Eukaryotic chromosome
structure
Objectives
At the end of this
lecture, student will be able to
• Describe the structure of mitotic chromosome involving
centromere, telomeres
• Describe heterochromatin and euchromatin
• Explain DNase hypersensitivity
• Explain the chemical modification by CpG methylation and
histone variants
Content
Eukaryotic chromosome structure
– Mitotic chromosome
– Centromere
– Telomeres
– Interphase chromosome
– Heterochromatin
– Euchromatin
– DNase I hypersensitivity
– CpG methylation
– Histone variants and modification
Eukaryotic
chromosome structure
The mitotic
chromosome
• Paired sister chromatids at mitosis represents highly
condensed state of chromatin
• Sister chromatids – products of replication of a single
chromosome joined at centromeres
• Telomeres – tip of chromosome
• Chromosomal loops fan out from nuclear matrix
• Loops comprise chromatin in 30-nm fibre form
The
centromere
• Region where two chromatids are joined
• Site of attachment through kinetochore to the mitotic
spindles
• Mitotic spindles pulls apart sister chromatids at anaphase
• Characterized by specific short DNA sequence
• In mammalian cells, there may be involvement of satellite
DNA
Structure
of chromosome and centromere
Telomeres
• Specialized DNA sequence that form the ends of linear DNA
molecules
• Consists of hundreds of copies of short repeated sequence
• Synthesized by enzyme telomerase
• Mechanism of formation is independent of DNA replication
• Sequence adopts a characteristic G-quadruplex structure
Interphase
chromosome
• In interphase, the genes on chromosome are transcribe
• DNA replication takes place
• Chromosome adopt much more diffuse structure
• Structure cannot be visualized individually
• Chromosomal loops remain attached to nuclear matrix
Heterochromatin
• Highly compacted portion of chromatin in interphase and
not so compacted portion in metaphase
• Dense region at the periphery of the nucleus
• Consists of closely packed regions of 30-nm fibre
• Transcriptionally inactive
• Constitutive
heterochromatin – always compacted; has repeated units of satellite DNA
sequences
• In some cases entire chromosome may remain heterochromatin
• Eg. One of two X chromosome in mammals
• Facultative
heterochromatin – chromatin compacted in some tissue; genes are not
expressed
Euchromatin
• Bulk of chromatin that is not visible as heterochromatin
• Region where all transcription takes place
• Not homogenous, contains relatively inactive region
• Active region has been dissociated to the ‘beads on
string’ structure
• Parts of this may be depleted of nucleosomes
Heterochromatin
and euchromatin
DNase I
hypersensitivity
• Short regions of chromatin that are readily attacked by
Dnase I
• Acts on the backbone of DNA unless it is protected by
bound protein
• Used to map the regions of transcriptionally active
chromatin in cells
• Longer regions of sensitivity represents sequence where
transcription is taking place
DNase I
hypersensitive site
CpG
methylation
• Methylation of C-5 in the cytosine base of 5’-CG-3’
sequence
• CpG sites are normally methylated in mammalian cell
• Methylation of CpG is associated with transcriptionally
inactive regions of chromatin
• Islands of unmethylated CpG occur near promoters of
frequently transcribed gene
• Forms regions of high Dnase sensitivity
• Patterns of CpG methylation can persists through multiple
cell division
Histones
variants and modification
• Chemical modification of histone protein controls the
degree of condensation of chromatin
• Involves changes in charge, interaction with other
proteins during cell cycle
• Uses histone variants in particular cell types or during
development
Summary
• Mitotic chromosomes are paired sister chromatids at
mitosis represents highly condensed state of chromatin
• Centromeres is the region where two chromatids are joined
and is the site of attachment through kinetochore to the mitotic spindles
• Telomeres is the specialized DNA sequence that form the
ends of linear DNA molecules
consisting of hundreds of copies of short repeated sequence and synthesized by
enzyme telomerase
• Highly compacted portion of chromatin in interphase and
not so compacted portion in metaphase is heterochromatin
• Euchromatin is the region where all transcription takes
place
• Dnase hypersentivity is the phenomenon where short regions
of chromatin that are readily attacked by Dnase I if not protected by proteins
• CpG methylation is the methylation of C-5 in the cytosine
base of 5’-CG-3’ sequence
• Histone modification is the Chemical modification of
histone protein controls the degree of condensation of chromatin