robertsonian translocation
jillery
expert in these things and I am truly confused. It seems there are
several ways that chromosomes can break and re-join According to
Hersheyh, the type of translocation described by Ken Miller is called
a Robertsonian Translocation. PZ Myers also wrote this in his
Pharnygula blog.
However, according to the Wikipedia cite given by Hersheyh, this type
breaks and re-joins right at the centromere. So, if I understand
Wikipedia's description, this would NOT preserve a secondary
centromere nor telomere-like sections in the middle of the newly
translocated chromosome.
So, what distinguishes a Robertsonian Translocation from other types
of chromosome fusions? If the fusion event Ken Miller described is
NOT called this, what IS its actual name?
Again, I have no problem understanding and agreeing with Dr. Miller's
point, I am just trying to pin down the details.
hersheyh
> This is somewhat OT, and is basically semantic in scope, but I am no
> several ways that chromosomes can break and re-join �ｿｽAccording to
> Hersheyh, the type of translocation described by Ken Miller is called
> Pharnygula blog.
>
> However, according to the Wikipedia cite given by Hersheyh, this type
> Wikipedia's description, this would NOT preserve a secondary
> centromere nor telomere-like sections in the middle of the newly
> translocated chromosome.
The fusion does not have to be *exactly* at the centromere. As long
as the new 'centromere' does not *significantly often* act as a
dicentric chromosome where the two centromeres can regularly go in
opposite directions (which leads to random breaks leaving chromosomes
with extra and missing coding information). The main feature of
Robertsonian fusions (Robertsonian fissions are the reverse process,
producing two acrocentrics from a single metacentric) is that it has
to preserve *most* of the information content (coding genes) and
regulatory sequences of the two acrocentric genes that fused to form a
metacentric gene. That makes it more likely that when a cell
acquires two copies of the new 'fusion' chromosome, it will have all
the information needed to form a functional organism.
For the fusion chromosome in humans (human #2 fom chimp 2A and 2B), no
information was lost.
http://en.wikipedia.org/wiki/Chimpanzee_genome_project#Genes_of_the_Chromosome_2_fusion_site
Even the requirement for no major loss or gain of coding information
is not necessary if the genome is polyploid or the chromosomes are
small or have little information.
Telomeric sequences (often degraded) internal to a chromosome is
usually a sign of an ancestral inversion (where the end flips over and
re-fuses; a new telomere sequence gets added on the other end) rather
than a translocation.
> So, what distinguishes a Robertsonian Translocation from other types
> of chromosome fusions? �ｿｽ
Balanced translocations (exchange of chromatid parts without any loss,
as would occur if one produced a dicentric and a non-centric
chromosome) can also be perpetuated through generations in ways that
can become fixed, but the reduction in fertility for such
translocations is over 50%, whereas the reduction for centric fusions
is around 5-15%. That doesn't mean it is impossible to fix a
translocation in a population. But you either need stronger selective
pressure for the change or you need a small population (bottleneck) to
allow it to happen by chance.
http://www.vivo.colostate.edu/hbooks/genetics/medgen/chromo/translocations.html
Sapient Fridge
<b09a8b58-9b3c-41bd...@19g2000vbq.googlegroups.com>,
jillery <69jp...@gmail.com> writes
<snip>
>However, according to the Wikipedia cite given by Hersheyh, this type
>breaks and re-joins right at the centromere. So, if I understand
>Wikipedia's description, this would NOT preserve a secondary
>centromere nor telomere-like sections in the middle of the newly
>translocated chromosome.
The fusion point in Robersonian translocation only has to be *near* the
two centromeres, if two centromeres are close together then they act as
a single one.
I did a college project on this if you are interested:
http://www.sapientfridge.org/chromosome_count
Have you seen the Ken Miller video discussing chromosome fusion?
http://www.youtube.com/watch?v=zi8FfMBYCkk
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jillery
> On Jan 6, 7:30 pm, jillery <69jpi...@gmail.com> wrote:
>
> > This is somewhat OT, and is basically semantic in scope, but I am no
> > several ways that chromosomes can break and re-join According to
> > Hersheyh, the type of translocation described by Ken Miller is called
> > Pharnygula blog.
>
> > However, according to the Wikipedia cite given by Hersheyh, this type
> > Wikipedia's description, this would NOT preserve a secondary
> > centromere nor telomere-like sections in the middle of the newly
> > translocated chromosome.
>
> The fusion does not have to be *exactly* at the centromere. �As long
> as the new 'centromere' does not *significantly often* act as a
> dicentric chromosome where the two centromeres can regularly go in
> opposite directions (which leads to random breaks leaving chromosomes
> with extra and missing coding information). � The main feature of
> Robertsonian fusions (Robertsonian fissions are the reverse process,
> producing two acrocentrics from a single metacentric) is that it has
> to preserve *most* of the information content (coding genes) and
> regulatory sequences of the two acrocentric genes that fused to form a
> metacentric gene. �That makes it more likely that �when a cell
> acquires two copies of the new 'fusion' chromosome, it will have all
> the information needed to form a functional organism.
