AQA BIOL2 Biology Unit 2 Exam - 26th May 2011

i dont understand, wouldnt the number of chromosomes have to double at some point to have now two cells with the same number of chromosomes, so when you replicate dna arnt you replicating chromosomes? ahhh confused

Yeah, the DNA replicates during interphase, that's why the chromosomes appear as the 'X' shape. Originally, it's just one 'strand', one chromatid. After replication, you have two sister chromatids, which are then separated during mitosis.

Reply 561

TlanTlan

6

Original post by liviaaa

Please can someone help with any of the following? :smile:

-Difficulties in defining a species?

-Importance of taxonomy?

-Ethics for antibiotic resistance?

Thanks!

A species is a group of organisms that can reproduce together to produce fertile offspring. The difficulty in establishing this could result from species being rare and hard to make mate in isolation, and the ethical implications of interfering with the organism to get the DNA sample or what-not.

Taxonomy is the classification of organisms into a hierarchy. The hierarchy simply being large groups containing smaller groups with no overlap, and allowing organisms to be classified by their common ancestor and evolutionary relationship(phylogenetic classification), or grouping into groups of similar phenotypes/characteristics and alleles.

The ethical implications of antibiotic resistance mean that bacteria have gained a selection advantage as a result of their mutated resistance gene, and due to this modern antibiotics may not work on them, and without use of multiple antibiotics can result in strains of bacteria resistant to multiple antiobiotics. This can result in more suffering for patients and animals who are infected with these strains as they cannot be treated as effectively as well as leading them to die in this time.

Reply 562

TlanTlan

6

Original post by oHellno

Yeah, the DNA replicates during interphase, that's why the chromosomes appear as the 'X' shape. Originally, it's just one 'strand', one chromatid. After replication, you have two sister chromatids, which are then separated during mitosis.

During interphase can the DNA be seen as a single chromatid or is it diffused throughout the nucleus as chromatin and thus not visible?

Reply 563

??????????????????

8

Original post by liviaaa

Please can someone help with any of the following? :smile:

-Difficulties in defining a species?

-Importance of taxonomy?

-Ethics for antibiotic resistance?

Thanks!

-Might be extinct, nocturnal.
-Because some countries have different names for the same organisms so a scientific name helps to identify what they are talking about.
-Not sure what you mean? Like testing antibiotics on animals?

Reply 564

Stratos

15

Original post by kingsmod1

LOOL wat u get the last 3 times

this is my third time and ive had 2 Ds

I understand all stuff now and have lots of experience lol
need A

I got a d last time >__>. But yeah I'm revising properly now last time I actually forgot I had the exam and didn't revise a single minute.

Reply 565

jessplease

Original post by oHellno

Yeah, the DNA replicates during interphase, that's why the chromosomes appear as the 'X' shape. Originally, it's just one 'strand', one chromatid. After replication, you have two sister chromatids, which are then separated during mitosis.

oh okay, thanks! so the number of chromosomes doesn't change unless its a haploid cell?

Reply 566

lolo-x

Original post by ewoo3148

im so bored of revision now! i just want the exam to be now!! any good revision techniques guys? :\

so am I, I have lost all my motivation today, bored is the only way to describe it. god knows what I'll be like tomorrow night after stats and biology, knowing that I have a chemistry exam the next day too :| i wish they were a bit more spread out

Reply 567

LifeIsGood

12

Original post by oHellno

Yeah, the DNA replicates during interphase, that's why the chromosomes appear as the 'X' shape. Originally, it's just one 'strand', one chromatid. After replication, you have two sister chromatids, which are then separated during mitosis.

Thank you, thank you! I understand it a lot more now :biggrin:

Reply 568

parallal

I'm not sure about the structures of cellulose and starch. Could someone please explain?

I know that cellulose is made up of beta glucose and starch is made up of alpha glucose. Cellulose has many Hydrogen bonds between its chains for strength and it forms criss-crossing micro fibrils (also for strength). Starch is spiral-shaped to be compact so it takes up less space, and it is branched so that it can be readily hydrolyzed.

Is there anything else I need to know?

Reply 569

oHellno

12

Original post by jessplease

oh okay, thanks! so the number of chromosomes doesn't change unless its a haploid cell?

Exactly

Reply 570

??????????????????

8

Original post by parallal

I'm not sure about the structures of cellulose and starch. Could someone please explain?

I know that cellulose is made up of beta glucose and starch is made up of alpha glucose. Cellulose has many Hydrogen bonds between its chains for strength and it forms criss-crossing micro fibrils (also for strength). Starch is spiral-shaped to be compact so it takes up less space, and it is branched so that it can be readily hydrolyzed.

