DNA and tree vascular tissues

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DNA and tree vascular tissues

Post by mercuryy » Thu Jul 20, 2006 2:24 am

If a molecule of mRNA, prior to splicing or polyadenylation, has 15% A, 20% G, 30% U, and 35%
C, what is the composition of the double-stranded DNA that it was transcribed from?

a question from a past competition. and the answer is
22.5% T, 22.5% A, 27.5% G, 27.5% C

You cut a live twig from a tree and examine the cut surface of the twig with a magnifying glass. You
locate the vascular tissue and observe a growing droplet of fluid exuding from the cut surface. Where
is this fluid most likely to be from?
another question that i don't get
and the answer is phloem. why doesn't it come from xylem.

any help of explaination is appreciated :lol:

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Post by canalon » Thu Jul 20, 2006 3:29 am

A hint for question 1: mRNA is single stranded.

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

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It's phloem

Post by chloe18 » Thu Jul 20, 2006 11:28 am

The reason the answer is phloem and not xylem is that phloem sieve tubes transport liquid materials under very low pressure while in xylem vessels, the water moves up the narrow tubes by capillary action which keeps it at high pressure. Because of this the liquid is more likely to escape the low pressure pull of phloem tubes than the high pressure pull of xylem vessels.

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Post by SU_reptile » Thu Jul 20, 2006 11:29 am

As far as the second question is concerned, it's phloem indeed.
It is so because phloem transport nutrients from leaves to the other organs. Leaves present on that twig still suck the watery solution through xylem so it (water) doesn't leak from the cut. However, if you examined the cut from the side of the tree, then you could observe the solution coming from xylem.
That is the way I understand the problem.

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Post by SU_reptile » Thu Jul 20, 2006 3:06 pm

Woa, I think I got the genetic task too.
mRNA is single stranded while DNA is double stranded.
Let's say that in the mRNA there are 100 bases, so percents refer to the amount of bases.

You have mRNA with
15% A, 20% G, 30% U, and 35% C

You need to remember that every type of base occurs in both DNA strands, so that:
A occuring in 1st strand gives U (30) in mRNA. Then A occurs in 2nd strand and is complementary to T in 1st strand, which in turn gives A in mRNA (15) - now you have 45 adenines in DNA. But double stranded DNA has 200 bases - 100 per strand, hasn't it? Now you can estimate the percentage of A bases from proportion: if 200 bases is 100%, then 45 bases is X%. X is then 22,5%, isn't it?.
The same thing you do with other bases - The easiest way is to sum up complementary bases in mRNA (here I show you that on A and U axample) and divide it by 2 or use the proportion. It may look obscure, but I don't know how to explain it in other way.

Here is the example:

mRNA is comprised by: A=30%, G=20%, C=40%, U=10%



A: 30%+10%=40%; 40%/2=20%

or: 3 A bases constitute 30% and 1 U base constitute 10%;
20 bases - 100%
4 (3+1) bases - X%
400=20X%; X=20%


G: 20%+40%=60%; 60%/2=30%

or: 2 G bases constitute 20% and 4 C bases constitute 40%;
20 bases - 100%
6 (2+4) bases - X%
600=20X%; X=30%


In DNA there are: A=20% (4 bases), G=30% (6 bases), C=30% (6 bases), T=20% (4 bases).
If you still don't understand that task I will try to explain it more clearly and work that your task . :wink:

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