Eukaryotic chromosome structure

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