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HELP! Questions on mitosis and meiosis

Genetics as it applies to evolution, molecular biology, and medical aspects.

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HELP! Questions on mitosis and meiosis

Postby robotnick » Tue Jun 10, 2008 12:11 pm

Hope somebody can help, I've got an A level biology exam on friday and whatever I read on the subject of meiosis and mitosis, I keep going round in circles!

Ok, can somebody please answer the following questions:

1. In meiosis, what happens to the chromatid in each daughter cell at the end of Telophase 2? Do they replicate their sister chromatid? So that resulting egg + sperm cells actually have chromatids [chromosomes?] that look like - X ?

2. Do gametes ever divide/replicate?

3. If sister chromatids are separated in Anaphase 2 of meiosis, does that mean that 2 of the resulting gametes are genetically identical to each other [provided crossing-over hasn't taken place]?

4. So even though that we have 32 pairs of chromosomes, 32 are purely derived from the sperm cell and 32 are purely derived from the egg cell - i.e. the 32 pairs are not genetically identical to each other?

5. During fertilisation, does a sperm donate a - | [chromatid] or a - X [chromosome]?

6. During Mitosis, does each chromosome of each homologous pair replicate?

7. If so, what causes the homologous chromosomes to stay together? Why don't they get mixed up with other homologous pairs? What is stopping both homologues from going into one daughter cell?

8. If there is some force holding homologous chromosomes together - when a pair of homologous chromosomes line up on the equator of the spindle during mitosis/meiosis, do they have to separate first? So that each homologue is on top of each other [on top - I mean from a birds eye view] instead of side by side? Are they always side by side when the cell is not replicating?


Would really appreciate somebody clearing this up for me.

Kind Regards!

Jonathan
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Postby Darby » Tue Jun 10, 2008 1:01 pm

Whoever's writing the exam has some weird terminology - once chromosomes separate, they are just chromosomes - chromatids only exist when the copies are still attached.

I know biology people play a bit loose with our terms, but that just makes it more confusing.

You ought to be able to think these through - take a shot and we'll tell you where you're on and off-target.
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Postby robotnick » Tue Jun 10, 2008 3:40 pm

Ok, thats a good idea.

1. Unfortunately, I can't guess. As this seems to be where I'm confused. I just don't know when/if all chromosomes are in the form of - X and - | . I suppose gametes' chromosomes are never in the form of X as they do not divide [my guess at Q 2].

3. My guess is that 2 resulting gametes ARE genetically identical to each other [provided crossing-over of genetic information has not occurred].

4. I would guess that the 32 pairs ARE actually genetically different to each other.

5. Following on from my guess at Q1+2, I would say sperms donate a chromosome that does not have sister chromatids and so donates - [ | ] instead of [ X ]. I'm feeling a little stupid here, maybe thats wrong..?

6. My guess is YES.

7. I have absolutely no idea!

8. My guess is that they do separate and arrange themselves top-bottom, and that they do exist side by side when the cell is not replicating.

Did I get at least a C ?

Jonathan
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Postby mith » Tue Jun 10, 2008 10:27 pm

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Postby Cat » Tue Jun 10, 2008 11:59 pm

First of all to Darby: chromatid is one, chromosome is a structure of either 2 chromatids that are identical (replicated chromosome) or one chromatid (not replicated chromosome). Technically, chromosome means the condensed form of chromatid, so both chromosomes and chromatids exist only during cell division. No "weird terminology" here.

Now for the questions:

1. At the end of Telophase 2 chromatids uncoil etc... Meiosis is complete -- daughter cells haploid. No replication takes place.

2. NO.

3. Yes.

4. Correct.

5. | [chromatid]

6. Yes.

7. Centromeres, kinetochores.

8. If there is some force holding homologous chromosomes together - when a pair of homologous chromosomes line up on the equator of the spindle during mitosis/meiosis, do they have to separate first? -- No. They align first, than separate.

So that each homologue is on top of each other [on top - I mean from a birds eye view] instead of side by side? -- not sure what is meant here.

Are they always side by side when the cell is not replicating? No.
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Postby Darby » Wed Jun 11, 2008 2:19 am

Sorry, I can't find a definition that where a chromatid is anything other than 1 strand of a double-stranded chromosome - once separate, they are single-stranded chromosomes.

I do agree with the other bits, though, except that it's 23 pairs rather than 32.
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Postby MrMistery » Wed Jun 11, 2008 5:36 am

Cat is right. Once the sister chromatids have separated in metaphase, we call the resulting chromosomes "monochromatidic"
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
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Re: HELP! Questions on mitosis and meiosis

Postby Darby » Wed Jun 11, 2008 7:54 pm

Except that actually disagrees. It isn't really saying that the new chromosome is a chromatid (after all, it's being called a chromosome), it's just clarifying the recent origin.

It's ironic that I'm on this side of the discussion, since I don't tend to get too particular about terminology. It just seems to be adding to the confusion here, though.
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Postby Cat » Thu Jun 12, 2008 6:39 pm

I agree that definitions are confusing, but that is the fact that cannot be changed. Look up meiosis 1st division. After it cells have 46 chromatids but considered to be 1N - 23 chromosomes (by the way 32 must have been a type-o) because two chromatids in a pair are identical to each other (cell can have 2Y chromatids!).
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Re: HELP! Questions on mitosis and meiosis

Postby YAZ » Wed Apr 22, 2009 3:06 am

i also need help. if you know the answer please help.
I need help on explaining how alleles are transmitted differently in mitosis vs. meiosis and why both processes are necessary.
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