Botany speak!

[David Nicholson]

My scientific education came to an end when I was 14, as far as I can remember the main reason being that I was hopeless at it. In every monograph I read the discussion always turns to Chromosome counts with various figures involved, for example 2n=20 and so on. Can anyone explain to me please in words of little more than one syllable what these figures mean?

[Maggi Young]

I know the feeling, David. What we need, I thought, is a Chromosomes for Dummies book.... and guess what...... such a book exists!
See here http://www.dummies.com/WileyCDA/DummiesArticle/Genetics-Examining-the-Basics-of-Chromosomes.id-4097.html

I think I/We will still need a dictionary for the long words, though!

 
   
 

 

[David Nicholson]

Could be far too advanced for me Maggi, I'm well below the dummy stage!

[Maggi Young]

Okay, then try this:
I'm working my way through it right now!

[David Nicholson]

That's about my level, but it stops at the fish picture? Time to go for a Friday pint now but I shall look at this again tomorrow.

[Anthony Darby]

David, 'n' is the haploid number which represents the number of chromosomes in a gamete (pollen or ovule). Fertilisation gives us '2n' as pollen and ovule nuclei fuse to produce a fertilised ovule or zygote. This is diploid and therefore '2n'. As chromosomes occur in homologous pairs, 'n' also represents the number of pairs of chromosomes in a normal cell of the plant in question. If '2n' - 20, then a normal (somatic as opposed to sex) cell has 20 chromosomes as 10 pairs. As genes also occur in pairs, one on each of two homologous chromosomes, they can be the same (homozygous) or different (heterozygous) for individual characteristics.

[David Nicholson]

Well thank you Anthony. The words in that I have never seen before were:- haploid;gamete;zygote;homologous;somatic and homozygous. So I shall have a cup of coffee, open my dictionary and off we go!

[Lvandelft]

Can anyone explain to me please in words of little more than one syllable what these figures mean?

David, in this case I think the TEACHER did not quite understand your question.... 

[David Nicholson]

Now you see why science rules the world Luit, no-one, apart from Scientists, understands a word of it!

[Michael]

David, i think it Genetic talk topic would be more apropriate 
Well i will try to explain you what "2n=20" means... , i will try to simplify it the most.

Take this image for now, and pretend that it refers to a specific plant:

link:http://www.ikkeweer.net/cats-otherfiles/chromo.gif

Now count the X's...


When you finish you can see that you have 38 X's . Some are big, other are small, but they all represent chromosomes. Each X is a chromosome. If you notice, they are all in pairs, the two most alike pair together to form what we call an homologous pair, this means they are "brothers", and in normal conditions each chromosome only pairs with the respective brother.

Now, if you have 38 X's (38 chromosomes) then you have 19 pairs of X's (19 pairs of chromosomes). Regarding the plant itself now, each cell of the plant (except ovules and pollen) has 19 pairs of X's (or 38 X´s). then you can say that this plant is: 2n=38.
2n represents the diploid phase. Diploid means that the plant has the 2 chromosome brothers (pairs) together in the same cell. Now when the plant wants to set seeds, it needs the pollen (or ovules) of other plants from the same species. In both pollen and in unfertilized ovules, the cells only have 19 chromosomes (no pairs) instead of having 38 chromosomes (or 19 pairs) because, in simple terms, from one mother cell with 38 chromosomes (19 pairs), when it divides by a special process, you "obtain" 2 grains of pollen or 2 ovules with 19 chromosomes (no pairs) each.
 (there is only present one brother chromosome of each pair, because the brother pairs (homologous pairs) are broken while the cell splits in 2, and then each brother goes to a different cell). (this is not the way it happens in the real life, in fact one cell gives you 4 daughter cells, and not 2 cells, but it is only to simplify).

Now look at the grain of pollen (or ovule) with 19chromosomes (no pairs). When you look at it you can say that n=19. n represents the haploid phase.
 Haploid means that only one brother chromosome of each pair is present on the cell. If only one brother is present, it cannot pair with the restant chromosomes.

Now when a grain of pollen with 19 chromosomes fuse with an ovule with 19 chromosomes, then one chromosome from the pollen pairs with the respective brother of the ovule. Then you return again to the dipliod phase (2n= 38).

I hope you understood my poor english. I tried to simplify the process the best i could!
Now with your own example:

2n=20 means that in the dipliod phase (plant phase) each cell of the plant has 20 chromosomes, and that they are paired (2n) You have 10 pairs of chromosomes (or 20 chromosomes).

In pollen or ovules of that same plant you have n=10. This is the haploid phase. In the pollen or ovule cells you only have one chromosome of each pair (n).

[ranunculus]

Understood perfectly!  Many thanks.

[Maggi Young]

I do understand a lot of this, thank you, Michael! 

If only one brother is present, it cannot pair with the restant chromosomes.

I need a little more explanation for this, however... what are "restant chromosomes",, please/

[Casalima]

I do understand a lot of this, thank you, Michael! 

If only one brother is present, it cannot pair with the restant chromosomes.

I need a little more explanation forthis, however... what are "restant chromosomes",, please/

Remaining, I would imagine ...

Thanks for the explanation Michael!
I actually almost understood Anthony's too! 

Chloë

[Maggi Young]

Yes, Chloë,  I would guess "remaining" too, but remaining from where....why?

[rob krejzl]

Strictly, restant = persistent.

Mike is saying that the chromosomes in sex (haploid) cells don't form pairs because each is different from the other. Pairing only occurs again at fertilisation.

And all of this is important because it give clues to the origin and evolution of plant species. There's a chapter on the asian trilliums in the Case's book which talks about the varying ploidy levels of these plants - one can make inferences about their likely history from this. The discussion on Primula x kewensis in Richard's book is another example, especially since it deals with the emergence of fertility from what was originally an infertile diploid hybrid.