when does a cell become haploid in meiosis?

[sundaymorning7]

OK so I guess this specifically is unlikely to come up but I would love some clarification.
My understanding was that the cell was haploid after telophase I, since it only has DNA from one parent, just that the dna content was doubled. A TPR review sheet I have, however, says that the cell is haploid after telophase II.
Thoughts?

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[ilovemcat]

OK so I guess this specifically is unlikely to come up but I would love some clarification.
My understanding was that the cell was haploid after telophase I, since it only has DNA from one parent, just that the dna content was doubled. A TPR review sheet I have, however, says that the cell is haploid after telophase II.
Thoughts?

TPR is wrong then. A diploid cell becomes haploid during Meiosis I and is completed after Telephase I.

You start off with a duplicated copy of all the chromosomes (2 sister chromatids); all your mom's chroms are dulicated & all your dad's chroms are duplicated
These homologous chromosomes (from mom & dad, all duplicated) pair up during prophase I forming tetrads.
The pairs of homologs line up on the metaphase plate during metaphase I.
Homologs are pulled apart by the meotic spindles during anaphase I.
Telephase I follows shortly after with the reformation of nucleolus and the nucelus.

In Meosis II, the recombinant sister chromatids for each chromosome in the haploid cell are then pulled apart forming 4 unreplicated haploid cells.

 

[sundaymorning7]

OK I think it must be a typo then, thanks!

 

[ilovemcat]

OK I think it must be a typo then, thanks!

No probs. I encountered quite a few errors by TPR while I was studying. It was annoying but in a way it solidified my understanding of certain things because I'd always try to make sense of their incorrect answers.

 

[ModusProbandi]

TPR is wrong then. A diploid cell becomes haploid during Meiosis I and is completed after Telephase I.

You start off with a duplicated copy of all the chromosomes (2 sister chromatids); all your mom's chroms are dulicated & all your dad's chroms are duplicated
These homologous chromosomes (from mom & dad, all duplicated) pair up during prophase I forming tetrads.
The pairs of homologs line up on the metaphase plate during metaphase I.
Homologs are pulled apart by the meotic spindles during anaphase I.
Telephase I follows shortly after with the reformation of nucleolus and the nucelus.

In Meosis II, the recombinant sister chromatids for each chromosome in the haploid cell are then pulled apart forming 4 unreplicated haploid cells.

I concur, although it's called telophase from where I'm from.

 

[ilovemcat]

I concur, although it's called telophase from where I'm from.

That's embarrassing. Thanks for the correction

 

[BHaus9]

I hate to say it, but I think your book is right. Right after replication and prior to meiosis, the chromosome number is technically 4n. Not until after completion of meiosis II is the complete chromosome number "n." Each sister chromatid is counted as one chromosome - it doesn't matter if all chromatids in a cell following meiosis I are derived from a single parent. Each cell at this point still has a full complement of 46 chromatids/chromosomes.

 

[Rabolisk]

I hate to say it, but I think your book is right. Right after replication and prior to meiosis, the chromosome number is technically 4n. Not until after completion of meiosis II is the complete chromosome number "n." Each sister chromatid is counted as one chromosome - it doesn't matter if all chromatids in a cell following meiosis I are derived from a single parent. Each cell at this point still has a full complement of 46 chromatids/chromosomes.

I love to say it, and I believe you are wrong. What you have after meiosis I is 23 chromosomes and 46 sister chromatids. Each sister chromatid would not be counted as one chromosome. If that were true, then do somatic cells have 92 chromosomes right before mitosis?

 

[BHaus9]

I love to say it, and I believe you are wrong. What you have after meiosis I is 23 chromosomes and 46 sister chromatids. Each sister chromatid would not be counted as one chromosome. If that were true, then do somatic cells have 92 chromosomes right before mitosis?

The pleasure is all mine. Strictly speaking, no, most people would probably not define a chromatid as identical to a chromosome. That said, a human somatic cell undergoing mitosis is considered quadraploid after replication and up to/including metaphase. This is the mechanism whereby cells that undergo DNA replication, but are arrested before division, give rise to polyploid cells.

If both daughter cells are diploid, and all the genetic material included in the daughter cells was included in the parent, how could the parent also be diploid? Obviously, it was diploid prior to synthesis, but once you duplicate each gene you double the chromosome number.

