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AS Bio Concept...DNA

help! am really stuck on this bio question on dna...we havnt dun this topic at college yet so i'm not really gettin 2 grips with it...wud appreciate ne help...
basically theres a table with proportions of the four organic bases in a piece of dna frm a squirrel, a shark and mRNA frm human.

Squirrel DNA = 29%A 21%G 22%C 28%T
Shark DNA = 28%A 21%G 21%C 30%T
Human mRNA = 40%A 15%G 30%C U15%

1. explain why the proportion of A + G is approx. equal to the proportion of T + C in both pieces of DNA.

2. explain why a similar equality does not exist btw the proportion of A+G and the proportion of U+C in the RNA.
cos of the fact that rna remain as single strands of polynucleotide & form. v. diff structure??

3. Despite being v.different organisms, the squirrel & the shark hav similar proportions of the 4 diff. bases in their DNA. explain how this is possible.
sumthin 2 do with dna replication??
1 - Well that might be something to do with the fact that A and T always go together and C and G always go together.
Reply 2
thats true...but i dont think its that simple...cos its a 3 mark question.....
Reply 3
urban_flavaz
help! am really stuck on this bio question on dna...we havnt dun this topic at college yet so i'm not really gettin 2 grips with it...wud appreciate ne help...
basically theres a table with proportions of the four organic bases in a piece of dna frm a squirrel, a shark and mRNA frm human.

Squirrel DNA = 29%A 21%G 22%C 28%T
Shark DNA = 28%A 21%G 21%C 30%T
Human mRNA = 40%A 15%G 30%C U15%

1. explain why the proportion of A + G is approx. equal to the proportion of T + C in both pieces of DNA.

2. explain why a similar equality does not exist btw the proportion of A+G and the proportion of U+C in the RNA.
cos of the fact that rna remain as single strands of polynucleotide & form. v. diff structure??

3. Despite being v.different organisms, the squirrel & the shark hav similar proportions of the 4 diff. bases in their DNA. explain how this is possible.
sumthin 2 do with dna replication??



For the last one you can say that it is not the content of the dna that determines the organisms charecteristics (phenotype) but it is the sequence in which the dna is ordered.

1st one is defo sumthing to do with the fact that A and T and C and G are paired. Just elaborate on this.
Reply 4
urban_flavaz
help! am really stuck on this bio question on dna...we havnt dun this topic at college yet so i'm not really gettin 2 grips with it...wud appreciate ne help...
basically theres a table with proportions of the four organic bases in a piece of dna frm a squirrel, a shark and mRNA frm human.

Squirrel DNA = 29%A 21%G 22%C 28%T
Shark DNA = 28%A 21%G 21%C 30%T
Human mRNA = 40%A 15%G 30%C U15%

1. explain why the proportion of A + G is approx. equal to the proportion of T + C in both pieces of DNA.

2. explain why a similar equality does not exist btw the proportion of A+G and the proportion of U+C in the RNA.
cos of the fact that rna remain as single strands of polynucleotide & form. v. diff structure??

3. Despite being v.different organisms, the squirrel & the shark hav similar proportions of the 4 diff. bases in their DNA. explain how this is possible.
sumthin 2 do with dna replication??

1. Perhaps this is a 3 mark question because they use the word approximately? The proportions are close because A only pairs with T and G with C. However they are not exactly the same, and you would expect them to be identical due to complementary base pairing. I don't know if you've covered, or will cover, telomeres, but these could be an explanation for the slight differences. In humans (and other mammals) the telomeres are at either end of each chromosome and basically the DNA extends from the coding bit of the chromosome as a single strand of DNA, which loops back on itself to close the loop. However within the loop there's no complementary base pairing, and so the proportions of the bases can be quite different here. (The telomeres exist, by the way, to protect the coding DNA in the chromosomes from degradation during replication.)

2. Yup. mRNA is created as a single strand and so you've no complementary pairing, so you can have any proportions of the bases.

3. Yeah, what the previous poster said about the arrangement of the bases being important is right. Let me know if you want some explanations about codons and things :smile:
Reply 5
thanks 4 the help