The Science of Existence (104) 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.