Heterochromatin and Euchromatin PDF

Heterochromatin and Euchromatin  Heterochromatin and Euchromatin Session Learning Outcomes (SLO) SLO# 1 : Describe how the packing of chromatin changes during the course of the cell cycle. SLO# 2 : Distinguish between heterochromatin and euchromatin. SLO# 3 : Describe the process of X in...

Heterochromatin and Euchromatin  Heterochromatin and Euchromatin Session Learning Outcomes (SLO) SLO# 1 : Describe how the packing of chromatin changes during the course of the cell cycle. SLO# 2 : Distinguish between heterochromatin and euchromatin. SLO# 3 : Describe the process of X inactivation in mammals and explain its function.  Chromosomes chromatin fibers Interphase chromosome DNA is very dispersed so it can be accessed for replication and transcription. form become in the Mitotic chromosome DNA is in its most highly condensed of chromosome state & favors delivery of an intact DNA package to each daughter cell. will be separated chromosome each daughter chromatids one for each into two cell  Chromati (Heterochromatin n and Euchromatin) - Different forms of chromatin show different gene active chromatin activity notistatin a Heterochromatin Euchromatin packed less chromatin that remains chromatin that returns to compacted during interphase dispersed state after each mitosis stains darkly 99  Heterochromatin and Euchromatin Interphase chromosomes Heterochromatin Euchromatin Highly condensed areas Less compacted areas Inaccessible to Accessible to transcription enzymes transcription enzymes Tility silent gene nucleosomes are close 60 nucleosomes very each other are open interaction no them between  Heterochromatin vs Heterochromatin Euchromatin Euchromatin it compacted because is less a - Stains darkly - Stains lightly with specific genes many - Repetitive sequences a replic - Single copy sequences (genes) amen each gene for if -Replicates later in the cell cycle -Replicates early in the cell cycle -DNA hypermethylated -DNA hypomethylated hiL insphase - Little or no recombination - Recombines -Transcriptionally repressive: - Transcriptionally active: methylation of silences gene expression permissive for gene expression means silence 55 the gene - Localitazed to telomers and regions flanking centromers I IT telomers end of the chromosome heterochromatin can't be expressed are chromosome toprotect the become shorter gene because with age  Heterochromatin and Euchromatin – Heterochromatin divided into 2 classes depending on whether it's permanently or transiently sometimes active and compacted sometimes inactive and somecells active in othercells Facultative inactive in Constitutive Tecomers and regions heterochromatin heterochromatin flanking centromers specifically inactivated stays condensed in all cells at all times (certain phases of organism's life) permanently or silenced DNA (certain types of differentiated cells) the embryo needs someprotein to beexpressed for example in the expressed at thisstage so the cell will be another protein nextstage the embryo needs the first stage will be activatedin thegene whichwere inactivated  Heterochromatin Constitutive heterochromatin In mammalian cells - most is found in regions flanking centromeres in a few other sites (Y chromosome distal arm in males). 2. In many plants, telomeres also consist of blocks of constitutive heterochromatin 3. Consists primarily of repeated DNA sequences & contains relatively few genes  Heterochromatin Constitutive heterochromatin 4. If normally active genes move into site adjacent to constitutive heterochromatin (change position via transposition or translocation) —> tend to become transcriptionally silenced (position effect). 5. May have components whose influence spreads out a certain distance, affecting nearby genes. 6. The spread of heterochromatin along a chromosome is normally blocked by specialized barrier sequences in genome responsibleof red prevent the effect of heterochromatin the color of the eye give color separate heterchromatin on the Euchromatin from Euchromatin sexprission s.im present of barrierto prevent heterochromatin the effect of the on the euchromatin normal case redeye no barrier a IN close to the gene is heterochromatin without barrier silent Heterochromatin will affectthegene white eye and it will be silent gett the white color  Heterochromatin heterochromatin it will be affectedby the and it will be silent  inactive Heterochromatin could be active or ofthe cell depending on the stage Facultative heterochromatin example: X chromosome in mammals female Cells have 2 X Male cells have a chromosomes Y chromosome & X chromosome Two copies of most genes carried onsingle copy of most genes carried on sex chromosomes sex chromosomes only one X chromosome is transcriptionally active The other X chromosome is condensed as heterochromatic mi clump more than one chromosome one is active Barr body bodies are Barr the remaining chromosomes Female with 3 in active bodies 2x barr Heterochromatin Facultative heterochromatin Barr body is present female Barr body is not present male chromosome in the m Barr body Barr body ensures that cells of both males & females have the same number of active X chromosomes synthesize equivalent amounts of products encoded by X-linked genes  Heterochromatin Facultative heterochromatin X chromosome inactivation – The Lyon hypothesis: Heterochromatization inactivation of genes on of X chromosome in female that chromosome mammals occurs during early embryonic development  Heterochromatin X chromosome inactivation-The Lyon