BIO SCI 98 Chapter Notes - Chapter 1-5: Agarose Gel Electrophoresis, Histone H1, Spindle Apparatus
19 Jun 2018
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297-299, 331-342, 359, 405
Chromosomal Function Relies of Specialized
Genomic Sequences
-Bacterial chromosomes are circular
-Eukaryotic chromosomes are linear
-Some gene sequences are for maintaining the chromosomal
-Replication, segregation, protection, maintenance
-Bacteria + eukaryotes: origins of replication = start sites for replication
-Eukaryotes have more than bacteria
-Centromere: segment of each eukaryotic chromosome for proteins to attach to link
chromosomal to mitotic spindle during metaphase (cell division)
-Yeast: 130 bp long, A=T rich
-Eukaryotes: 5-10 bp, thousands of copies
-Site of kinetochore assembly → kinetochore anchors spindle fibers when
chromosomes segregate into daughter cells
-Stabilizes chromosome segregation during cell division
-Telomere: sequences at ends of eukaryotic chromosomes
-Protect ends from nucleases
-Polymerases can't synthesize DNA at ends of linear chromosome
-5' - (TG) repeats
-3' - (AC) repeats
-Serve as binding sites for specialized proteins that package telomeres and help in
cell division
-Yeast artificial chromosomes (YACs) developed to study functions of centromeres +
telomeres
-Need only centromere + telomere + replication origins
Nucleosomes: The Basic Units of DNA
Condensation
-chromatin = 50% DNA + 50% protein (material of chromosomes)
-SMC proteins, topoisomerases, transcription regulators
-Histones are largest component of chromatin
-Highly conserved, basic proteins that assemble into octameric complexes
-DNA wraps around histones to form condensed nucleosomes, composed of 200
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bp of DNA wrapped around histone octamer
Histone Octamers Organize DNA into
Repeating Units
-Evidence
-DNA was cut by nuclease
-DNA fragments analyzed for size in agarose gel
1) If proteins packaged DNA by size, nuclease would cut between these
units, DNA segments would migrate in gel as ladder of unit sized bands
2) If there is no repeating pattern of protein-packaging, units would be
random, and nuclease digestion would smear DNA fragments
-Results: regularly spaced DNA bands of 200 bp apart = DNA is packaged by
proteins into 200 bp units
-Histones
-4 histone units in equimolar ratios: H2A, H2B, H3, H4
-H1 was found in half the amount
-Histones rich in Arg and Lys
-Highly conserved in eukaryotes = strict conservation of function
-Protein cross-linking
-A chemical is used to react with a protein complex
-Can only react with 2 proteins that are close together reveals which proteins are⇒
next to each other
-Most of the 200 bp of DNA is wrapped around a histone octamer
-Contains H2A, H2B, H3, H4: core histones
-H1 binds to remaining DNA which serves as a linker between nucleosomes
-DNA is bound tightly to beads of protein
-Formation of histone octamer requires DNA
-Without DNA, H3 and H4 form heterotetramer and H2A and H2B forms
heterodimer → do not form into octamer
-With DNA, two H2A-H2B dimers + one H3-H4 heterotetramer + DNA
forms nucleosome
DNA Wraps Nearly Twice around a Single
Histone Octamer
-Nucleosome: crystal structure of histone octamer with 146 bp of DNA
-DNA wrapped tightly around octamer in left-handed solenoidal supercoil
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Some gene sequences are for maintaining the chromosomal. Bacteria + eukaryotes: origins of replication = start sites for replication. Centromere: segment of each eukaryotic chromosome for proteins to attach to link chromosomal to mitotic spindle during metaphase (cell division) Site of kinetochore assembly kinetochore anchors spindle fibers when chromosomes segregate into daughter cells. Telomere: sequences at ends of eukaryotic chromosomes. Polymerases can"t synthesize dna at ends of linear chromosome. Serve as binding sites for specialized proteins that package telomeres and help in cell division. Yeast artificial chromosomes (yacs) developed to study functions of centromeres + telomeres. Need only centromere + telomere + replication origins. Chromatin = 50% dna + 50% protein (material of chromosomes) Highly conserved, basic proteins that assemble into octameric complexes. Dna wraps around histones to form condensed nucleosomes, composed of 200 bp of dna wrapped around histone octamer. Results: regularly spaced dna bands of 200 bp apart = dna is packaged by proteins into 200 bp units.
