Dna Organization In Eukaryotic Chromosomes
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DNA Organization in Eukaryotic Chromosomes Chapter 12: Section 12.4
Chapter 12: Organization in Chromosomes
1
Eukaryotic chromosomal organization
Many eukaryotes are diploid (2N) The amount of DNA that eukaryotes have varies; the amount of DNA is not necessarily related to the complexity (Amoeba proteus has a larger amount of DNA than Homo sapiens) Eukaryotic chromosomes are integrated with proteins that help it fold (protein + DNA = chromatin) Chromosomes become visible during cell division DNA of a human cell is 2.3 m (7.5 ft) in length if placed end to end while the nucleus is a few micrometers; packaging/folding of DNA is necessary
Chapter 12: Organization in Chromosomes
2
Eukaryotic chromosomal organization
2 main groups of proteins involved in folding/packaging eukaryotic chromosomes
Histones = positively charged proteins filled with amino acids lysine and arginine that bond Nonhistones = less positive
Chapter 12: Organization in Chromosomes
3
Model for Chromatin Structure
Chromatin is linked together every 200 bps (nuclease digestion) Chromatin arranged like “beads on a string” (electron microscope) 8 histones in each nucleosome 147 bps per nucleosome core particle with 53 bps for linker DNA (H1) Left-handed superhelix Chapter 12: Organization in Chromosomes
4
Eukaryotic chromosomal organization
Histone proteins
Abundant Histone protein sequence is highly conserved among eukaryotes—conserved function Provide the first level of packaging for the chromosome; compact the chromosome by a factor of approximately 7 DNA is wound around histone proteins to produce nucleosomes; stretch of unwound DNA between each nucleosome
Chapter 12: Organization in Chromosomes
5
Eukaryotic chromosomal organization
Nonhistone proteins
Other proteins that are associated with the chromosomes Many different types in a cell; highly variable in cell types, organisms, and at different times in the same cell type Amount of nonhistone protein varies May have role in compaction or be involved in other functions requiring interaction with the DNA Many are acidic and negatively charged; bind to the histones; binding may be transient
Chapter 12: Organization in Chromosomes
6
Eukaryotic chromosomal organization
Histone proteins
5 main types
H1—attached to the nucleosome and involved in further compaction of the DNA (conversion of 10 nm chromatin to 30 nm chromatin) H2A H2B Two copies in each nucleosome H3 H4 ‘histone octomer’; DNA wraps
around this structure1.75 times This structure produces 10nm chromatin
Chapter 12: Organization in Chromosomes
7
Fig. 8.17 A possible nucleosome structure
Chapter 12: Organization in Chromosomes Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
8
Fig. 8.18 Nucleosomes connected together by linker DNA and H1 histone to produce the “beads-on-a-string” extended form of chromatin
H1
Histone octomer
Linker DNA 10 nm chromatin is produced in the first level of packaging. Chapter 12: Organization in Chromosomes Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
9
Eukaryotic chromosomal organization
Histone proteins
DNA is further compacted when the DNA nucleosomes associate with one another to produce 30 nm chromatin Mechanism of compaction is not understood, but H1 plays a role (if H1 is absent, then chromatin cannot be converted from 10 to 30 nm) DNA is condensed to 1/6th its unfolded size Chapter 12: Organization in Chromosomes
10
Fig. 8.20b Packaging of nucleosomes into the 30-nm chromatin fiber
Chapter 12: Organization in Chromosomes Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
11
Eukaryotic chromosomal organization
Compaction continues by forming looped domains from the 30 nm chromatin, which seems to compact the DNA to 300 nm chromatin Human chromosomes contain about 2000 looped domains 30 nm chromatin is looped and attached to a nonhistone protein scaffolding DNA in looped domains are attached to the nuclear matrix via DNA sequences called MARs (matrix attachment regions) Chapter 12: Organization in Chromosomes
12
Fig. 8.21 Model for the organization of 30-nm chromatin fiber into looped domains that are anchored to a nonhistone protein chromosome scaffold
Chapter 12: Organization in Chromosomes
13
Eukaryotic chromosomal organization
MARs are known to be near regions of the DNA that are actively expressed Loops are arranged so that the DNA condensation can be independently controlled for gene expression
Chapter 12: Organization in Chromosomes
14
Fig. 8.22 The many different orders of chromatin packing that give rise to the highly condensed metaphase chromosome
Chapter 12: Organization in Chromosomes
15
DNA compaction
Level of DNA compaction changes throughout the cell cycle; most compact during M (see fig 8.22 bottom) and least compact during S 2 types of chromatin; related to the level of gene expression
Euchromatin—defined originally as areas that stained lightly Heterochromatin—defined originally as areas that stained darkly Chapter 12: Organization in Chromosomes
16
DNA compaction
Euchromatin—chromosomes or regions therein that exhibit normal patterns of condensation and relaxation during the cell cycle
Most areas of chromosomes in active cells Usually areas where gene expression is occurring
Chapter 12: Organization in Chromosomes
17
DNA compaction
Heterochromatin—chromosomes or regions therein that are condensed throughout the cell cycle Provided first clue that parts of eukaryotic chromosomes do not always encode proteins.
Chapter 12: Organization in Chromosomes
18
DNA Cell Cycle At the G1/S, G2/M, and M checkpoints, cells decide whether to proceed to the next stage of the cell cycle. Regulation of cell cycle progress is mediated by cyclins and cyclin-dependent kinases (CDKs) that regulate synthesis and destruction of cyclin proteins.
