WEBVTT

00:00:04.371 --> 00:00:08.755
Identical twins are genetic carbon copies

00:00:09.094 --> 00:00:14.248
The DNA sequences of their genes are exactly the same

00:00:14.248 --> 00:00:19.453
Yet physically, identical twins become increasingly different over time...

00:00:19.453 --> 00:00:21.573
why is this so?

00:00:27.327 --> 00:00:29.029
Genes provide the instructions

00:00:29.029 --> 00:00:33.517
for the development and maintenance of the body.

00:00:37.271 --> 00:00:40.174
Yet a second set of instructions known as the epigenome

00:00:40.174 --> 00:00:42.543
interacts with DNA

00:00:42.543 --> 00:00:44.745
to activate or suppress

00:00:44.745 --> 00:00:48.348
the expression of particular genes.

00:00:48.348 --> 00:00:49.507
Certain chemicals

00:00:49.507 --> 00:00:52.519
known as <i>epigenetic tags</i>

00:00:52.519 --> 00:00:54.505
turn genes OFF

00:00:58.859 --> 00:01:00.227
or ON

00:01:00.227 --> 00:01:02.993
without changing the underlying genetic code.

00:01:02.993 --> 00:01:03.493


00:01:04.064 --> 00:01:06.259
During fertilization

00:01:06.259 --> 00:01:09.036
one set of chromosomes from Dad

00:01:09.036 --> 00:01:11.505
and one set of chromosomes from Mum

00:01:11.505 --> 00:01:13.445
are combined to form an embryo.

00:01:22.583 --> 00:01:24.651
Epigenetic tags are erased

00:01:24.651 --> 00:01:26.753
from Mum and Dad's chromosomes

00:01:26.753 --> 00:01:31.058
during the first days after fertilization.

00:01:31.058 --> 00:01:34.394
However, on some genes the tags remain

00:01:34.394 --> 00:01:38.132
these are known as IMPRINTED genes.

00:01:38.532 --> 00:01:40.434
Identical twins result

00:01:40.434 --> 00:01:43.814
when a single embryo splits in two.

00:01:48.460 --> 00:01:50.844
Each embryo has the same genome

00:01:50.844 --> 00:01:53.557
and the same epigenome.

00:01:58.819 --> 00:02:00.721
As the embryos develop

00:02:00.721 --> 00:02:03.600
cells differentiate into distinct types

00:02:03.600 --> 00:02:07.094
such as bone, muscle and skin.

00:02:07.094 --> 00:02:10.063
As each type specialises epigenetic tags

00:02:10.063 --> 00:02:13.500
activate and silence specific genes

00:02:13.500 --> 00:02:17.137
leading the cells to specialise further.

00:02:17.137 --> 00:02:19.473
Each cell type gradually takes on

00:02:19.473 --> 00:02:23.486
a unique epigenetic profile.

00:02:25.194 --> 00:02:27.681
Because the twins share the same environment

00:02:27.681 --> 00:02:31.018
their epigenomes are very similar at birth

00:02:31.018 --> 00:02:34.521
and through the early years of life.

00:02:34.521 --> 00:02:35.923
As the twins age,

00:02:35.923 --> 00:02:39.193
their environments begins to differ.

00:02:44.331 --> 00:02:47.000
Their genomes remain the same

00:02:47.000 --> 00:02:49.343
but signals from the environment

00:02:49.343 --> 00:02:51.171
act on the twins epigenomes

00:02:51.171 --> 00:02:53.140
to activate and silence

00:02:53.140 --> 00:02:56.399
different genes.

00:03:06.153 --> 00:03:07.721
Diet, for example

00:03:07.721 --> 00:03:10.023
is an environmental factor

00:03:10.023 --> 00:03:12.636
that can have an effect

00:03:12.667 --> 00:03:14.480
on the epigenome.

00:03:24.034 --> 00:03:26.139
Differences in physical activity

00:03:26.139 --> 00:03:28.942
can also cause epigenetic differences

00:03:28.942 --> 00:03:30.967
between the twins.

00:03:48.629 --> 00:03:50.597
Exposure to toxins

00:03:50.597 --> 00:03:53.580
can influence the epigenome.

00:04:04.811 --> 00:04:07.223
Stress also plays a role

00:04:07.223 --> 00:04:09.746
in shaping the epigenome.

00:04:20.961 --> 00:04:22.529
By the time the twins

00:04:22.529 --> 00:04:24.531
are well into adulhood

00:04:24.531 --> 00:04:28.268
their epigenomes, thus gene expression patterns,

00:04:28.268 --> 00:04:30.103
are very different...

00:04:30.103 --> 00:04:32.484
... making each twin unique!