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Biology SL/HL help


Mahuta ♥

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  • 2 weeks later...

I feel like I'm never, EVER going to remember meiosis and mitosis. Anytime I read it, I immediately forget it. Please, do you have any advice for how to remember all the stages and what happens in there? Can you talk about it in a little bit of an interesting way to me? I'd be grateful. Exams are slowly, but surely approaching...!!!!

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*2.5- Cell division*

Okay we are going to take this step by step. I am not going to explain meiosis before you understand mitosis. Before you start, bring your book/study guide with the mitosis steps picture and follow it step by step.

Keep it simple! You have to understand the simplest form then add more information.

To start off: as the IB syllabus requests: The replicated DNA= Sister chromatids. Each chromatid is called chromosome.

Here's an overall plan:

cell_cycle.jpg

0. Interphase:

*You have to understand this, otherwise none of the below is going to make sense.*

a. G1: This is the normal cell life, where it carries out all the necessary functions.

b. S: DNA replication- This is the most important part! The 46 chromosomes are doubled, becomes 92. That’s why when the cell divides, we end up with the same number of chromosomes. You have to understand this very well, we only call them sister chromatids because they are double what they’re supposed to be!

c. G2: This is like a check point. The cell makes sure it has all the necessary organelles. In other words, it makes sure it has enough protein for two cells, the centrioles are formed..etc..etc.

1. Prophase:

- The DNA starts coiling. Imagine having two copper wires and swirling them around each other, the more you coil, the thicker and shorter it becomes. That’s exactly what happens to the DNA double helix (Sister chromatids).

- The centrioles move to the poles of the cell to start preparing themselves to shoot out the spindle fibers that are going to pull those DNA strands apart.

- The nuclear envelope disappears. (Obviously because it’s going to divide)

Don’t go to the next step before you pretty much understand this one.

2. Metaphase:

-The centrioles are going to shoot the spindle fibers that are going to attach to the middle of the chromosomes. The sister chromatids are two strands attached at one point (centromere). Since you have one at each pole, each chromosome (a single strand) is going to be attached to 2 fibers, one from each side. It’s like the fibers are fighting about who gets the sister chromatid.

- Before they divide, they are going to organize themselves. They are going to line up in the middle of the cell (equator). It’s like standing at the start line in a race.

3. Anaphase:

- The spindle fibers are going to pull the sister chromatids apart, like tug of war.

- Both sides win. So at the end, each half sister chromatid (called chromosome) is going to go to one side. You are going to have identical sets on each pole (That’s the whole point).

- The spindle fibers are going to disappear. Imagine them being pulled back into the centriole.

4. Telophase:

- Now that you have two identical sets, all you need to do is to separate them. But you can’t break the cell into two without protecting the information (DNA). Therefore, the nuclear envelope is going to reform around each set. It’s like after a tie at tug of war, each pole of the cell is trying to keep what it won safe.

- After the nuclear envelopes develop, the cytoplasm (with the organelles) is going to divide into two.

- The new daughter cell needs to be like the original one, so the chromosomes (They are not chromatids anymore) are going to uncoil, thus becoming thinner and longer.

Okay, now lets check we got everything covered on the syllabus about mitosis:

2.5.1: Outline the stages in the cell cycle,including interphase (G1, S, G2), mitosis and cytokinesis.: We already discussed that above. Cytokinesis is just the process of dividing the cytoplasm: cyto-kinesis (divide)

2.5.3: State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts: Just like we said, the G1,S,G2 include the normal cell life (all the metabolic reactions)

2.5.4: "Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase)."- "Include supercoiling of chromosomes, attachment of spindle microtubules to centromeres, splitting of centromeres, movement of sister chromosomes to opposite poles, and breakage and re-formation of nuclear membranes. Textbooks vary in the use of the terms chromosome and chromatid. In this course, the two DNA molecules formed by DNA replication are considered to be sister chromatids until the splitting of the centromere at the start of anaphase; after this, they are individual chromosomes. The term kinetochore is not expected.

