Lecture 5: Epigenetics Flashcards
What does Epi mean?
Epi : from Greek – “on” or “upon”
What is Epigenetics?
Study of REVERSIBLE ‘HERITABLE’ changes in gene function that occur WITHOUT A CHANGE in the sequence of DNA.
i.e. “upon” or “in addition to” the genetic code
Genetic code in each cell is the same – EPIGENETIC CODE IS TISSUE AND CELL SPECIFIC
Epigenetic: definition by Adrian Bird…without heritability
Adrian Bird proposed a modern definition that
avoids requirement for heritability:
“the structural adaptation of chromosomal
regions so as to register, signal or
perpetuate altered activity states”
define epigenetic - according to lecture
Epigenetics = study of the factors that cause STABLE &
HERITABLE, yet REVERSIBLE, changes in the way genes are expressed without changing their original DNA
sequence
Epigenetic changes are by…
+EXAMPLES
Epigenetic changes are by ADDING or SUBTRACTING various
CHEMICAL TAGS on DNA nucleotides/histones
EXAMPLES
* DNA methylation, histone acetylation/methylation +
OTHERS
Defining Epigenetics:
Persistence of epigenetic marks.
- Alterations that last less than one cell cycle (green asterisk, a) do not
qualify as epigenetic under the definition that strictly requires heritability. - Whereas non-mutational changes that are transmitted from one cell to
its daughters (red asterisk, b) - or
between generations of an organism
(blue asterisk, c) do qualify.
Epigenetic processes: 3
- Histone modification & chromatin remodeling
- DNA methylation
- Non-coding RNA-mediated regulation
Epigenetic processes: examples = 5
1 * Cooling tulip bulbs before planting
2 * Maternal nurture
3 * Calico cats
4 * Honey bees – workers vs queens
5 * Host immunity e.g.
against retrovirus
what is chromosomes?
sizes of DNA TO CHROMOSOME…
- DNA + protein (chromatin) = chromosomes
- DNA DOUBLE HELIX = 2nm
- NUCLEOSME CORE OF ‘8 HISTONE MOLECULES’
- CHROMATOSOME
- Histone H1 = 11nm
- 30nm
- 300nm
- 250nm wide fibre
- 700nm
- chromosome = 1400nm
look at slide 9
DNA sequences are commonly compared to … epigenetics can be viewed similarly to..
DNA sequences are commonly compared to a text of written letters.
Epigenetics can be viewed
similarly to punctuation that can modify the meaning of text.
Cut And Paste.. LOOK at slide 12
- DNA methylation
- Histone Modification
- Small non-coding RNAs
A typical eukaryotic promoter
* Gene transcription: vs Histone proteins
Look at slide 13 image
‘Gene transcription’ in eukaryotes occurs in the context of DNA packaged
into chromatin
‘Histone proteins’ are important components of the “Epigenetic machinery”
that package genomic DNA
The nucleotides of DNA?
VERSIONS OF CYTOSINE?
PURINES = Adenine + Guanosine
Pyrimidines = thymine, cytosine
– Cytosine (C)
— 5-methylcytosine (mC)
—- 5-Hydroxymethylcytosine (hmC)
LOOK AT CHEMICAL IMAGE ON SLIDE 14
What are Histones? What is Histone H1? 4
1 * Nucleosomes arranged as an octamer of histone
proteins with protruding N-terminal ends.
2 – 147 bp of coiled DNA wrapped around the histones.
3 – Two each of the four core histones H2A, H2B, H3 and H4.
4 * Histone H1, the linker protein, is bound to DNA
between nucleosomes.
LOOK AT HISTONE IMAGE SLIDE 5
Histone modifications and the histone code hypothesis: 4
- Modifications of the HISTONE TAILS act as EPIGENETIC MARKERS that control the expression
or replication of chromosomal regions. - The epigenetic marks in the histones are HERITABLE.
- The pattern of histone modifications (the histone code) can determine how
histones behave. - Modifications of the histone tails act as epigenetic marks that control the expression of chromosomal regions (controlled by transcription factors).
