The document details the cell cycle, a process through which a cell duplicates its contents and divides, essential for reproduction in living organisms. It explores phases of the cell cycle including interphase (G1, S, G2), mitosis, cytokinesis, and meiosis, and highlights the role of cyclins and cyclin-dependent kinases in regulating these processes. Each phase is characterized by specific activities and checkpoints that ensure proper cell growth, DNA replication, and division, maintaining genetic integrity.

1. CELL DIVISION

2.  A cell reproduces by carrying out an orderly sequence of events in which it duplicates its
contents and then divides in two. This cycle of duplication and division, known as the cell
cycle
 This is an essential mechanism by which all living things reproduce.
 The details of the cell cycle vary from organism to organism and at different times in an
individual organism’s life. In unicellular organisms, such as bacteria and yeasts, each cell
division produces a complete new organism, whereas many rounds of cell division are
required to make a new multicellular organism from a fertilized egg.
 Certain features of the cell cycle, however, are universal, as they allow every cell to perform
the fundamental task of copying and passing on its genetic information to the next generation
of cells.

3. Cell cycle
 The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in
the chromosomes and then to segregate the DNA into genetically identical daughter cells such
that each cell receives a complete copy of the entire genome
 In most cases, a cell also duplicates its other macromolecules and organelles and doubles in
size before it divides; otherwise, each time a cell split it would get smaller and smaller. Thus,
to maintain their size, dividing cells coordinate their growth with their division

4.  Seen in a microscope, the two most dramatic events in the cell cycle are when the
nucleus divides, a process called mitosis, and when the cell later splits in two, a
process called cytokinesis.
 These two processes together constitute the M phase of the cycle. In a typical
mammalian cell, the whole of M phase takes about an hour, which is only a small
fraction of the total cell-cycle time

5.  The period between one M phase and the next is called interphase. Viewed with a
microscope, it appears, deceptively, as an uneventful interlude during which the
cell simply increases in size.
 Interphase, however, is a very busy time for a proliferating cell, and it
encompasses the remaining three phases of the cell cycle.
 During the S phase (S = synthesis), the cell replicates its DNA.
 S phase is flanked by two “gap” phases—called G1 and G2—during which the cell
continues to grow

6. Cell cycle Control
Cyclins:
• Definition: Cyclins are a family of proteins that undergo a cycle of synthesis and
degradation during the cell cycle. Their levels rise and fall in a characteristic
manner, peaking at specific points in the cell cycle.
• Function: Cyclins bind to and activate cyclin-dependent kinases (CDKs), forming
complexes that regulate the progression of the cell cycle.
• Classification: Cyclins are categorized based on the phases of the cell cycle in
which they are active. Examples include G1 cyclins, S-phase cyclins, and M-phase
cyclins.

7. Cyclin-Dependent Kinases (CDKs):
• Definition: CDKs are a family of protein kinases that, when bound to cyclins, become
activated and play a central role in regulating the cell cycle.
• Function: CDKs phosphorylate specific target proteins, thereby regulating their activity and
controlling the progression of the cell cycle.
• Activation: CDKs are enzymatically inactive on their own. They must bind to a cyclin to form
an active complex. The binding of cyclins induces a conformational change in the CDK,
activating its kinase activity.

9. G0 PHASE
 The G0 phase, sometimes called the resting phase, is a phase in the cell cycle where cells
exit the active cell cycle and temporarily or permanently stop dividing. Cells in the G0 phase
are not actively preparing to divide; instead, they are in a quiescent state.
 Not all cells go through the G0 phase. Some cells, like nerve cells and muscle cells, become
terminally differentiated and enter G0, where they can remain for extended periods or even for
the rest of the organism's life. Other cells, such as those in the liver, can exit the cell cycle
temporarily and re-enter when needed.

10.  The G0 phase is essentially a non-dividing state where cells are metabolically
active but not actively progressing through the cell cycle. Cells may enter G0 in
response to signals that indicate the absence of the necessary conditions for
division or in response to specific signals that induce cell cycle arrest.
 For example, if a cell detects DNA damage during the cell cycle, it may enter a G0-
like state to allow time for repair before proceeding with division. Cells in G0 can
re-enter the cell cycle if they receive the appropriate signals, indicating that
conditions are favorable for division.
 Understanding the G0 phase is crucial for studying cell cycle control, especially in
the context of development, tissue maintenance, and diseases like cancer where
cell cycle regulation is disrupted.

11. G1 PHASE
The G1 phase, or Gap 1 phase, is the first phase of the cell cycle, occurring after a cell has
divided and before it proceeds to the synthesis phase (S phase). The G1 phase is a period of cell
growth, protein synthesis, and preparation for DNA replication
 Cell Growth: During G1, the cell increases in size, synthesizes new proteins, and produces
additional organelles. This phase is focused on ensuring that the cell has the necessary
resources and components to support the upcoming rounds of cell division.
 Metabolic Activity: G1 is a metabolically active phase where the cell carries out its normal
functions and prepares for the challenges of the cell cycle. This includes energy production,
maintenance of cellular structures, and other biochemical processes.