We talked about all that in details. If there's something in the paragraph above you don't understand, let me know.

2.5.5: "Explain how mitosis produces two genetically identical nuclei": Simple. Think about where the resulting chromosomes in both cells came from. The original set was doubled before you even start. It's like when you are trying to give the same type and amount of candy to two kids so neither gets upset.


You are not going to understand meiosis if you don't understand mitosis. So if you don't understand something I said above, make sure you ask me.

I hope I made it a little more interesting easier to imagine. :)

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OMG! Hey Mahuta...

You are REALLY helpful! All your posts were really helpful for me too.. I wonder, how much knowledge do you have and you remember all of it ! :D

Amazing...Btw I love biology too, but many of my concepts are not clear..( no good teacher to explain properly)

Need your help..

For now, just the basic concept of...

endocytosis, exocytosis, pinocytosis, phagocytosis...etc...all of that..i get very confused as to what is what??? Can you explain me, with some example???

Thanks ALOT!!!

Edited by Mahuta ♥
No text speak please. :)
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OMG! Hey Mahuta...

You are REALLY helpful!!!!!!! All your posts were really helpful for me too.. I wonder, how much knowledge do you have and you remember all of it ! :D

Amazing...Btw I love biology too, but many of my concepts are not clear..( no good teacher to explain properly)

Need your help..

For now, just the basic concept of...

endocytosis, exocytosis, pinocytosis, phagocytosis...etc...all of that..i get very confused as to what is what??? Can u explain me, with some example???

Thanks ALOT!!!

Haha thank you. I am glad I could help. I only remember a lot because I am in love with biology and studying medicine. That being said, I do use the syllabus to make sure I give just the information required.

Okay. First of all, pinocytosis and phagocytosis are two types of endocytosis.

Endocytosis:

Basically, as the name suggests, allows substances such as proteins to be absorbed into the cell. It's like pinching in a little piece of the membrane. Imagine a plastic bag full of liquid, and a small object..say a ball is trying to push in. For endocytosis, it is going to keep pushing in until it is surrounded by the membrane. Then that little part of the membrane englobing the substance will be detached from the membrane itself, which returns to it's normal form This may be a bad example, but this is one of the ways I learnt it. Helps to use the image below.

endocytosis.gif

Pinocytosis:

All you need to know is that it concerns very small substances, because pinocytosis means "cell-drinking". This may not be 100% accurate, but the way to remember it is that the substances are so small that you have to take in a lot of liquid in order to absorb it and the absorbed subtance + the liquid is called a vesicle.

Exocytosis:

It's like endocytosis, but this time you are pinching off. The only difference is, this time, you are starting with step 6 in the image above. The subtance is already englobed before the process starts. The surrounding membrane (or whatever you want to call it) is going to fuse with the plasma membrane. Basically going backwards with the image above. I don't know if this is going to make sense to you, but imagine having a round piece of dough, and you are pinching it towards the end to take another piece. I know this is going to sound so weird but every time I see someone doing that, it reminds me of exocytosis. If it doesn't make sense, let me know.

4027063449_32d9d86026.jpg

I did this pretty quickly, so I may have not made some points clear. Let me know if something is vague. :)

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OMG! Hey Mahuta...

You are REALLY helpful!!!!!!! All your posts were really helpful for me too.. I wonder, how much knowledge do you have and you remember all of it ! :D

Amazing...Btw I love biology too, but many of my concepts are not clear..( no good teacher to explain properly)

Need your help..

For now, just the basic concept of...

endocytosis, exocytosis, pinocytosis, phagocytosis...etc...all of that..i get very confused as to what is what??? Can u explain me, with some example???

Thanks ALOT!!!

Haha thank you. I am glad I could help. I only remember a lot because I am in love with biology and studying medicine. That being said, I do use the syllabus to make sure I give just the information required.

Okay. First of all, pinocytosis and phagocytosis are two types of endocytosis.

