The division of cytoplasm to form two daughter cells after nuclear division is called cytokinesis.
Definitions [5]
Definition: Centromere
The two sister chromatids remain attached to each other at a small region called centromere.
Definition: Spindle Fibres
A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.
Definition: Karyokinesis
All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).
Definition: Cytokinesis
Definition: Cell Cycle
The cell cycle is a series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells.
Key Points
Key Points: Phases of Cell Cycle
- The cell cycle has two main phases: Interphase (growth and DNA replication) and M phase (cell division).
- Interphase is the longest phase, covering about 95% of the cell cycle.
- Interphase includes G₁ (growth), S (DNA replication), and G₂ (preparation for mitosis).
- In S phase, DNA content doubles (2C → 4C) but chromosome number remains the same (2N).
- M phase involves karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division).
- Some cells enter G₀ phase, where they become inactive and do not divide (e.g., heart cells).
- Duration of cell cycle varies: human cells ~24 hours, yeast ~90 minutes.
Key Points: Karyokinesis
Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:
- Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
- Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
- Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
- Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.
Key Points: Cytokinesis
- Cytokinesis is the division of cytoplasm into two daughter cells after karyokinesis (nuclear division).
- In animal cells, a furrow appears in the plasma membrane, deepens, and joins in the centre to divide the cytoplasm.
- In plant cells, a cell plate forms in the centre and grows outward to meet the lateral walls, forming a new cell wall. The cell plate represents the middle lamella between two adjacent cells.
- During cytokinesis, organelles like mitochondria and plastids are distributed between the two daughter cells.
- In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition called syncytium (e.g., liquid endosperm in coconut).
Key Points: Cell Cycle
- The cell cycle is the sequence of events in which a cell grows, duplicates its DNA, and divides into two daughter cells.
- Cell division is essential for growth and reproduction in all living organisms, starting from a single cell.
- A single cell forms a large organism through repeated cycles of growth and division, producing many cells.
- DNA replication and cell growth must be coordinated to ensure that daughter cells receive a complete and correct genome.
- Cell growth is continuous, but DNA synthesis occurs only at a specific stage of the cell cycle, followed by the distribution of chromosomes during division.