At some point this is just a game of semantics. But, if you think about it, why would meiosis II need to occur at all if a complete haploid gamete were produced after meiosis I?

EDIT: If anyone can find a conclusive answer to this (preferably with reference) it would be immensely appreciated. I tried simply researching this question but I found equal numbers of responses favoring each camp. My views and explanation are based entirely upon what I have been taught. I am also interested in how the specific review question was phrased: If it said "when does a cell FIRST become haploid during mitosis?" then the question is admittedly ambiguous. However, if it simply asks when the cell is haploid, you should know that it is 100% positively haploid at the end of meiosis II and eliminate all other answers.

 

[attixx]

BHaus, that's very much untrue.

Ploidy refers to the # of sets of chromosomes in a cell. Humans are diploid because we have 23 homologous pairs of chromosomes, two complete haploid sets (one set from the father and one set from the mother).

At metaphase each of the 46 chromosomes (members of the 23 pairs) consists of two sister chromatids that had formed after replication in S phase. So there is a total of 92 chromatids, but that does not mean there is 92 chromosomes. It's easy to follow chromosome number by keeping track of the number of centromeres. Sister chromatids become individual chromosomes during anaphase, when the centromeres split.

So you start with 23 pairs (46 chromosomes) in interphase/mitosis, and each member of the pair replicates itself. Technically speaking, you have double the amount of genetic material but still 46 chromosomes since the replicated material shares a centromere. The only time you really have "92 chromosomes" is the moment after anaphase. The sister chromatids that were previously held together by the centromere split, and each become their own chromosome -- 46 chromosomes are pulled to one side and 46 to the other side. After cytokinesis, there are two new cells with 46 chromosomes (23 pairs like at the beginning), and this ends the cycle. The cell is never 4n because there is never 4 homologs (or members of a homologous family).

Consider meiosis. The goal is to get 4 haploid cells from one diploid cell. With your 23 pairs (46 chomosomes) in your diploid cell, you replicate to 23 tetrads. A tetrad consists of 4 chromatids, two sets of sisters, and technically 46 chromosomes because the sisters are held together by centromeres.
Meiosis I happens, and the tetrads are pulled apart (but no centromeres are pulled apart). So a cell with 46 chromosomes becomes 2 cells, each with 23 chromosomes (reductive division). This cell is haploid because there is no longer a homologous pair in any cell. Each of the two new cells only consists of the sister chromatids that were a result of replication. Meosis II serves to split the sister chromatids apart. Remember, this splitting results in each chromatid becoming its own chromosome, so you go from 23 chromosomes in one cell to 23 chromosomes in each cell. This happens in each of the two cells that resulted from meiosis I, so you have 4 cells that contain 23 chromosomes = haploid.

Long winded I know, but this is a complicated topic. It's definitely a game of semantics, but the MCAT likes to prety on semantics when it comes to this topic.

 

[BHaus9]

@attixx:

I don't disagree in the least bit with any part of your description of how meiosis proceeds, and I'm definitely not proposing any alternative theories here.

All I know is that in both of the bio text books that I have at my disposal, mitosis is listed as proceeding 2n -> 4n -> 2n, and meiosis is said to proceed 2n -> 4n -> 2n -> n. I have been looking for evidence to either confirm or refute this, but my search has turned up short thus far.

Whatever the case, though, if a question asks when a cell undergoing meiosis is haploid, there is no reason whatsoever NOT to choose telophase II.

 

[attixx]

I'm not hating on your materials, but I've never seen it explained like like.

Absolutely there is no reason to say that a cell is not haploid after telophase II, but if a question asks when a cell becomes haploid in meiosis, the answer is after meiosis I.

 

[BHaus9]

So at what point does one cell officially become two? It seems that if the cytoplasm is still shared, as it is in telophase, then the parent cell remains intact, and ergo there can be no haploid daughter cells regardless of how chromosome # is defined. Doesn't cytokinesis technically occur between meiosis I and meiosis II?

 

[Catburr]

This diagram sums it up pretty well, I think. [insert number here]c refers to number of chromosomes, and [insert number here]n refers to ploidy.

http://upload.wikimedia.org/wikipedia/commons/5/54/Meiosis_diagram.jpg

As I understand it, telophase I is when you see new nuclear membranes around each haploid set, and cytokinesis occurs.