Endocytosis:

Basically, as the name suggests, allows substances such as proteins to be absorbed into the cell. It's like pinching in a little piece of the membrane. Imagine a plastic bag full of liquid, and a small object..say a ball is trying to push in. For endocytosis, it is going to keep pushing in until it is surrounded by the membrane. Then that little part of the membrane englobing the substance will be detached from the membrane itself, which returns to it's normal form This may be a bad example, but this is one of the ways I learnt it. Helps to use the image below.

endocytosis.gif

Pinocytosis:

All you need to know is that it concerns very small substances, because pinocytosis means "cell-drinking". This may not be 100% accurate, but the way to remember it is that the substances are so small that you have to take in a lot of liquid in order to absorb it and the absorbed subtance + the liquid is called a vesicle.

Exocytosis:

It's like endocytosis, but this time you are pinching off. The only difference is, this time, you are starting with step 6 in the image above. The subtance is already englobed before the process starts. The surrounding membrane (or whatever you want to call it) is going to fuse with the plasma membrane. Basically going backwards with the image above. I don't know if this is going to make sense to you, but imagine having a round piece of dough, and you are pinching it towards the end to take another piece. I know this is going to sound so weird but every time I see someone doing that, it reminds me of exocytosis. If it doesn't make sense, let me know.

4027063449_32d9d86026.jpg

I did this pretty quickly, so I may have not made some points clear. Let me know if something is vague. :)

OH GREAT....YES it does make alot of sense...I've understood the concept! Yipee...! Thank you soooooooo much! :):) :)

The way you explain things are so clear and easy to understand, and with that easy to remember, with your examples....

Really, thanks alot....!!!!! :D

Will come back to you..if any more doubts...Thanks..

P.S. Biology mock exam tom..:/

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  • 2 weeks later...

No problem. Glad you understand it. :)

Good luck tomorrow, just use logic and common sense to remember things, with your keen interest, you're going to ace the exam.

I need your help again. And its a bit urgent. Only few weeks for the exams..

I am not understanding the concept of dihybrid crosses and linked and unlinked genes at all..! Genetics (HL) is so confusing..And do we always get that 9:3:3:1 ratio???? Please help.

Also,

These objective statements:

1) Explain that female carriers are heterozygous for X-linked recessive alleles.

2) Analyse a human karyotype to determine gender and whether non disjunction has occurred.

3) 4.4.6 Outline three outcomes of the sequencing of the complete human genome.Explain that female carriers are heterozygous for X-linked recessive alleles.

What is the exact explanation for these? :(

Please help.

Thank you.

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I am not understanding the concept of dihybrid crosses and linked and unlinked genes at all..! Genetics (HL) is so confusing..And do we always get that 9:3:3:1 ratio???? Please help.

Also,

These objective statements:

1) Explain that female carriers are heterozygous for X-linked recessive alleles.

2) Analyse a human karyotype to determine gender and whether non disjunction has occurred.

3) 4.4.6 Outline three outcomes of the sequencing of the complete human genome.Explain that female carriers are heterozygous for X-linked recessive alleles.

What is the exact explanation for these? :(

Please help.

Thank you.

The 9:3:3:1 ratio is for classical Mendelian inheritance only - i.e. unlinked genes. It's just logical really. Draw out AaBb AaBb into a punnet square and you should be able to see it for yourself that there's a 9:3:3:1 ratio - PROVIDED the alleles are distributed discretely. If A and B were linked genes, you'd not expect to see A without B very often at all, as that would require recombination of chromosomes to have occurred. The probability of that happening to split up those two exact genes is unlikely. Also if it's an X-linked gene - then only half the population are capable of having it, short of being very very unlucky!

Of course, not all genes will lead to a 9:3:3:1 ratio in the population. For instance if a gene is lethal if inherited recessively, any 'aa' individuals will die so you won't find them in the population.

1) Explain that female carriers are heterozygous for X-linked recessive alleles.

This is a bit of a no-brainer really. Men have one X and one Y chromosomes. Women have two X chromosomes. If a woman has a recessive allele on her X chromosome, but has the dominant allele on her other X chromosome, she's going to express the dominant allele. However if a man has a recessive allele on his X chromosome then he's screwed because he only actually has that one X chromosome to begin with.

