Drona Mission Jeet: Cell Cycle and Cell Division - Lecture 01

Definition: Cell division is a biological process by which a parent cell divides and distributes its internal content into two daughter cells.
Historical Context: The concept of cell division was provided by Rudolf Virchow in the year $1855$ .
Division Components: During the process, the parent cell ( $P$ ) divides to produce daughter cells ( $D$ ), distributing materials including:
- $DNA$
- Cell Organelles
- Proteins

Growth:
- Every multicellular organism begins its life as a single cell known as a Zygote.
- The progression follows a hierarchical structure: Single Cell (Zygote) $\rightarrow$ Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism (Multicellular human being).
Reproduction:
- Asexual Reproduction: Seen in processes such as binary fission.
- Sexual Reproduction: Involved in the formation of gametes.
Repair and Replacement:
- Cell division is essential for replacing dead, damaged, or old cells.

Division Capacity: Cells may follow different paths regarding their ability to divide:
- Some cells retain the capacity for division.
- Other cells lose the capacity for division to perform specific functions, becoming what is known as a Differentiated Cell.

Integrated Processes: During cell division, $DNA$ replication and cell growth occur in a highly coordinated manner.
Objectives of Coordination:
- To ensure the proper division of a cell.
- To ensure the distribution of an Intact Genome to daughter cells.
- An Intact Genome is defined as one that is physically and functionally undamaged ( $DNA$ ).
Biological Characteristic: Cell growth and reproduction are fundamental characteristic features of all living organisms.

Biosynthesis: A cell grows in size due to biosynthesis. This involves the formation of proteins, lipids, and carbohydrates within a cell at a rate faster than their degradation.
Shift in Ratios:
- As the cell grows, its volume increases.
- This leads to an increase in the Cytoplasmic Ratio (Nucleo-cytoplasmic ratio).
- Eventually, the nucleus can no longer effectively control the metabolic activities of the enlarged cell.
Outcome: To restore control and efficiency, cell division takes place, dividing the cell into two or more daughter cells.
Exponential Growth: Two daughter cells can divide further, allowing millions of cells to be formed from a single original cell through exponential increase.

Definition: The cell cycle is a sequence of events in which a cell duplicates its genome ( $DNA$ ), synthesizes other constituents of the cell, and eventually divides into two daughter cells.
Key Features:
- It is a highly coordinated process.
- It is under strict genetic control.
- $DNA$ Replication: This occurs specifically once in a cycle during a particular phase.
- Cell Growth: This refers to the increase in cytoplasm and is a continuous process throughout the cell cycle.
- Distribution: Duplicated chromosomes are distributed equally to daughter cells via the division process.
Cycle of Growth: These cycles of growth and division allow a single cell to form structures consisting of millions of cells, which is the basis of development for all organisms, including the largest ones.

Study Model: In Eukaryotes, the cell cycle is best studied using human cells in culture.
Duration Variations: The time required to complete a cell cycle varies between different organisms and cell types:
- Human Cells: Completed in approximately $24\,hours$ .
- Yeast (Saccharomyces cerevisiae): Completed in only about $90\,mins$ .
Primary Phases: The cell cycle is divided into two basic phases:
- Interphase: Also known as the Resting phase, Non-Dividing phase, or Preparatory phase. This accounts for roughly $95\%$ of the duration of the cell cycle.
- M-Phase (Mitotic Phase): The actual dividing phase where the cell divides into two cells.

Interphase Sub-phases:
- $G_1$ Phase (Gap 1 Phase): The first growth phase.
- $S$ Phase (Synthesis Phase): The phase where $DNA$ synthesis/replication occurs.
- $G_2$ Phase (Gap 2 Phase): The second growth phase preparing for mitosis.
M-Phase Components:
- Karyokinesis: The division of the nucleus, consisting of Prophase, Metaphase, Anaphase, and Telophase ( $PMATC$ ).
- Cytokinesis: The division of the cytoplasm, resulting in the formation of two distinct cells.
$G_0$ Phase: A quiescent stage where cells exit the cell cycle and stop dividing.

Question 1 (Assertion/Reason):
- Assertion (A): Daughter cells lose their ability to divide after formation.
- Reason (R): Cell cycle stops permanently after one division.
- Answer: (d) Both A and R are false. Cells often retain division capacity, and the cycle does not necessarily stop permanently.
Question 2: Which statement best explains why the cell cycle is described as a coordinated process?
- Answer: (b) Each event depends upon proper completion of previous events.
Question 3: The term "cell cycle" includes:
- Answer: (c) Genome duplication, synthesis of cell constituents, and division.
Question 4: Which process is continuous throughout the cell cycle?
- Answer: (c) Cytoplasmic growth.
Question 5: If chromosome distribution becomes unequal during division, the daughter cells formed would:
- Answer: (b) Have genomic imbalance.
Question 6 (Assertion/Reason):
- Assertion (A): Cell cycle events occur in a highly coordinated manner.
- Reason (R): Proper coordination ensures accurate genome transmission.
- Answer: (a) Both A and R are true and R is the correct explanation.

Nuclear Anatomy (from Homework EX-1):
- Nuclear Pores: Help in the exchange of materials between the nucleus and cytoplasm. They specifically allow for the movement of $RNA$ and proteins.
- Nucleolar Organizer Region (NOR): Identified as a secondary constriction on a chromosome.
- Centromere: The structure that provides the shape to chromosomes.
- Chromatin Composition: Contains $DNA$ , basic histone proteins, non-histone proteins, and $RNA$ .
- Active Nucleus Characteristics: Shows a large nucleolus, diffused chromatin, and many nuclear pores.