Many histone tags work together to control histones. These include: 4
- Acetyl
- Phosphate
3.Methyl - Ubiquitin
Active & repressive marks
- Different amino acids
constituting HISTONE TAILS are represented along with the DIFFERENT COVALENT MODIFICATION SPECIFIC OF EACH RESIDUE.
2.ACTIVE MARKS are represented in
the SUPPER SECTION, and
REPRESSIVE MARKS in the LOWER SECTION
LOOK AT SLIDE 17 IMAGE
Understanding Histone acetylation..8
- DNA is NEGATIVELY CHARGED , whilst HISTONES ARE POSITIVELY CHARGED
- Acetylation of histones OCCUR IN LYSINE RESIDUES OF HISTONE TAILS
- Acetylation NEUTRALISES THE POSITIVE CHARGE AND DECREASES THEIR AFFINITY FOR DNA.
- DNA IS LESS TIGHTLY WOUND AND PERMITS TRANSCRIPTION.
- “Acetylated lysine residues” = transcriptional ACTIVATION (gene
expression) - “Deacetylated lysine residues” = transcription REPRESSION (gene
silencing) - Histone acetylase (HAT) & histone deacetylase (HDAC) ENZYMES ADD/REMOVE ACETYL GROUPS
- HISTONES NEAR ACTIVE GENE ARE HYPERACETYLATED.
Histone de/acetylation
hdac vs hat
HDAC = for Deacetylation
HAT = ACETYLATION
LOOK AT IMAGE ON SLIDE 19
Understanding DNA Methylation…
1 * A well studied example of an EPIGENETIC MECHANISM involved
in GENE REGULATION (AND OTHER PROCESSES)
2 *OCCURS IN BACTERIA (RESTRICTION ENZYME SITES), …NONE IN YEAST OR WORMS
3 * FOR VERTEBRATES=
- 5-methyl C residues within the CpG dinucleotide common (70-80% methylated CpG in mammals)
4 * CpG, CpNpG and CpHpH (H=A, T or C) methylation of C
COMM IN PLANTS (RARE IN ANIMALS)
- 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine appear as intermediates of active DNA-demethylation
during embryogenesis
- 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine appear as intermediates of active DNA-demethylation
What appear as intermediates of active DNA-demethylation
during embryogenesis? 3
1….5-hydroxymethylcytosine,
2….. 5-formylcytosine,
3…. 5-carboxylcytosine
Understanding CpG DNA methylation… 4
- Forward equation:
Cytosine — DNA methyltransferase (DNMT) —> 5 - Methylcytosine
(AdoMet —–> AdoHcy) - Reverse Equation: 5-methylcytosine —– DNA demethylase —–> Cytosine
- CpG RARE in most Vertebrate genomes —- convert to TpG
- CpG: p refers to the phosphate between bases
Look at CpG DNA Methylation Equation …
slide 21
Methylation - Normal Processes
CAN BE FOUND IN = 4
1 * embryonic development
2 * X chromosome inactivation
3 * Imprinting
4 * Gene silencing
Conrad Waddington’s Epigenetic Landscape (1946)…
WHAT IS WADDINGTON LANDSCAPE?
Waddington landscape: A metaphor of development,
in which valleys and ridges illustrate the epigenetic
landscape that guides a PLURIPOTENT CELL to a WELL-DEFINED, DIFFERENTIATED STATE…., represented by a ball rolling down the landscape.
…Ball rolls down to spectator, corresponds to the developmental history of a particular part of the egg.
… first alternative = towards left or right.
… second alternative : along the former path..along the path to the left, the main channel continues leftwards but there is an alternative path which, however can only be reached over a threshold.
‘the complex system of interactions underlying the epigenetic landscape…”
- the pegs in the ground represent genes
- strings leading from them the chemical tendencies which the genes produce
- the modelling of epigenetic landscape, which slopes down from above one’s head towards the distance, is controlled by the power of numerous guy-ropes which are ultimately anchored the genes.
Stages of Conrad Waddington’s Epigenetic Landscape (1946)
- STEM CELLS
- NORMAL DEVELOPMENT OR AT NEOPLASIA
- NORMAL DEVLEOPMENT = DIFFERENTIATED STATES
- EPIGENETIC MODULATORS ..(HILLS AND VALLEYS)..
- EPIGENETIC MODIFIERS