Cellular Biology: From Nucleus to Apoptosis
The Nucleus
Structure
- Envelope: A double membrane with nuclear pores enabling communication between nucleus and cytoplasm.
- Nucleoplasm: The internal environment containing DNA, RNA, proteins, and the nucleolus.
- Cellular Matrix: A protein network organizing chromatin. The nuclear lamina, made of intermediate filaments, contacts the inner membrane.
- Nucleolus: Concentrates ribosomal genes encoding ribosomal RNA.
Chromatin and Chromosomes
During interphase, chromatin forms nucleosome filaments or 30 nm fibers. In cell division, chromatin condenses into chromosomes. The 30 nm fibers coil into 700 nm filaments. Proteins play a crucial role in this condensation, forming a scaffold around the filaments. Chromosomes condense, coil, and become visible under a light microscope.
Parts
- Centromere: Divides the chromosome into two arms. The centromere contains the kinetochore, where microtubules attach for chromosome movement during mitosis.
- Telomeres: Chromosome ends with repetitive DNA sequences essential for replication.
A chromosome consists of two identical sister chromatids joined at the centromere.
The Cell Cycle
Growth Phase (Interphase)
The longest phase, where the cell doubles its components, including DNA. This occurs in the following stages:
- G1 Phase: General growth and organelle reproduction.
- S Phase: DNA synthesis.
- G2 Phase: Continued growth and preparation for mitosis.
Cell Division
The process creating two identical daughter cells from a mother cell. It includes mitosis and cytokinesis (M phase).
Mitosis
Equal distribution of duplicated chromosomes between daughter cells.
- Prophase: Chromosomes condense and become visible. Each chromosome has two chromatids. The mitotic spindle forms between separating centrosomes.
- Prometaphase: The nuclear envelope breaks down. Chromosomes interact with and attach to spindle microtubules via kinetochores.
- Metaphase: Chromosomes align at the spindle equator, attached to microtubules from opposite poles.
- Anaphase: Sister chromatids separate and move to opposite poles. Each chromatid is now a chromosome.
- Telophase: Daughter nuclei reform. Nuclear envelopes form from fragments of the parent cell’s nuclear envelope.
Cytokinesis
Division of the cytoplasm.
- Animal Cells: A contractile ring of actin and myosin filaments divides the cytoplasm.
- Plant Cells: A new cell wall forms in the equatorial plane from Golgi vesicles.
Meiosis
Cell division producing four haploid daughter cells from a diploid cell (sexual reproduction).
1st Division
- Prophase I:
- Leptotene: Homologous chromosomes begin pairing.
- Zygotene: Synapsis occurs – gene-for-gene pairing of homologous chromosomes, forming bivalents.
- Pachytene: Crossing over occurs – exchange of DNA fragments between homologs.
- Diplotene: Homologous chromosomes separate slightly, revealing chiasmata (crossing over points).
- Diakinesis: Chromosomes condense further. Other events similar to mitotic prophase occur.
- Metaphase I, Anaphase I, Telophase I, and Cytokinesis: Similar to mitosis but with homologous chromosomes separating.
2nd Division
Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis, similar to mitosis, resulting in four haploid daughter cells.
Life Cycles
- Haplontic Organisms (algae and fungi): Meiosis occurs after zygote formation. The organism is haploid, with the zygote being the only diploid cell.
- Diplontic Organisms (animals): Meiosis occurs during gamete formation. Gametes are haploid, and the zygote is diploid.
- Diplohaplontic Organisms (plants and some algae): The diploid zygote forms a diploid sporophyte, which produces haploid spores through meiosis. Spores give rise to haploid gametophytes.
Autophagy
An immune defense and maintenance mechanism. Autophagosomes engulf damaged cellular components, viruses, and bacteria, acting as a cleaning mechanism and removing dysfunctional mitochondria.
Apoptosis
Programmed cell death. It eliminates excess cells and cancerous cells.