Different stages of mitosis

Karyokinesis

The word karyokinesis is derived from two words:

Karyon - Means nucleus

Kinesis - Means movement

This stage of mitosis involves division of nucleus which occurs through following stages:

• Prophase
• Metaphase
• Anaphase
• Telophase

Cytokinesis

The word Cytokinesis is derived from two words:

Cytos - Means cell

Kinesis - Means movement

This stage of mitosis involves division of cytoplasm. After the segregation of chromosomal material, the next step is the division of cytoplasmic material of the cell. The process of cytokinesis differs in a plant cell and animal cell. In animal cells it occurs by furrowing of the cell membrane whereas in plant cells it occurs by cell plate formation.

GIF: Cytokinesis in animal cell

Regulation of cell cycle

The cell cycle is regulated by some biochemical switches which control the transition from one phase to another. These regulatory molecules are of two types -

• Cyclins - act as regulatory subunits which are formed in the cell when it gets a signal to divide.
• Cyclin Dependent Kinases (CDKs) - are catalytic subunits which are present in the cell in inactive forms. They are activated when cyclins bind to them.

Together the cyclin and CDKs trigger the phosphorylation of proteins which push the cell to move from one phase to another. 

The various cell cycle checkpoints are -

• G1/S checkpoint - controls transition of cell from G1 to S phase.
• G2/M checkpoint - controls transition of cell from G2 to M phase.
• M checkpoint - controls transition from metaphase to anaphase during the M phase.

Practice problems

Q1. A cell has 12 chromosomes in its nucleus. What will be the number of chromosomes in the cell after DNA replication at the end of S phase?

(a) 24
(b) 12
(c) 6
(d) 36

Solution: During the S phase of the cell cycle the DNA replicates and doubles itself. However the chromatin fibres in the nucleus of the cell do not duplicate and hence even though the cell has double the amount of DNA, the number of chromosomes in the nucleus remains the same. Thus the cell will have 12 chromosomes at the end of S phase.

Hence, the correct option is b.

Q2. Duplication of mitochondria and chloroplasts occurs during which phase of the cell cycle?

(a) G1 phase
(b) G2 phase
(c) S phase
(d) both (a) and (b)

Solution: Both G1 and G2 phases involve the synthesis of RNA and proteins and division and duplication of cell organelles such as mitochondria and chloroplasts. S phase is involved only with the replication of DNA.

Thus, the correct option is d.

Q3. Which of these is the most eventful period of the cell cycle?

(a) G1 phase
(b) G2 phase
(c) S phase
(d) M phase

Solution: Most of the visible events and changes in the cell take place during M phase. In spite of being the shortest phase of the cell, the cell undergoes maximum amount of change in its nucleus and cytoplasm during this phase. The M phase is further divided into karyokinesis (division of nucleus) and cytokinesis (division of cytoplasm). The karyokinesis involves four phases - prophase, metaphase, anaphase and telophase, during which the chromosomes duplicate themselves, divide to form daughter chromosomes and get distributes into the two daughter nuclei that are formed.

Thus, the correct answer is option d.

Q4. Find out the correct statement:

(a) A somatic cell that has just completed the S phase of its cell cycle has twice the number of chromosomes and twice the amount of DNA compared to a gamete of the same species.
(b) A somatic cell that has just completed the S phase of its cell cycle has the same number of chromosomes but twice the amount of DNA compared to a gamete of the same species.
(c) A somatic cell that has just completed the S phase of its cell cycle has twice the number of chromosomes and four times the amount of DNA compared to a gamete of the same species.
(d) A somatic cell that has just completed the S phase of its cell cycle has four times the number of chromosomes and twice the amount of DNA compared to a gamete of the same species.

Solution: Suppose, a somatic cell has 2n number of chromosomes in its normal diploid condition and the amount of DNA in the cell is C. A gamete of the same species will have n number of chromosomes and ½ C amount of DNA. At the end of S phase, the somatic cell will have double the amount of DNA, that is, 2C but the number of chromosomes will be the same. Thus, at this point, the cell has four times the amount of DNA and double the number of chromosomes compared to a gamete of the same species.

Thus, the correct option is c.

FAQs

Question 1. Being the resting phase, is interphase a metabolically inactive phase?
Answer: The interphase is wrongly called as the resting phase as this phase is metabolically highly active and actively synthesises DNA, RNA, proteins, ATP and all that it would need for cell division.

Question 2. What is cell cycle arrest?
Answer: Cell cycle arrest refers to the stopping of the cell cycle at one of the checkpoints. Cell cycle arrest can be triggered by lack of nutrients, DNA damage, certain chemicals, etc.

Question 3. What does cell cycle arrest at the G2/M phase indicate?
Answer: If the cell cycle is arrested at the G2/M phase checkpoint, then it indicates that the cell has some errors in DNA replication or DNA damage. At this point, the cell either tries to repair the damage or undergoes a programmed cell death if the damage is irreparable.

Question 4. How do cells die?
Answer: Cells can die in four different ways -

• Apoptosis or programmed cell death is activated due to the release of certain chemicals known as caspases which destroy the cell from within and the cell contents are not released outside. Thus, it does not trigger the immune system.
• Necrosis is the death of the cell due to release of toxins or lack of blood supply. Cell contents are released into the blood stream and the immune system is triggered.
• Necroptosis is a programmed cell death triggered by certain proteins but the cell contents may leak out.
• Pyroptosis is the highly inflammatory death of virus or bacteria infected cells due to release of cytokines.

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