A level Biology: Post your doubts here!

[Alice123]

Can any1 explain what a frameshift is

 

[Irfan1995]

But wait, what's the difference in "intermediate" and "both"?

Stabilizing selection = If you have a (bell-shaped) graph whose peak is near the centre, the graph will be squeezed so that it will be taller but thinner.
Disruptive selection = You will end up with two peaks, with one peak on each extreme. Both extremes have a high chance of survival, whereas the intermediate/average intensity allele will not be viable.

The most famous example of disruptive selection is the beak size in the Galapagos islands. Over there, there were only two types of seeds: big and small. Those with small beaks can easily pick up the small seeds, and those with big beaks can easily crush the big seeds to digest them. Those with medium-sized beaks are too big to pick up the small seeds, but they're also too small/weak to crush the big seeds. Therefore the medium-sized beak birds are left with a disadvantage. The intermediate (medium-sized beaks) is not viable (disadvantageous) and the two extremes (big beaks and small beaks) are preferred.

 

[Irfan1995]

But why did they refer to the volume of air absorbed as the volume of air exchanged , because the more air was exchanged in each breath , the more would be absorbed right? oh ok I think I got it. Thank you so much for the help

Introductory notes:

• Amino acids are translated from codons
• Each codon is made up of 3 bases

Frameshift:
Let's say you have a sentence (polypeptide) of three letter words (since each codon/word is made up of three letters/bases)
THE FAT CAT ATE HIS RAT. (It's just a sentence I made up to explain the situation)
Assume that you have a base addition or deletion. In this example, I'm going to delete the "A" in "FAT". You'll be left with this:
THE FTC ATA TEH ISR AT.
Remember that the aminoacyl tRNA synthase (enzyme that translates mRNA into amino acids) reads the sentence in groups of three, so every "word" must have three "letters"
As you can see, every word after the base deletion makes no sense, so completely different amino acids (than the original) will be added to the polypeptide.
Another problem is that one of the new codons could be a stop codon which will stop translation immediately giving an incomplete polypeptide.
From this, it is clear that frameshifts are deadly (compared to point mutations/base substitution)

 

[Irfan1995]

But why did they refer to the volume of air absorbed as the volume of air exchanged , because the more air was exchanged in each breath , the more would be absorbed right? oh ok I think I got it. Thank you so much for the help

Let's say I lost a bet to a friend of mine, so I had to give him $150.
So I lost $150 and my friend gained $150.
The total amount of money exchanged is $150 NOT $300.
So the amount of money exchanged = amount of money I lost = amount of money my friend gained.

Taking this example to the question:
If I take in 5 litres of air and exhale another 5 litres of air, the amount of air exchanged is 5 litres (not 10 litres)

Hope that makes sense

 

[MÁťT PŭמҚ]

can any1 tell me how to measure cells using a graticule?? :/

 

[PANDA-]

can any1 tell me how to measure cells using a graticule?? :/

I'm guessing you need that so you can draw the cell to scale.
First measure, choose the limits of your drawing, but on one axis only, say you take the height for example, measure a reasonable height on your paper, you can choose this height, but make sure it's taking most of the space given, measure it, and draw two end points, now measure the height of the cell on the graticule, now by ratio, you know how many graticules gives you the height you chose, make this your scale, by which you'll draw the length as well, and the layers in the plan diagram. (I hope you understand)

 

[MÁťT PŭמҚ]

I'm guessing you need that so you can draw the cell to scale.
First measure, choose the limits of your drawing, but on one axis only, say you take the height for example, measure a reasonable height on your paper, you can choose this height, but make sure it's taking most of the space given, measure it, and draw two end points, now measure the height of the cell on the graticule, now by ratio, you know how many graticules gives you the height you chose, make this your scale, by which you'll draw the length as well, and the layers in the plan diagram. (I hope you understand)

how to calibrate the graticule units??

 

[daredevil]

how to calibrate the graticule units??

when u place a stage micrometer on the stage and look at it through the microscope u see big lines in the eyepiece lie a sscale ryt?
now the eypiece scale is much smaller...
u know for a fact that each division o the micrometer is 0.01mm (i think but anyways it is given on the micrometer). adjust the eyepiece and the stage micrometer in such a way that they are aligned. to adjust the eyepiece scale u can rotate the eyepiece and for the stage micrometer u can move the stage until one of its lines is exactly superposed on a line of the eyepice graticule scale (prefer to align a zero from the eyepiece with the line of the micrometer. it makes calliberation easier.)
now see how many divisions of the eyepiece it takes for the next division of the micrometer to come in line with the eyepice.
like this : -
| | | | ...> micrometer lines u see [ the larger ones]
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii ....> eyepiece lines

now from the above shown u can gather that the first line of the eyepiece meets a micrometer line at 0. then count upto when the next two lines get aligned... its kinda like in the vernier calipers but there u just have to look for one alignment. now in the above example 12 lines of the eyepice graticule are equal to 1 division of the micrometer scale. that means that 0.01mm is equal to 12 divisions on the eyepiece scale. so one division on the eyepiece = (0.01/12)mm


now replace the stage micrometer with the slide of the sample to be measured. align the sample on the slide with the eyepiece scale in such a way that again the 0 of the eyepiece scale is where the sample starts. count how many divisions of the eyepice is the sample in length. if let's say it is 24 divisions then do this: -

(0.01/12) * 24

this will give u the size of ur sample

i hope i helped =)

 

[daredevil]

Can any1 explain what a frameshift is

this is an A2 thing ryt??

