Re: Mutations

On Thu, 26 Apr 2007 23:06:01 GMT, John Harshman
<jharshman.diespamdie@xxxxxxxxxxx> wrote:

Richard Harter wrote:

On 26 Apr 2007 11:07:51 -0700, Throwback <throwback9@xxxxxxxxx> wrote:


http://www.talkorigins.org/faqs/mutations.html#types

"Types of mutations:

Point mutations
The most common type of copying error is the point mutation. In this
form of mutation the nucleotide at a site is replaced by a different
nucleotide. When people talk about mutation rates they are usually
talking about rates of point mutations.

Effects of point mutations: Point mutations in junk DNA are common but
have no effect. Sometimes point mutations in regulatory regions have
no effect and sometimes they alter the expression of some genes.

Additions and deletions
During copying a segment of DNA may be deleted or a new segment may be
inserted. Typically this happens as a result of chromosome breakage or
realignment. (See below.) Additions and deletions can also be produced
by certain types of horizontal transfer.

Effects of additions and deletions: If the length of the new or
deleted segment is not a multiple of three the translation will be
garbled after the point at which the insertion/deletion occurred
because the frame reading is now misaligned. This is known as a
frameshift mutation. In some genes there are segments that may be
duplicated as a block. This is known as tandem duplication.

Chromosomal duplication
Sometimes one or more chromosomes are duplicated during reproduction;
the offspring get extra copies of those chromosomes.

Effects of chromosomal duplication: Duplicating only one chromosome is
generally disadvantageous; an example in human beings is Down's
syndrome. Having multiple copies of all of the chromosomes is known as
polyploidy. Polyploidy is rare in fungi and animals (although it does
occur) and is common in plants. It has been estimated that 20-50% of
all plant species arise as the result of polyploidy.

Gene duplication is very common; it is important because it provides a
way to evolve new capabilities while retaining the old capabilities.
All intermediate stages can be found in nature, from a single gene
with alternate alleles to nearly identical duplicated genes with
slightly different functional alleles to gene families of
evolutionarily related genes with different functionalities.

Chromosomal breakage and realignment
During reproduction a chromosome may break into two pieces or two
chromosomes may be joined together. A section may be moved from one
part of the chromosome to another or may be flipped in orientation
(inverted). This is the mechanism by which deletions, duplications and
transpositions my occur.

Effects of chromosomal breakage and realignment: Quite often these
types of changes do not affect the viability of the organism (the
genes are still there; they're just in different places) but, in
sexually reproducing species, they may make it less likely for the
organism to produce viable, fertile offspring.

Retroviruses
Certain viruses have the ability to insert a copy of themselves into
the genome of a host. The chemical that make this possible (reverse
transcriptase) is widely used in genetic engineering.

Effects of retroviruses: Usually this is a way for the virus to get
the host to do the work of reproducing the virus. Sometimes, however,
the inserted gene mutates and becomes a permanent part of the host
organism's genome. Depending on the position of the viral DNA in the
host genome, genes may be disrupted or their expression altered. When
insertions occur in the germline of multicellular organisms, they can
be passed on vertically.

Plasmids
Plasmids are little pieces of circular DNA that are passed from
bacterium to bacterium. Plasmids can be transferred across species
lines.

Effects of plasmid transfer: Plasmid transfer is an important way of
spreading useful genes such as those which confer resistance to
antibiotics. Plasmid transfer is an example of horizontal transfer.

Bacterial DNA exchange
Bacteria can exchange DNA directly. They often do this in response to
environmental stress.

Effects of bacterial DNA exchange: Exchange is often fatal to one or
both of the bacteria involved. Sometimes, however, one or both of the
partners acquires genes which are essential for the current
environment.

Higher level transfer
Some parasites can pick up genetic material from one organism and
carry it to the next. This has been observed in fruit flies in the
wild.

Effects of higher level transfer: When this happens novel alleles can
spread much more rapidly through a species than they would for
ordinary gene flow.

Symbiotic transfer
When two organisms exist in a close symbiotic relationship one may
"steal" genes from the other. The most notable example of this are
mitochondria. In most organisms with mitochondria most of the original
mitochondrial genes have moved from the mitochondria to the nuclear
genome.

Effects of symbiotic transfer: A major effect is that the symbiotic
relationship changes from being optional to be obligatory.

Transposons
Transposons are genes that can move from one place in the genome to
another.

Effects of transposons: Depending on the position of insertion,
transposons can disrupt or alter the expression of host genes. In some
species most mutations due to transposon insertion. For example, in
Drosophila, 50-85% of mutations are due to transposon insertions."

Which one of the above would explain how large blocks
of genes of varying sizes transposed themselves throughout
a DNA sequence?


What an odd way to phrase your question. I suppose transposons might be
said to transpose themselves, so I suppose that would count as an
answer. More importantly, there are several ways in which large scale
change can occur, e.g., chromosomal duplication, horizontal tranfer via
retroviruses, gene duplication, and, of course, transposons.

By the way, you really should thank the author of the material you
quoted - it's quite clear and very well written. Perhaps if you read
more by him your mind would be greatly improved.

I believe the exact answer to his question is type #5 above
("Chromosomal breakage and realignment"): "During reproduction a
chromosome may break into two pieces or two chromosomes may be joined
together. A section may be moved from one part of the chromosome to
another or may be flipped in orientation (inverted). This is the
mechanism by which deletions, duplications and transpositions my occur."

Now what does the fact that he posts the answer and then asks the
question tell us about his reading skills?

His what? He has what? I don't know whether the quoted passage answers
his question but it certainly says something about my proofreading.



.

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