So females are by default the only people capable of 'carrying' rather than expressing the allele because only they can have a second chromosome capable of compensating for their recessive allele and not show it. They'd have to be very unlucky to inherit 2 faulty X chromosomes and as a rule, X-linked diseases are expressed by men only. The woman can't possibly be homozygous for a recessive allele and not express it - so if she's a carrier, she must be heterozygous, logically.

2) Analyse a human karyotype to determine gender and whether non disjunction has occurred.

Disjunction is the correct separation of chromosomes during cell division. You want one chromosome to go to one cell, and the other to go to the other daughter cell - that way both cells have their full set of chromosomes and are healthy. If non-disjunction occurs, then this process has failed and the chromosomes are unevenly distributed. For instance, one cell might have the 2 normal chromosomes PLUS one that was meant to go to its sister cell when the parent cell divided (so it has 3 copies instead of 2!). The cell which was meant to receive the extra chromosome now only has 1. So it's deficient. Down's Syndrome is a popular example of this. It's also known as Trisomy 23 = literally, 3 copies of chromosome 23. People can still be born with this defect because chromosome 23 hasn't got too much important stuff on it, but they're disabled.

As for gender, the Y chromosome is a tiddly little thing so if your karyotype has unmatching chromosomes where one is massive and the other is a midget, it's a male. If you see two matching massive chromosomes (i.e. all the chromosomes in the karyotype match in size) then it's female.

http://www.biology.iupui.edu/biocourses/N100/2k2humancsomaldisorders.html <--- see this site.

3) 4.4.6 Outline three outcomes of the sequencing of the complete human genome.

Well I did the job on your behalf of googling this and the answers include:

  • It is now easier to study how genes influence human development.
  • It helps identify genetic diseases.
  • It allows the production of new drugs based on DNA base sequences of genes or the structure of proteins coded for by these genes.
  • It will give us more information on the origins, evolution and migration of humans.

Pick 3.

Also you can actually find a lot of factual information just by googling. Obviously if it's conceptual and needs explanation than do ask, but otherwise you'll probably find it about 100000% times more efficient to do a bit of searching yourself if it's just getting statements off the syllabus.

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Hi,

Do I need to know how to draw starch, glycogen, or cellulose? E.g. the condensation reactions for the formation of glycogen? Also, do I need to know how to draw the condensation of two amino acids to form a dipeptide (+water)? Then a polypeptide? Similar question for glycerol and triglycerides?

Thanks,

brofessional

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I am not understanding the concept of dihybrid crosses and linked and unlinked genes at all..! Genetics (HL) is so confusing..And do we always get that 9:3:3:1 ratio???? Please help.

Also,

These objective statements:

1) Explain that female carriers are heterozygous for X-linked recessive alleles.

2) Analyse a human karyotype to determine gender and whether non disjunction has occurred.

3) 4.4.6 Outline three outcomes of the sequencing of the complete human genome.Explain that female carriers are heterozygous for X-linked recessive alleles.

What is the exact explanation for these? :(

Please help.

Thank you.

The 9:3:3:1 ratio is for classical Mendelian inheritance only - i.e. unlinked genes. It's just logical really. Draw out AaBb AaBb into a punnet square and you should be able to see it for yourself that there's a 9:3:3:1 ratio - PROVIDED the alleles are distributed discretely. If A and B were linked genes, you'd not expect to see A without B very often at all, as that would require recombination of chromosomes to have occurred. The probability of that happening to split up those two exact genes is unlikely. Also if it's an X-linked gene - then only half the population are capable of having it, short of being very very unlucky!

Of course, not all genes will lead to a 9:3:3:1 ratio in the population. For instance if a gene is lethal if inherited recessively, any 'aa' individuals will die so you won't find them in the population.

1) Explain that female carriers are heterozygous for X-linked recessive alleles.