hypothesis: Heterochromatized X chromosome is reactivated in Birr body germ cells before meiosis both X chromosomes are active during oogenesis all gametes get a euchromatic X chromosome  Heterochromatin X chromosome inactivation-The Lyon hypothesis: both chromosome of chromosomes the process inactivation is random are active Heterochromatization derived or matern either thepaternally in embryo is chromosome can be derived random process cell inactivated inactivated in any given The paternally-derived & maternally- derived X chromosomes have equal chances of being inactivated in any given cell Once X chromosome is Iss inactivated, same X chromosome is inactivated in all of the cell's descendants IIwewei w.is xgwnojwji.am t gwosswsidwsj.iq w̅ isn't genetic mosaics iajiswvissbsiswwi.SI ange because The consequences of X- ii na it a inactivation ternally Adult mammalian females are genetic mosaics (with chromosome fathers different alleles functioning in different cells) black frommother orange from becue pigmentation genes chromosome are carried on Yegion notte calico cats inactivated paternally derived chromosome Males don't have skin either black paternal & maternal X chromosomes may have different colors different alleles for same trait. or orange X-linked pigment genes in cats are calico. Pigmentation genes in humans are not found on X chromosome so there are no calico women  The interaction between the If the interaction is very str can affect The Histone Code if fyd a tin of Ine a so the DNA ismore compacted no activity - Histones are subjected to a variety of post less the D translational modifications (most often on the terminal If the interaction is is less compacted open more activi tails) - Theses modifications are generated by specific we in theprevious said flexible ta chapter that thelong enzymes that can add or remove chemical groups to or interacts HY N terminal tail from amino acid residues in the histone tails. with the linker DNA betweenthe nucleosomes and H2A H2B in the Agtenine lysine adjacent nucleosome - Histones are acetylated and phosphorylated, altering their ability to bind to DNA. - These modifications can influence gene expression and other chromatin functions The Histone Code - The state & activity of a particular region of chromatin depends upon the specific modifications, or combination of modifications, to the histone tails in that region - The pattern of modifications on the tails of the core histones contains encoded information governing the properties of the nucleosomes containing them  The Histone Code I Two interrelated chromatin properties were shown to depend upon histone modification patterns: 1. The degree of compaction – most importantly, whether a region of chromatin is heterochromatic or euchromatic F 2. The likelihood that a gene or cluster of genes will be transcribed  chemical modification the tails will be on Chemical modification of histone tails changes the shape of nucleosomes.  acetylation activate DNA methylation inactivate DNA When methyl, acetyl, or phosphate groups are attached to the tails, the tails change shape, altering access to the DNA wrapped around the core particle. In most cases, these modifications restrict access to the underlying DNA.  inactivate DNA more make methyst p compacted 9 natieasonites etylated a silent 9ᵗʰ Five Heterochromatin S it need to be inactive in the nextstage deacetylase remove acetylegroup Histone modification can silence DNA to form heterochromatin. Histone Post-translational Regulatory Modifications All 5 histones can Covalent Modification of core histone tails: Acetylation of lysines (K), Mythylation of lysines, Phosphorylation of serines be modified by methylation (S) acetylation phosphorylation Histone acetyl transferase (HAT) Histone deacetylase (HDAC) acetylation on specific phlysin amino acids lysine methylation lysine serine phosphorylation Histone Post-translational Regulatory Modifications acetylation deposition inecleosome chang session position will be more open minromal of thegene activation silencing methylation inactivation the gene Heterochromatin cell devision phosphorylation meiosis mitosis combination of modification unknown result Transcriptional Regulation by Histone Modification Histone acetyltransferase is component of transcriptional modification activation complexes no silent gene Heterochromatin will an DNA will necleosome become more position open and shape Acetylation RNA Ipperaction more accessible to polymerases Transcription factors synthesize of mRNA  H1 Promotes Nucleosome Compaction H1Binds the complex. It inhibits new transcription Cause the creation Of new mRNA. H1 function thought to be regulated by phosphorylation on N and C terminal tails -H1 +H1  Modifications to histones and DNA direct formation of heterochromatin emove group Deacetylation of H4 Methylation of CpG islands cetyle Methylation of H3 inversion a am i b 2 remove silent become acetyle gfferoinromat group Position Effects on Gene Expression position effect  Summary Chromosomes are decondensed during interphase and hard to visualize see Gene expression needs the decondensation of chromosome loops Chromatin can be catagorized into heterocrhomatin and euchromatin depending on its activity Heterochromatin can be subdivided according to whether it is always inactive or only inactive some of the time Be familiar with the reasoning and consequences of X- inactivation The histone code hypothesis suggests that chemical modification of histone tails effects the activity regions of chromatin