Homologous Chromosomes Help! - Need by today!!!

OP

Original post by zed963

Chromosomes are chromosomes. Chromatids are also considered chromosomes.

There are 3 pairs.

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okay, that helped!

just one more question: so when I consider meiosis, and I'm dealing with chromatids, 2 chromatids will be my homologous pairs? And each 'chromosome' (one chromatid) will replicate and split into 4 chromatids? Will all 4 have the same gene loci?

Reply 4

zed963

16

Original post by ps1265A

okay, that helped!

just one more question: so when I consider meiosis, and I'm dealing with chromatids, 2 chromatids will be my homologous pairs? And each 'chromosome' (one chromatid) will replicate and split into 4 chromatids? Will all 4 have the same gene loci?

If you have one chromatid and that replication you have two chromatids so how can that split into 4 ? It simply doesn't.

What happens is the cell replicates, so you have two chromosomes, that cell then splits into two hence one chromosome in each cell. Them two cells split again forming four cells which each contain a chromatid ( chromosome)

Reply 5

OP

Original post by zed963

If you have one chromatid and that replication you have two chromatids so how can that split into 4 ? It simply doesn't.

What happens is the cell replicates, so you have two chromosomes, that cell then splits into two hence one chromosome in each cell. Them two cells split again forming four cells which each contain a chromatid ( chromosome)

So you can look at it from two perspectives:
1) One homologous pair (XX), replicated, each X in one gamete
OR
2) One homologous pair (Two chromatids), replicate, each chromatid in one gamete

Reply 6

Original post by ps1265A

http://filestore.aqa.org.uk/subjects/AQA-BIOL2-QP-JUN13.PDF

the pictures in question 1 and their descriptions are confusing!

the first pictures shows 2 chromosomes (2 separate X shapes) and says that it is "one pair of homologous chromosomes".
then the second picture doesn't make sense based on this. It says that there are "3 pairs of homologous chromosomes"
but there is only a single chromosome of each type rather than pairs for each one as shown in the first picture???

so what's going on :s ?

OP as the other posters have said, two chromatids make one chromosome.

The two chromosomes are then split in Anaphase II as the centromeres divide. Keep in mind that in Meiosis II there are two cells, each with one chromosome/two chromatids). So as each chromosome splits at the centromere, each of the two chromatids are pulled to the opposite poles of the cell that they are in, by the centrioles.

Once each chromatid is at each pole of the two cells (so four chromatids are at four separate poles across two cells and there are two poles in a cell) a nuclear envelope reforms around the chromatid and nucleolus reappears. The result is four haploid daughter cells (each daughter cell has ONE chromosome) and so half of the parents DNA (haploid).

Someone please correct me if I am wrong!

But I hope this helps OP. :smile:

(edited 9 years ago)

Reply 7

zed963

16

Original post by ps1265A

So you can look at it from two perspectives:
1) One homologous pair (XX), replicated, each X in one gamete
OR
2) One homologous pair (Two chromatids), replicate, each chromatid in one gamete

I'd go for number 2). This is because you can't really have an X in a gamete as such, ( well you can but... ) You'd be saying that the two chromatids have joined to form X hence 23/2 = 11.5 pair which doesn't really occur. Do you understand?

Let me take you through the Human process.

Every diploid cell in the body has 46 chromosomes.

What happens is these 46 replicate forming 92. Meiosis 1 occurs, the cell divides into two hence each cell has 46 chromosomes.
These two cells then divide again forming 4 cells each containing 23 chromatids hence now haploid.

It's another way of looking at it. There's more to it such as Prophase, Anaphase, Metaphase... but the Spec doesn't require it.

Reply 8

Hype en Ecosse

20

Original post by ps1265A

http://filestore.aqa.org.uk/subjects/AQA-BIOL2-QP-JUN13.PDF

the pictures in question 1 and their descriptions are confusing!

the first pictures shows 2 chromosomes (2 separate X shapes) and says that it is "one pair of homologous chromosomes".
then the second picture doesn't make sense based on this. It says that there are "3 pairs of homologous chromosomes"
but there is only a single chromosome of each type rather than pairs for each one as shown in the first picture???

so what's going on :s ?

"Pair" doesn't mean the chromosones are stuck together! In fig. 2, you still have 3 pairs of homologous chromosomes. "Homologous" simply means they're the same type! So the maternal chromosome 1 and paternal chromosome 1 are homologous chromosomes, whereas chromosome 1 and chromosome 7 aren't homologous.

A chromosome is not the same thing as a chromatid, zed963 is giving you some poorly communicated information. A chromatid is one half of a duplicated chromosome (one-half of the X shaped chromosome). Chromosomes are only X-shaped after they've duplicated, but when they're unduplicated they look like they do in fig. 2 (something called "interphase"). The things shown in figure 1 and figure 2 are both chromosomes (duplicated and unduplicated chromosomes, respectively), but only figure 1 contains chromatids (half of the X).

