mtDNA and other DNA

["v.h.villarreal" <raices_regias@...>]

Josie,

At the risk of oversimplifying this whole thing, I'm posting a few
essential concepts about DNA. Please excuse me if I sound patronizing.

We all have heard that DNA is the blueprint of life.  And it is!  Or
to be more exact, each organism's DNA is its own blueprint of life. 
Just think how it is possible to clone an animal (make an exact
genetic replica) from the DNA found in the nucleus of a single cell. 
And notice that in theory a clone could be made from practically every
cell in the body, as most cells have a copy of all their DNA, their
full genome.

Although each cell with a nucleus has their full genome it only has a
small section of it activated, the specific section that contains the
instructions that each cell must carry out.  Imagine a huge theater
performance with many, many actors, where every actor has a copy of
the whole script but some actors only have to read a few lines or a
few pages at most.  So it is with cells; each one has differentiated
to play a specific role in the body.  

We could say, for practical purposes, that for each cell's role or
function a set of instructions are written and stored in a gene. 
There are many things that cells do and therefore there are many
genes.  These genes in turn are packaged into chromosomes of which
human cells have 46.  These cells with 46 chromosomes are called
diploid cells. 

Since we're not clones, half of those 46 chromosomes come from each of
our progenitors.  23 from the father and 23 from the mother to make 23
pairs.  Every chromosome is of equal size to its pair except in males
where the 23rd pair or the XY is an uneven pair.  There's a section in
the Y chromosome that is unmatched in the X chromosome and it is
called a non-recombining section or segment.  

Like I said above, every cell in the body has two sets of chromosomes
or a total of 46, but there's an exception: reproductive cells. 
Reproductive cells only carry 23 single chromosomes or half the number
of other cells and they are called haploid cells.  But each of these
chromosomes in reproductive cells are a combination of the
corresponding chromosome pair in diploid cells.

Since Y chromosome has an unmatched segment that doesn't recombine
with the X chromosome, this segment passes almost unaltered from
generation to generation (but only in the male line or patrilineal
ancestry).  These non recombining sections of the Y chromosome are
what genetic labs analyze in the now famous YDNA tests.

The other 22 pairs of chromosomes or the autosomal DNA from both
progenitors do recombine and shuffle their genes before creating a
single set of chromosomes in reproductive cells.  In other words, out
of our full genetic material our reproductive cells can only carry on
and pass to our children half of it. But in contrast to what we
receive: half from our mother and half from our father, we do not
necessarily pass on to our progeny the same proportions we receive
from each.  And furthermore, each reproductive cell can have a
different proportion or combination of genetic material from our
progenitors, otherwise all our children would be identical. Makes
sense?  That is why it is so difficult to trace the genetic lineage of
any other of our ancestors besides the Y lineage.  Yet, we can tell
when a child gets a trait from his/her grandpa or her/his grandma, etc.

Mitochondrial DNA is a whole different story.  To start with, some
people think that mtDNA is part of our chromosomes but it is not. Like
chromosomal DNA, mtDNA is also in the cells but it is found only in
the protoplasm not in the nucleus.  Mitochondrial DNA is not strictly
human.  Biologists believe that millions of years ago human cells and
mitochondrial cells created a symbiosis or mutually beneficial
association.  Human cells provide the nutrients mitochondria need and
these in turn provide the energy the human cells need to carry out all
their functions.  None can live without the other.

I started out saying that DNA is the blueprint of life and I was
referring to chromosomal DNA from both parents.  But to build a human
being, like building a house or anything else, you not only need a
blueprint but also the raw materials and a lot of energy.  These raw
materials and energy always come from the mother, because only females
get pregnant and give birth.  In fecundation and conception the male
only provides one cell; all the mitochondrial "machinery" is provided
by the female.

So you can see, mtDNA is strictly inherited thru the maternal line. 
And when a mutation ocurrs in mtDNA all offspring inherit the same
distinctive mutation.  But mutations in mtDNA are much more rarer than
mutations in YDNA, therefore a single mismatch in mtDNA can mean a
shared ancestor many, many generations ago, beyond the conventional
genealogical framework.  This is our deeper ancestry link!

Saludos,

Victor


--- In ranchos@yahoogroups.com, "Josie T. Trevino" <Josiett3@s...> wrote:
>
> Hi!  
>  
> Someone once explained to me that the mtDNA results were, in some
cases, the
> result of Spaniards marrying the "local girls" ........is this what
you are
> referring to, Arturo?    I do not understand these numbers.....I am just
> trying to make heads or tails out of it......and to justify the $$$$
I spent
> to get these numbers!  :-)   
> Seriously, I don't mind spending the money if it will give me some
clues as
> to my ancestors, the migration pattern they followed, etc.  I have only
> researched about three or four generations on my Dad's maternal line and
> three or four on my Mom's maternal and paternal lines so I have a
long way
> to go.  I have been focusing on my Dad's paternal Trevino lineage and go
> "way back" to about the 1400s in Spain.  
>  
> I really enjoy reading the messages everyone is sending regarding
Y-DNA and
> mtDNA because I am slowing learning a little here and there.  What I
really
> like is finding out that I have matches to a few in this group, Linda,
> Victor, Arturo and an exact match with Ed!  Even though I have never
met any
> of you in person  I do feel a connection!  The only grandparent I
knew was
> my Mom's father and he died when I was 7 years old.  My other
grandparents
> died before I was born.  I only have one brother so you can see that my
> familia is very small.......maybe that is why I am always looking
for "mi
> gente!"   I have met several primos and primas (live ones!) through
> genealogy and it feels good when we share information and discover
that our
> great-grandparents were brothers/sisters and here we are digging up
their
> information!  
>  
> It would be wonderful if a group were started concentrating on mtDNA.
>  
> Josie in San Antonio
>