Meiosis and Apoptosis: Cellular Processes Explained
Meiosis
Each organism has a characteristic chromosome number. Gametes (cells specializing in sexual function) have half the chromosome number of somatic cells (which form the rest of the body). These are referred to as haploid (n) and diploid (2n), respectively. The diploid cell produced by the fusion of two gametes is called a zygote. The purpose of meiosis is to reduce chromosome number and establish restructuring in homologous chromosomes through the exchange of genetic material.
Phases of Meiosis I
Prophase I
This is the longest and most complex phase. The nuclear envelope is preserved until the end of this phase, at which point it disintegrates. The nucleolus disappears, and the spindle is formed. Prophase I is subdivided into five stages:
- Leptotene: Chromosomes are longer and thinner than in mitosis, appearing as rosaries due to structures called chromomeres. Two chromatids can be identified together. Some chromosome ends are joined to the nuclear envelope at sections called gussets.
- Zygotene: Homologous chromosomes synapse (come together) but do not fuse, remaining 150 to 200nm apart. The synaptonemal complex forms here, representing the pairing of chromosomes with two lateral and one central component. Dense, ellipsoidal structures called recombination nodules also appear. Chromatin condensation becomes more obvious.
- Pachytene: Homologous chromosome pairing completes, and the chromosomes shorten and thicken. Though not easily distinguished, homologous chromatids undergo crossing-over, which will later appear as chiasmata. This is the longest stage of Prophase I. Each chromosome pair is called a bivalent, and the combination of four chromatids is called a tetrad.
- Diplotene: It becomes clear that each bivalent is made up of two chromatids. Chiasmata, evidence of crossing-over, are visible.
- Diakinesis: Chromosomes condense further, preparing for metaphase. Chiasmata persist, and terminalization occurs, where chiasmata move towards the chromosome ends. The nucleolus begins to fragment.
Prometaphase I
- Maximum condensation of chromosomes.
- Progressive diminution and disappearance of the nucleolus.
- Disintegration of the nuclear membrane.
- Microtubules attach to kinetochores.
Metaphase I
Bivalents align at the spindle equator, with both kinetochores of a chromosome oriented towards the same pole, and the kinetochores of its homologue oriented towards the opposite pole.
Anaphase I
Homologous chromosomes, each with two chromatids, separate and move to opposite poles. Chiasmata disappear as a result of chromosome disjunction.
Telophase I
Two daughter cells are formed, each with half the original chromosome number (n), but each chromosome still consisting of two chromatids.
Meiosis II
In Anaphase II, the sister chromatids of each chromosome in the two daughter cells separate, resulting in four cells, each with n chromosomes, each consisting of a single chromatid.
Genetic Significance of Meiosis
- Reduction of chromosome number from diploid to haploid.
- Genetic recombination.
- Segregation of maternal and paternal chromosomes.
Comparison of Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Energy Level | Exact distribution of genetic material. | Random segregation of chromosomes and crossing-over as a source of genetic variation. |
| Cellular Level | Genetically identical cells are formed. | Leads to a reduced set of chromosomes (exactly half the homologous chromosomes). |
| Organismal Level | Used for asexual reproduction in unicellular organisms and for development, growth, repair, and regeneration of tissues and organs in multicellular organisms. | Used for the formation of sexual reproductive cells (gametes) or asexual spores. |
Apoptosis
In multicellular organisms, cell number is tightly regulated. Cells that are no longer necessary undergo programmed cell death, or apoptosis. The intracellular machinery of apoptosis is similar in all animal cells and depends on a family of proteases called caspases.
Caspases Involved in Apoptosis
- Initiator caspases: Caspases 2, 8, 9, 10
- Executioner caspases: Caspases 3, 6, 7
Bcl-2 and IAP proteins are intracellular proteins that regulate procaspases. BH123 proteins play a role in the release of intermembrane space proteins from mitochondria in the intrinsic pathway of apoptosis.
Cellular Necrosis
Necrosis is accidental, non-apoptotic cell death. It is a passive form of cell death resulting from bioenergetic catastrophe due to ATP depletion.
Morphological and Biochemical Characteristics
| Feature | Necrosis | Apoptosis |
|---|---|---|
| Morphology | Loss of membrane integrity, chromatin flocculation, cell dilatation and lysis, vesicle formation, organelle disintegration. | Membrane deformation without loss of integrity, chromatin aggregation, cell shrinkage, limited membrane vesicle formation, intact organelles. |
| Biochemistry | Loss of ionic homeostasis regulation, no energy required, random DNA digestion, post-lytic DNA fragmentation. | Highly regulated process requiring ATP, non-random DNA fragmentation into mono- and oligonucleosomes, pre-lytic DNA fragmentation. |
| Physiological Significance | Group cell death caused by physiological stimuli, macrophage phagocytosis, significant inflammatory response. | Individual cell death induced by physiological stimuli, phagocytosis by adjacent cells or macrophages, no inflammatory response. |
Autophagy
Autophagy is a process where cytoplasmic components are sequestered in double-membrane vesicles called autophagic vacuoles and degraded by fusion with lysosomes.
- Non-selective autophagy: Induced in response to changes in nutrient availability to maintain bioenergetic homeostasis.
- Selective autophagy: Used in cellular processes requiring remodeling and can be induced in response to toxic stimuli to remove damaged components.
Control of Cell Growth, Division, and Apoptosis
- Mitogenic factors
- Growth factors (kinases)
- Survival factors