Cell Cycle

Do you know the cause of cancer is related to cell cycle? The cancer

cells cannot be regulated by the cell cycle and therefore constantly

divides even in the absence of the required growth factors. This is the

reason why it is difficult to kill cancer cells. Let us learn more about

the cell cycle.

Cell Cycle

Cell cycle and cell division form the basis of life. Growth,

regeneration, and reproduction depend upon cell cycle and cell

division. However, what happens when this cell cycle is disturbed or

when the cells fail to regulate the cell cycle? It can lead to

uncontrolled cell growth and the division which can finally lead to

cancer

Scientists and microscopists have known cell cycle and cell division

for over hundred years. However, it was the pioneering and

revolutionary work of Alma Howard and Stephen Pelc in the 1950s

that revealed DNA replication took place at a particular phase of the

cell cycle. They demonstrated that cell cycle is distinct and separate

from the mitosis process.

Browse more Topics under Cell Cycle And Cell Division

● Meiosis

● Mitosis

Definition

Cell cycle (cell division cycle) is an ordered sequence of events

occurring in a cell. Cell cycle results in cell growth and DNA

replication thereby forming two daughter cells. It is an essential

process for the formation of a mature organism from single-celled

fertilized eggs. The process of cell cycle promotes renewal and

regeneration of hair, blood cells, skin, and certain internal organs. The

formation of daughter cells by cell division indicates the start of a new

interphase cycle.

Different Stages of a Cell Cycle

Stages of Cell Cycle

I = Interphase, M = Mitosis; inner ring: M = Mitosis, G1 = Gap 1, G2

= Gap 2, S = Synthesis; not in ring: G0 = Gap 0/Resting.

(Source Credit: Wikipedia)

A cell splits after completing a sequence of events. For example, a cell

grows, replicate its DNA (genetic material), and then finally divides

into two daughter cells. The cell cycle is termed a cycle because the

events repeat itself. After completing one complete round the newly

formed daughter cells begin the same process all over again. The two

key phases of a cell cycle are interphase and M phase or the mitotic

phase.

Every phase will be successfully activated on proper progression and

completion of the previous phases. However, if a cell is temporarily

stopped progressing or somehow stopped dividing then the cell enter

into another state termed as G0 phase, also called a “state of

quiescence.” Let’s discuss the steps or phases in details.

Interphase

In this phase, the cell grows and produces a copy of the genetic

material (DNA). Interphase can be further subdivided into three

distinct phases: G1 phase, S phase (synthesis), G2 phase. The cell

cycle begins after the division of mother cell into two new daughter

cells. If the newly formed cell wants to move on then it must divide

itself.

However, there are certain initial steps that occur before the actual

division. In these phases, the new daughter cell prepares itself for the

division. The interphase usually seems like a resting phase between

the cell divisions but on contrary, it is a phase with a number of

diverse activities. The duration of interphase may vary from 12 to 24

hours in the mammalian tissues.

Sub-phases of Interphase ● Gap 0 (G0) Phase: At times the cell will leave the cycle and

temporarily stop dividing. This is called a resting period. It can

be for a short time or long more permanent period. For example

neurons after reaching the end stage of development stop

dividing and enter into a more permanent resting phase.

● Gap 1 (G1) Phase: It is also termed as the first gap phase. In

this phase, the cell starts growing and enlarges physically. It

forms the copy of organelles, produces all the necessary

molecular building blocks such as RNA and also synthesizes

proteins that are essential in later stages. At this point, a control

mechanism is activated to ensure proper DNA synthesis. The

control mechanism is termed as the G1 checkpoint.

● S Phase: In this phase, a cell produces a complete copy of DNA

in the nucleus to produce two similar daughter cells. DNA

replication begins in the S phase or the synthesis phase. The

microtubule-organizing structure (centrosome) is also copied in

this phase. The centrosome is the structure that helps in

dividing the DNA during M phase.

