Exploring the World of Cells
1. Cell Theory
– A cell is the structural unit of living things.
– A cell is the functional unit of living things. It performs all metabolic processes.
– Every cell comes from an existing one.
– A cell is the genetic unit of all living beings. It contains the hereditary material.
2. Types of Cellular Organization
– Prokaryotic cells: lack a nucleus. The genetic material is dispersed in the cytoplasm. Ex: bacteria.
– Eukaryotic cells: possess a nucleus containing the genetic material. Plants and animals have this type of organization.
3. General Structure of Eukaryotic Cells
– The plasma membrane: a layer surrounding the cell.
– Nucleus: contains the genetic material, separated from the rest of the cell by the nuclear envelope.
– Cytoplasm: the part of the cell between the plasma membrane and the nucleus. It consists of an aqueous medium and a network of protein fibers (cytoskeleton) containing cellular organelles.
– Centrosome: involved in cell division.
– Organelles: endoplasmic reticulum, Golgi apparatus, ribosomes, mitochondria, lysosomes, and vacuoles.
4. Two Models of Eukaryotic Cells
– Animal Cells: have two centrioles in the centrosome, composed of tubular protein.
– Plant Cells: have a cell wall surrounding the plasma membrane, plasmodesmata channels, large vacuoles, and chloroplasts.
5. The Nucleus
The nucleus is the largest structure in eukaryotic cells, containing most of the cellular DNA and genetic information. Core components:
- Nuclear envelope: formed by a double membrane (outer and inner) separated by an intermembrane space. Nuclear pores allow substance exchange between the nucleus and cytoplasm.
- Nucleoplasm: the internal aqueous medium where DNA replication occurs.
- Nucleolus: a spherical corpuscle without a membrane, responsible for ribosome formation.
- Chromatin: consists of DNA strands dispersed throughout the nucleoplasm. During cell division, chromatin condenses into chromosomes.
6. Chromosomes
Chromosomes are thread-like structures appearing during cell division. They distribute genetic information (DNA) from the parent cell to daughter cells. Each chromosome consists of two chromatids joined at the centromere. Each chromatid contains a DNA molecule identical to its sister chromatid. Each chromatid has two arms.
Each species has a characteristic number of chromosomes:
- Haploid organisms: have one set of chromosomes (represented by ‘n’).
- Diploid organisms: have two sets of chromosomes (represented by ‘2n’).
7. The Karyotype
The karyotype is the set of chromosomes of a species. Two types of chromosomes:
- Sex chromosomes (heterosomes): differ between sexes and determine sex (X and Y). Females are XX, and males are XY.
- Autosomes: the remaining chromosomes, identical in both sexes.
8. The Cell Cycle
The cell cycle is the sequence of changes a cell undergoes from its formation to division into two daughter cells.
- Interphase: the initial, long phase where the nucleus is visible. DNA replication occurs at the end of this phase.
- M phase (division phase): the final, short phase where the nucleus disappears, and chromosomes become visible. Includes nuclear division (mitosis) and cytoplasmic division (cytokinesis).
9. Cell Division: Mitosis and Cytokinesis
During the M phase, a parent cell produces two daughter cells with the same number of chromosomes. Before nuclear division, DNA and centrosomes duplicate. Centrioles also duplicate in animal cells. Mitosis has four phases:
- Prophase: chromatin condenses into chromosomes. The nucleolus and nuclear membrane disappear, and the spindle apparatus begins to form.
- Metaphase: chromosomes align at the cell’s center (equatorial plate), with centromeres attached to spindle fibers.
- Anaphase: sister chromatids separate and move to opposite poles, pulled by spindle fibers.
- Telophase: the nucleolus and nuclear membrane reappear. The spindle apparatus disappears, and chromosomes decondense back into chromatin.
Cytokinesis differs in plant and animal cells:
- Animal cells: cytokinesis occurs by constriction of the cytoplasm at the cell’s equator.
- Plant cells: a cell plate (fragmoplast) forms between daughter cells.
10. Meiosis
Meiosis is a reductional division producing haploid daughter cells (half the DNA content) from a diploid parent cell. It occurs in the gonads, producing gametes. Meiosis results in four haploid daughter cells different from the parent cell.
11. Comparison Between Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Cell Type | Somatic cells | Germ cells |
| Process Length | Short | Long |
| Ploidy | Occurs in haploid or diploid cells | Occurs in diploid cells |
| Number of Divisions | One | Two |
| Crossing Over | No | Yes (in Prophase I) |
| Anaphase Separation | Sister chromatids separate | Homologous chromosomes separate in Anaphase I, sister chromatids separate in Anaphase II |
| Daughter Cells | Two identical daughter cells with the same chromosome number as the parent cell | Four genetically distinct haploid daughter cells |
12. Significance of Mitosis and Meiosis
In multicellular organisms, mitosis enables growth through successive cell divisions from the zygote. In unicellular organisms, mitosis is a form of asexual reproduction. Meiosis produces haploid gametes, maintaining a constant chromosome number in sexually reproducing organisms. It also ensures genetic variability in offspring through genetic information exchange during meiosis.