 

[MÁťT PŭמҚ]

when u place a stage micrometer on the stage and look at it through the microscope u see big lines in the eyepiece lie a sscale ryt?
now the eypiece scale is much smaller...
u know for a fact that each division o the micrometer is 0.01mm (i think but anyways it is given on the micrometer). adjust the eyepiece and the stage micrometer in such a way that they are aligned. to adjust the eyepiece scale u can rotate the eyepiece and for the stage micrometer u can move the stage until one of its lines is exactly superposed on a line of the eyepice graticule scale (prefer to align a zero from the eyepiece with the line of the micrometer. it makes calliberation easier.)
now see how many divisions of the eyepiece it takes for the next division of the micrometer to come in line with the eyepice.
like this : -
| | | | ...> micrometer lines u see [ the larger ones]
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii ....> eyepiece lines

now from the above shown u can gather that the first line of the eyepiece meets a micrometer line at 0. then count upto when the next two lines get aligned... its kinda like in the vernier calipers but there u just have to look for one alignment. now in the above example 12 lines of the eyepice graticule are equal to 1 division of the micrometer scale. that means that 0.01mm is equal to 12 divisions on the eyepiece scale. so one division on the eyepiece = (0.01/12)mm


now replace the stage micrometer with the slide of the sample to be measured. align the sample on the slide with the eyepiece scale in such a way that again the 0 of the eyepiece scale is where the sample starts. count how many divisions of the eyepice is the sample in length. if let's say it is 24 divisions then do this: -

(0.01/12) * 24

this will give u the size of ur sample

i hope i helped =)

THNKX..

 

[daredevil]

THNKX..

np
and hey u giving AS or A levels??
and who else is giving just AS here??
could we like make a separate thread or sth bcz things get confusing and mixed up wen talking along with the A levels students cz we're never sure if sth is in our syllabus or not....
watdya say Alice123??

 

[MÁťT PŭמҚ]

np
and hey u giving AS or A levels??
and who else is giving just AS here??
could we like make a separate thread or sth bcz things get confusing and mixed up wen talking along with the A levels students cz we're never sure if sth is in our syllabus or not....
watdya say Alice123??

em giving AS only..

 

[PANDA-]

np
and hey u giving AS or A levels??
and who else is giving just AS here??
could we like make a separate thread or sth bcz things get confusing and mixed up wen talking along with the A levels students cz we're never sure if sth is in our syllabus or not....
watdya say Alice123??

Make a thread yourself
I am AS as well.

 

[narutogirl]

I am in A2 and I haven't solved a single paper 5 exam. I don't even know anything about it. Is this going to affect my paper 4, is there a probability they might ask something from p5?

 

[Alice123]

np
and hey u giving AS or A levels??
and who else is giving just AS here??
could we like make a separate thread or sth bcz things get confusing and mixed up wen talking along with the A levels students cz we're never sure if sth is in our syllabus or not....
watdya say Alice123??

Im givin a2......Alredy done wid AS

 

[Alice123]

this is an A2 thing ryt??

yep

 

[Irfan1995]

I am in A2 and I haven't solved a single paper 5 exam. I don't even know anything about it. Is this going to affect my paper 4, is there a probability they might ask something from p5?

Occasionally, they ask a question relating to the chi-squared test on paper 4, so I think it's best to be prepared.

 

[strangerss]

So w

Let's say I lost a bet to a friend of mine, so I had to give him $150.
So I lost $150 and my friend gained $150.
The total amount of money exchanged is $150 NOT $300.
So the amount of money exchanged = amount of money I lost = amount of money my friend gained.

Taking this example to the question:
If I take in 5 litres of air and exhale another 5 litres of air, the amount of air exchanged is 5 litres (not 10 litres)

Hope that makes sense

so what you mean is the volume of air exchanged is the also the volume of air inhaled? but how can the air inhaled be equal to the air exhaled shouldn't it be less ? X_X

 

[strangerss]

I'm guessing you need that so you can draw the cell to scale.
First measure, choose the limits of your drawing, but on one axis only, say you take the height for example, measure a reasonable height on your paper, you can choose this height, but make sure it's taking most of the space given, measure it, and draw two end points, now measure the height of the cell on the graticule, now by ratio, you know how many graticules gives you the height you chose, make this your scale, by which you'll draw the length as well, and the layers in the plan diagram. (I hope you understand)

I measure the height of the cell by calibrating it? this is necessary in plan diagrams? I should always do it?

 

[strangerss]

What do they mean when they say 70S for ribosomes? what's the meaning of the "S"?