Cell Division, Genetics, and Inheritance Fundamentals
Mitosis: Nuclear Division Process
Mitosis is a fundamental process of cell division where the nucleus divides, resulting in two genetically identical daughter cells. It is crucial for growth, repair, and asexual reproduction. This nuclear division is followed by cytokinesis, the division of the cytoplasm, which differs in animal and plant cells:
- In animal cells, cytokinesis occurs via a constriction furrow.
- In plant cells, a phragmoplast forms, leading to a new cell wall.
DNA duplication occurs during the interphase, preparing the cell for mitosis.
Prophase
During prophase, the chromatin condenses, making the chromosomes visible as distinct structures. Each chromosome consists of two sister chromatids joined at the centromere. The nuclear membrane gradually disappears, and the chromosomes disperse within the cell.
Metaphase
In metaphase, chromosomes become highly condensed, shorter, and thicker. They align precisely at the cell’s equatorial plate (metaphase plate), ensuring equal distribution to daughter cells.
Anaphase
Anaphase begins when the centromere of each chromosome cleaves, separating the sister chromatids. These individual chromatids, now considered full chromosomes, are pulled by spindle fibers towards opposite poles of the cell.
Telophase
During telophase, a new nuclear membrane forms around each set of chromosomes at the poles, effectively dividing the nucleus into two distinct parts. The chromosomes begin to decondense, returning to their chromatin state.
Cytokinesis
Cytokinesis is the final stage of cell division, where the cytoplasm divides, resulting in two separate daughter cells. Note: DNA duplication occurs during Interphase, prior to mitosis, not during cytokinesis.
Meiosis: Reproductive Cell Division
Meiosis is a specialized type of cell division that produces four daughter cells, each with half the number of chromosomes as the parent cell. This process is essential for sexual reproduction, creating gametes (sperm and egg cells) or spores. Meiosis involves two sequential rounds of division, Meiosis I and Meiosis II, each with distinct phases:
- Meiosis I:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I & Cytokinesis I
- Meiosis II:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II & Cytokinesis II
Key Genetic Terms
Gene
A gene is a specific segment of a chromosome that carries the genetic information for a particular trait or character.
Genome
The genome refers to the complete set of genes or genetic material present in an individual or a species.
Karyotype
A karyotype is an organized display of the complete set of chromosomes within a cell, arranged by size, shape, and number, used to identify chromosomal abnormalities.
Stem Cells: Regenerative Potential
Stem cells are unique cells with the remarkable ability to self-renew and differentiate into various specialized cell types. They are categorized by their differentiation potential:
- Totipotent Cells: These cells can generate all cell types, including embryonic and extraembryonic tissues (e.g., a zygote).
- Pluripotent Cells: Capable of generating all cell types of the three germ layers, but not extraembryonic tissues (e.g., embryonic stem cells).
- Multipotent Cells: Can differentiate into a limited range of cell types within a specific lineage or tissue (e.g., hematopoietic stem cells, which form blood cells).
Human Blood Groups (ABO System)
The ABO blood group system is a classification of blood based on the presence or absence of specific antigens on the surface of red blood cells and corresponding antibodies in the plasma.
| Phenotype | Antigens Present | Antibodies Present | Possible Genotypes |
|---|---|---|---|
| O | None | Anti-A, Anti-B | OO |
| A | A | Anti-B | AA or AO |
| B | B | Anti-A | BB or BO |
| AB | A and B | None | AB |
Mendelian Inheritance Principles
Mendelian inheritance describes the patterns by which traits are passed from parents to offspring, based on the work of Gregor Mendel.
First Law: Law of Uniformity
This law states that when two pure-bred individuals (homozygous) for a given character are crossed, all first-generation hybrids (F1) will be uniform. These F1 individuals are heterozygous, carrying alleles from both pure-bred parents: a dominant allele (which is expressed) and a recessive allele (which is not expressed in the F1 generation).
Second Law: Law of Segregation
Mendel then crossed individuals from the first filial generation (F1) with each other. This cross revealed that the recessive trait, which had seemingly disappeared in the F1 generation (e.g., green seeds), reappeared in the second filial generation (F2) in a predictable ratio. This demonstrates that alleles for a trait segregate (separate) during gamete formation and are then randomly reunited during fertilization.
Third Law: Law of Independent Assortment
Mendel crossed pea plants that differed in two characters, such as yellow, smooth seeds with green, rough seeds (both homozygous for both characters). The seeds from this cross (F1 generation) were all yellow and smooth, demonstrating the dominance of these traits. When these F1 dihybrid plants (e.g., AaBb) were crossed with each other, the F2 generation displayed new combinations of traits (e.g., wrinkled yellow peas and green smooth peas) that were not present in the parental (P) or F1 generations. This showed that alleles for different genes are transmitted independently of each other during gamete formation, provided they are on different chromosomes or far apart on the same chromosome.