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AQA As Biology June 2014 - [BIO2]

The founder effect is when a small group of individuals colonize in a new region. As they interbreed, they only carry a small gene pool, so genetic diversity decreases

Genetic bottle neck is when a population of a species suffer a dramatic drop due to an event (such as disease,disaster). Survivors have a small and reduced gene pool, therefore less genetic diversity

But the idea is the same, right? It's just that the population is reduced in different ways?

I feel good about this exam. Don't worry everyone, we'll be fine. We are smart people.

Reply 1581

3-methylbutane

8

Good luck everyone! X

Reply 1582

pearson1995

Whats the purpose of meiosis guys?

Mitosis is to replace dead/damaged cells, repair tissue and increase cell number

whats meiosis for? Creating variation.....

Reply 1583

JustBeing

3

Original post by Holly16263

Hey could someone please sum up what you would have to include in a 6 mark question about gas exchange in fish?!

thank you!!!

Depends on the question really
but everything you need to know is:

- The structure, so there's gill filaments, and at right angles to the filaments on top of them are gill lamellae.

- Blood and oxygen are moving in opposite directions this is called countercurrent exchange principle (there's a countercurrent flow)

Moving in opposite directions = maximum uptake of oxygen as opposed to if they moved in same direction

Countercurrent flow works like this:

- Blood with some oxygen in it, meets water with it's maximum conc. of oxygen. So there's diffusion of oxygen from water to blood
- Then blood with little or no oxygen, meets water which has had most, but not all oxygen removed, but since it still has more than blood, oxygen diffuses from water to blood.

Also, know that water is taken in from the mouth, and pushed out through the gills on the side of the fish

The one where blood and water is moving in same direction is a parallel flow, that is only 50% efficient (50% available oxygen absorbed) as opposed to the countercurrent flow being 80% efficient

Reply 1584

LxH

Can someone please explain what the lymphatic system is?

Reply 1585

Aardvark1998

18

Original post by JustBeing

Can someone explain everything they know about meiosis
organise it so it's easy to understand what happens at each stage
thanks!

1) DNA replicates so you end up with 2 copies of each chromosome (called chromatids)
2) The chromatids come together to form double armed chromosomes (from pairs of sister chromatids)
3) The chromosomes settle into homologous pairs, and then cell division occurs, so you end up HALVING the chromosome number
4) Another division occurs from the 2 cells formed in stage 3. The pairs of sister chromatids are separated You end up with 4 genetically different haploid daughter cells.

(in reality this occurs for 46 chromosomes, but they won't ask you to work with that because it's too complicated)

Reply 1586

Bustamove

19

Original post by LxH

But the idea is the same, right? It's just that the population is reduced in different ways?

I feel good about this exam. Don't worry everyone, we'll be fine. We are smart people.

yup

Reply 1587

JustBeing

3

Original post by pearson1995

Whats the purpose of meiosis guys?

Mitosis is to replace dead/damaged cells, repair tissue and increase cell number

whats meiosis for? Creating variation.....

To make sex cells. They're haploid cells, so half the genes of a diploid parent cell. This is so at fertalisation, 23+23 chromosomes = the full 46 in humans

Reply 1588

LxH

Original post by pearson1995

Whats the purpose of meiosis guys?

Mitosis is to replace dead/damaged cells, repair tissue and increase cell number

whats meiosis for? Creating variation.....

- Creates cells with haploid number of chromosomes so the resulting zygote contains the diploid number
- Variation as independent segregation and crossing over results in recombination of alleles (mixture of maternal and paternal DNA)

I think this is right, I can't think of any more, it is a bit of a weird question on its own.

Reply 1589

JustBeing

3

Original post by bad_moose

1) DNA replicates so you end up with 2 copies of each chromosome (called chromatids)
2) The chromatids come together to form double armed chromosomes (from pairs of sister chromatids)
3) The chromosomes settle into homologous pairs, and then cell division occurs, so you end up HALVING the chromosome number
4) Another division occurs from the 2 cells formed in stage 3. The pairs of sister chromatids are separated You end up with 4 genetically different haploid daughter cells.

(in reality this occurs for 46 chromosomes, but they won't ask you to work with that because it's too complicated)

When you halve the number of chromosomes in meiosis one, does it happen so maternal chromosomes all go into one cell, and the paternal chromosomes are all in the other, or are they mixed up?

Reply 1590

Mr.benr

Original post by iWoof

How long does it take for the cell cycle to complete?

Posted from TSR Mobile

There is no set time for cell cycles to complete. No question will ask you to say how long a cell cycle takes without any other information. You may have to work out from graphs and tables, but different cells in different species, and within same species in different areas of the body go through cell cycles over a wide range of time frames.

Reply 1591

pearson1995

Original post by JustBeing

To make sex cells. They're haploid cells, so half the genes of a diploid parent cell. This is so at fertalisation, 23+23 chromosomes = the full 46 in humans

so...

1) To introduce variation
2) to produce haploid cells

Thanks, have a feeling it might come up, we have had whats the purpose of mitosis in the past but never meiosis.

Reply 1592

LxH

Original post by JustBeing

When you halve the number of chromosomes in meiosis one, does it happen so maternal chromosomes all go into one cell, and the paternal chromosomes are all in the other, or are they mixed up?

Homologous chromosomes are split into separate cells, so yes, but before this happens the homologous chromosomes pair up and recombination of alleles occurs by crossing over, so often it is not just maternal in one cell and paternal in the other as some maternal and paternal alleles have been swapped over in Prophase I.
I hope this didn't confuse you, I tried to explain it as best as possible. Good luck in the exam!

Reply 1593

Aardvark1998

18

Original post by JustBeing

When you halve the number of chromosomes in meiosis one, does it happen so maternal chromosomes all go into one cell, and the paternal chromosomes are all in the other, or are they mixed up?

Are you asking if all of the maternal chromosomes end up in one individual cell, and all the paternal ones end up in another? If so, nope. It's called independent segregation, you get a different combination of maternal and paternal chromosomes in each cell. So yes, they are essentially all mixed up. Does that help?

Reply 1594

JustBeing

3

Original post by LxH

Can someone please explain what the lymphatic system is?

It's a system in the body that helps get rid of toxins, it just contains a load of fluid etc.

Reply 1595

Darkwarrior23

8

Original post by LxH

Can someone please explain what the lymphatic system is?

You don't need to know much... its a system of vessels that drain remaining issue fluid and they do not require the pumping heart to move .... its only worth a mark

Reply 1596

Mr.benr

Original post by pearson1995

so...

1) To introduce variation
2) to produce haploid cells

Thanks, have a feeling it might come up, we have had whats the purpose of mitosis in the past but never meiosis.

Variation is a result of it, not the reason for it.

The reason is simple, to create further generations (pass on genes to further generations)

Reply 1597

JustBeing

3

Original post by LxH

Homologous chromosomes are split into separate cells, so yes, but before this happens the homologous chromosomes pair up and recombination of alleles occurs by crossing over, so often it is not just maternal in one cell and paternal in the other as some maternal and paternal alleles have been swapped over in Prophase I.
I hope this didn't confuse you, I tried to explain it as best as possible. Good luck in the exam!

No, that really clarified it for me! Thanks!

Reply 1598

ahopson

Original post by bad_moose

All of that seems right except prophase. I was under the impression that the spindle fibres formed here rather than in metaphase? I may be wrong though, but that's what I've learned it as.