12.  Cellular Checkpoints: Throughout the G1 phase, the cell undergoes checkpoints
that monitor its readiness to progress to the next phase of the cell cycle.
Checkpoints assess factors such as DNA integrity, cell size, and the availability of
nutrients. If the conditions are favorable, the cell receives signals to proceed with
the cell cycle.
 Decision to Divide or Enter G0: Towards the end of the G1 phase, the cell makes
a critical decision regarding its fate. It can either enter the synthesis phase (S
phase) and initiate DNA replication, leading to cell division, or it can exit the active
cell cycle and enter a resting state known as the G0 phase. The decision depends
on various internal and external signals.

13. G1 Checkpoint (Restriction Point):
• Decision Point: Determines whether the cell will proceed with the cell cycle and
enter the S phase or exit the active cycle and enter the G0 phase.
• Criteria: The cell checks for the availability of nutrients, proper cell size, and the
integrity of the DNA. If conditions are favorable, the cell progresses to the S phase.

14. G1 Phase checkpoint:
• Cyclin D: Cyclin D forms complexes with CDK4 and CDK6. Together, Cyclin D-
CDK4/6 complexes play a role in the progression from the G1 phase to the S
phase.
• Cyclin E: Cyclin E binds to CDK2, and the Cyclin E-CDK2 complex is involved in
the G1 to S-phase transition, promoting DNA replication

15. S PHASE
The S phase, or Synthesis phase, is a critical stage in the cell cycle during which DNA replication
occurs. In this phase, the cell synthesizes a copy of its entire DNA content to prepare for
subsequent cell division
 DNA Replication: The primary activity of the S phase is the replication of DNA. Each
chromosome is duplicated, resulting in the formation of identical sister chromatids connected
by a centromere. DNA replication is a highly coordinated and complex process that involves
the unwinding of the DNA double helix, the synthesis of new complementary strands, and the
proofreading and correction of errors.
 Synthesis of Histones and Other Proteins: Along with DNA replication, the S phase
involves the synthesis of histone proteins, which are essential for packaging and organizing
DNA into chromosomes. Additionally, other proteins involved in cell division and DNA repair
are synthesized.

16.  Checkpoint Control: The cell undergoes checkpoints during the S phase to
ensure that DNA replication is proceeding accurately. Checkpoints monitor DNA
integrity, the presence of replication errors, and other factors. If any issues are
detected, the cell cycle may be temporarily halted to allow for repair.
 Preparation for Cell Division: The completion of the S phase marks the point at
which the cell has doubled its genetic material. This sets the stage for the
subsequent phases of the cell cycle, including G2 phase and cell division (mitosis
or meiosis).

17. S Phase checkpoint:
• Cyclin A: Cyclin A associates with CDK2 and CDK1. Cyclin A-CDK2 complexes
are active during the S phase, contributing to DNA replication, and they persist into
the G2 phase.
• Cyclin E: Cyclin E continues to play a role in promoting DNA replication during the
S phase

18. G2 phase
 Is the third phase of the cell cycle, occurring after the synthesis phase (S phase) and before
cell division (mitosis or meiosis). The G2 phase is a period during which the cell undergoes
further growth, prepares for division, and checks for any errors or damage in the newly
replicated DNA
 Cell Growth and Preparation for Mitosis/Meiosis: During G2, the cell continues to grow,
synthesizing additional proteins, organelles, and other cellular components necessary for the
upcoming process of cell division. This phase ensures that the cell is adequately prepared for
mitosis or meiosis.
 DNA Damage Checkpoints: The cell undergoes checkpoints during the G2 phase to assess
the integrity of the newly replicated DNA. If DNA damage is detected, the cell cycle may be
halted to allow for repair before proceeding to cell division. This helps maintain genomic
stability and prevents the transmission of genetic abnormalities to daughter cells.
 Cyclin-Dependent Kinase (CDK) Activation: G2 phase involves the activation of cyclin-
dependent kinases (CDKs), which play a crucial role in regulating the cell cycle. CDKs, along
with their associated cyclin proteins, control the progression of the cell cycle by
phosphorylating specific target proteins.

19. G2 Checkpoint:
• Decision Point: Assesses whether the cell is ready to enter the M phase (mitosis).
• Criteria: The cell checks for DNA replication completion, detects any DNA damage,
and ensures that the cell is of sufficient size. If conditions are met, the cell
proceeds to mitosis.

20. G2 Phase cyclins and CDKS:
• Cyclin A: Cyclin A continues to form complexes with CDK1 and CDK2 in the G2
phase, preparing the cell for mitosis.
• Cyclin B: Cyclin B associates with CDK1. Cyclin B-CDK1 complexes are essential
for the transition from the G2 phase to mitosis (M phase).

