NOTES FOR BIOLOGY 1201
Section 001
Spring 2005
DR. STEVEN POMARICO
The Eukaryotic Genome: DNA Packing - 6.9.1
The eukaryote genome is very large compared to prokaryotes
-More DNA is required for cellular differentiation
The larger amount of DNA in eukaryotes must be organized or packed to fit in the nucleus.
The packing of DNA is organized in levels:
1. DNA is wound around nucleosomes that are made of histone proteins.
2. The nucleosomes are then wound into chromatin.
3. The chromatin is arranged in looped domains.
4. The looped domains are folded liked an accordion.
---Histones
---Nucleosomes
All levels of packing are involved when chromosome condensation occurs prior to mitosis or meiosis.
Some of the DNA (heterochromatin) remains highly condensed even during interphase.
Heterochromatin is not transcribed.
The remaining chromatin (euchromatin) unfolds to the stage of looped domains during interphase.
Some of the euchromatin (about 3-5% of genome) is expressed at any given time
WHY SO LITTLE??
Because all the information is not need in all cell types.
>>>>>>The arrangement of genes
-Operons not found in eukaryotes
-Each gene has own promoter
Most eukaryotic genes are unique sequences and present as a single copy in the genome.
>>>>>>There are many opportunities for the control of gene expression
Eukaryotic Gene Control: Transcriptional Controls - 6.10.1
DNA availability (pretranscriptional)
DNA methylation: addition of -CH3 to bases after DNA synthesis
- inactivates DNA
- may cause packing
- long-term control of gene expression
Transcription regulation
Control elements and enhancers
Gene expression may be blocked or enhanced at any posttranscriptional step:
Eukaryotic Control Mechanisms: Posttranscriptional and Posttranslational
Controls - 6.10.2
Post transcription
-RNA processing or splicing
-RNA transport
-poly A tail
-RNA degradation
Translational control
Post-translational control
-protein modification
-protein degradation