Understanding Cell Division: Mitosis and Meiosis
Cell Division: Mitosis and Meiosis
Mitosis
Prometaphase
Prometaphase occurs after the fragmentation of the nuclear envelope, leading to the mixing of the cytoplasm and nucleoplasm. Chromosomes condense and remain within the cell. Kinetochores, protein complexes that attach to microtubules, develop on the centromeres. Microtubules pull the chromosomes, resulting in a chaotic movement known as the “dance of the chromosomes.”
Metaphase
During metaphase, the centromeres align at the equatorial plate of the cell. The chromatids of each chromosome face opposite poles.
Anaphase
Anaphase begins when proteins holding sister chromatids together are degraded. The chromatids are pulled towards opposite poles due to the shortening of microtubules attached to the kinetochores.
Telophase
Telophase reverses the changes that occurred during prophase and prometaphase. Nuclear envelopes reform around the separated chromosomes. Nucleoli reappear, and chromatin disperses.
Cytokinesis
Cytokinesis is a separate process that overlaps with telophase. It is not technically part of mitosis but is necessary to complete cell division. In animal cells, a cleavage furrow containing a contractile ring forms at the former metaphase plate, constricting the cytoplasm and separating the two new nuclei into daughter cells. In both animal and plant cells, vesicles from the Golgi apparatus move along microtubules to the cell’s equatorial zone. In plants, these vesicles coalesce into a cell plate within the fragmoplast, eventually forming a new cell wall that separates the two nuclei. The fragmoplast is a microtubule structure typical of higher plants, while some algae use a ficoplast during cytokinesis. At the end of this process, each daughter cell has a complete copy of the original cell’s genome.
Meiosis
Concept of Homologous Chromosomes
Homologous chromosomes are pairs of chromosomes that recombine during meiosis. They have the same structure and loci but may contain different alleles because one comes from each parent.
Sex Chromosomes
Sex chromosomes determine the sex of an organism. In humans, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Meiosis Process
Meiosis is a specialized cell division process that occurs only in germ cells, producing gametes (haploid cells) from diploid cells. It is essential for sexual reproduction and ensures that the chromosome number remains constant across generations. Meiosis involves two nuclear divisions but only one chromosome duplication. The process has two main stages:
- Reduction Division (Meiosis I): The chromosome number is reduced by half.
- Equational Division (Meiosis II): Similar to mitosis, sister chromatids separate.
Biological Importance of Meiosis
The most important consequence of meiosis is the genetic variability that arises from genetic recombination during crossing over. This variability contributes to the evolution of species.
Chromosome Theory of Inheritance
The chromosome theory of inheritance states that genes reside on chromosomes. This theory was developed in the late 19th and early 20th centuries, building upon Mendel’s work. Researchers observed parallels between the inheritance of traits and the behavior of chromosomes during meiosis and fertilization, leading them to conclude that genes are located on chromosomes.
Key Figures in Chromosome Theory
- Sutton and Boveri (1902): Noted the parallelism between gene inheritance and chromosome behavior.
- Johannsen (1909): Coined the term “gene” for Mendel’s hereditary factors.
- Morgan (1910): Studied fruit flies and identified sex chromosomes (X and Y).
Morgan’s work with fruit flies demonstrated that genes are arranged linearly on chromosomes and that genes on the same chromosome tend to be inherited together (linked genes). He also showed that crossing over leads to recombination and that chromosomes carry genetic information across generations through mitosis.
Genotype and Phenotype
Genotype: The specific genetic makeup of an individual, represented by their DNA sequence.
Phenotype: The observable traits of an individual, determined by the interaction between their genotype and the environment.
Mendel’s Laws
Mendel’s 1st Law: Law of Uniformity
When two purebred individuals are crossed for a specific trait, the offspring of the first generation (F1) will all be identical in both genotype and phenotype. This law reflects the concept of dominance, where one allele masks the expression of another.
Mendel’s 2nd Law: Law of Segregation
During gamete formation, each allele of a pair separates from the other, ensuring that each gamete receives only one allele for each gene. This law explains how parental alleles are combined in offspring, leading to variation.
Mendel’s 3rd Law: Law of Independent Assortment
Different traits are inherited independently of each other, meaning that the inheritance pattern of one trait does not affect the inheritance pattern of another. This law applies only to genes located on different chromosomes or genes that are far apart on the same chromosome.
Cell Cycle
The cell cycle is the series of events that lead to cell growth and division. It consists of two main phases: interphase and the mitotic (M) phase.
Interphase
stage s call. The interval in the cell cycle between the end of mitosis and early S phase is called G1 phase, and interval between the end of the phase and mitosis INCI g2.A phase is called mitosis, with the citonesis ocn framing it in the M phase, the duration of the phases is very desigual.Las phase g1 and g2 are muxo qe nerois more rest periods between S and M phases, phase ams g1 is active and it has the bulk of RNA synthesis. Its duration depends on the conditions exxternas and signals from other cells progression cell cycle is highly regulated by proteins called cyclin-dependent kinases are activated only if qe are associated with different cyclins, the action of Cdk depends qe checkpoints delay the entry on the next phase of the cycle if no upstream completed successfully or if the DNA has been damaged by radiation or chemical agents providing time for the activation of repair systems, poor mechanism of these control mechanisms can lead to anomalous Ceulen poliferacion of INA and cancer.