The Student Room Group

Meiosis - A-Level Biology

This may be a very silly question, but wouldn't it be more efficient if the DNA did not replicate in meiosis and consequently only had meiosis I take place, as it would take less time and perhaps less energy?

My thought process with this is that if there is just meiosis I with no replication, there are still gametes produced.

I'm not sure whether my point makes much sense, but any help/explanations would be appreciated.
Hey there!
I see what you're getting at with this question but the key point of meiosis is that we want to produce haploid gamete cells that can be used for reproduction. After meiosis 1 we are left with 2 daughter cells that are both diploid (having a full chromosome). These cells wouldn't be able to be used for reproduction as the process of fertilization requires half the genetic material to come from each parent. Consequently, having the daughter cells undergo Meiosis II will create 4 haploid cells that are then ready for fertilization.
Hope that makes sense! Let me know if you have any questions.
Dottie
Medic Mind
Apologies students, and I don't believe in criticizing people unnecessarily, but this is another instance where this "Rep" [Dottie?] is exhibiting his/her profound deficiency of knowledge in biology - I have seen some v good posts by this "Medic Mind" entity, so it appears to be this particular rep who needs to re-study simple biology [one other example of this serious misunderstanding was a recent post on the cardiac cycle, in which this person, with due respect, did not have the foggiest idea about the anatomy and physiology of the human heart, which places the acquisition of basic biological facts by students in serious jeopardy!]

Coming to the Q posed by OP, LET ME POINT OUT EMPHATICALLY that the halving of the number of chromosomes resulting in haploid daughter OCCURS IN MEIOSIS I NOT IN MIEOSIS II as this "rep" seems to think. Meiosis II is actually almost identical to mitosis in non-reproductive cells. Therefore, @Bex.:smile: is asking a very good Q. The answer is that as in mitosis in non-reproductive cells, the cells resulting from the division in meiosis need to undergo DNA replication AT SOME STAGE: if only Meiosis I were to occur with no Meiosis II, there would be no separation of the chromatids [as occurs in metaphase in mitosis] and there would be no separation of homologous chromosomes.

I would think that having Meiosis II plays a crucial role twofold in bringing about variation [crossing over and separation of homologous chromosomes] [the 3rd is random fertilization], which is absolutely crucial and fundamental for evolution by natural selection, and which is therefore the reason we humans are here - in the absence of variation, we would ALL still be amoebae or bacteria.

I hope this clarifies things somewhat!
Be safe!

M.
Reply 3
Original post by macpatgh-Sheldon
Apologies students, and I don't believe in criticizing people unnecessarily, but this is another instance where this "Rep" [Dottie?] is exhibiting his/her profound deficiency of knowledge in biology - I have seen some v good posts by this "Medic Mind" entity, so it appears to be this particular rep who needs to re-study simple biology [one other example of this serious misunderstanding was a recent post on the cardiac cycle, in which this person, with due respect, did not have the foggiest idea about the anatomy and physiology of the human heart, which places the acquisition of basic biological facts by students in serious jeopardy!]

Coming to the Q posed by OP, LET ME POINT OUT EMPHATICALLY that the halving of the number of chromosomes resulting in haploid daughter OCCURS IN MEIOSIS I NOT IN MIEOSIS II as this "rep" seems to think. Meiosis II is actually almost identical to mitosis in non-reproductive cells. Therefore, @Bex.:smile: is asking a very good Q. The answer is that as in mitosis in non-reproductive cells, the cells resulting from the division in meiosis need to undergo DNA replication AT SOME STAGE: if only Meiosis I were to occur with no Meiosis II, there would be no separation of the chromatids [as occurs in metaphase in mitosis] and there would be no separation of homologous chromosomes.

I would think that having Meiosis II plays a crucial role twofold in bringing about variation [crossing over and separation of homologous chromosomes] [the 3rd is random fertilization], which is absolutely crucial and fundamental for evolution by natural selection, and which is therefore the reason we humans are here - in the absence of variation, we would ALL still be amoebae or bacteria.

