©KNI Bell

Life cycle, ,chromosomes as formatted lists some words and definitions for algal lifecycles

The life cycle of an organism is the sequence of developmental events linking one generation to the next. The life cycle you are familiar with e.g. human has one adult multicellular stage ( diploid - 2 sets of 23 chromosomes for a total of 46). A special form of cell division called meiosis (my-oh-sis, a.k.a. 'reduction division' because it reduces ploidy) produces gametes ( eggs and sperm which are haploid - having a single single set of chromosomes ...23). Haploid gametes (one egg, one sperm) fuse to form diploid zygote.

Most multicellular autotrophic protists and all higher plants have two multicellular stages in their life cycles one haploid (single set of chromosomes called the gametophyte),  one diploid ( double set of chromosomes called the sporophyte). These alternate in the life cycle - alternation of generations.

Two linked concepts and their definitions:
CHROMOSOME NUMBER, often represented as n: the number of chromosomes in a single set of non-homologous chromosomes, e.g. the number of chromosomes in a gamete of ploidy = n. Humans have haploid n=23, diploid 2n=46.
PLOIDY: the number of complete sets of chromosomes per cell, e.g. a diploid organism has (2n) = one paternal set (n) and one maternal set (n). Diploid is literally "two-ploid". Gametes of all organisms are called haploid (from Greek haplos = simple, so "simple-ploid"; even though it sounds like it and it fits, the etymology has nothing to do with 'half') but in 2n organisms haploid=n.

Chromosomes as formatted lists

If chromosome number n, ploidy, are still giving you trouble, let me revisit the analogy I gave you already (Mar. 12 '03) whereby we compare a chromosome to a grocery list. Take an imaginary 'species' that with such a 'grocery list' (remember each copy is a double strand of DNA, and we're looking at a diploid individual so it has two copies in each cell) :

Format_of_LIST#1

copy A

 copy B individual is _-ozygous individual can manage
bread type brown white heter either
eggs type small small hom small only
milk type full cream 2% skim heter either
dish detergent type orange [defect, e.g. oringblxx] heter either except only one chromosome has a functional gene here, so it's orange liquid only
tomatoes canned type noname bestname heter either
cheese1 type cheddar gouda heter cheddar or gouda
cheese2 type cheddar cheddar hom cheddar or cheddar
household cleaner type [defect] [defect] no function* non-functional genes on both copies, this guy's apartment will be a mess (*can properly call individual homozygous only if same error exists on both sides)

If each chromosome is a list, then LIST#1 can represent one chromosome. If LIST#1 is all that a cell of this species has, then n=1 (one complete set of all the different lists is n=1 list), so an individual with two copies (e.g. A and B above) is a diploid or 2n individual. Supposing this individual produces gametes, then the gametes (neglecting swapping of A and B information by crossing-over) could be:
"brown,small,fullcream,orange,noname,cheddar,cheddar,[defect]", or:
"white,small,2%skim,[defect],bestname,gouda,cheddar,[defect]".

Each gamete has only a single copy of LIST#1, so it is haploid or n. (If that gamete grows to become a multicellular organism, it is still haploid or n.)

Now, if a species had a second list**, then n=2, and that would mean 4 copies in all for a 2n individual, 2 copies in all for a haploid or n individual.
**A second list, LIST#2, for e.g., might be clothing: "hat,scarf,shoes,biscuits" (no guarantee that all lists are perfectly organised), and the chromosome might specify "straw,none,[defect],ginger".

Definitions: think about these carefully - understanding of alternation of generations depends on understanding these terms

  1. Gamete - a cell (n) which fuses with another cell to form a zygote (2n)
  2. Spore - a reproductive cell that develops directly (i.e. no fusion with other cells)
  3. Gametangia - cells formed on gametophytes which produce gamete(s)
  4. Sporangia - cells formed on sporophytes which produce spore(s)
  5. Gametophyte - multicellular organism, part of a protist or plant life cycle, that produces gametes
  6. Sporophyte - multicellular organism, part of a protist of plant life cycle, that produces spores
  7. Alternation of generations is the alternation within a life cycle of a spore-producing sporophyte (2n, diploid) with a gamete-producing gametophyte (n, haploid). (Don't confuse it with 'alternation of generations' in Cnidarians, where it is not associated with change in ploidy.)

Note the above definitions involve issues of mitosis, meiosis, and levels of ploidy. (Jul 19 2002 summer semester*): we discussed in some detail binary fission, compared it with mitosis, compared that with meiosis, syngamy, ploidy vs. chromosome number, haploid/diploid, FORMAT as a way of thinking about a chromosome -- good questions from class about transfer of genetic material from one chromosome to another, syngamy/meiosis as a way of achieving genetic diversity, syngamy/meiosis always alternating except that if two syngamies occurred in sequence then chromosome number would be doubled and the next meiosis would only reduce ploidy to the previous diploid level and it would take two meiosis operations to reduce back to the original haploid level ... syngamy a bit pointless if gametes have same origin so yes there are recognition factors in some species that reduce the chance of self-fertilising and increase the chance of combining with gamete from a different source)

*some of these discussed in class around Mar 12 '03: the idea of chromosome as a formatted list