● Gap 2 (G2) phase: In G2 phase the cell grows further, produce

proteins and organelles and starts rearranging the constituents

of the cell for mitosis phase. At the end of the G2 phase,

another checkpoint is activated called as G2 Checkpoint. G2

Checkpoint ensures everything is ready for division and M

phase. The end of the G2 phase ends when the mitosis process

begins.

Interphase can be summarized as the phase between two M (mitotic)

phase.

M phase

n this phase, the cell splits its DNA into two copies. Additionally, the

division of the cytoplasm takes place thereby forming two daughter

cells. M phase can be categorized into karyokinesis (the division of

cell chromosome) and cytokinesis (the division of cell cytoplasm to

form new daughter cells). M phase is categorized into two distinct

phases: mitosis and cytokinesis. In this phase, the cell divides the

duplicated DNA and the cytoplasm into two new daughter cells.

● Mitosis: The cell’s “nuclear DNA” is condensed into

chromosomes. These visible chromosomes are pulled apart

with the help of mitotic spindles (the special structures formed

from microtubules). Mitosis is further subdivided into 4

separate stages including prophase, metaphase, anaphase, and

telophase.

● Cytokinesis: Cytokinesis begins after mitosis is complete. In

this phase, the cytoplasm of the cell is divided into two

daughter cells.

Cell Cycle Exit and Gap 0 Phase

By now we know the cell cycle results in the formation of two new

daughter cells. Now the question arises what happens to the newly

formed daughter cells after one complete round of cell cycle? This is

entirely dependent on what cells are dividing. There are certain kinds

of cells that divide quickly and in these types of cells, the new

daughter cells immediately enter into another round of cell division

cycle. Examples include embryo and tumour.

Alternatively, there are other types of cells that divide at a slow pace

and sometimes completely stop dividing and enter into another phase

called as G0 phase or resting phase. The cell will continue its usual

function. Such as neuron after the end stage does not divide but

continue to conduct signals.

Duration of the Cell Cycle

The cell cycle duration will vary in different types of cells. The G1

phase will continue for approximately 11 hours, S phase will continue

for 8 hours, G2 phase for nearly 4 hours and the M phase for nearly

one hour in a rapidly dividing human cell with cell cycle duration of

24 hours. Some cells may divide faster than human cells whereas

some cells may take more time to complete an entire cell cycle. For

example “budding yeast” will complete the entire cell cycle (4 stages

of the cell cycle) in about 90 minutes.

Solved Example for You

Q: Which is the longest phase of the cell cycle?

a. G2

b. G1

c. S

d. G0

Sol: The correct answer is option “B”. G1 phase is usually the longest

phase of cell cycle. G1 phase is the first gap phase where the cell is

preparing for the other stages of cell cycle. Moreover, G1 phase

follows the mitosis cell division. It is the time for the newly formed

cells to grow before the DNA replication. So, the G1 phase is the

longest. G1 phase can vary in the different type of cells. It can last for

minutes such as prokaryotic cells, hours such as yeast or sometimes

for years such as liver cells.

Meiosis

Sometimes when you’re nervous, you tend to bite your nails and at

times you also end up biting your finger. Yet have you noticed how

just in a span of days the nail regrows and so does the skin of the

finger. How does that happen? Well, it happens because of a process

of cell division known as ‘Meiosis’. But what is it? And how is

different from mitosis? Let us study more about it.

What is meiosis?

It is a specialized form of cell division process in which a single cell

divided twice to form four daughter cells that possess exactly half the

number of chromosomes of the parent cell. Therefore, all the four cells

are haploid and will contain half the original amount of the genetic

information. These cells are call sex cells: sperms in male whereas

eggs in females.

Browse more Topics under Cell Cycle And Cell Division

● Cell Cycle

● Mitosis

Meiosis vs Mitosis

Mitosis & Meiosis are the two major cell division processes.

However, meiosis is solely used for one purpose in a human body. It is

used for the production of gametes cells, also known as the sex cells.

Mitosis and meiosis are quite similar in processes with very few

differences. One of the differences is that mitosis has one division

whereas there are two divisions in meiosis.