Original post by ps1265A

So you can look at it from two perspectives:
1) One homologous pair (XX), replicated, each X in one gamete
OR
2) One homologous pair (Two chromatids), replicate, each chromatid in one gamete

Not really. An "X" is a duplicated chromosome. The XX structure is the homologous pair; a pair of chromatids is just one X (a single chromosome).

How meiosis works is:

We've got a cell chilling out like in figure 2. The chromosomes are all disorganised and not coiled up or duplicated. You can see that some of the chromosomes are the same size - the ones that are the same size are homologous (in real life we use the patterns on each chromosome to identify them, but they make it easier for you in the exam). So what happens is each unduplicated chromosome replicates itself (to make the X-shape; which is made up of 2 chromatids) and then pairs up with its homologous partner, to form a homologous pair. Then our first stage of cell division pulls the homologous chromosomes away from each other into their own cells, then the next stage of cell division pulls the chromatids that make up those chromosomes apart (so 2 X shaped chromosomes breaking into 4 chromatids), and after this, because they're in their own cells now, we call them chromosomes again, which look like they do in fig. 2 - in their unduplicated form.

Original post by TheStudent18

OP as the other posters have said, two chromatids make one chromosome.

The two chromosomes are then split in Anaphase II as the centromeres divide. Keep in mind that in Meiosis II there are two cells, each with one chromosome/two chromatids). So as each chromosome splits at the centromere, each of the two chromatids are pulled to the opposite poles of the cell that they are in, by the centrioles.

Once each chromatid is at each pole of the two cells (so four chromatids are at four separate poles across two cells and there are two poles in a cell) a nuclear envelope reforms around the chromatid and nucleolus reappears. The result is four haploid daughter cells (each daughter cell has ONE chromatid) and so half of the parents DNA (haploid).

Someone please correct me if I am wrong!

But I hope this helps OP. :smile:

You're right, but you have to be careful not to bog people down with information! OP's struggling to get their head around chromatids/chromosomes/homologous pairs, I think jumping into PMAT is a bit overwhelming at this stage. :tongue:

(edited 9 years ago)

Reply 9

Original post by Hype en Ecosse

X

Ah fair enough! I just want the OP to understand why it is that way, instead of just being told it is that way! I also thought because exams are so close, there is no time to waste :P Aha

(edited 9 years ago)

Reply 10

Hype en Ecosse

20

Original post by TheStudent18

Ah fair enough! I just want the OP to understand why it is that way, instead of just being told it is that way! Aha

It doesn't really explain why though, does it? It's just giving a name to a part of a process they didn't know had a name. :wink:

Are you familiar with Feynman's "name of that bird" anecdote? :tongue:

Reply 11

Original post by Hype en Ecosse

It doesn't really explain why though, does it? It's just giving a name to a part of a process they didn't know had a name. :wink:

Are you familiar with Feynman's "name of that bird" anecdote? :tongue:

Well I suppose not, but I was trying to get across the idea of the difference between a chromosome and a chromatid and then using PMAT to explain how the two terms are used appropriately :tongue:

. I thought that the OP would have been familiar with PMAT seeing as its so close to exams (unless its not on their spec.).

And I am! Aha :tongue:

Reply 12

Hype en Ecosse

20

Original post by TheStudent18

Well I suppose not, but I was trying to get across the idea of the difference between a chromosome and a chromatid and then using PMAT to explain how the two terms are used appropriately :tongue:

. I thought that the OP would have been familiar with PMAT seeing as its so close to exams (unless its not on their spec.).

And I am! Aha :tongue:

And you did a good job explaining it! Just have to keep jargon to the right level. :biggrin:

N.B: after a chromatid is in its own cell, it's a chromosome again. I don't think there's an "official" borderline, but after telophase II is usually a good point to "switch" vocabulary again.

I think PMAT isn't something that comes up until A2. That's how it is in Scotland!

Reply 13

Original post by Hype en Ecosse

And you did a good job explaining it! Just have to keep jargon to the right level. :biggrin:

N.B: after a chromatid is in its own cell, it's a chromosome again. I don't think there's an "official" borderline, but after telophase II is usually a good point to "switch" vocabulary again.

I think PMAT isn't something that comes up until A2. That's how it is in Scotland!

Ah nice, didn't know that it's called a chromosome again! Thanks.

Yeah I think PMAT is largely A2 content, but if I remember correctly, we kind of touched on it in AS (although not on the spec.) to help us really understand the difference between mitosis and meiosis as a lot of students in my class confused the two quite often.

I didn't look to if OP was on about AS or A2... oops. I assumed it was A2 as that's what we're doing now (different exam board -.-).

Reply 14

OP

Original post by zed963

I'd go for number 2). This is because you can't really have an X in a gamete as such, ( well you can but... ) You'd be saying that the two chromatids have joined to form X hence 23/2 = 11.5 pair which doesn't really occur. Do you understand?

Let me take you through the Human process.

Every diploid cell in the body has 46 chromosomes.

What happens is these 46 replicate forming 92. Meiosis 1 occurs, the cell divides into two hence each cell has 46 chromosomes.
These two cells then divide again forming 4 cells each containing 23 chromatids hence now haploid.

