Transcript chromatin fiber

Toyosi Coker, Maddie Irvin, Abby Keller,
Ryan Kilgore
Nucleosome


A bead like structure
which is a segment of
DNA that is wrapped
around a histone protein.
A good comparison is the
thread which is wrapped
around a spool. With the
thread being the DNA
and the spool the histone
proteins.
This helps to keep the
DNA compacted.
Histone


A protein which DNA is
wrapped around and
formed into structural
components called
nucleosomes. Histones are
the chief protein
components of chromatin.
It is the “spool” which DNA
or “thread” is wrapped
around.
DNA wraps around 8
histone molecules
approximately twice.
“Beads on a string”- Refers to the
looks of the string of DNA after
nucleosomes have been formed. The
nucleosomes resemble the beads on
the DNA string.
•
•
•
Scaffold proteins- Attached
to DNA at specific regions
(scaffold attachment
regions). Form loops in the
unwound form of the
chromatin (“beads-on-astring”).
The chromatin fiber and
associated scaffold proteins
coil into a helical structure
which may be observed as a
chromosome.
In looped domains, the
loops are attached to a
scaffold of non-histone
proteins.






Higher Levels of DNA packing:
There are several orders of
chromatin coiling, each in
order of increasing
compactness.
DNA- coils around histones to
form a nucleosome. Look like
“beads on a string” formation.
The beaded string coils/folds
to form chromatin fiber (30
nm).
The chromatin fiber forms
loops called looped domains.
They are in the extended form
of the chromosome.
The looped domains condense
tightly into small portions of
the mitotic chromosome.
Heterochromatin (tightly-packed)
Euchromatin (loosely-packed)
◦ Found in regions of a
chromosome where there
are few to no genes
◦ Two types: constitutive and
facultative
◦ Functions as a regulator of
gene expression
◦ Found in regions of o
chromosome where there
are many genes
◦ Unfolded structure
◦ Functions in the active
transcription of DNA to
mRNA
DNA methylation
Attachment of methyl
groups (-CH3)
Activate DNA
Cellular
differentiation in
embryo
Genomic imprinting
Histone Acetylation
Attachment of acetyl
groups (-COCH3) to
the amino acids in
histone proteins
Deactylation

http://www.youtube.co
m/watch?v=0D54U2CK
5ek&feature=related





Positive and Negative charges- helps form the
nucleosome. Histone proteins have a positive charge
due to it’s amino acids. DNA has a negative charge
due to its phosphate backbone.
Amino Acid- The amino acid is altered in methylation
and acetylation.
Protein- structural proteins attach with DNA to form
the structure of the chromatin.
DNA- segments form genes. Heterochromatin effects
gene expression.
Chromosome- the very condensed form of
chromatin.




1. What is the role of histones
in DNA packaging (the
compacting of DNA so it can fit
into a smaller space) and what
do they help form?
2.Describe the Nucleosome
“beads”.
3. What are the levels of
chromatin packing from lowest
to highest order?
4. What is the most
fundamental form of
chromatin?




1. To provide a structure DNA
can wrap around which forms
nucleosomes. They help in the
first levels of DNA packing.
2. The nucleosomes form the
“beads” along the string of
DNA in the “Beads on a String”
analogy.
3. Nucleosomes - chromatin
fiber-looped domainsmetaphase chromosome.
4. DNA -histone complex
(beads on a string formation)
DNA is wound around
histones.




5. What is the main difference
between heterochromatin and
euchromatin?
6. In what areas of a
chromosome would you
generally find heterochromatin?
Euchromatin?
7. What three significant
functions of DNA methylation?
8. What is the effect of histones
changing shape in histone
acetylation?




5. Heterochromatin is tightlypacked chromatin within the
nucleus (more dense), while
euchromatin is loosely-packed
chromatin within the nucleus
(less dense)
6. Heterochromatin is found in
areas where there are few to no
genes present (specifically the in
and around the centromere and
the telomeres, while
euchromatin is found in areas
where there are many genes
7. activate DNA, cellular
differentiation in embryo, and
genomic imprinting.
8. They grip the DNA less tightly,
and transcription proteins have
easier access to genes in the
acetylated region