I hope this clarifies things somewhat!
Be safe!

M.

Thank you so so much! I'm going to write out some more thoughts, so would you mind just checking my understanding on the points that I make please :smile:

So the reason why meiosis II MUST occur is because this is the part where the chromatids separate? And so if this did not occur, only the homologous chromosomes would be separated - would this cause the cell to remain diploid, or would it then be haploid? IF it would be haploid at this point, why does separation of chromatids need to occur? (so sorry if you have already answered this!!)

And my apologies for asking the same question again - why must DNA replication occur to begin with? Is it so that there is enough DNA for separation of chromatids to actually occur? (without replication, would this be impossible)

One final question related to my understanding - is the reason why there must be 2 divisions because in meiosis I there is crossing over when bivalents form? As in, is this the primary reason as to why both meiosis I and II is needed?

I'm so sorry if this does not make sense, for the long reply and the huge number of questions - I have my A-Levels this year, so need to make sure that I have a clear understanding of the topic.

Thank you :smile:
Original post by Medic Mind
Hey there!
I see what you're getting at with this question but the key point of meiosis is that we want to produce haploid gamete cells that can be used for reproduction. After meiosis 1 we are left with 2 daughter cells that are both diploid (having a full chromosome). These cells wouldn't be able to be used for reproduction as the process of fertilization requires half the genetic material to come from each parent. Consequently, having the daughter cells undergo Meiosis II will create 4 haploid cells that are then ready for fertilization.
Hope that makes sense! Let me know if you have any questions.
Dottie
Medic Mind

I am, trying to reply to either OP or "Medic Mind"; However, I am not sure what is happening here - I hold a Masters in CS yet this ultra-simple concept is trying to make me visit confused.com - shameful, is it not? :mad:Apologies students, and I don't believe in criticizing people unnecessarily, but this is another instance where this "Rep" [Dottie?] is exhibiting his/her profound deficiency of knowledge in biology - I have seen some v good posts by this "Medic Mind" entity, so it appears to be this particular rep who needs to re-study simple biology [one other example of this serious misunderstanding was a recent post on the cardiac cycle, in which this person, with due respect, did not have the foggiest idea about the anatomy and physiology of the human heart, which places the acquisition of basic biological facts by students in serious jeopardy!]

Coming to the Q posed by OP, LET ME POINT OUT EMPHATICALLY that the halving of the number of chromosomes resulting in haploid daughter OCCURS IN MEIOSIS I NOT IN MIEOSIS II as this "rep" seems to think. Meiosis II is actually almost identical to mitosis in non-reproductive cells. Therefore, Bex. is asking a very good Q. The answer is that as in mitosis in non-reproductive cells, the cells resulting from the division in meiosis need to undergo DNA replication AT SOME STAGE: if only Meiosis I were to occur with no Meiosis II, there would be no separation of the chromatids [as occurs in metaphase in mitosis] and there would be no separation of homologous chromosomes.

I would think that having Meiosis II plays a crucial role twofold in bringing about variation [crossing over and separation of homologous chromosomes] [the 3rd is random fertilization], which is absolutely crucial and fundamental for evolution by natural selection, and which is therefore the reason we humans are here - in the absence of variation, we would ALL still be amoebae or bacteria.

I hope this clarifies things somewhat!
Be safe!

M.
Original post by Bex.:)
Thank you so so much! I'm going to write out some more thoughts, so would you mind just checking my understanding on the points that I make please :smile:

So the reason why meiosis II MUST occur is because this is the part where the chromatids separate? And so if this did not occur, only the homologous chromosomes would be separated - would this cause the cell to remain diploid, or would it then be haploid? IF it would be haploid at this point, why does separation of chromatids need to occur? (so sorry if you have already answered this!!)

And my apologies for asking the same question again - why must DNA replication occur to begin with? Is it so that there is enough DNA for separation of chromatids to actually occur? (without replication, would this be impossible)

One final question related to my understanding - is the reason why there must be 2 divisions because in meiosis I there is crossing over when bivalents form? As in, is this the primary reason as to why both meiosis I and II is needed?