This is the reason meiosis is also known as a reduction division

process. Thus, in meiosis germ cells are divided by two fissions of the

nucleus which result in four gametes or sex cells. Each cell possesses

half the number of chromosomes of the parent cell. The process of

meiosis occurs in almost all sexually reproducing single-celled and

multicellular eukaryotic organisms like animal, plants, and fungi.

Stages of Meiosis

Meiosis can be majorly categorized into 9 different stages. As there

are two divisions in meiosis, the two divisions are called meiosis I

(where the cell divides for the first time) and meiosis II (where the cell

divides for the second time).

Meiosis starts with duplication of chromosomes. Then the parent cell

undergoes two rounds of nuclear divisions (meiosis I and meiosis II).

Thus in an overall meiosis process, four daughter cells are produced

from the parent cell. However, before entering the stages of meiosis

the parent cell go through a phase called interphase. Similar to mitosis,

the cells growth begins in the G1 phase of interphase, the

chromosomes are duplicated in the S phase and then in G2 phase the

parent cells prepares for cell division.

Before the start of meiosis I, the chromosomes are duplicated and they

are fused together. The duplicated chromosomes are called sister

chromatids. The point at which the duplicated chromosomes (sister

chromatids) are joined is called centromere. The complete structure

looks like a “letter X”. The chromosomes condense and become

compact at the time of every nuclear division. At this point, they are

visible under the microscope.

Meiosis I

Prophase I

In this stage, the sister chromatids or the chromosomes of the maternal

set combines together with their homologs of the paternal

chromosomes set. The duplicated chromosomes condense and in

combination, they look like two X’s present next to each other.

Every chromosome is made up of two sister chromatids that contain

similar genetic information. The chromosomes pair up in a pattern that

copies of particular chromosomes are paired together. The maternal

and paternal chromatids exchange their parts of DNA and recombine

to develop genetic variations.

The process of exchange of genetic information is called

recombination or crossing over. However, in the case of a male in

humans the sex chromosomes X and Y are not homologs but they can

pair together and exchange DNA. Recombination happens in a small

part of two chromosomes where homology is present.

The nuclear membrane of the cell breaks down releasing the

chromosomes by the end of prophase I. The meiotic spindle that

constitutes microtubules and other proteins that is stretched across the

cell in between the centrioles.

Metaphase I

The chromosome sets arrange themselves next to each other along the

equator of the cell. The meiotic spindle is positioned on both the sides

of the cell. At that time the centrioles are present in the opposite ends

of the cell and the meiotic spindles stretch from them.

The meiotic spindle attaches to the combined sister chromatids (one

chromosome of each pair). At the end of metaphase I, the fused sister

chromatids are latched to the centromere and line up in the center of

the cell. The homologs still represent X sitting next to each other.

Anaphase I

The chromosome pairs are gradually pulled apart when the meiotic

spindle fibres start to contract. As a result, one chromosome is pulled

to one end of the cell and the other chromosome to the opposite end.

Thus each X shape structure moves away from each other towards the

opposite pole.

Telophase I and Cytokinesis

In this stage, the chromosomes completely move apart and are present

in the opposite poles of the cell. Each pole of the cell contains a full

set of chromosomes arrange together. A membrane develops around

each chromosome pair. At this point, the single cell splits in the

middle to create two separate daughter cells by a process called

cytokinesis.

Each daughter cell is composed of a full set of chromosomes present

inside the nucleus. At the end of Meiosis I, two daughter cells each

having a full set of chromosomes are present. The genetic makeup of

each newly formed cells is different because of the DNA exchange

taking place between the homologs earlier.

The newly formed daughter cells enter into meiosis II without further

duplication of chromosomes.

Meiosis II

Meiosis II process is similar to an equational division. Thus by the end

of meiosis II, the chromosome number of cells that are undergoing the

meiosis II process and in the resultant daughter cells remain

unchanged. There are four stages of meiosis II.

Prophase II

After meiosis I, there are two cells with the same number of

chromosomes or the same number of chromatid pairs. The

chromosomes in each daughter cell once again condense into visible X

shaped structure. The nuclear membrane around the nucleus of the

daughter cells disintegrates releasing the chromosome. The meiotic

spindles start forming again and the centrioles duplicate.

