Understanding Cell Division: From Binary Fission to Mitosis
Binary Fission in Prokaryotes
Binary fission is the process used by most prokaryotes for asexual reproduction. Here are the major phases:
- DNA Replication: The circular DNA molecule replicates, starting at the origin of replication and proceeding in both directions.
- Chromosome Attachment: The two resulting chromosomes attach to the cell membrane.
- Cell Elongation: The cell grows and elongates, pushing the chromosomes apart.
- Cytokinesis: The cell membrane pinches inward, and a new cell wall forms, dividing the cell into two daughter cells.
Prokaryotic vs. Eukaryotic Cell Division
- Prokaryotes: No nucleus, no membrane-bound organelles, binary fission.
- Eukaryotes: Nucleus, membrane-bound organelles, mitosis (for somatic cells) and meiosis (for gametes).
Role of FtsZ Protein
FtsZ is a protein that forms a ring-like structure (the Z-ring) at the future division site in prokaryotes. It recruits other proteins involved in cell wall synthesis and helps constrict the cell membrane during cytokinesis.
Antibiotics and Binary Fission
Some antibiotics target binary fission to inhibit bacterial growth. For example, penicillin interferes with cell wall synthesis, which is essential for bacterial division.
Bacterial Resistance
Mutations during DNA replication can lead to variations in daughter cells. Some mutations may confer resistance to antibiotics, allowing bacteria to survive and multiply in the presence of the drug.
Factors Influencing Bacterial Growth
Environmental factors like temperature, pH, and nutrient availability significantly influence bacterial growth rates. A bacterial growth curve typically shows four phases: lag, exponential (log), stationary, and death.
Spectrophotometry and Bacterial Growth
A spectrophotometer can be used to measure bacterial growth by measuring the absorbance or transmittance of light through a bacterial culture. Absorbance is the amount of light absorbed by the sample, while transmittance is the amount of light that passes through it.
The Eukaryotic Cell Cycle and Mitosis
The eukaryotic cell cycle consists of two main phases: interphase and the mitotic (M) phase.
Interphase
- G1 Phase (Gap 1): The cell grows, replicates organelles, and synthesizes molecules needed for DNA replication.
- S Phase (Synthesis): DNA replication occurs, resulting in two identical sister chromatids for each chromosome.
- G2 Phase (Gap 2): The cell continues to grow, synthesizes proteins needed for mitosis, and prepares for cell division.
M Phase (Mitosis)
- Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and the mitotic spindle forms.
- Metaphase: Chromosomes align at the cell’s equator (metaphase plate).
- Anaphase: Sister chromatids separate and move to opposite poles of the cell.
- Telophase: Chromosomes decondense, the nuclear envelope reforms around each set of chromosomes, and the spindle disassembles.
Cytokinesis
The cytoplasm divides, resulting in two daughter cells. In animal cells, a cleavage furrow forms. In plant cells, a cell plate forms.
Cell Cycle Checkpoints
Checkpoints regulate the cell cycle and ensure that each stage is completed correctly before the next one begins.
- G1 Checkpoint: Checks for DNA damage, cell size, and growth factors.
- G2 Checkpoint: Checks for DNA replication completion and DNA damage.
- M Checkpoint (Spindle Checkpoint): Ensures that all chromosomes are correctly attached to the spindle fibers.
Apoptosis and the p53 Protein
Apoptosis is programmed cell death that eliminates damaged or unwanted cells. The p53 protein is a tumor suppressor that plays a crucial role in cell cycle arrest, DNA repair, and apoptosis. If DNA damage is irreparable, p53 can trigger apoptosis.
Chromosomes and Related Structures
- Chromosome: A thread-like structure made of DNA and proteins that carries genetic information.
- Chromatin: The complex of DNA and proteins that makes up chromosomes. It condenses during cell division to form visible chromosomes.
- Centromere: A specialized region on a chromosome that connects sister chromatids and serves as the attachment point for spindle fibers during cell division.
Haploid vs. Diploid
- Haploid (n): Having a single set of chromosomes. Gametes (sperm and egg cells) are haploid.
- Diploid (2n): Having two sets of chromosomes, one from each parent. Somatic cells (body cells) are diploid.
Studying Mitosis in Blastula
A blastula is an early stage of embryonic development where cells are rapidly dividing. It’s an ideal model for studying mitosis because the cells have large, easily visible chromosomes.