History of epigenetics
- The term epigenetics in its contemporary usage appeared
in the 1990s
- It was used with slightly different meanings
- In 2008 a special concept was reached in epigenetic
inheritance Cold Spring Harbor Laboratory.
- British embryologist Conrad Waddington coined the term
epigenetics in 1942 by analogy to the term phenogenetics
coined by Valentine Hecker.
History of epigenetics
- When Waddington coined the term the physical nature of genes
and their role in heredity was unknown Instead he used it as a
conceptual model for how genetic components might interact with
the environment to produce a phenotype.
- Nowadays Waddington's idea of an 'epigenetic landscape' is
firmly established within the context of the system dynamics
approach to the study of cell fate.
What is Epigenetics?
A study that investigates alterations to gene expression or phenotype in
an organism that occur without changing the nucleotide sequence of a
What is Epigenetics?
Epigenetic inheritance explains the manufacture of proteins that
distinguish a particular cell from other cells
-For example: nerve cells make proteins that differ from those made by
other cells in the body although all human body cells have the same
sequence of nucleotides in the DNA molecule.
There is a relationship between Epigenetics and
Gene expression can be altered
1- active gene That is, activating the gene in
order to build proteins through the process of
transcription and translation
2- silent gene Inhibiting the gene and
preventing the transcription and translation
process, so the protein building process does
The DNA will not
The DNA will
1- Histone acetylation and deacetylation:
- Histone acetylation: histone acetyltransferase (HATs) adds acetyl groups (Ac) onto histone tails
which results in a nucleosome opening thus allowing for transcription factors to access DNA and
initiate gene transcription.
- Histone deacetylation: histone deacetylases (HDACs) remove acetyl groups (Ac) from the histone
tails, leading to a closed chromatin structure.
Acetylation removes the positive charge on the
histones thereby decreasing the interaction of the
N termini of histones (Lysine) with the negatively
charged phosphate groups of (DNA).
As a consequence the condensed chromatin is
transformed into a more relaxed structure that is
associated with greater levels of
This relaxation can be reversed by
deacetylation catalyzed by HDAC activity.
Relaxed transcriptionally active DNA is referred to
as euchromatin. More condensed (tightly packed)
DNA is referred to as heterochromatin
Condensation can be brought about by processes
including deacetylation and methylation
2- DNA methylation
It involves adding a methyl group to:
the fifth position of the pyrimidine ring of cytosine, generating 5-methylcytosine (5meC)
the number-6 nitrogen of the purine ring of adenine
NOTE: Methylation usually occurs at the gene promoter and typically inhibits gene
transcription ,this change can be inherited through cell differentiation, and it is related to the
pathogenesis of various diseases
De novo methylation
is a process by which the addition of methyl groups to unmethylated DNA takes
place at specific CpG sites
Normally CpG sites are unmethylated and DNA methylation occurs in these sites
CpG sites are the specific regions of DNA in which cytosine nucleotide is
followed by a guanine nucleotide in the linear sequence of bases along its 5′ → 3′
De novo methylation is catalyzed by two different methylates: DNMT3A and
5-Methylcytosine (5mC) is an important epigenetic modification that serves
as a marker for gene expression X-chromosome inactivation, and
transposon silencing, among other developmental processes
• Are small noncoding RNAs approximately 18-25 nucleotides in length now recognized as one of the
major regulatory gene families in eukaryotes
• MicroRNAs are transcribed by RNA polymerases II and III
• The regulatory functions of microRNAs are accomplished through the RNA-induced silencing
• MicroRNA assembles into RISC, activating the complex to target messenger RNA (mRNA)
specified by the microRNA
• This will either lead to degradation of mRNA strand
Study the effect of methyl group on rats
• Two groups of pregnant rat mothers were brought, both carrying embryos
with homozygous genotype (AA) Then a different diet was established for
both mother mice:
The first group: the diet of the mothers in this group contained folic acid, which
is a source of a methyl group so the resulting mice had brown fur and were not
The second group: the diet was devoid of folic acid so the
resulting mice had yellow fur, were obese, and were infected
with other diseases.
Conclusion: The scientists explained this (experiment) by saying
that the methyl group contained in the diet of the mice of the first
group represents a factor of epigenetic inheritance.
Epigenetic explains the difference in traits between identical
twins as follows:
One of the twins may suffer from certain diseases that the other does not
One may become a sporter and the other a painter
They may differ in personal characteristics, such as one being shy,
unlike the other
It is true that they carry the same nucleotide arrangement in the
DNA molecule, but the gene expression of each of them differs
1- They may differ in diet
2- They may differ in physical and social activities
3- They may differ in medical care
4- There is an association of epigenetic factors with one of them that
differ from those associated with the other at any stage of their lives
Some studies have shown that the older a person gets, the more
differences appear in epigenetic factors between identical twins.
Epigenetic inheritance has created a scientific precedent with regard
to explaining the causes of cancer
Epigenetic factors may affect tumor suppressor genes, causing them to
become inactive (silent), which leads to the spread of tumors.
• Epigenetics is an inheritable phenomenon that affects gene expression without base pair
• Epigenetic phenomena include DNA methylation, histone modifications, and chromatin
• Chromatin is quite dynamic and is much more than a neutral system for packaging and
condensing genomic DNA, it is a critical player in controlling the accessibility of DNA for
• Modifications of chromatin structure can give rise to a variety of epigenetic effects.
• Due to its reversible character, epigenetics is now considered an attractive field of nutritional
• During our lifetime, nutrients can modify physiologic and pathologic processes through epigenetic
mechanisms that are critical for gene expression (summarized in Table 1).
• Modulation of these processes through diet or specific nutrients may prevent diseases and maintain health
However, it is very hard to delineate the precise effect of nutrients or bioactive food components on each
epigenetic modulation and their associations with physiologic and pathologic processes in our body,
because the nutrients also interact with genes, other nutrients, and other lifestyle factors.