Is there anything else I need to know?

Celluose has straight glysocidic bonds.
Starch has branches. I think so it can hydrolyse them easier as they are shorter chains?
Starch has amylose and amylopectin. Amylose is the non branched part and amylopectin is the branch. Starch is insoluble in water which is useful as it does not affect water potential so doesn't affect osmosis.

Reply 571

LifeIsGood

12

Original post by parallal

I'm not sure about the structures of cellulose and starch. Could someone please explain?

I know that cellulose is made up of beta glucose and starch is made up of alpha glucose. Cellulose has many Hydrogen bonds between its chains for strength and it forms criss-crossing micro fibrils (also for strength). Starch is spiral-shaped to be compact so it takes up less space, and it is branched so that it can be readily hydrolyzed.

Is there anything else I need to know?

Starch
-Made from alpha glucose
-Joined by condensation [forms glycosidic bonds]

Features:
-Compact - able to store a large amount in a small space
-Insoluble - no effect on water potential
-Has branches which can be hydrolysed to release glucose

Cellulose
-Made from Beta-Glucose
-Joined by condensation
-Carbons 1 & 4, 1 & 6
-Flipping over of alternate molecules

Features:
-Unbranched chains lie side by side
-To form microfibrils
-Hydrogen bonds link the cross-linkages of chains to give tensile strength

Starch - Plants
Glycogen - Animals [Muscles/Liver]

Reply 572

HaimishaParekh

3

I think it's clear to say its the HSW questions that f us all over.

Reply 573

ScienceGeek3

Original post by parallal

I'm not sure about the structures of cellulose and starch. Could someone please explain?

I know that cellulose is made up of beta glucose and starch is made up of alpha glucose. Cellulose has many Hydrogen bonds between its chains for strength and it forms criss-crossing micro fibrils (also for strength). Starch is spiral-shaped to be compact so it takes up less space, and it is branched so that it can be readily hydrolyzed.

Is there anything else I need to know?

You may also want to add that starch is the storage compound in plants and it is suitable for this role as it does not affect the water potential of the cell and therefore being insoluble it does not draw water into/out of cells, and it itself does not diffuse out of cells.

Reply 574

parallal

Original post by LifeIsGood

Starch
-Made from alpha glucose
-Joined by condensation [forms glycosidic bonds]

Features:
-Compact - able to store a large amount in a small space
-Insoluble - no effect on water potential
-Has branches which can be hydrolysed to release glucose

Cellulose
-Made from Beta-Glucose
-Joined by condensation
-Carbons 1 & 4, 1 & 6
-Flipping over of alternate molecules

Features:
-Unbranched chains lie side by side
-To form microfibrils
-Hydrogen bonds link the cross-linkages of chains to give tensile strength

Starch - Plants
Glycogen - Animals [Muscles/Liver]

Thanks so much!

Reply 575

parallal

Original post by ??????????????????

Celluose has straight glysocidic bonds.
Starch has branches. I think so it can hydrolyse them easier as they are shorter chains?
Starch has amylose and amylopectin. Amylose is the non branched part and amylopectin is the branch. Starch is insoluble in water which is useful as it does not affect water potential so doesn't affect osmosis.

Original post by ScienceGeek3

You may also want to add that starch is the storage compound in plants and it is suitable for this role as it does not affect the water potential of the cell and therefore being insoluble it does not draw water into/out of cells, and it itself does not diffuse out of cells.

Thanks to you two as well. (:
I'm feeling a bit more confident about this exam.

Reply 576

mclovin123

15

Hey all, I have just created a thread in the revision forum. It is just a place for people who want to cram this evening and maybe late into the night. Luckily for us our exam is in the afternoon meaning we can sleep in. Please just join and you can ask others for help on concepts or learning things. Join and we can get through the next evening/night to get the best gread we can get. HERE IS THE LINK.

http://www.thestudentroom.co.uk/showthread.php?p=31700415#post31700415

Reply 577

??????????????????

8

Original post by mclovin123

Hey all, I have just created a thread in the revision forum. It is just a place for people who want to cram this evening and maybe late into the night. Luckily for us our exam is in the afternoon meaning we can sleep in. Please just join and you can ask others for help on concepts or learning things. Join and we can get through the next evening/night to get the best gread we can get. HERE IS THE LINK.

http://www.thestudentroom.co.uk/showthread.php?p=31700415#post31700415

Lucky you getting to sleep in :frown:

Most people here have got S1 in the morning I think?