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Author Topic:   Biology Question
compmage
Member (Idle past 5152 days)
Posts: 601
From: South Africa
Joined: 08-04-2005


Message 1 of 8 (11863)
06-20-2002 7:49 AM


Hopefully there will be someone here who can explain this to me as I haven't been able to find a book that does.
In reproduction the sperm and the egg cell join to form an embrio. Each gametes cell countained half the number of chromosomes thereby giving the emrio a full set as it were. Here is where my lack of knowedge comes in. During meiosis how is the chromosome split? Is it down the middle, kinda like each side of a zipper?
If this is the case you should then be left with half (ie left or right for the lack of a better term) of a DNA strand. Lets say that the egg cell contained the following strand:
ATGTGCATCA
The sperm contained this corresponding strand:
ATTAGCATCA
Now, if I am correct so far and I am right in saying that adenine and thymine always bond together, and guanine and cytosine always bond together (i.e. your base pairs).
I can't see how these combine, adenine won't bond with adenine etc. Also the second strand differs slightly from the first, a mutation if you will, how does this come into play when the two strands attempt to join.
Could someone please explains this to me. I am fairly sure that I am missing or misunderstanding something here which is why I can't picture the joining of the egg and sperm cells once you move lower than the chromosome level.
------------------
compmage
[This message has been edited by compmage, 06-20-2002]

Replies to this message:
 Message 2 by mark24, posted 06-20-2002 8:13 AM compmage has replied
 Message 3 by Quetzal, posted 06-20-2002 8:25 AM compmage has replied

  
mark24
Member (Idle past 5194 days)
Posts: 3857
From: UK
Joined: 12-01-2001


Message 2 of 8 (11865)
06-20-2002 8:13 AM
Reply to: Message 1 by compmage
06-20-2002 7:49 AM


Hi compmage,
A diploid cell (ie us) has 2 PAIRS of chromosomes per cell. Early in meiosis each chromosome doubles, giving 2 chromatids, joined at the centromere (the familiar X shape). Homologous chromosomes (two chromosomes of each, remember)pair up & crossing over/recombination takes place. The doubled up chromosomes (note there are 4 homologous chromatids at this point, at the middle of the cell) are then randomly assorted, with 23 doubled up homologous chromosome pairs being pulled apart (in each case), & cell division occurs. This means there are 23 doubled up chromosomes (46 chromatids) in each daughter cell. The sister chromatids then split (becoming chromosomes in their own right), each homologous sister being pulled to opposite ends of the cell, which again divides. Note that there is only ONE chromosome set now per cell, the cells have divided twice, giving four haploid (1 chromosome set) cells, ie sperm or egg.
Fertilisation makes a diploid embryo out of two haploid cells.
When chromosomes replicate & form chromatids you have two strands, each of which forms a new strand based on nucleotide compatibility.
EG 1 chromosome.
ATAGGTTCCA
TATCCAAGGT
becomes two strands
ATAGGTTCCA & TATCCAAGGT
The other "missing strand" is copied using base pair complementing, from nucleotides in the cytoplasm. ie.
ATAGGTTCCA & TATCCAAGGT
TATCCAAGGT & ATAGGTTCCA
Giving 2 chromatids (note this happens for each homologous chromosome, giving four chromotids/somes). The chromatids are then passed on as a discrete unit, one ending up in each haploid cell. Thats two chromosomes becoming 4, being separated into 4 cells, by 2 cell divisions. In fertilisation, two chromosomes (from each parent) have different sequences, which is where I think you're going wrong. These two (sperm/egg) chromosomes do not have to interact. This is why we have two copies of each gene (one from sperm, on from the egg (sex chromosomes excepted)), but they aren't always the same allele (version of same gene).
Put differently, you have two copies of chromosome 20, both are different. The splitting/copying of the chromosomes occurs in the parents, not during fertilisation.
Hope this helps.
Mark
[This message has been edited by mark24, 06-20-2002]

This message is a reply to:
 Message 1 by compmage, posted 06-20-2002 7:49 AM compmage has replied

Replies to this message:
 Message 4 by compmage, posted 06-20-2002 10:53 AM mark24 has not replied

  
Quetzal
Member (Idle past 5871 days)
Posts: 3228
Joined: 01-09-2002


Message 3 of 8 (11866)
06-20-2002 8:25 AM
Reply to: Message 1 by compmage
06-20-2002 7:49 AM


compmage: the chromosome "splitting" (which is kind of a misnomer) takes place during the creation of the gametes - before fertilization. Here's a link to a pretty good, and relatively easy to understand, explanation. IOW, you don't have to worry about lining up base pairs - what gets separated are the two pairs of chromosomes. The chromatids aren't actually cut up. (See also "crossing over").
[This message has been edited by Quetzal, 06-20-2002]

This message is a reply to:
 Message 1 by compmage, posted 06-20-2002 7:49 AM compmage has replied

Replies to this message:
 Message 5 by compmage, posted 06-20-2002 10:56 AM Quetzal has replied

  
compmage
Member (Idle past 5152 days)
Posts: 601
From: South Africa
Joined: 08-04-2005


Message 4 of 8 (11875)
06-20-2002 10:53 AM
Reply to: Message 2 by mark24
06-20-2002 8:13 AM


quote:
Originally posted by mark24:

Put differently, you have two copies of chromosome 20, both are different. The splitting/copying of the chromosomes occurs in the parents, not during fertilisation.