I am PRETTY sure that "the end of telophase I, and thus meiosis I" coincides with the splitting up into two haploid daughter cells. These cells are 2c, 1n. At least that's how the materials I've studied from always seem to show it.

 

[donkeykong1]

TPR is correct. if u have the kaplan bio review notes, take a look at p.83 for a clearer pic.

the first cytokinesis occurs at the end of telophase 1. so this means if we have a diploid number of 2 [meaning 2 chromosomes with each comprising 2 chromatids], that each cell at metaphase 2 has one chromosome or 2 chromatids attached together at the centromere, so its haploid in the CHROMOSOME sense but not chromatid. next after the second cytokinesis we separate the chromatids which basically comprise that single haploid chromosome and get the haploid number once again. this time with bare chromatids

an easy trick to get the difference between chromosome and chromatid is to locate the centromere.

 

[Catburr]

haploid in the CHROMOSOME sense but not chromatid

Eh? Don't the terms diploid and haploid refer to how many homologous chromosomes you have, without regard to whether each of those chromosomes is made up of one chromatid, or two sister chromatids?

Let's say my example cell is in the middle of Anaphase I, of Meiosis I. It's in the process of pulling it's homologous chromosome pairs (which just finished with crossing over) to opposite ends of the cell. So each chromosome, when lined up together for crossing over, is facing it's homologue. Together, they're diploid. As they're being pulled apart, you're making two haploid sets. Each newly separated homologue is made up of two sister chromatids, but that doesn't make each set a diploid set.

Maybe this really is just a difference in how various sources are using the term ploidy.

EDIT: Campbell's bio (textbook) has it this way, saying "At the beginning of telophase I, each half of the cell has a complete haploid set of chromosomes, but each chromosome is still composed of two sister chromatids. Cytokinesis usually occurs simultaneously with telophase I, forming two haploid daughter cells."

 

[attixx]

Eh? Don't the terms diploid and haploid refer to how many homologous chromosomes you have, without regard to whether each of those chromosomes is made up of one chromatid, or two sister chromatids?

Yes. Because you have two sisters chromatids attached together does not mean you are diploid. That refers entirely to chromosomes.

 

[Ig5021]

What truly matters is the definition of haploid that you are using. There are two definitions for a haploid cell, if you state the definition you are using than it can be either the end of Meiosis I or II.

 

[Episome]

So at what point does one cell officially become two? It seems that if the cytoplasm is still shared, as it is in telophase, then the parent cell remains intact, and ergo there can be no haploid daughter cells regardless of how chromosome # is defined. Doesn't cytokinesis technically occur between meiosis I and meiosis II?

Cytokinesis occurs twice in meiosis: after telophase I, then again after telophase II. The monoploid number of chromosomes is reached after the very first cytokinesis event. This is confirmed by the application of terms reductional division and equational division.

Reductional division refers to meiosis I, because the ploidy of the cell is reduced from diploid to monoploid. Equational division refers to meiosis II (and mitosis) because the ploidy of the cell is left unchanged in these events.

The term haploid, by definition, refers to the number of chromosomes in each chromosome set of an individual's gametes. Because diploid creatures have monoploid gametes, a monoploid number of chromosomes can correctly be referred to as haploid in these creatures. Thus, the two resultant daughter cells from meiosis I can be described as haploid.

 

[adrakdavra]

Oogenesis starts with the process of developing oogonia, which occurs via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis.
[2] Oocytogenesis is complete either before or shortly after birth. So at birth you have
46XX. In other words, primary oocytes reach their maximum development at ~20[3] weeks of gestational age, when approximately seven million primary oocytes have been created; however, at birth, this number has already been reduced to approximately 1-2 million.
For those primary oocytes that continue to develop in each menstrual cycle, however, synapsis occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I, the primary oocyte has now developed into the secondary oocyte and the first polar body. so it went from 2n to 1n
Meiosis II
Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this process is also halted at the metaphase II stage until fertilization, if such should ever occur. When meiosis II has completed,23 x and another polar body have now been created
the 23 X is fritlized by 23X or 23Y spermatids .

 

[MedPR]

Why are there so many complicated answers? Homologues separate at Anaphase 1. At the end of Telophase 1 all normal cells are haploid.

 

[ReggieD6868]

Why are there so many complicated answers? Homologues separate at Anaphase 1. At the end of Telophase 1 all normal cells are haploid.

Exactamundo!