Metaphase II- The pair of sister chromatids arrange themselves in line

along the equator of the cell. The centrioles are now present at the

opposite ends in each of the daughter cells. The developed meiotic

spindle fibres at each end attach onto the centromere present in the

sister chromatids of the cell.

Anaphase II

The meiotic spindle fibres start contracting and the sister chromatids

are pulled to the opposite ends. At this point, the chromatids are

separated and they form individual chromosomes.

Telophase II

The chromosomes finally move to the opposite ends of the cell. At

each end or pole, an entire set of chromosomes are arranged together.

Again the nucleus membrane develops around each pair of

chromosomes and two new cell nuclei are formed. This marks the last

phase of meiosis but the cell division process remains incomplete

without the cytokinesis process. So the cell undergoes cytokinesis and

after the process, four granddaughter cells are formed.

Each newly formed cell contains half a set of chromosomes or haploid

chromosomes. Each chromosome is distinct and unique. It is

composed of a mix of genetic material from the paternal as well as

paternal chromosomes of the original cell.

For example in humans, these special cells are known as germ cells.

The germ cells undergo meiosis and create sperms (male) or eggs

(female). Germ cells constitute a total of 46 chromosomes (23

maternal and 23 paternal). Finally, after meiosis, the reproductive cells

or the gametes formed contain 23 genetically distinct and unique

chromosomes each.

Question For You

Q. What does the word homologous means?

1. Different traits with same information

2. Different traits with different information

3. Same traits with different information

4. Same traits with same information

Ans: The correct answer is 3.

Homologous chromosomes, also termed as homologs, is chromosome

pairs from the paternal and maternal chromosome that form a pair

with each other during meiosis. Homologous chromosomes have

similar traits such as length, gene position, and location of centromere

but they have different information. In each homologous chromosome,

the position of the gene is same but the genes may have different

alleles.

Mitosis

Have you noticed how a cut on your finger is gradually repaired in a

few days? This is enabled by a process called Mitosis. Mitosis is the

process responsible for regeneration and repair. Mitosis helps in cell

growth and development. Cells can grow old and wear off or they can

get bruised and injured but eventually, they repair and regenerate. 

Mitosis

Mitosis constitutes a comparatively small portion of a complete cell

cycle but it is one of the imperative parts of the cell cycle. German

Physician and cell biologist “Walther Flemming” coined the term

“mitosis” in the year 1882. He explained the process of how cells split

and separate their chromosome.

The process of cell division that results in the formation of two new

daughter cells is termed as Mitosis. The newly formed daughter cells

are genetically identical to the parent cell and to each other. It plays a

crucial role in a living organism’s life cycle. However, the level of

significance may vary depending on the type of organism

(multicellular or single-celled).

In unicellular organisms such as bacteria, mitosis helps in asexual

reproduction as it produces an identical copy of the parent cell.

Another example of the Eukaryotic unicellular organism is “Amoeba.”

An amoeba uses cell division for the production of new individuals. In

the case of multicellular organisms, mitosis helps in growth and repair

by producing more number of identical cells. For example plants,

animals depend on cell division for their growth by addition of new

cells. It is also used for repairing the injured tissues or replacing the

worn-out tissue by regenerating new cells.

Mitosis refers to the splitting of chromosomes in the eukaryotic cells

during the cell division process. The parent cell divides into two

daughter cells that are identical to the parent cell during the process of

cell division. During the mitosis process, the cell’s nucleus along with

the chromosome is divided to form two new daughter cell nuclei. The

daughter nuclei inherit the same number of chromosomes as that of

the parent nucleus.

Browse more Topics under Cell Cycle And Cell Division

● Cell Cycle

● Meiosis

Learn more about Cell Cycle here.

Importance of Mitosis in Living Process

● Genetic stability- Mitosis helps in the splitting of chromosomes

during cell division and generates two new daughter cells.

