Naked Science Forum

Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: hamdani yusuf on 17/11/2017 06:31:05

Title: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: hamdani yusuf on 17/11/2017 06:31:05
https://en.wikipedia.org/wiki/Chromosome_2_(human)

Quote
Chromosome 2 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 2 is the second-largest human chromosome, spanning more than 242 million base pairs[5] (the building material of DNA) and representing almost 8% of the total DNA in human cells.

Chromosome 2 contains the HOXD homeobox gene cluster.

Quote
All members of Hominidae except humans, Neanderthals, and Denisovans have 24 pairs of chromosomes.[7] Humans have only 23 pairs of chromosomes. Human chromosome 2 is a result of an end-to-end fusion of two ancestral chromosomes.[8][9]

The evidence for this includes:
The correspondence of chromosome 2 to two ape chromosomes. The closest human relative, the chimpanzee, has near-identical DNA sequences to human chromosome 2, but they are found in two separate chromosomes. The same is true of the more distant gorilla and orangutan.[10][11]
The presence of a vestigial centromere. Normally a chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere in the q21.3q22.1 region.[12]
The presence of vestigial telomeres. These are normally found only at the ends of a chromosome, but in chromosome 2 there are additional telomere sequences in the q13 band, far from either end of the chromosome.[13]

According to researcher J. W. IJdo, "We conclude that the locus cloned in cosmids c8.1 and c29B is the relic of an ancient telomere-telomere fusion and marks the point at which two ancestral ape chromosomes fused to give rise to human chromosome 2."

What will happen if chromosome 2 in human zygote splits at its vestigial telomere?
Or alternatively, what if 2 chromosomes in chimpanzee's zygote corresponding to human chromosome 2 somehow fuse?
Is there any research about this?
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: evan_au on 17/11/2017 09:19:35
It sounds like not many genes were lost in the fusion. So essentially all the RNA sequences and proteins could still be produced.

There would probably have been reduced fertility between individuals having or not having the fused chromosome.
But higher fertility if there were two copies of the fused gene.
So this probably would have accelerated speciation.
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: hamdani yusuf on 17/11/2017 11:20:50
It sounds like not many genes were lost in the fusion. So essentially all the RNA sequences and proteins could still be produced.

There would probably have been reduced fertility between individuals having or not having the fused chromosome.
But higher fertility if there were two copies of the fused gene.
So this probably would have accelerated speciation.
So, if a human zygote with split chromosome 2 lives and grows in a human womb, it will grow like normal human. But he/she will have difficulty in reproducing sexually, because the difference in chromosome number will prevent him/her to have offspring. Unless he/she can find a partner from opposite gender who also has split chromosome 2.

Is there record of fertile hybrid from parents with different number of chromosomes?
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: hamdani yusuf on 18/11/2017 07:30:56
Can anyone share a reference on how chromosomes fuse or split? Can it happen without any mutation of nucleic-bases in the edge of DNA?
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: evan_au on 18/11/2017 11:12:26
Quote from: hamdani yusuf
Can anyone share a reference on how chromosomes fuse or split?
See: https://en.wikipedia.org/wiki/Robertsonian_translocation
https://en.wikipedia.org/wiki/Dicentric_chromosome

Quote
Is there record of fertile hybrid from parents with different number of chromosomes?
The most common case is Down Syndrome, where there is an extra chromosome. Males tend to be infertile, while females have reduced fertility.
https://en.wikipedia.org/wiki/Down_syndrome#Fertility
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: evan_au on 18/11/2017 20:32:30
There are many ways that chromosomes can be damaged.
Most of them would result in non-viable embryos.

See: https://en.wikipedia.org/wiki/Chromosome_abnormality

Cancerous cells, once they have lost their proofreading machinery, display many of these chromosomal defects.
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: hamdani yusuf on 21/11/2017 04:27:16
There are many ways that chromosomes can be damaged.
Most of them would result in non-viable embryos.

See: https://en.wikipedia.org/wiki/Chromosome_abnormality

Cancerous cells, once they have lost their proofreading machinery, display many of these chromosomal defects.
Unfortunately, the reference doesn't mention about chromosomal defects similar to fusion of human chromosome 2. What will happen if other chromosomes also merge, creating a human individual with fewer pairs of chromosome?
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: evan_au on 21/11/2017 20:25:01
Quote from: hamdani yusuf
Unfortunately, the reference doesn't mention about chromosomal defects similar to fusion of human chromosome 2.
The reference gives common chromosomal abnormalities that are seen in living humans today.

I expect that few instances of chromosome fusion would result in a viable organism, let alone a viable population of such organisms. Humans are thought to have split from chimpanzees around 6 million years ago, so it evidently doesn't happen very often.

To see the effects of changing chromosome counts, look at a group that has been apart for a longer period, the Canids, which includes dogs, wolves, foxes, jackals, etc.

There are a wide variety of chromosome counts in this group; just search the following list for "fox" (I get 8 hits, varying between 78 & 34 chromosomes):
See: https://en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count

Having a different number of chromosomes really does have a negative impact on fertility:
See: https://en.wikipedia.org/wiki/Canid_hybrid
Title: Re: What is the effect of fusion of chromosome 2 in human nuclear cell?
Post by: hamdani yusuf on 22/11/2017 03:28:30
Thanks, Evan, you've been very helpful as usual.
I'm curious how those species get different number of chromosomes. How to determine if they were result of splitting or fusion?
Are split chromosomes get new telomere at splitting points? Do they generate new centromeres?