>
> For the fusion chromosome in humans (human #2 fom chimp 2A and 2B), no
> information was lost.
>
>
> Even the requirement for no major loss or gain of coding information
> is not necessary if the genome is polyploid or the chromosomes are
> small or have little information.
>
> Telomeric sequences (often degraded) internal to a chromosome is
> usually a sign of an ancestral inversion (where the end flips over and
> re-fuses; a new telomere sequence gets added on the other end) rather
> than a translocation.
>
> > So, what distinguishes a Robertsonian Translocation from other types
> > of chromosome fusions?
>
> as would occur if one produced a dicentric and a non-centric
> chromosome) can also be perpetuated through generations in ways that
> can become fixed, but the reduction in fertility for such
> translocations is over 50%, whereas the reduction for centric fusions
> is around 5-15%. �That doesn't mean it is impossible to fix a
> translocation in a population. �But you either need stronger selective
> pressure for the change or you need a small population (bottleneck) to
> allow it to happen by chance.
>
>
>
>
> > If the fusion event Ken Miller described is
> > NOT called this, what IS its actual name?
>
> > Again, I have no problem understanding and agreeing with Dr. Miller's
>
> - Show quoted text -
Thank you for taking the time to explain this. Please bear with me a
little longer, but I really need to beat this into the ground just to
make sure I grok it.
As I understand it, the fusion in question did not require any
chromosomes to break. Instead, the ancestral ape chromosomes 2A and
2B were, in Dr. Miller's words, "scotch-taped" end-to-end, thus
preserving an inactive centromere and degraded telomeres in the middle
of the newly-fused chromosome. If I understand you, none of this is
meaningful to labeling this a Robertsonian fusion. Instead, the only
characteristic feature that makes this a Robertsonian fusion is that
it preserved all/most of the important genetic information.
Presumably, this is because there are other translocation mechanisms
that cause significant destruction/duplication/loss of genetic
information, and microbiologists find it useful to classify the
different mechanisms by that single feature. Is that right? If so, I
am at a loss to understand why the cites seem to focus on just about
everything else but this end-result. In fact, nothing that I read
about Robersonian translocations even mention it. Very confusing.
Is there a name for the class of translocation mechanisms that are non-
Robertsonian translocations?
Ron O
> little longer, but I really need to beat this into the ground just to
> make sure I grok it.
>
> As I understand it, the fusion in question did not require any
> 2B were, in Dr. Miller's words, "scotch-taped" end-to-end, thus
> preserving an inactive centromere and degraded telomeres in the middle
> meaningful to labeling this a Robertsonian fusion. 锟絀nstead, the only
> characteristic feature that makes this a Robertsonian fusion is that
> it preserved all/most of the important genetic information.
> Presumably, this is because there are other translocation mechanisms
> that cause significant destruction/duplication/loss of genetic
> information, and microbiologists find it useful to classify the
> am at a loss to understand why the cites seem to focus on just about
> about Robersonian translocations even mention it. 锟絍ery confusing.
>
> Is there a name for the class of translocation mechanisms that are non-
The fusion event that resulted in human chromosome 2 is not considered
to be a Robertsonian centric fusion event. It is an end to end fusion
event, that as far as I know, does not have a special name.
Robertsonian fusion events usually result in loss of genetic
information on one side of the centromere. They can probably be end
to end fusion events for acrocentric chromosomes (chromosomes with the
centromers at one end of the chormosome). Often light microscope
cytology could not determine if both centromeres were on the fused
chromosome or not if both centromeres were near the ends of their
respective chromosomes. In the case of the common chromosome 21
Robertsonian fusion events part of chromosome 21 is usually lost when
it fuses with another chromosome.
Ron Okimoto
hersheyh
In this particular case, the two chromosomes did not result from a
centromeric fusion with the loss of telomeres from the "small" arm of
each chromosome. Rather, it resulted from a fusion of telomere
regions of the two "small" arms, resulting in telomeric sequences
between two centromeres separated by a small distance including these
telomere sequences. Because the two centromeres were near one
another, they acted (most of the time) as if they were one centromere,
with little loss of fertility. One of the two centromeres eventually
became inactive (selection would favor this) as did the telomere
sequences (again, there would be selective pressure to favor this, as
undegraded telomere sequences can produce fragile sequences.
IOW, rather than a true Robertsonian, the human #2 probably started
off as a pseudodicentric chromosome, which is not uncommon for
acrocentric fusions, although often with loss of information from the
small arms.
http://en.wikipedia.org/wiki/Dicentric_chromosome
> If I understand you, none of this is
> meaningful to labeling this a Robertsonian fusion. �Instead, the only
> characteristic feature that makes this a Robertsonian fusion is that
> it preserved all/most of the important genetic information.
