Question

1) What is the event that marks the transition from haploid to the diploid stage of the life cycle and what event marks the transition back to the haploid stage again. Describe the timing of these two cellular even in life cycles that are dominated by haploid cells, dominated by diploid cells or that have alternation of generations.

2) What are the closest relatives of land plants? How do you know this? What do these relatives tell us about the ancestors of green plants -- where did they live? How big were they?

3) Why was it the green algae that gave rise to land plants -- why don’t we have relatives of red or brown algae living on land?

Answer 1

First you have to have a background of what is the cell cycle, is composed of the interphase (which is divided by the phase G1, S and G2) and the mitosis (which is divided into Prophase, metaphase, anaphase and telophase.)

In the G1 interval there is cell growth, proteins, hydrocarbons and cell lipids are being elaborated, in this phase the DNA has not yet been replicated (unduplicated chromosomes) (means 1n or haploid).

Once the S phase begins, DNA replication begins and until the end of this phase we have duplicate chromosomes (means 2n or diploid). This gives rise to the G2 phase in which proteins that will boost mitosis are synthesized.

Now in mitosis we start with the prophase, remember that the chromosomes were already duplicated in the S phase of the interface, at this point the sister chromatids are joined by a centromere. Here the chromosomes are visible under the microscope because the DNA associated with their proteins condense. In addition, the centrioles begin to align.

In the metaphase the chromosomes are aligned at the midpoint of the cell and is a very important part of mitosis.

In the anaphase the sister chromatids of each chromosome are directed towards the opposite poles of the spindle so that they separate more and more and more until they reach the Telophase. The telophase begins when each type of chromosome reaches a pole of the spindle, two genetically identical groups of chromosomes are now at the opposite ends of the cell.

The chromosomes are decondensed and take the form of thread. at this point the membranes are already forming that will define each new cell, each one. In this example the mother cell had a diploid number of chromosomes and each nucleus of the daughter cells has the same number of chromosomes as the nucleus of the cell mother. (although in a haploid state).

Now the alternation of generations between haploid and diploid is something that happens in certain types of organisms (known as foraminifera) and they do it in such a way that they can have asexual reproduction (by binary fission in which they only do mitosis) and sexual reproduction in the which is fusion of nuclei and this is through meiosis.

For second question

The closest relatives of the plants are karyophytes, since they have chloroplasts with chlorophylls a and b, store sugars in the form of starch possess cellulose fibers that strengthen their cell wall. Of these follow the liver plants or mosses also known as bryophytes these latter are somewhat more similar to the ground plants but lack structures such as vascular system or roots. These ancestors tell us how the plants evolved. Although these ancestors (Karyphytes) began in swamps and still waters, we can now find bryophytes in these plants are of varied habitats but mainly in humid habitats, on soil, rocks, trunks and branches of trees and shrubs or fallen trunks, on bones, manure, etc. These plants are small as grasses or mosses, they do not have a vascular system to drive water and nutrients to great heights.

For question 3.

There are theories that say that chlorophyll is one of the most efficient pigments to capture light, while other pigments such as anthocyanins or carotenoids do not do so well and that is why we have more green plants.