2. THE CELL CYCLE
• The process by which new cells arise from other living cells is called cell division.
• Although cell division occurs in all organisms, it takes place very differently in prokaryotes and
eukaryotes.
• There are two distinct types of cell divisions: Mitosis and Meiosis.
Mitosis leads to production of cells that are genetically identical to their parent. It serves as the basis for
producing new cells
Meiosis leads to production of cells with half the genetic content of the parent. It forms the basis for
producing new sexually reproducing organisms.
Together, these two types of cell division form the links in the chain between parents and their offspring.
4. The cell cycle can be divided into several distinct phases, each with specific events and checkpoints to ensure
accurate cell division. These phases are:
1.Interphase:
1. G1 Phase (Gap 1): This is the first phase of the cell cycle, during which the cell grows in size and
synthesizes various molecules necessary for DNA replication. It's also a period of checking for any
damage or errors in DNA before proceeding to the next phase.
2. S Phase (Synthesis): In this phase, DNA replication takes place. Each chromosome is duplicated,
resulting in two sister chromatids held together by a centromere.
3. G2 Phase (Gap 2): After DNA replication, the cell continues to grow and prepares for mitosis. During
this phase, the cell checks for DNA damage and completes the synthesis of proteins and organelles
needed for cell division.
6. The M phase consists of several subphases, each with specific events and processes:
I. Prophase:
1. During prophase, chromatin (a complex of DNA and proteins) condenses and becomes visible as
individual chromosomes under a light microscope. Each chromosome consists of two identical sister
chromatids connected at the centromere.
2. The nuclear envelope, which surrounds the nucleus, begins to break down, allowing the spindle
fibers to access the chromosomes.
3. Centrosomes, structures located near the nucleus, start to move toward opposite poles of the cell.
These centrosomes organize microtubules into the mitotic spindle, a structure that will help
segregate the chromosomes.
Fig. Formation of the Mitotic Spindle
7. II. Metaphase:
1. In metaphase, the chromosomes align at the cell's equatorial plane, known as the metaphase plate.
This alignment ensures that the chromosomes will be evenly distributed to the two daughter cells.
2. Spindle fibers, which extend from the centrosomes to the centromeres of the chromosomes, attach to
the kinetochores, specialized protein structures found on the centromeres of each chromatid. These
attachments ensure that each chromatid will be pulled to the appropriate pole during anaphase.
III. Anaphase:
3. Anaphase is characterized by the separation of sister chromatids. The proteins holding the
chromatids together are degraded, allowing the spindle fibers to pull each chromatid toward the
opposite pole.
4. As the chromatids are pulled apart, they become individual chromosomes.
Fig. Anaphase
8. IV. Telophase:
1. Telophase marks the near end of mitosis. During this phase, the separated chromosomes reach their
respective poles of the cell.
2. A new nuclear envelope begins to form around each set of chromosomes, creating two distinct nuclei
within the cell.
3. Chromosomes start to de-condense back into chromatin as they return to their more extended and less
condensed state.
4. The cell ensures that each daughter cell will receive an identical set of chromosomes, ensuring
genetic stability.
9. V. Cytokinesis:
1. Cytokinesis is not technically part of mitosis but is the final step of the M phase. It involves the
division of the cell's cytoplasm and organelles into two separate daughter cells.
2. In animal cells, a contractile ring, primarily composed of actin filaments, pinches the cell
membrane, creating two separate daughter cells.
3. In plant cells, a structure called the cell plate forms between the daughter nuclei. This cell plate
gradually develops into a new cell wall, separating the two daughter cells.
Fig. Cytokinesis
10. 3. G0 Phase:
1. Not all cells go through the entire cell cycle. Some cells, such as nerve cells and muscle cells, exit the
cell cycle and enter a non-dividing state called the G0 phase. They remain in this phase until
stimulated to re-enter the cell cycle.
Fig. Cell Cycle showing G0 phase
11. MEIOSIS
• Meiosis is a specialized type of cell division that occurs in sexually reproducing organisms, resulting in
the formation of haploid gametes (sperm and egg cells) with half the number of chromosomes as the
parent cell.
• This reduction in chromosome number is essential for maintaining genetic diversity during sexual
reproduction. Meiosis consists of two sequential divisions: meiosis I and meiosis II, each with specific
stages and processes.
13. Meiosis I: Reduction Division Meiosis I is the first division in meiosis and includes the following stages:
1.Prophase I:
1. Prophase I is a lengthy and complex stage. It can be further subdivided into five phases: leptotene,
zygotene, pachytene, diplotene, and diakinesis.
2. During leptotene, chromatin condenses into visible chromosomes. Each chromosome consists of two
sister chromatids.
3. In zygotene, homologous chromosomes start pairing up through a process called synapsis, facilitated
by the formation of the synaptonemal complex.
4. Pachytene is marked by complete synapsis, where homologous chromosomes are fully aligned.
5. Crossing over occurs during pachytene, where chromatids of homologous chromosomes exchange
genetic material, resulting in genetic recombination.
6. Diplotene follows, and the synaptonemal complex starts to disassemble, but homologous
chromosomes remain attached at points called chiasmata.
7. Diakinesis is the final phase of prophase I, where the nuclear envelope begins to break down, and
spindle fibres start forming.
15. 2. Metaphase I:
1. In metaphase I, homologous chromosome pairs align at the metaphase plate.
2. Spindle fibres from opposite poles attach to the centromeres of each homologous chromosome.
3. Unlike in mitosis, where individual chromosomes align, in meiosis I, homologous pairs align
together.
3. Anaphase I:
4. Anaphase I is characterized by the separation of homologous chromosome pairs.
5. Spindle fibres pull one chromosome of each homologous pair to opposite poles of the cell.
6. The reduction in chromosome number occurs as each daughter cell receives only one member of
each homologous pair.
4. Telophase I:
7. During telophase I, the separated chromosomes reach the opposite poles.
8. Nuclear envelopes form around each set of chromosomes, creating two haploid daughter cells.
9. Cytokinesis may follow, resulting in two distinct haploid cells.
16. Meiosis II: Equational Division Meiosis II closely resembles a simplified version of mitosis, but it involves
haploid cells. It consists of two stages:
1.Prophase II:
1. Prophase II is similar to prophase in mitosis.
2. Chromosomes condense, spindle fibers form, and the nuclear envelope begins to break down.
3. Each of the two haploid daughter cells from meiosis I enters prophase II.
2.Metaphase II:
1. Chromosomes align individually along the metaphase plate in each of the two haploid daughter cells.
2. Spindle fibers attach to the centromeres of the chromosomes.
3.Anaphase II:
1. Anaphase II involves the separation of sister chromatids.
2. Spindle fibers pull the sister chromatids of each chromosome apart and towards opposite poles of the
cell.
4.Telophase II:
1. Telophase II marks the end of meiosis, resulting in four haploid daughter cells.
2. Nuclear envelopes reform around each set of chromosomes.
3. Cytokinesis occurs, leading to the formation of four genetically distinct haploid gametes.
17. Fig. Separation of Homologous Chromosomes During Meiosis I and Separation of
Chromatids During Meiosis II.
(a) Schematic diagram of a pair of homologous chromosomes at metaphase I.
(b) At anaphase I, the cohesin holding the arms of the chromatids is cleaved,
allowing the homologues to separate from one another.
(c) At metaphase II, the chromatids are held together at the centromere, with
microtubules from opposite poles attached to the two kinetochores
(d ) At anaphase II, the cohesin holding the chromatids together has been
cleaved, allowing the chromosomes to move to opposite poles.
