Chapter 11: Reproduction and sex
Reproduction is an essential component of organisms—organisms are ‘living things’ that can replicate themselves, i.e., reproduce. While some other ‘living things’ (e.g., cells, some organelles) can reproduce, many forms of life cannot (e.g., tissues, organs (usually) , many communities). We have defined organisms as biological entities that are discrete in time, i.e., they have a beginning and ending; organisms originate as the result of a reproduction event carried out by some already existing organism(s). Although sex is often associated with reproduction, it is a distinct phenomenon. And many organisms, especially non-animal organisms, usually reproduce in ways that do not involve sex and some have no sexual process at all.
TOPICS
- Reproduction — single celled organisms
- Reproduction —multi-cellular organisms
- Reproduction and dispersal in space
- Sex
- Examples from the green, red and brown algae
- Chlamydomonas
- Oedogonium
- Fucus
- Saprolegnia
- Lamanaria
- Ulva
- Porphyra
- Polysiphonia
Reproduction—single celled organisms
For single celled organisms, reproduction is a familiar and conceptually simple process (although the cellular details are far from simple!)—cell division. The original cell is partitioned by the synthesis and deposition of a new boundary (Fig. 1). For all cells, the boundary includes two plasma membranes (one for each cell) and for organisms with a cell wall the new boundary also has wall material between the two membranes. Note that for cell division to be a means of reproduction the daughter cell needs to detach from its parent cell either immediately following cell division or at some point in the future. Otherwise the new cell is simply increasing the size of the existing organism.
The original cell need not be divided equally, but both cells must be provisioned with whatever is required for the cell to maintain itself, in particular it needs genetic information in the form of DNA. Consequently, before the cell divides it needs to replicate its DNA and give each daughter cell a copy. For eukaryotic organisms, the DNA is packaged in units, chromosomes, that replicate before division, with one copy going to each daughter cell, i.e. the process of mitosis. Also present in eukaryotic cells are organelles (e.g. mitochondria) that need to replicate before cell division, with at least one ending up in each daughter cell. For photosynthetic organisms the same would be true for plastids that develop into chloroplasts. Remember that most biologists consider mitochondria and plastids to be remnants of unicellular organisms and their ability to divide is support for this idea.
Reproduction has to involve growth at some point but, as discussed in Chapter 7, growth might come after cell division (a cell is cut in half and then each half grows to full size) or before (a cell grows to twice its normal size and divides); ultimately reproduction requires that the organism acquire material, but thi