All *balanced* translocations preserve all/most of the genetic
information in the genome. Robertsonian translocations have greater
fertility than other types of balanced translocations because
quadravalents do not form during mitosis. Translocations that produce
acentric and dicentric chromosomes, however, almost always lose and
gain significant amounts of genetic information. The acentric
fragments get lost because of the absence of a centromere and the
dicentric ones break often during mitosis and meiosis.
> Presumably, this is because there are other translocation mechanisms
> that cause significant destruction/duplication/loss of genetic
> information, and microbiologists
Geneticists, not microbiologists. Mitosis and the type of chromosome
abnormality described here is eucaryotic.
> find it useful to classify the
> different mechanisms by that single feature. �Is that right? �
Translocation refers to *all* transfers of chromosome fragments
between different chromosomes. Reciprocal translocations involve
exchanges that do not involve loss of much information. Robertsonian
fusions and fissions (or events like that that produced #2 which are
close to being Robertsonian) are a subset of reciprocal translocations
that are relatively common in nature (because they are less likely to
be lethal). Needless to say, they were first described by W. R. B.
Robertson in 1916 (in grasshoppers).
The other common type of genomically balanced chromosome rearrangement
seen in evolution involves inversions. There are two types:
Paracentric (inversions that do not include the centromere) and
pericentric (inversions that include the centromere). Like
translocations, there is *reduced* fertility in inversion
heterozygotes, but the reduction is affected by the size of the
inversion since problems only occur when there is crossing-over within
the inverted region. The amount of reduced fertility is often less
than would be expected on theory.
It is, in fact, possible to identify patches of chromosomes that have
their genes in the same order in even organisms as distantly related
as mice and humans. We have most of the same genes, and, within those
patches, the same genes in the same order. Despite a large difference
in the number and size of chromosomes, it is clear that what we are
seeing is repeated rearrangements of ancestral sequences rather than
independent invention of a new genome.
> If so, I
> am at a loss to understand why the cites seem to focus on just about
> everything else but this end-result. �In fact, nothing that I read
> about Robersonian translocations even mention it. �Very confusing.
>
> Is there a name for the class of translocation mechanisms that are non-
> Robertsonian translocations?
Reciprocal translocations that do not produce acentric/dicentric
chromosomes are described in the site I mentioned before:
http://www.vivo.colostate.edu/hbooks/genetics/medgen/chromo/translocations.html
jillery
wrote:
> In message
> <b09a8b58-9b3c-41bd-b021-c882f5f2b...@19g2000vbq.googlegroups.com>,
> jillery <69jpi...@gmail.com> writes
>
> <snip>
>
> >However, according to the Wikipedia cite given by Hersheyh, this type
> >breaks and re-joins right at the centromere. �So, if I understand
> >Wikipedia's description, this would NOT preserve a secondary
> >centromere nor telomere-like sections in the middle of the newly
> >translocated chromosome.
>
> The fusion point in Robersonian translocation only has to be *near* the
> two centromeres, if two centromeres are close together then they act as
> a single one.
>
> I did a college project on this if you are interested:
>
> http://www.sapientfridge.org/chromosome_count
>
> Have you seen the Ken Miller video discussing chromosome fusion?
>
> http://www.youtube.com/watch?v=zi8FfMBYCkk
> --
> Grok:http://spam.abuse.net�http://www.cauce.org* nuke a spammer �*
> Find:http://www.samspade.orghttp://www.netdemon.net�* � �today � �*
By following everybody's suggested links, I believe I can distill a
working consensus of the relevant features of a Robertsonian
Translocation:
1. It must involve two non-homologous acrocentric chromosomes. By non-
homologous, I mean the chromomsomes come from separate pairs. By
acrocentric, I mean the centromere asymetrically divides the
chromosome into a long arm and a short arm.
2. The two long arms must break off and join together at or near the
centromere. The fate of the short arms is excluded from the
definition. The presence or absence of a duplicate centromere is
excluded from the definition.
3. A direct consequence to the cell is a reduction by one of the
number of chromosomes in the cell.
So, if the above is correct, and if I understand Dr. Miller's
analysis, a contrasting feature is the fusion of the complete
ancestral ape chromosomes 2a and 2b, as evidenced by the presence of
degraded telomeres and an inactive centromere in the middle of human
chromosome two. When someone describes this as a Robertsonian
Translocation, perhaps they are focusing on the consequence only,
without regarding the equally relevant cause.
Thank you, Ron O, for correctly identifying the relevant
specialization is cytology. After all, when I obsess about semantics,
I should at least use the right words.
Thank you all for your time. I grok.