This is a bit of a no-brainer really. Men have one X and one Y chromosomes. Women have two X chromosomes. If a woman has a recessive allele on her X chromosome, but has the dominant allele on her other X chromosome, she's going to express the dominant allele. However if a man has a recessive allele on his X chromosome then he's screwed because he only actually has that one X chromosome to begin with.

So females are by default the only people capable of 'carrying' rather than expressing the allele because only they can have a second chromosome capable of compensating for their recessive allele and not show it. They'd have to be very unlucky to inherit 2 faulty X chromosomes and as a rule, X-linked diseases are expressed by men only. The woman can't possibly be homozygous for a recessive allele and not express it - so if she's a carrier, she must be heterozygous, logically.

2) Analyse a human karyotype to determine gender and whether non disjunction has occurred.

Disjunction is the correct separation of chromosomes during cell division. You want one chromosome to go to one cell, and the other to go to the other daughter cell - that way both cells have their full set of chromosomes and are healthy. If non-disjunction occurs, then this process has failed and the chromosomes are unevenly distributed. For instance, one cell might have the 2 normal chromosomes PLUS one that was meant to go to its sister cell when the parent cell divided (so it has 3 copies instead of 2!). The cell which was meant to receive the extra chromosome now only has 1. So it's deficient. Down's Syndrome is a popular example of this. It's also known as Trisomy 23 = literally, 3 copies of chromosome 23. People can still be born with this defect because chromosome 23 hasn't got too much important stuff on it, but they're disabled.

As for gender, the Y chromosome is a tiddly little thing so if your karyotype has unmatching chromosomes where one is massive and the other is a midget, it's a male. If you see two matching massive chromosomes (i.e. all the chromosomes in the karyotype match in size) then it's female.

http://www.biology.i...ldisorders.html <--- see this site.

3) 4.4.6 Outline three outcomes of the sequencing of the complete human genome.

Well I did the job on your behalf of googling this and the answers include:

  • It is now easier to study how genes influence human development.
  • It helps identify genetic diseases.
  • It allows the production of new drugs based on DNA base sequences of genes or the structure of proteins coded for by these genes.
  • It will give us more information on the origins, evolution and migration of humans.

Pick 3.

Also you can actually find a lot of factual information just by googling. Obviously if it's conceptual and needs explanation than do ask, but otherwise you'll probably find it about 100000% times more efficient to do a bit of searching yourself if it's just getting statements off the syllabus.

Ok thanks alot! That was very helpful.

Yes, i do search for the information and refer textbooks before asking, but the explanation was not very clear for those statements, hence, wanted to get them cleared....and yes, now they are. Thank you.

Also, understood that 9:3:3:1 ratio thing but,

Still my understanding of constructing dihybrid crosses and which gametes are produced when linked genes are involved is not cleared. I am not able to understand how to go about it,when i start solving past paper questions. When there are two different characterisitcs involved, then we use the dihybrid cross, right? And how does the cross between linked genes happen? The recombinants????

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Still my understanding of constructing dihybrid crosses and which gametes are produced when linked genes are involved is not cleared. I am not able to understand how to go about it,when i start solving past paper questions. When there are two different characterisitcs involved, then we use the dihybrid cross, right? And how does the cross between linked genes happen? The recombinants????

I don't really know what you mean by constructing. You get a dihybrid by breeding a homozygous dominant and a homozygous recessive together. AABB x aabb can, assuming no chromosome recombination occurs, only give AaBb as an outcome - if that's what you mean.

As for linked genes, you should basically consider those a write-off of normal Mendelian genetics. They'll give ratios which aren't normal but are heavily skewed to particular gene combinations always being together. You may wish to think back to basics for a minute with linked genes. If 2 genes are nearly always found together on a chromosome, then they're going to reappear together in combination all the time. The only way for them not to appear together would be if part of the chromosome was spliced out at exactly the site between the two linked genes, separating them from each other. This is the basis of chromosome recombination - 2 chromosomes come together during replication, create a chiasma where they overlap each other, and then swap genetic material in that spot. e.g. figure19.gif

So short of that happening, linked genes will always be inherited together.

