You have asked a marvelous and wonderful
question! Scientists called "Developmental
Biologists" study this question. And guess, what,
my area of research is exactly this question! In
fact, the whole reason I became a scientist was
because I was fascinated by the very question you
I am a professor at UCSB and I run a research
lab in addition to teaching classes. My favorite
class to teach is called Developmental Biology!
As you know, when a sperm fertilizes an egg,
that starts the "development" of a new organism.
Now, that newly fertilized egg, called a "ZYGOTE,"
has to divide to create lots of smaller cells. In
some organisms, these new cells are "instructed"
as to what they will become later in development;
that is, what will be their "fate?" Brain? Heart?
In other kinds of organisms, the instruction or
"fate specification" does not occur until much
later, once many thousand of cells have been
formed from that one big zygote. Regardless of
when this specification occurs, the instructions
are the key thing. How does a cell "know" what to
become and at what time and in what place in the
developing embryo? That is another way to state
your question. No one really knows the complete
answer to this, but we do have ideas about it
based on studying "model organisms" that are easy
to see as they develop. These model organisms,
like fruit flies, frogs, mice, nematode worms and
sea urchins, also can be used in genetic and
molecular studies to ask the question of "how."
Let's start with this: every cell in your body
has the exact same DNA in its nucleus. That is
your "genome" -- all of the genes are the same in
each cell. Yet, each kind of cell is different. A
brain cell is very different from a heart cell.
Why? Well, as you probably know, a gene (DNA) is
"expressed" in a cell in a form called mRNA. This
mRNA is then "translated" to make a protein. It is
the protein that does the "work" and actually
functions in a cell. So, if a heart cell has a
different function than a brain cell, what do you
predict about the proteins in the two cells? Will
they be the same or different? Try to answer the
question before you read further-- I bet you can
figure it out.
Right -- they will be different! So, the DNA
(genome) is the same, but WHICH GENES ARE
EXPRESSED AND MAKE PROTEIN IS DIFFERENT IN
DIFFERENT KINDS OF CELLS. Scientists call this
"differential gene expression" and understanding
how it works and is regulated is a big area of
research. Not only is it important to understand
from the point of view of development (how an egg
becomes a new organism) but also in terms of human
disease -- in many diseases, like cancer, the
pattern of gene expression is changed in an
affected cell. Identifying the "molecular
regulators" of gene expression is a very big goal
for many scientists.
How a new organism develops specific cell types
is very complex, because it happens in a very
regulated way in space and time (in other words,
it wouldn't be very good if the heart formed
before the chest cavity was sculpted or if the
heart formed in the wrong place). We have a long
way to go before we really understand how this
happens. If you can, visit the educational link
from the home page of the Society for
You can learn much more about this and you can
also watch movies of eggs turning into new
organisms. I encourage you to keep asking
questions like this -- maybe some day you will be
the one who figures it out!
I'm not a biologist but I suspect that no one
has all the answers.Basically, what is known is
that in the egg there is an embryo which is made
up of what are known as stem cells. Stem cells
are immature cells that have the ability to
develop into many different kinds of cells like
brain, bone, blood, or liver cells for example.
The DNA inside the cells is a code that determines
how the embryo will develop.
A lot of scientists are working on figuring
out how stem cells are told what kind of cell to
turn in to. The hope is that we could use stem
cells to create a new liver or other organs for
people who need them. Today, people sometimes get
a stem cell transplant when they have a problem in
their blood producing bone marrow such as
leukemia, lymphoma, or certain types of anemia.
Somehow, the new stem cells know to go to the
right place and start producing blood cells.
This is a complicated question that people are
still trying to figure out,but here's the general
idea. We all started as a fertilized egg that
divided to make more of itself. As you noted,
that's not enough to make a person. The cells
have to specialize and organize to make a baby (or
a corn plant, or a salmon). Each cell has the
full set of recipes for making the whole
individual, the DNA. Some cells start using one
part of the DNA,while other cells use another
part. This means the cells specialize as muscle
or liver cells for example. They also have to
organize into eyes, muscles and liver.
Here's how the brain and eyes get started. We
were all basically a flat disc of cells when we
were about 2 weeks old. Picture a groove forming
at the top of the disk. This could form by the
cells in the groove dividing faster than the cells
on either side of it. Now picture the sides of
the groove gradually coming together to form a
tube. This is called the neural tube and it will
become the brain and spinal cord. This all happens
in the first few weeks after fertilization. The
eyes form from out-pockets of the brain.
In general, a lot of things form as tubes and
fold up or get thicker. The main things that seem
to shape us are differences in the actual cell
types specialization) and differences in how fast
cells divide, which causes shape changes. Cell
death is important too. We started off with
paddle-shaped hands and feet, but some cells were
programed to die, leaving us with separated
fingers and toes. Some cells have to move around
to find their targets. A nerve cell may have to
grow from the spinal cord to a finger muscle.
The fields of "embryology" and "developmental
biology" study questions like these. You might
want to explore a career in one of these fields.
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