Chapter 19 Organization Chapter 19 Organization and Control of Eukaryotic and Control of Eukaryotic

GenomesGenomes

……Or How To Fit All of the Junk In Or How To Fit All of the Junk In the Trunkthe Trunk

Eukaryotic Chromatin StructureEukaryotic Chromatin Structure

Order of Chromatin StructureOrder of Chromatin StructureDNA double helixDNA double helixNucleosomesNucleosomes30-nm chromatin fiber30-nm chromatin fiberLooped domainsLooped domainsFolding of looped domainsFolding of looped domains

Details, Details, DetailsDetails, Details, DetailsNucleosomes are made up of four Nucleosomes are made up of four

Histones. H2A, H2B, H3, H4Histones. H2A, H2B, H3, H4Histones are mostly postitivly charged amino Histones are mostly postitivly charged amino

acids.acids.DNA (negitivly charged is attracted to DNA (negitivly charged is attracted to

Histones)Histones)DNA is wrapped around the Nucleosomes DNA is wrapped around the Nucleosomes

twice. An extra Histone (H1) Binds to the twice. An extra Histone (H1) Binds to the DNA just after the Nucleosome.DNA just after the Nucleosome.

Figure 3-D-2.  Each Figure 3-D-2.  Each nucleosome consists nucleosome consists of 146 bp DNA and 8 of 146 bp DNA and 8 histones: two copies histones: two copies for each of H2A, H2B, for each of H2A, H2B, H3 and H4.  The DNA H3 and H4.  The DNA is wrapped around is wrapped around the histone core, the histone core, making nearly two making nearly two turns per turns per nucleosome.nucleosome.

Even More Details!Even More Details!30-nm Chromatin fiber—Coils of the Fiber 30-nm Chromatin fiber—Coils of the Fiber

with the Nucleosome that forms folds that with the Nucleosome that forms folds that are 30 nm in sizeare 30 nm in size

Looped domains—30nm fiber folds and Looped domains—30nm fiber folds and attaches on to nonhistone protein scafold.attaches on to nonhistone protein scafold.

When Chromatin is condensed into When Chromatin is condensed into chromosomes the looped domains coil chromosomes the looped domains coil and form a tightly packed Chromosome.and form a tightly packed Chromosome.

Genome Organization at the DNA Genome Organization at the DNA LevelLevel

Repetitive DNA (non-coding)Repetitive DNA (non-coding)Accounts for aprox 97% of human DNAAccounts for aprox 97% of human DNATandemly Repetitive DNA—Short sequences Tandemly Repetitive DNA—Short sequences

repeted in a series.repeted in a series.Can cause Genetic disorders.Can cause Genetic disorders.Typically found in centromeres and telomeres Typically found in centromeres and telomeres

so it is thought to be used for structure.so it is thought to be used for structure. Interspersed Repetitive DNA—Copies of Interspersed Repetitive DNA—Copies of

similar sequences but not repetitive.similar sequences but not repetitive.

Gene Change in Somatic CellsGene Change in Somatic CellsGene Amplification—Selective replication Gene Amplification—Selective replication

of specific genes.of specific genes.Example in egg cells increasing the amount of Example in egg cells increasing the amount of

rRNA will increase the number of ribosomes rRNA will increase the number of ribosomes which increases the protein output of the cell which increases the protein output of the cell when it is fertilized.when it is fertilized.

Transposons and Retrotransposons—Transposons and Retrotransposons—DNA that can move from one location to DNA that can move from one location to another. RT utilizes RNA and reverse another. RT utilizes RNA and reverse transcriptase to move.transcriptase to move.

Gene ExpressionGene Expression

Specialized Cells and tissues are selective Specialized Cells and tissues are selective in the genes they express. Genes turned in the genes they express. Genes turned on and off in response to signals.on and off in response to signals.

Pg 363 to see general overview of gene Pg 363 to see general overview of gene expression.expression.

Chromatin ModificationsChromatin Modifications

Chromatin Modifications affect the gene Chromatin Modifications affect the gene availability for transcriptionavailability for transcription

DNA Methylation—Attachment of methyl DNA Methylation—Attachment of methyl groups (CH3) to DNA bases after DNA groups (CH3) to DNA bases after DNA synthesis.synthesis.

Inactive DNA is usually highly methylated Inactive DNA is usually highly methylated when compared to highly active when compared to highly active transcription regions of DNA.transcription regions of DNA.

More ModificationsMore Modifications

Histone Acetylation—attachement of Histone Acetylation—attachement of Acetyl groups (COCH3) to amino acids of Acetyl groups (COCH3) to amino acids of histone groups. Deacetylation is the histone groups. Deacetylation is the removal of acetyl groups from histones.removal of acetyl groups from histones.

Histones that have been acelylated have a Histones that have been acelylated have a looser bond with DNA and transcription looser bond with DNA and transcription proteins have easier access to DNA.proteins have easier access to DNA.

Left for youLeft for you

Control elements and transcription factors Control elements and transcription factors pg. 364-368pg. 364-368

Biology of Cancer pg. 369-372Biology of Cancer pg. 369-372