Perhaps if you posted the past paper questions you find confusing it would be a lot easier for me to understand what it is that you're stuck with.

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Brofessional:

All that falls under this:

Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides"

The syllabus then says: "This can be dealt with using equations with words or chemical formulas." That's your answer. In other words you know when 2 molecules of glucose condense they release one molecule of water. When asked a question about this, you could either use the formula showing how water was made or just simply using a word equation. Like it said, chemical formulas, so no need to learn how to draw them. That goes for all of them.

As for the peptide bond, you have to know how it's formed (if I remember correctly) and assuming you take chemistry, it's pretty easy:

pp3.jpg

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Hi,

Mahuta i need your help.

Can you pls help me understand how allosteric control of metabolic pathway by end-product inhibition includes negative feedback and non-competitive inhibition? What us an allosteric enzyme'?

Thanks.

Hi,

A metabolic pathway is made up a series of chemical reactions, each catalysed by an enzyme. However, sometimes the product formed from the last reaction will inhibit the enzyme of the first reaction; this is known as end-product inhibition and involves non-competitive inhibitors.

If the end product binds to the enzyme of the first reaction, it will act as a non-competitive inhibitor and will NOT bind to the enzyme's active site but to an allosteric site. The effect of it binding to the allosteric site is that it distorts the shape of the enzyme's active site, meaning that that enzyme is unlikely to be able to catalyse any more chemical reactions. (Note: removing the inhibitor from the allosteric will allow the enzyme's active site to return to its original shape.)

In terms of uses, end-product inhibition is useful to control metabolic pathways, e.g. when there is an excess of a product, the metabolic pathway can be shut down until more of the product is required, at which stage the inhibitor is removed from the allosteric site to restart the metabolic pathway. The term negative feedback is often used as the process opposes a change in the environment and therefore is vital for homeostasis.

Hope this helped,

brofessional (not Mahuta, unfortunately)

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  • 3 months later...
  • 4 weeks later...

By the way, my friend who went to Oxford Revision Courses was told to expect evolution and classification to feature largely in the exam, because apparently it's Darwin's 100th birthday this year or something with 100 years and him anyways.

Another question, this time about the general syllabus: what are the identifying features for bryophyta, coniferophyta, filicinophyta and angiospermophyta? My book has this nowhere.

Also, are we required to know an entire food web (for ecology) and dichotomous key (for classification) by heart?

So it is about that some of the features of a bird song are innate and some are learned from other bird of the same species. In my book the example is the chaffinch. They compared the song that a, from birth onwards, isolated chaffinch sings to the song free chaffinches sing (- the song varies a bit between the free male chaffinches because that allows identification of individuals). They used soundproof boxes to ensure the birds don't hear any other bird sing. The song of the isolated birds will still have features of the song free birds sing: the isolated bird's song had the correct length and the correct number of notes - these are innate. It had however a narrower range of frequencies and as they call it, fewer distinctive phases - these are learned from other chaffinches.

That's it. smile.gif

(I recommend the IB study guide for biology.)

Thank you for answering. I read some stuff online about white-crowned sparrows, similar stuff. Now I know what to say.

Hey there,

I don't know if this is what you're after but my teacher taught us the simple external recognition features of bryophyta, coniferophyta, filicinophyta and angiospermophyta as:

bryophyta= mosses and liverworts

- small

- terrestrial

- no true roots, stem or leaves

- live in clusters

- leaf-like structures are spiral shaped

coniferophyta= conifers

- woody plants, mostly trees with one trunk and many branches

- green needle thin leaves arranged in spirals

- seeds are in cones (eg. pinecones)

filicinophyta= ferns

- true leaves

- new leaves unroll

- have rhizomes (underground creeping stem)

angiospermophyta=flowering plants

- have flowers that can be small

- seeds are in the ovaries which turn into fruit

- leaves are usually blade like with a stalk and visible veins

Hope this helps, enjoy studying!

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