Biology: using the life cycle of an angiosperm as an example explain the characteristics...?

Question

and significance of the alteration of generations as seen in this particular plants group?

A group of friends and I are studying for a biology test and going through our bio book trying to find connections....please please please help us with the above question...=]

Answer 1

The important part is that in the alternation of generations, there's a diploid generation (2n) and a haploid generation (n).

So looking at the life cycle of an angiosperm (flowering plant), you can see that the dominant generation - the one we see - is the diploid sporophyte. The stem, flower, leaves, roots - all of those cells are diploid.

The haploid generation is the "fertilizing" generation - the gametophyte.

Inside the anther of a flower, the microsporocyte develops and undergoes meiosis, becoming haploid. Each daughter cell undergoes mitosis (the number of chromosomes stays the same) and makes a generative cell and a tube cell. The generative cell mitotes again to make 2 sperm cells, so you end up with a pollen grain consisting of 2 sperms cells and a tube cell.

In the ovule, a megasporocyte undergoes meiosis, and then all but 1 of the daughter cells die, so you get one haploid megaspore. It undergoes mitosis (same # chromosomes) 3 times to make an embryo sac.

When a pollen grain from an anther lands on a stigma, the tube cell grows down into the embryo sac, and the sperm cells go down it.

One fuses with the egg, and one fuses with the two polar nuclei (the "fake" extra nuclei) to make the endosperm (which is the food for the embryo, and makes up what we think of as the seed).

Soooo the embryo is diploid again, since the sperm and the egg were haploid and fused.

Yay!

Except the gametophyte in this case (the microsprocyte and the macrosporocyte) are really reduced, almost to nothing, and are extremely dependent on the sporophyte. This is the complete opposite from mosses, where the gametophyte is dominant and the sporophyte grows on top of the gametophyte.