[課外genetics]望大學生指教, 謝.

dolphin_ice

有關DNA shortening的問題 (只是好奇, 和功課無關)

Given that: When DNA replicates, it is possible DNA shortening happens
Many scientists belief that this is the key to aging.

Example: Dolly the clone sheep has 未老先sui 現象due to DNA shortening.

Question 1: Why does it only happen in mitosis, and not meiosis?
Question 2: Why won't it happen in asexual reproduction? e.g. budding of bacteria.

[ 本帖最後由 dolphin_ice 於 23/2/2008 10:46 AM 編輯 ]

fish

原帖由 dolphin_ice 於 23/2/2008 09:13 AM 發表

                               
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有關DNA shortening的問題 (只是好奇, 和功課無關)

Question 1: Why does it only happen in mitosis, and not meiosis?
Question 2: Why won't it happen in asexual reproduction? e.g. budding of bacteria.

Telomeres are neucleotide sequences at the end of  chromosomal DNA. They are responsible for delaying the erosion of DNA during series of replication.

1.
In germ cells, presence of telomerase helps lengthening telomeres so to avoid shortening of DNA. However, telomerase is inactive in somatic cells. As a result, DNA in somatic cells would be shortened in successive rounds of replication but not germ cells.

2.
I am not totally sure about this as there are many modes of asexual reproduction.
But it is obvious to prokaryotes that since prokaryotes are of circular DNA, DNA shortening won't be a problem for them. For eukaryotes like fungi and plants, I believe there should be some regulators that are capable of maintaining the length of telomeres as I have read an article about this before.

Also, I don't agree with the word "budding" here as budding would produce an offspring that is relatively smaller than the parent while the binary fission of prokaryotes generates offspring of similar size to parents.

還有嘛.....不要指名好吧.....很大壓力的

dolphin_ice

sorry, should have used "binary fission" for bacteria.
Yes, telomerase was what I was looking for
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
But...

What about plants in this situation? Will they have the problem of aging?
I used to plant 紫羅蘭, and a trick to clone many of them is to cut some of their old leaves, and plant the leaves separately on different pots. Eventually, the leaves grow into individual new 紫羅蘭s.
Will they experience aging?

Also, the Amazon Molly issue
http://en.wikipedia.org/wiki/Amazon_molly (for more information)

The Amazon molly, Poecilia formosa, is a freshwater fish which reproduces throughgynogenesis. This means that although females must mate with a male,genetic material from the male is not incorporated into the already diploid egg cells that the mother is carrying(except in extraordinary circumstances), resulting inidentical clones of the mother being produced en mass. This unusual characteristic has led to the Amazon molly becoming an all female species.

Since the offsprings are genetically identical to its mother,
why won't they have an aging problem?

[ 本帖最後由 dolphin_ice 於 1/3/2008 10:55 AM 編輯 ]

fish

原帖由 dolphin_ice 於 1/3/2008 10:54 AM 發表

                               
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What about plants in this situation? Will they have the problem of aging?

For the case of plant, I am really unsure about that since I haven't started Botany yet. In fact, I am taking Botany next quarter.
Perhaps it is good for us to do some research at school =].

Since the offsprings are genetically identical to its mother,
why won't they have an aging problem?

What I believe is that aging should happen slowly during subsequence replication by mitosis of the child after they were given birth.
For the case of Amazon Molly, as their mode of reproduction(gynogenesis) is a type of parthenogenesis, I would use the example of genus Cnemidophorus ,which reproduce by parthenogenesis, to try to explain it as I learnt it this Monday (Yep, I am taking Zoology this quarter.).

Many Cnemidophorus have only female and they sometimes mate with males form different species. (An example of interbreed between mates NOT from the same species.)
But in reality, they actually "mate" with female as well. (They still practise the "mating dance" and other mating behavior with each other.)

In parthenogenesis,meiosisis actually going on. However, after the diploid duplicated, the cell does NOT divide to form gametes. The reason why it is still considered as meiosis is that there is pairing of homologous chromosome and crossover happening. As it is a meiosis process, the telomerase is still active and can prevent aging.

算了....edit了2次color code都只有一個meiosis 變紅
小心一下我特意bold掉的 mitosis 及 meisosis吧

[ 本帖最後由 fish 於 1/3/2008 01:58 PM 編輯 ]

dolphin_ice

Wow!
You are so pro!
I've heard about those lizards before, but I didn't know the mechanism.
I admire your knowledge and ability to answer difficult questions!

fish

原帖由 dolphin_ice 於 2/3/2008 09:41 AM 發表

                               
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Wow!
You are so pro!
I've heard about those lizards before, but I didn't know the mechanism.
I admire your knowledge and ability to answer difficult questions!

If my teacher doesn't mention about them as a mode of asexual reproduction, I even wouldn't try do understand anything about them.
I should thank you for posting this topic and let me try to explain concepts which would not be examined =].