BIO130H1 Lecture 6: Packaging of Nucleosomes & DNA Replication

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25 Jan 2019
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BIO130 Lecture 6 24/01/19
Packaging of nucleosomes:
Cohesion is involved in forming chromatin loops
Loops may be attached to a nuclear scaffold to enable increased compaction
Chromatin levels of organization:
The overall result is that each DNA molecule is packaged into a mitotic
chromosome that is 10 000 times shorter than its extended length
Ends of chromosomes are called telomeres
Condensing hold the “daisy”-like structures in the middle
Centromere area on replicating chromosomes
Heterochromatin:
Highly condensed chromatin
Meiotic and mitotic chromosomes
Centromeres and telomeres regions with very few genes
One X chromosome in females (Barr body)
o X chromosome has more genetic material than Y
o One X chromosome is suppressed in females in order to reduce
expression to prevent disparity between males and females
Heterochromatin regions of interphase chromosomes are areas where gene
expression is suppressed
Euchromatin:
Relatively non-condensed chromatin
Regions of interphase chromosomes where genes tend to be expressed
It’s reversible going from euchromatin and heterochromatin
o Very dynamic
Implications for gene expression:
The nucleus is far more organized than we thought
Distinct colours show the organization of the nucleus chromosomes occupy
discrete regions in the nucleus
The middle of the nucleus tends to be more favourable for transcription
Chromosomes with more active genes tend to be located in the middle
whereas those with less active genes tend to be on the outside
NOTE: chromatin is remodelled during transcription to alter access to DNA
Transcription factories:
Areas of the nucleus which are very favourable for transcription
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