Identical twins are genetic carbon copies
The DNA sequences of their genes are exactly the same
Yet physically, identical twins become increasingly different over time...
why is this so?
Genes provide the instructions
for the development and maintenance of the body.
Yet a second set of instructions known as the epigenome
interacts with DNA
to activate or suppress
the expression of particular genes.
Certain chemicals
known as epigenetic tags
turn genes OFF
or ON
without changing the underlying genetic code.
During fertilization
one set of chromosomes from Dad
and one set of chromosomes from Mum
are combined to form an embryo.
Epigenetic tags are erased
from Mum and Dad's chromosomes
during the first days after fertilization.
However, on some genes the tags remain
these are known as IMPRINTED genes.
Identical twins result
when a single embryo splits in two.
Each embryo has the same genome
and the same epigenome.
As the embryos develop
cells differentiate into distinct types
such as bone, muscle and skin.
As each type specialises epigenetic tags
activate and silence specific genes
leading the cells to specialise further.
Each cell type gradually takes on
a unique epigenetic profile.
Because the twins share the same environment
their epigenomes are very similar at birth
and through the early years of life.
As the twins age,
their environments begins to differ.
Their genomes remain the same
but signals from the environment
act on the twins epigenomes
to activate and silence
different genes.
Diet, for example
is an environmental factor
that can have an effect
on the epigenome.
Differences in physical activity
can also cause epigenetic differences
between the twins.
Exposure to toxins
can influence the epigenome.
Stress also plays a role
in shaping the epigenome.
By the time the twins
are well into adulhood
their epigenomes, thus gene expression patterns,
are very different...
... making each twin unique!