I was aware of this. I only just realized that the way the question was put made it sound as though I did not know that the splitting takes place in each parent.
From your explanation I realise that my mistake came from thinking that the DNA itself split to form an egg or sperm cell, this was then causing me problems when I tried and join the egg and sperm cells in my mind and combine the half DNA strands. Thanks for the clarification.
These things happen when you didn't take biology after std. 7 (grade 9) and even that was a decade ago.
------------------
compmage

This message is a reply to:
 Message 2 by mark24, posted 06-20-2002 8:13 AM mark24 has not replied

Replies to this message:
 Message 7 by Brad McFall, posted 07-09-2002 10:55 PM compmage has not replied

  
compmage
Member (Idle past 5152 days)
Posts: 601
From: South Africa
Joined: 08-04-2005


Message 5 of 8 (11876)
06-20-2002 10:56 AM
Reply to: Message 3 by Quetzal
06-20-2002 8:25 AM


quote:
Originally posted by Quetzal:
compmage: the chromosome "splitting" (which is kind of a misnomer) takes place during the creation of the gametes - before fertilization. Here's a link to a pretty good, and relatively easy to understand, explanation. IOW, you don't have to worry about lining up base pairs - what gets separated are the two pairs of chromosomes. The chromatids aren't actually cut up. (See also "crossing over").
[This message has been edited by Quetzal, 06-20-2002]

Thanks. I do think 'splitting' is what put me on the wrong road here.
------------------
compmage

This message is a reply to:
 Message 3 by Quetzal, posted 06-20-2002 8:25 AM Quetzal has replied

Replies to this message:
 Message 6 by Quetzal, posted 06-20-2002 11:34 AM compmage has not replied
 Message 8 by Brad McFall, posted 07-09-2002 10:59 PM compmage has not replied

  
Quetzal
Member (Idle past 5871 days)
Posts: 3228
Joined: 01-09-2002


Message 6 of 8 (11879)
06-20-2002 11:34 AM
Reply to: Message 5 by compmage
06-20-2002 10:56 AM


Glad we could help. It is hard to remember all the details after a long time. (Don't even ask me to define a differential equation, fr'instance.
)

This message is a reply to:
 Message 5 by compmage, posted 06-20-2002 10:56 AM compmage has not replied

  
Brad McFall
Member (Idle past 5032 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 7 of 8 (13201)
07-09-2002 10:55 PM
Reply to: Message 4 by compmage
06-20-2002 10:53 AM


The problem occurs when one tries to find a morph type of a ancestor parent (or transitional form if one was assuming it was fair to speak of species no matter how fancy the conversation got)(there is such a notion of "return to the parent form" which is somewhat similar between Darwin and Wallace but do not ask for the citation i have that not here now).
I had thought about this before I began to read Pascal and became acutally even more confused than the time it took me too realize some of the language barrier which I find is usually 75% of the reason I do not understand something in biology and having absoutley nothing in that probability to do with the facts as they are/were etc...when the concept of this process in plants is connecting to the OPEN HABITAT (when I was trying to track with morphology fern forms and chromosomoe numbers AND geographic locations) because this notion menat that the split splits space from the perspective of the aninmal unless .... up to date , a third division that Pascal could not think of in communicating with Fermat that facial as well as a dice and stake division exists even if this is a corruption of the history of infinite divisibility that Cantor tried to purely clear in the Mess of greek I may have done to translation of Anaxagoras when they come out.

This message is a reply to:
 Message 4 by compmage, posted 06-20-2002 10:53 AM compmage has not replied

  
Brad McFall
Member (Idle past 5032 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 8 of 8 (13202)
07-09-2002 10:59 PM
Reply to: Message 5 by compmage
06-20-2002 10:56 AM


I joined Bovine egg and sperm cells in sterile dishes in 91 92. I do not think it is acutally possible to join them in one's mind but a differential equation, on the other hand with Acrchimedes axiom and some upper bound....that might in truth have been thought.

This message is a reply to:
 Message 5 by compmage, posted 06-20-2002 10:56 AM compmage has not replied

  
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