What happens to chromatin during replication

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What happens to chromatin during replication /or
gene expression (transcription)?

• Some stretches of the gene (promoter region) are
  devoid of histones (nucleosomes) during active
  gene expression BUT
• Nucleosomes are present on the rest of the gene.
• Promoter regions are hypersensitive to DNAse I
  (sequence independent endonuclease)

Gene
Promoter
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• Histones are NOT displaced by RNA Polymerase
• During DNA Replication, as the replication fork
  approaches a stretch of chromatin, nucleosomes on
  the parental duplex are displaced and new ones
  (containing both re-cycled and new histones) are
  re-formed on the newly synthesized daughter
  duplexes

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Model for Chromatin Replication Mathews Fig.
28.17
Both old and new histones are used for chromatin
on both daughter duplexes
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Non-Histone DNA Binding Proteins Other
Chromatin-Associated proteins
- DNA Polymerases - ssDNA binding proteins -
Topoisomerase I II - Helicase - Primase, etc. -
RNA Polymerases - Transcription Factors -
activators suppressors - Factors for ribosomal
assembly - ribosomal proteins - Nuclear RNPs
(RiboNucleoProteins) involved in mRNA
processing - Telomere associated proteins -
Centromere associated proteins - Scaffolding
proteins - Recombination Proteins - Other
Replication
Transcription
RNA Processing
Others
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Chromosome Structure, Function Transmission
Basic structure of eukaryotic chromosomes
Centromere
Telomere
Telomere
Telomere
Metaphase Chromosome
Centromere ?
Replication - Multiple Origins of replications -
Eukaryotic chromosomes are too large to replicate
from ONE origin of replication.
It would require over ten days to replicate a
chromosome with one ORI.
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To be fully functional a eukaryotic chromosome
needs - at least one Origin of replication - a
centromere - telomeres 1. Origin of Replication
in YEAST ARS Autonomous Replication
Sequence 11 base ARS consensus
sequences 5-(A/T)TTTAT(A/G)TTT(A/T)-3 This
sequence is repeated multiple times in a 100 bp
region
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2. Centromere - contains an unique sequence,
usually (but not always) at the midpoint of the
chromosome, (plus repetitive DNA) - The
attachment site for spindle fibers in MEIOSIS and
MITOSIS. - Binding site for kinetochore
proteins
spindle fiber
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• 3. Telomere
• - Highly repetitive sequences
• DNA sequence that stabilizes the ends of
  chromosomes
• - 3 OH is unprotected ? susceptible to
  exonucleases
• Helps to solve the problem of replicating the
  ends of a linear DNA molecule.
• - RNA primer required for DNA replication. How
  to put DNA at the new 5 end?

Confocal microscope image of chromasomes using a
general DNA probe (BLUE) along with telomere
specific probes (PINK).
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- So after every round of DNA replication, the
chromosomes will get a little shorter -Unless
some other process occurs to lengthen the ends
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• How telomeres accomplish these tasks?
• Telomeres are repetitive Guanine-rich sequences
  that often contain a 3 overhangs
• - These sequences can fold back to form unusual
  GG base pairs Guanine Quartets
• (G-quartet)
• - These structures or the single stranded
  overhang itself may be capped by proteins to
  stabilize the ends

GG


GG
5
TTTTGGGGTTTTGGGG-3
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(No Transcript)
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II. Recruit an enzyme, telomerase- a special type
of reverse transcriptase uses an RNA Template
(part of the enzyme) to synthesize new telomere
sequences on the ends of existing telomeres