Therefore the chromosomes form from the parent

chromosomes by copying the exact DNA. Therefore, the

daughter cells formed as genetically uniform and identical to

the parent as well as to each other. Thus mitosis helps in

preserving and maintaining the genetic stability of a particular

population.

● Growth- Mitosis help in increasing the number of cells in a

living organism thereby playing a significant role in the growth

of a living organism.

● Replacement and regeneration of new cells- Regeneration and

replacement of worn-out and damaged tissues is a very

important function of mitosis in living organisms. Mitosis helps

in the production of identical copies of cells and thus helps in

repairing the damaged tissue or replacing the worn-out cells.

But the degree of regeneration and replacement in multicellular

organisms vary from one another. For example, mitosis process

is used in order to regrowth the legs of newts and crustaceans.

However, the degree of regrowth may vary.

● Asexual reproduction- Mitosis is used in the production of

genetically similar offspring. For example budding of hydra

and yeast, binary fission in amoeba, etc.

Five Basic Stages of Mitosis

Prophase

In this phase the DNA supercoils, chromatin fibres become coiled and

condense into chromosomes. The chromosomes consist of two

chromatids connected at the centromere. The microtubule spindles

fibres start forming at the opposite ends of the cell. The mitotic

spindle is composed of microtubule proteins that slowly increase in

length during the prophase which eventually initiate the cell division

process by elongating it.

The pairs of centrioles (two pairs) move away from each other

towards the poles during the microtubule lengthening process. Finally,

the nuclear envelope disintegrates and the nucleus dissolves.

Late Prophase

Late prophase is also known as the prometaphase. The nuclear

envelope dissolves. The microtubules composed of spindle fibres

move from the pole to the centre of the cell (cells equator).

Kinetochores attach themselves to specialized microtubules called

kinetochore fibres.

Kinetochores are special protein structures that develop on a

chromatid during the process of cell division. It helps in attaching the

spindle fibre to the chromosome. The kinetochore fibres combine with

spindle polar fibres. Finally, the chromosomes start migrating toward

the centre of the cell.

Metaphase

In metaphase, the spindle entirely develops. The nuclear membrane

dissolves completely. Polar fibres keep on extending from the poles to

the centre of the cell. Chromosomes assemble and disassemble

themselves and try to find the centromere of sister chromatids. The

chromosomes arrange themselves in the metaphase plate at 90 degrees

to the spindle poles. The polar fibres produce equal forces and push

the centromere of chromosome thus holding the chromosomes

together at the metaphase plate.

Anaphase

The pairs of centromeres present in the chromosome start to move

away in this phase. The sister chromatids, paired chromosomes,

separate and form a complete chromosome, also termed as daughter

chromosomes. The daughter chromosomes start moving towards the

poles in the opposite ends through the spindle apparatus.

The centromere move first and gradually the kinetochores decreases in

length as the chromosomes move closer to the pole. The two cell poles

migrate further away during the anaphase and prepare for telophase.

At the end of this stage, both the pole contains a complete set of

chromosomes. Cytokinesis starts at this stage and continues through

the next stage.

Telophase

The polar fibres keep on lengthening. Nuclei forms at the opposite

end. Nuclear envelopes start developing from the leftover pieces of the

nuclear envelope of the parent cell and from the endomembrane

system. Nucleoli start reappearing. Chromatin fibres of the

chromosome unwind. At this point, the process of mitosis is almost

complete and the genetic material of the parent cell is equally divided

into two.

Cytokinesis

The division of the cell cytoplasm is termed as cytokinesis. It starts

before the anaphase stage and ends just after telophase. Two

genetically identical daughter cells are formed after the end of

cytokinesis. The new daughter cells are identical diploid cells. Each

cell contains a full set of chromosome.

Solved Example for You

Q: Name the structure responsible for moving the chromosomes

during the mitosis process?

a. Cytoplasm

b. Nuclear membrane

c. Nucleolus

d. Spindle

Sol: The correct answer is option “d”. The microtubules of the spindle

are responsible for moving and arranging the chromosomes during

mitosis.