I'm so sorry if this does not make sense, for the long reply and the huge number of questions - I have my A-Levels this year, so need to make sure that I have a clear understanding of the topic.

Thank you :smile:

Hi watch this space v early t-rrow!
Original post by Bex.:)
Thank you so so much! I'm going to write out some more thoughts, so would you mind just checking my understanding on the points that I make please :smile:

So the reason why meiosis II MUST occur is because this is the part where the chromatids separate? And so if this did not occur, only the homologous chromosomes would be separated - would this cause the cell to remain diploid, or would it then be haploid? IF it would be haploid at this point, why does separation of chromatids need to occur? (so sorry if you have already answered this!!)

And my apologies for asking the same question again - why must DNA replication occur to begin with? Is it so that there is enough DNA for separation of chromatids to actually occur? (without replication, would this be impossible)

One final question related to my understanding - is the reason why there must be 2 divisions because in meiosis I there is crossing over when bivalents form? As in, is this the primary reason as to why both meiosis I and II is needed?

I'm so sorry if this does not make sense, for the long reply and the huge number of questions - I have my A-Levels this year, so need to make sure that I have a clear understanding of the topic.

Thank you :smile:

So the reason why meiosis II MUST occur is because this is the part where the chromatids separate? And so if this did not occur, only the homologous chromosomes would be separated - would this cause the cell to remain diploid, or would it then be haploid? IF it would be haploid at this point, why does separation of chromatids need to occur? (so sorry if you have already answered this!!)[no need to apologize - I am here to help :colondollar:]
Since halving of chromosome no. has already occurred in meiosis I, yes the daughter cells [only TWO at this stage] are HAPLOID; remember separation of CHROMATIDS does NOT result in any reduction in chromosome number ONLY THAT the DNA is shared between the 2 daughter cells AND IT CAN BE REPLICATED, yeah? [each CHROMATID is like only half a chromosome - it is almost like half of EACH chromosome separates, so the NO of chromosomes remains the same i.e. the haploid number created in meiosis I where WHOLE chromosomes moved to the poles , half of them to one pole half to other pole, with me?

And my apologies for asking the same question again - why must DNA replication occur to begin with? Is it so that there is enough DNA for separation of chromatids to actually occur? (without replication, would this be impossible)
When a cell divides into two cells [at an empirical evolutionary level as in a bacterium becoming two in 20 minutes] each cell must have a full complement of genetic information [tho meiosis is concerned with producing gametes for reproduction, the cell is still dividing to make first two then four cells] - DNA replication after the unwinding of the double helix [catalysed by DNA helicase and separation of the two strands] ensures that each daughter cell acquires the full complement of genetic info - consider the pathological situation as follows:in certain diseases where there is an error in meiosis so that halving of chromosome no. occurs twice [in both meiosis I and in meiosis II[, the gametes have quarter the diploid no - you can imagine the drastic consequences in the foetus/infant - [you do not need to know any more on this at A level].

One final question related to my understanding - is the reason why there must be 2 divisions because in meiosis I there is crossing over when bivalents form? As in, is this the primary reason as to why both meiosis I and II is needed?
Sorry but I do not see any link between these two items in bold - I WOULD SUGGEST YOU COMPLETELY IGNORE THIS POINT!

GENERAL TIPs - ABSOLUTELY CRUCIAL:-
1. Tho it is a v good sign that you are asking Qs after putting some thought into the material and trying to explain [=why?] the reasons behind phenomena, I would strongly advise you to USE THE OBVIOUS BASIC points in your answers in exams - ALL the marks in the mark schemes [except Edexcel,s who are very illogical and unfair AND ACTUALLY SOMETIMES DOWNRIGHT INCORRECT I shall venture out and say!] are assigned to simple obvious facts [or deductions in data Qs] rather than to profound convoluted rocket science type of pedantic detail. It will save you a lot of time and effort and result in a better mark if you do not get bogged down as partly on this topic. What matters is mark, mark, mark as in hockey [do you play? - big in medicine] where what matters is goal, goal goal i.e. the end result not whether you deflected an aerial pass from your defender into the net or you dribbled past three oppo defenders and creamed a flying bullet into the top right corner.
2. TRy to read the examiner's mind and work out what is needed i.e. try to filter out unnecessary facts tho they might be correct BUT WILL EARN ZERO MARKS!