It's another way of looking at it. There's more to it such as Prophase, Anaphase, Metaphase... but the Spec doesn't require it.

yes, I understand it by looking at the number of chromosomes as you've stated. However, I can't seem to understand AQA's diagram of meiosis in reference to meiosis 1 and meiosis 2. I'm finding it difficult to understand how, at the end of meiosis 2, there will be the same genes in each of the 'chromatids'

Reply 15

OP

Original post by Hype en Ecosse

"Pair" doesn't mean the chromosones are stuck together! In fig. 2, you still have 3 pairs of homologous chromosomes. "Homologous" simply means they're the same type! So the maternal chromosome 1 and paternal chromosome 1 are homologous chromosomes, whereas chromosome 1 and chromosome 7 aren't homologous.

A chromosome is not the same thing as a chromatid, zed963 is giving you some poorly communicated information. A chromatid is one half of a duplicated chromosome (one-half of the X shaped chromosome). Chromosomes are only X-shaped after they've duplicated, but when they're unduplicated they look like they do in fig. 2 (something called "interphase"). The things shown in figure 1 and figure 2 are both chromosomes (duplicated and unduplicated chromosomes, respectively), but only figure 1 contains chromatids (half of the X).

Not really. An "X" is a duplicated chromosome. The XX structure is the homologous pair; a pair of chromatids is just one X (a single chromosome).

How meiosis works is:

We've got a cell chilling out like in figure 2. The chromosomes are all disorganised and not coiled up or duplicated. You can see that some of the chromosomes are the same size - the ones that are the same size are homologous (in real life we use the patterns on each chromosome to identify them, but they make it easier for you in the exam). So what happens is each unduplicated chromosome replicates itself (to make the X-shape; which is made up of 2 chromatids) and then pairs up with its homologous partner, to form a homologous pair. Then our first stage of cell division pulls the homologous chromosomes away from each other into their own cells, then the next stage of cell division pulls the chromatids that make up those chromosomes apart (so 2 X shaped chromosomes breaking into 4 chromatids), and after this, because they're in their own cells now, we call them chromosomes again, which look like they do in fig. 2 - in their unduplicated form.

You're right, but you have to be careful not to bog people down with information! OP's struggling to get their head around chromatids/chromosomes/homologous pairs, I think jumping into PMAT is a bit overwhelming at this stage. :tongue:

Okay, so let's break this down:

"Not really. An "X" is a duplicated chromosome. The XX structure is the homologous pair; a pair of chromatids is just one X (a single chromosome)."

Well, this is the diagram I'm having some difficulty understanding. I understand how chromosomes duplicate so that they can divide into 4 gametes. But I'm having difficulty understanding this diagram in particular. The diagram clearly states that the original cell consists of a homologous pair. Therefore, each chromosome must have the same genes. As you said, each chromosome is made up of 2 chromatids, by the end of meiosis 2, you have 4 chromatids. Clearly, they cannot ALL have the same gene loci as you split up a chromosome.
[Only referring to this diagram and nothing else]

They would have the same genes if they replicated, but this diagram does not include replication.
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There are so many contradictions about chromatids. Yes, chromotids make up a chromosome, but I also read that a chromatid can be classed a chromosome, and 2 chromatids make up a homologous pair. With this idea, the diagram does make sense again, however you previously posted that they're not the same thing and the diagram also recognises the pictures solely as chromosomes.
-----------------------------------------------------------------------
Back to this: http://filestore.aqa.org.uk/subjects/AQA-BIOL2-QP-JUN13.PDF
The diagram says that it is the cell "at the end of cell division" (i.e. meiosis)
"A chromosome is not the same thing as a chromatid, zed963 is giving you some poorly communicated information. A chromatid is one half of a duplicated chromosome (one-half of the X shaped chromosome). Chromosomes are only X-shaped after they've duplicated, but when they're unduplicated they look like they do in fig. 2 (something called "interphase"). The things shown in figure 1 and figure 2 are both chromosomes (duplicated and unduplicated chromosomes, respectively), but only figure 1 contains chromatids (half of the X)."

So only figure one contains "duplicated chromosomes" and therefore, there are chromatids present, whilst in figure 2 there aren't any chromatids present.
So when does this "duplication" happen?
Is the "duplicated cell" a normal body cell?

You see, this stuff is not in my specification so I'm having a lot of difficulty trying to understand it

EDIT:
The cell is "at the end of cell division". According to the diagram again, there should only be one chromatid of each size in the cell.

And by duplicate, do you mean when the cell is undergoing division and when the chromatin actually forms chromatid?

And when are chromosomes NOT homologous?

Soooo confused!

(edited 9 years ago)

Reply 16

OP

Original post by Hype en Ecosse

Here's a diagram showing how the gene loci would not be shared amongst all of the gametes

Does this mean that my first diagram (in the previous post) is missing chromosome replication?

(edited 9 years ago)

Reply 17

OP