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8/10/2019 14-Unit4_Chromatin_transcription_Oct7_post.pptx
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Welcome to Biology 200 Section 101
Dr. Marcia GravesMWF 10-10:50
Wesbrook 1001
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Unit 3: Nuclei and Chromatin
Three topics:
1. The interphase nucleus
2. Nucleic acids review (self-study)3. Chromatin and chromosomes
2
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1. Naked DNA (2nm)
Levels of DNA packing in the nucleus
2. Nucleosomes (10-11nm)
3. Chromatin (30nm)
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Levels of DNA packing in the nucleus
3. Chromatin (30nm)
Fig. 5-21
A chain of nucleosomes can be arranged in a 30 nm fiber
formation of the 30nm fiber is dependent on the presence
of the H1 histone.
H1 pulls nucleosomes together to form a regular repeating
array of stacked nucleosomes
There are competing ideas about how the 30nm is arranged
This is the level of packing found in
the interphase nucleus
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This is one model of the 30nm fiber
Where every 6th-8thnucleosome curl on themselves to
form a compacted structure
This interphase chromatin structure can be loosened by
remodeling complexes to allow expression of genes
Levels of DNA packing in the nucleus
3. Chromatin (30nm)
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1. Naked DNA (2nm)
Levels of DNA packing in the nucleus
2. Nucleosomes (10-11nm)
3. Chromatin (30nm)
Interphase chromatin canform higher-ordered loops
of the 30nm fiber
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1. Naked DNA (2nm)
Levels of DNA packing in the nucleus
2. Nucleosomes (10-11nm)
3. Chromatin (30nm)
Additional folding beyond the30nm loops, only happens
when chromatin condenses into
chromosomes during mitosis
4. Chromosome condensation
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Levels of DNA packing in the nucleus
4. Chromosome condensation
Recall: The structure of chromatin varies along a single interphasechromosome
Loops of 30nm chromatin Formed by non-histone
chromatin proteins that
form a scaffold
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Levels of DNA packing in the nucleus
4. Chromosome condensation
Mitotic chromosomes: After DNA replication, 2 copies of each
chromosome
It is poorly understood how mitotic
chromosomes become so compact
10 000 fold shorter than its length
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Important structural components of every eukaryotic
chromosome
Most important function of chromosomes is to carry genes
Mitosis produces genetically identical daughter cells
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Levels of organization:
From DNAgene-chromosome-genome
Human Genome: ~25000 genesonly
10% of total DNA!
23 pairs of
chromosomes
Multiple genes per
chromosomes Genes are a segment
of DNA that encodes
for a functional
productthrough
transcription!
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Unit 4: From Gene to Protein
Three topics:
1. Nuclear FunctionRNA transcription
2. Nuclear FunctionRNA Processing
3. Genetic Code and Tranlation
12
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Unit 4: Topic 1Nuclear Function RNA Transcription
Learning Objectives:
8/10/2019 14-Unit4_Chromatin_transcription_Oct7_post.pptx
14/28Figure 7-1 Essential Cell Biology ( Garland Science 2010)
Biological Information Flow
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RNAs as Intermediates in the Flow of Genetic Information
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Anatomy of a Transcription Unit (gene): codes for RNA
A Transcription unit Consists of
TWO major parts:
Regulatory region Coding region
16
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TranscriptionStart site
Transcription
Stop site5
53
3
Template strand
Coding strandEnhancer
regionPromoter
Regulatory region
Promoter: Regulatory region of DNA near the
transcriptional start site Binds RNA polymerase, transcription
factors
Enhancers/repressors : Bind activators and repressors to
control transcription
may be far away from the actual gene
Transcribed region
Coding region
A Transcription Unit Consists of TWO major parts:
Upstream Downstream
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Fig. 7-2
Regulatory regions regulate gene expression:
Some genes are expressed a lot, some only a little
Different genes aretranscribed at different
rates
mRNA can be used to
direct the synthesis of
many identical protein
molecules (amplifies)
A cell can change how
much gene expressiondepending on its needs
Different tissues express
different genes, defines
cell specialization
(differentiation) 18
Regulatory region
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TranscriptionStart site
Transcription
Stop site5
53
3
Template strand
Coding strandEnhancer
regionPromoter
Regulatory region
Promoter: Regulatory region of DNA near the
transcriptional start site Binds RNA polymerase, transcription
factors
Enhancers: Bind activators and repressors to
control transcription
may be far away from the actual gene
Transcribed region
Coding region
A Transcription Unit Consists of TWO major parts:
Gene sequence
Code is transcribed with RNA
Polymerasemoving along the DNAtemplate 3 to 5
RNA is the gene product of
transcription
New ribonucleotides are added to
the 3 end of the growing RNA
Upstream Downstream
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Type of polymerase Genes transcribed
RNA polymerase I Most rRNA genes
RNA polymerase II All protein-coding genes, plus some
genes for small RNAs (ie. those in
spliceosomes)
RNA polymerase III tRNA genes
55 rRNA gene
Genes for some small structural RNAs
The three RNA Polymerases in Eukaryotic Cells
20
TranscriptionStart site
Transcription
Stop site5
53
3
Template strand
Coding strandEnhancer
regionPromoter Transcribed region
Table 8-1
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RNA Polymerase reads the template strand
moving from 3 to 5
RNA
Polymerase II
5 end of RNA is
synthesized first
New ribonucleotidesare added on the 3 end
h h l h h h h
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1. 3 end of transcript
2. 5 end of transcript
3. 3 end of template strand
4. 5 end of template strand
Match the letters on the micrograph with the
numbered terms.
A. D, C, B, AB. B, A, D, C
C. B, A, C, D
D. C, D, A, B
C
A
B D
DNA
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A single gene (Transcription unit) visualized with TEM
TranscriptionStart site
Transcription
Stop site5
53
3
Template strand
Coding strandEnhancer
regionPromoter Transcribed region
C
A
B D35
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RNA polymerase reads 3 to 5 on the template DNA strand
The 5 end of the RNA is made first (due to complimentary base pairing)
Promoter region determines which DNA strand is the template
Template
strand
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bl l h d ( ) b
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Problem 4.1.2: Explain how adjacent transcription units (TU) can be
transcribed in different directions relative to the chromosome.
(Template strand always read 3 to 5)
A. Genes can invert due to viruses.
B. mRNA is usually produced from both strands
simultaneously within each transcription unit.
C. Genes can be encoded on either strand of the DNA.
D. mRNA synthesis can occur in either direction on the
template strand
E. Genes on either strand are complementary
Transcription can occur in both directions onProblem 4 1 2
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Transcription can occur in both directions on
chromosomes.
Genes are transcribed on both DNA strands, but
are always read 3 to 5 Any one transcription unit is always transcribed in
the same direction.
RNA polymerase
Problem 4.1.2
How does transcription occur?
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For next day.
1. Compare and contrast prokaryotic and eukaryotic
gene transcription
Give it some thought before class on
Wednesday
2. We will finish transcription and begin discussing
RNA processing
Pre-read on-line notes for Unit 4: Topic 2