Check out my numerous tips and exam tactics herein [on TSR] particularly from 2016 to 2017]

Best of luck!
M
Reply 7
Original post by macpatgh-Sheldon
So the reason why meiosis II MUST occur is because this is the part where the chromatids separate? And so if this did not occur, only the homologous chromosomes would be separated - would this cause the cell to remain diploid, or would it then be haploid? IF it would be haploid at this point, why does separation of chromatids need to occur? (so sorry if you have already answered this!!)[no need to apologize - I am here to help :colondollar:]
Since halving of chromosome no. has already occurred in meiosis I, yes the daughter cells [only TWO at this stage] are HAPLOID; remember separation of CHROMATIDS does NOT result in any reduction in chromosome number ONLY THAT the DNA is shared between the 2 daughter cells AND IT CAN BE REPLICATED, yeah? [each CHROMATID is like only half a chromosome - it is almost like half of EACH chromosome separates, so the NO of chromosomes remains the same i.e. the haploid number created in meiosis I where WHOLE chromosomes moved to the poles , half of them to one pole half to other pole, with me?

And my apologies for asking the same question again - why must DNA replication occur to begin with? Is it so that there is enough DNA for separation of chromatids to actually occur? (without replication, would this be impossible)
When a cell divides into two cells [at an empirical evolutionary level as in a bacterium becoming two in 20 minutes] each cell must have a full complement of genetic information [tho meiosis is concerned with producing gametes for reproduction, the cell is still dividing to make first two then four cells] - DNA replication after the unwinding of the double helix [catalysed by DNA helicase and separation of the two strands] ensures that each daughter cell acquires the full complement of genetic info - consider the pathological situation as follows:in certain diseases where there is an error in meiosis so that halving of chromosome no. occurs twice [in both meiosis I and in meiosis II[, the gametes have quarter the diploid no - you can imagine the drastic consequences in the foetus/infant - [you do not need to know any more on this at A level].

One final question related to my understanding - is the reason why there must be 2 divisions because in meiosis I there is crossing over when bivalents form? As in, is this the primary reason as to why both meiosis I and II is needed?
Sorry but I do not see any link between these two items in bold - I WOULD SUGGEST YOU COMPLETELY IGNORE THIS POINT!

GENERAL TIPs - ABSOLUTELY CRUCIAL:-
1. Tho it is a v good sign that you are asking Qs after putting some thought into the material and trying to explain [=why?] the reasons behind phenomena, I would strongly advise you to USE THE OBVIOUS BASIC points in your answers in exams - ALL the marks in the mark schemes [except Edexcel,s who are very illogical and unfair AND ACTUALLY SOMETIMES DOWNRIGHT INCORRECT I shall venture out and say!] are assigned to simple obvious facts [or deductions in data Qs] rather than to profound convoluted rocket science type of pedantic detail. It will save you a lot of time and effort and result in a better mark if you do not get bogged down as partly on this topic. What matters is mark, mark, mark as in hockey [do you play? - big in medicine] where what matters is goal, goal goal i.e. the end result not whether you deflected an aerial pass from your defender into the net or you dribbled past three oppo defenders and creamed a flying bullet into the top right corner.
2. TRy to read the examiner's mind and work out what is needed i.e. try to filter out unnecessary facts tho they might be correct BUT WILL EARN ZERO MARKS!

Check out my numerous tips and exam tactics herein [on TSR] particularly from 2016 to 2017]

Best of luck!
M

Thank you so much for all of your time and help! I really appreciate it!

There are still a couple of parts that I do not quite understand unfortunately, but I will try to keep focussed on the required information for the exams.

Thanks again!
My pleasure & forgot to say earlier - tysm for the rep!

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