Cell Division
The cell cycle is the life cycle of cell, though the term is often used to refer to the mitotic phase, when cell division occurs. Between divisions, a cell is in interphase, which is its ordinary life.
Cell division is a daedal process of cell replication. There are a variety of regimes for cell division, depending upon the type of cell and organism.
Cell division is dynamically regulated, with much decision-making. Cells decide to divide based upon the environment they sense they are in, cell size, and the time of day. Changing cell fluid pressure helps drive cell division.
Prokaryotes asexually reproduce via fission: splitting their cells into 2 (binary fission) or more (multiple fission). Some organelles in eukaryotic cells also replicate via fission, as do some single-celled eukaryotes.
In eukaryotic cell division, a parent cell divides into daughter cells. The cell nucleus divides first (mitosis), followed by the cytoplasm splitting in 2 (cytokinesis). For unicellular eukaryotes, such as amoeba, cell division is equivalent to reproduction: a new organism is created.
Cell division in multicellular eukaryotes varies by cell type. Somatic and stem cells undergo mitosis. As part of sexual reproduction, germline cells divide via meiosis.
The primary concern of cell division is maintaining the parent cell’s genome. Prior to division, the genomic encyclopedia stored in chromosomes (or prokaryotic genophores) must be precisely replicated, and the duplicated genome cleanly bifurcated between cells.
Considerable cellular infrastructure is involved in ensuring genomic consistency from one generation to the next. The intricacy of cell division illustrates the amazing intelligence in biological processes.
Mitosis
The fact that mitosis is a universal eukaryotic property suggests that it arose at the base of the eukaryotic tree. ~ English biologists Adam Wilkins & Robin Holliday
Mitosis results in somatic or stem daughter cells, each with a full set of chromosomes. Daughter cells have the same genome as their mother cell. A crucial process within mitosis is chromosomal replication, which is regulated epigenetically.
A chromosome is a genetic package in the nucleus of a eukaryotic cell, comprised of DNA base pairs. A DNA base pair comprises 2 complementary nucleobases on opposite sides of a DNA double helix, linked by hydrogen bonds.
A chromosome is a single macromolecule, the largest in Nature. Human DNA has 46 chromosomes. Chromosome 1 in the human body, the biggest, has 10 billion atoms. The average human chromosome has ~140 billion base pairs.
Mitosis begins with chromosomal DNA base pairs being cleaved in half: the hydrogen bonds holding each ladder rung broken by enzymes. This happens in several sections simultaneously. To keep things tidy, proteins neatly stack the chromosomes in layers, forming multilaminar plates.
Then each half-ladder has its complement rebuilt. The result is 2 DNA ladders: half each of the original nucleotide string. The other side of each ladder is reconstructed.
Then these 2 sets of chromosomes are pulled apart from the center of the cell, in opposite directions, by protein ropes. These microtubules are part of the cytoskeleton, and play numerous roles in cell upkeep, mostly in moving bits about the cell.
Mitosis is an incredibly intricate process: an orchestration of many thousands of proteins moving through space, finding their exact positions at just the right time. While the proteins remember where they are supposed to go, and have an innate affinity to the right location, innumerable decisions must be made for cell division to succeed.
For normal tissue structure and function, cells exert strict control over growth versus differentiation. The distribution of organelles influence the outcome of stem cell division. ~ American cytologist Elaine Fuchs
Epigenetic influences take effect during mitosis. Hence, life experiences are passed on during each cell replication.
Mitosis takes 15–30 minutes per chromosome. Replication of a human cell DNA takes up to 10 hours. Yet human cells may divide as often as every day.
When the whole DNA molecule has been duplicated, the chromosomes move to opposite ends of the cell. Then cytokinesis kicks in: the cytoplasm of a eukaryotic cell divides; whence 2 cells are constituted.
Meiosis
Meiosis is the special cell division for sexual reproduction, producing germline gametes (sperm or eggs). Meiosis also refers to the cell division process for making spores. Meiosis evolved 1.4 bya as a derivative of mitosis.
In meiosis, the chromosomes replicate, but rather than pulling apart, the duplicate (homologous) chromosomes stay together. These doubled chromosomes appear as 4 lengths of ladder, side by side.
RNA mediates recombination of homologous DNA during meiosis. Ribosomal RNA molecules search for and sense homologous sequences.
The matched pairs then separate and move to opposite ends of the cell, as in mitosis. The chromosomes divide again, then replicate, thereby producing 4 daughter cells.
Because there have been 2 divisions with only 1 replication, these gamete cells have only half the complement of chromosomes, one of each matching pair. A human gamete has 23 chromosomes.
While meiosis produces gametes in animals, meiosis generates spores for fungi, bacteria, and other simpler life forms.
Spores are patient hibernators. Unlike seeds, which are stocked with food, spores have minimal energy supply. Still, spores keep their wits about them.
Once conditions become favorable, a spore comes to life by mitotic division, turning the haploid mitospore into a diploid living organism. Ferns, especially those adapted to dry habitats, produce diploid spores.
Sporulation is one process of asexual reproduction. Other asexual reproductions include: binary fission, where 1 parent becomes 2 daughters; budding (e.g. baker’s yeast, hydra): a mother creating a smaller daughter; and fragmentation: a new organism grows from a fragment of the parent.
Vegetative reproduction is floral fragmentation. Herbaceous and woody perennial plants often practice vegetative propagation.
Single-celled organisms, such as the archaea, bacteria, and protists, reproduce asexually. Many multicellular plants reproduce asexually as well.
Sexual reproduction starts with fertilization. An egg recognizes whether the sperm before it is proper, beginning with it being species compatible. Unsuitable suitors are rejected.
A human sperm delivers its 23-chromosome package to an egg. 1 chromosome per pair is inherited from each parent.
The chromosome pairing process is not necessarily a neat, exact matchup of the genetic sequences from each parent. Genetic recombination of DNA segments occurs. This further increases genetic variability.
For humans, with 46 chromosomes per cell, there are over 8 million (223 = 8,388,608) possibilities for the chromosomal pair in 1 gamete. Fertilization squares that to the neighborhood of 70 trillion.
But heredity is not a genetic numbers game. There is no direct relationship between chromosomal quantity and organism sophistication. For one thing, as a byproduct of evolution, the number of genes per chromosome varies among species.
Bacteria have 1 circular DNA molecule, but regularly complement their DNA with plasmids (gene packets) found in the ambient environment, often provided by other bacteria. This is how antibiotic resistance spreads among bacteria. Numerous strains of cave bacteria, cut off from the surface for millions of years, are resistant to several antibiotics. They remember the bacterial wars of old. These bacteria can easily pass antibiotic resistance to other bacteria.
Meiosis is not the only time that gene shuffling occurs. Diploid somatic cells of fungi, plants, and animals occasionally undergo mitotic recombination during mitosis. Recessive genes may be expressed through this genic jumble.
