1 00:00:04,371 --> 00:00:08,755 Identical twins are genetic carbon copies 2 00:00:09,094 --> 00:00:14,248 The DNA sequences of their genes are exactly the same 3 00:00:14,248 --> 00:00:19,453 Yet physically, identical twins become increasingly different over time... 4 00:00:19,453 --> 00:00:21,573 why is this so? 5 00:00:27,327 --> 00:00:29,029 Genes provide the instructions 6 00:00:29,029 --> 00:00:33,517 for the development and maintenance of the body. 7 00:00:37,271 --> 00:00:40,174 Yet a second set of instructions known as the epigenome 8 00:00:40,174 --> 00:00:42,543 interacts with DNA 9 00:00:42,543 --> 00:00:44,745 to activate or suppress 10 00:00:44,745 --> 00:00:48,348 the expression of particular genes. 11 00:00:48,348 --> 00:00:49,507 Certain chemicals 12 00:00:49,507 --> 00:00:52,519 known as epigenetic tags 13 00:00:52,519 --> 00:00:54,505 turn genes OFF 14 00:00:58,859 --> 00:01:00,227 or ON 15 00:01:00,227 --> 00:01:02,993 without changing the underlying genetic code. 16 00:01:02,993 --> 00:01:03,493 17 00:01:04,064 --> 00:01:06,259 During fertilization 18 00:01:06,259 --> 00:01:09,036 one set of chromosomes from Dad 19 00:01:09,036 --> 00:01:11,505 and one set of chromosomes from Mum 20 00:01:11,505 --> 00:01:13,445 are combined to form an embryo. 21 00:01:22,583 --> 00:01:24,651 Epigenetic tags are erased 22 00:01:24,651 --> 00:01:26,753 from Mum and Dad's chromosomes 23 00:01:26,753 --> 00:01:31,058 during the first days after fertilization. 24 00:01:31,058 --> 00:01:34,394 However, on some genes the tags remain 25 00:01:34,394 --> 00:01:38,132 these are known as IMPRINTED genes. 26 00:01:38,532 --> 00:01:40,434 Identical twins result 27 00:01:40,434 --> 00:01:43,814 when a single embryo splits in two. 28 00:01:48,460 --> 00:01:50,844 Each embryo has the same genome 29 00:01:50,844 --> 00:01:53,557 and the same epigenome. 30 00:01:58,819 --> 00:02:00,721 As the embryos develop 31 00:02:00,721 --> 00:02:03,600 cells differentiate into distinct types 32 00:02:03,600 --> 00:02:07,094 such as bone, muscle and skin. 33 00:02:07,094 --> 00:02:10,063 As each type specialises epigenetic tags 34 00:02:10,063 --> 00:02:13,500 activate and silence specific genes 35 00:02:13,500 --> 00:02:17,137 leading the cells to specialise further. 36 00:02:17,137 --> 00:02:19,473 Each cell type gradually takes on 37 00:02:19,473 --> 00:02:23,486 a unique epigenetic profile. 38 00:02:25,194 --> 00:02:27,681 Because the twins share the same environment 39 00:02:27,681 --> 00:02:31,018 their epigenomes are very similar at birth 40 00:02:31,018 --> 00:02:34,521 and through the early years of life. 41 00:02:34,521 --> 00:02:35,923 As the twins age, 42 00:02:35,923 --> 00:02:39,193 their environments begins to differ. 43 00:02:44,331 --> 00:02:47,000 Their genomes remain the same 44 00:02:47,000 --> 00:02:49,343 but signals from the environment 45 00:02:49,343 --> 00:02:51,171 act on the twins epigenomes 46 00:02:51,171 --> 00:02:53,140 to activate and silence 47 00:02:53,140 --> 00:02:56,399 different genes. 48 00:03:06,153 --> 00:03:07,721 Diet, for example 49 00:03:07,721 --> 00:03:10,023 is an environmental factor 50 00:03:10,023 --> 00:03:12,636 that can have an effect 51 00:03:12,667 --> 00:03:14,480 on the epigenome. 52 00:03:24,034 --> 00:03:26,139 Differences in physical activity 53 00:03:26,139 --> 00:03:28,942 can also cause epigenetic differences 54 00:03:28,942 --> 00:03:30,967 between the twins. 55 00:03:48,629 --> 00:03:50,597 Exposure to toxins 56 00:03:50,597 --> 00:03:53,580 can influence the epigenome. 57 00:04:04,811 --> 00:04:07,223 Stress also plays a role 58 00:04:07,223 --> 00:04:09,746 in shaping the epigenome. 59 00:04:20,961 --> 00:04:22,529 By the time the twins 60 00:04:22,529 --> 00:04:24,531 are well into adulhood 61 00:04:24,531 --> 00:04:28,268 their epigenomes, thus gene expression patterns, 62 00:04:28,268 --> 00:04:30,103 are very different... 63 00:04:30,103 --> 00:04:32,484 ... making each twin unique!