ATAR Notes: Forum

Archived Discussion => 2010 => Mid-year exams => Exam Discussion => Victoria => Biology => Topic started by: matt123 on October 10, 2010, 02:21:35 pm

Title: some 1 please help on bio exam question ( multi choice question)
Post by: matt123 on October 10, 2010, 02:21:35 pm: Question 15
The position where a restriction enzyme can cut a piece of DNA into smaller fragments is referred to as a

A.
receptor site.
B.
receptor sequence.
C.
recognition sequence.
D.
restriction site.

answer is C . why cant it be D ? ,.. whats the diff?

thanks
Title: Re: some 1 please help on bio exam question ( multi choice question)
Post by: matt123 on October 10, 2010, 02:24:53 pm: also

Question 19
Uranium-235 is a radioactive isotope which has a half life of 700 million years. Sedimentary rocks that contain fossils are found to have ⅛ of their original amount of uranium-235. From the information provided, how old are the fossils in the sedimentary rocks?

A.
700 million years.
B.
1400 million years.
C.
2100 million years.
D.
2800 million years.

how would you work that out

thanks alot in advance :)
Title: Re: some 1 please help on bio exam question ( multi choice question)
Post by: Russ on October 10, 2010, 02:55:05 pm: Quote
answer is C . why cant it be D ? ,.. whats the diff?

Formally it's C (which is "more" correct than D). People use D because it's easier to say/think of but it's not a great question tbh

Quote
how would you work that out

Every 700 million years, half the uranium in the fossil decays. So if there are 256 units originally, after 700 million years: 128. After 1400 million years: 64 After 2100 million years: 32 etc.

Basically, the fraction of uranium remaining is equal to $1/2^n$ where $n$ is however many half-lives have passed. (there's a proper formula for radioactive decay but you don't need it)

You have $1/8 = 1/2^n$ , so by solving you get $n=3$ or 2100 million years. I hope. :P

This is really a physics question not a biology one.
Title: Re: some 1 please help on bio exam question ( multi choice question)
Post by: matt123 on October 10, 2010, 03:00:27 pm: Quote from: Russ on October 10, 2010, 02:55:05 pm
Quote
answer is C . why cant it be D ? ,.. whats the diff?

Formally it's C (which is "more" correct than D). People use D because it's easier to say/think of but it's not a great question tbh

Quote
how would you work that out

Every 700 million years, half the uranium in the fossil decays. So if there are 256 units originally, after 700 million years: 128. After 1400 million years: 64 After 2100 million years: 32 etc.

Basically, the fraction of uranium remaining is equal to $1/2^n$ where $n$ is however many half-lives have passed. (there's a proper formula for radioactive decay but you don't need it)

You have $1/8 = 1/2^n$ , so by solving you get $n=3$ or 2100 million years. I hope. :P

This is really a physics question not a biology one.

im still a tiny bit lost on how u got 2100

1/2^8 isnt 3 lol.... what am i missing here

i feel like an idiot for asking btw haha
Title: Re: some 1 please help on bio exam question ( multi choice question)
Post by: Russ on October 10, 2010, 03:05:12 pm: If we accept that we have 1/8th of the total uranium left and that the decay is represented by the formula above, we say

$1/8 = 1/2^n$

We can just flip everything and rewrite that as

$8 = 2^n$

Which we can then rewrite as

$2^3 = 2^n$

Therefore, n is 3. Since n represents how many "half lives" have passed the total time elapsed is

$n * half life length$

We know that n is 3, we know that one half life is 700 million -> 2100 million
Title: Re: some 1 please help on bio exam question ( multi choice question)
Post by: matt123 on October 10, 2010, 03:07:11 pm: OHHHHHHHHHHHH
cheers mate
yeah get it now:)

thanks