Chapter 12: Organization in Chromosomes
19
Chapter 12: Organization in Chromosomes
20
Chapter 12: Organization in Chromosomes
21
Chapter 12: Organization in Chromosomes
22
Chapter 12: Organization in Chromosomes
1
Eukaryotic chromosomal organization
Many eukaryotes are diploid (2N) The amount of DNA that eukaryotes have varies; the amount of DNA is not necessarily related to the complexity (Amoeba proteus has a larger amount of DNA than Homo sapiens) Eukaryotic chromosomes are integrated with proteins that help it fold (protein + DNA = chromatin) Chromosomes become visible during cell division DNA of a human cell is 2.3 m (7.5 ft) in length if placed end to end while the nucleus is a few micrometers; packaging/folding of DNA is necessary
Chapter 12: Organization in Chromosomes
2
Eukaryotic chromosomal organization
2 main groups of proteins involved in folding/packaging eukaryotic chromosomes
Histones = positively charged proteins filled with amino acids lysine and arginine that bond Nonhistones = less positive
Chapter 12: Organization in Chromosomes
3
Model for Chromatin Structure
Chromatin is linked together every 200 bps (nuclease digestion) Chromatin arranged like “beads on a string” (electron microscope) 8 histones in each nucleosome 147 bps per nucleosome core particle with 53 bps for linker DNA (H1) Left-handed superhelix Chapter 12: Organization in Chromosomes
4
Eukaryotic chromosomal organization
Histone proteins
Abundant Histone protein sequence is highly conserved among eukaryotes—conserved function Provide the first level of packaging for the chromosome; compact the chromosome by a factor of approximately 7 DNA is wound around histone proteins to produce nucleosomes; stretch of unwound DNA between each nucleosome
Chapter 12: Organization in Chromosomes
5
Eukaryotic chromosomal organization
Nonhistone proteins
Other proteins that are associated with the chromosomes Many different types in a cell; highly variable in cell types, organisms, and at different times in the same cell type Amount of nonhistone protein varies May have role in compaction or be involved in other functions requiring interaction with the DNA Many are acidic and negatively charged; bind to the histones; binding may be transient
Chapter 12: Organization in Chromosomes
6
Eukaryotic chromosomal organization
Histone proteins
5 main types
H1—attached to the nucleosome and involved in further compaction of the DNA (conversion of 10 nm chromatin to 30 nm chromatin) H2A H2B Two copies in each nucleosome H3 H4 ‘histone octomer’; DNA wraps
around this structure1.75 times This structure produces 10nm chromatin
Chapter 12: Organization in Chromosomes
7
Fig. 8.17 A possible nucleosome structure
Chapter 12: Organization in Chromosomes Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
8
Fig. 8.18 Nucleosomes connected together by linker DNA and H1 histone to produce the “beads-on-a-string” extended form of chromatin
H1
Histone octomer
Linker DNA 10 nm chromatin is produced in the first level of packaging. Chapter 12: Organization in Chromosomes Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
9
Eukaryotic chromosomal organization
Histone proteins
DNA is further compacted when the DNA nucleosomes associate with one another to produce 30 nm chromatin Mechanism of compaction is not understood, but H1 plays a role (if H1 is absent, then chromatin cannot be converted from 10 to 30 nm) DNA is condensed to 1/6th its unfolded size Chapter 12: Organization in Chromosomes
10
Fig. 8.20b Packaging of nucleosomes into the 30-nm chromatin fiber
Chapter 12: Organization in Chromosomes Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
11
Eukaryotic chromosomal organization
Compaction continues by forming looped domains from the 30 nm chromatin, which seems to compact the DNA to 300 nm chromatin Human chromosomes contain about 2000 looped domains 30 nm chromatin is looped and attached to a nonhistone protein scaffolding DNA in looped domains are attached to the nuclear matrix via DNA sequences called MARs (matrix attachment regions) Chapter 12: Organization in Chromosomes
12
Fig. 8.21 Model for the organization of 30-nm chromatin fiber into looped domains that are anchored to a nonhistone protein chromosome scaffold
Chapter 12: Organization in Chromosomes
13
Eukaryotic chromosomal organization
MARs are known to be near regions of the DNA that are actively expressed Loops are arranged so that the DNA condensation can be independently controlled for gene expression
Chapter 12: Organization in Chromosomes
14
Fig. 8.22 The many different orders of chromatin packing that give rise to the highly condensed metaphase chromosome
Chapter 12: Organization in Chromosomes
15
DNA compaction
Level of DNA compaction changes throughout the cell cycle; most compact during M (see fig 8.22 bottom) and least compact during S 2 types of chromatin; related to the level of gene expression
Euchromatin—defined originally as areas that stained lightly Heterochromatin—defined originally as areas that stained darkly Chapter 12: Organization in Chromosomes
16
DNA compaction
Euchromatin—chromosomes or regions therein that exhibit normal patterns of condensation and relaxation during the cell cycle
Most areas of chromosomes in active cells Usually areas where gene expression is occurring
Chapter 12: Organization in Chromosomes
17
DNA compaction
Heterochromatin—chromosomes or regions therein that are condensed throughout the cell cycle Provided first clue that parts of eukaryotic chromosomes do not always encode proteins.
Chapter 12: Organization in Chromosomes
18
DNA Cell Cycle At the G1/S, G2/M, and M checkpoints, cells decide whether to proceed to the next stage of the cell cycle. Regulation of cell cycle progress is mediated by cyclins and cyclin-dependent kinases (CDKs) that regulate synthesis and destruction of cyclin proteins.
Chapter 12: Organization in Chromosomes
19
Chapter 12: Organization in Chromosomes
20
Chapter 12: Organization in Chromosomes
21
Chapter 12: Organization in Chromosomes
22
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