21. Interphase:
1. G1 Phase (Gap 1): This is the phase during which the cell grows and carries out its normal metabolic
activities. It prepares for DNA synthesis.
2. S Phase (Synthesis): DNA replication occurs during this phase. The genetic material is duplicated,
resulting in two identical sister chromatids attached at the centromere.
3. G2 Phase (Gap 2): The cell continues to grow and prepares for cell division. It synthesizes proteins
and organelles needed for the upcoming division.
Cell Division:
1. Mitosis (in somatic cells): The nucleus of the cell divides into two nuclei, each with the same
number of chromosomes as the original cell. It is divided into stages: prophase, metaphase,
anaphase, and telophase.
2. Cytokinesis: The division of the cytoplasm and other organelles occurs, resulting in two distinct
daughter cells.

22. Mitotic phase
Stages
Prophase:
1. Chromosomes condense and become visible under the microscope.
2. The nuclear envelope begins to break down.
3. The mitotic spindle, a structure composed of microtubules, starts to form. The spindle fibers extend
from structures called centrosomes, which were duplicated during the G2 phase.
Metaphase:
1. Chromosomes align at the cell's equator, known as the metaphase plate.
2. Spindle fibers from opposite poles of the cell attach to the centromeres of each chromosome. This
ensures that each chromatid will be pulled to opposite ends of the cell during later stages.

23. Anaphase:
1. Sister chromatids are pulled apart toward opposite poles of the cell by the shortening of
spindle fibers.
2. Each chromatid, now an individual chromosome, is pulled toward the centrosome to
which it is attached.
Telophase:
1. Chromosomes reach opposite poles and begin to de-condense.
2. The nuclear envelope reforms around each set of chromosomes, resulting in two distinct
nuclei.
3. Cytokinesis, the division of the cytoplasm, typically begins during late telophase or
shortly afterward.

24. Cytokinesis:
In animal cells, a cleavage furrow forms and pinches the cell membrane, dividing the cell into two
daughter cells.
 After cytokinesis is complete, each daughter cell enters the G1 phase of the cell cycle, and the
cycle begins anew. The M phase ensures that genetic material is equally distributed between
the daughter cells, maintaining the proper chromosome number in each cell.
 It's important to note that in germ cells (sperm and egg cells), the M phase is part of meiosis, a
specialized type of cell division that produces haploid cells for sexual reproduction

26. Mitosis (M Phase):
• Cyclin B: Cyclin B continues to be important in promoting the events of mitosis. Its
levels rise during the G2 phase and peak during mitosis, ensuring the proper
progression through mitotic stages.
• Cyclin A: Cyclin A associates with CDK1 and is involved in the control of mitotic
events, contributing to the transition from G2 to M phase.

27. MEIOSIS (IN GERM CELLS):
• Meiosis is a specialized form of cell division that occurs in germ cells (sperm and
egg cells) and results in the formation of haploid cells (gametes).
• It involves two rounds of division: Meiosis I and Meiosis II, each with prophase,
metaphase, anaphase, and telophase.
• The end result is four non-identical haploid cells, each with half the number of
chromosomes as the original cell

28. Meiosis I:
Prophase I:
1. Chromosomes condense, becoming visible under the microscope.
2. Homologous chromosomes (chromosomes with the same genes but potentially different alleles) pair
up in a process called synapsis.
3. Chromatids of homologous chromosomes exchange genetic material in a process called crossing
over.
4. The nuclear envelope begins to break down, and spindle fibers start to form.
Metaphase I:
1. Homologous chromosome pairs align at the metaphase plate.
2. Spindle fibers attach to each chromosome at its centromere.
Anaphase I:
1. Homologous chromosomes are separated and pulled to opposite poles of the cell.
2. Unlike in mitosis, the sister chromatids remain attached during this phase.

29. Telophase I:
1. Chromosomes reach the poles and de-condense.
2. Nuclear envelopes may form around each set of chromosomes.
3. Cytokinesis occurs, resulting in two daughter cells, each with half the chromosome number of the
original cell.
Interphase (between Meiosis I and Meiosis II):
• In some cases, a brief interphase occurs. However, there is no DNA replication during this
interphase.

30. Meiosis II:
 Meiosis II is similar to mitosis but involves the division of haploid cells produced in Meiosis I.
Prophase II:
1. Chromosomes condense again.
2. If there was an interphase, the nuclear envelope may reform and break down again.
Metaphase II:
1. Chromosomes align at the metaphase plate in each of the haploid cells.
Anaphase II:
1. Sister chromatids are separated and pulled to opposite poles.
Telophase II:
1. Chromosomes reach the poles, de-condense, and nuclear envelopes may form.
2. Cytokinesis occurs, resulting in a total of four haploid daughter cells, each with a unique combination
of genetic material.

31.  The end result of meiosis is the production of four non-identical haploid cells, each with half
the chromosome number of the original cell. These cells are the gametes (sperm or egg cells)
that can fuse during fertilization to restore the diploid chromosome number in the zygote.