3.2 Cell Nucleus and Gene Expression Flashcards
Histone Variants
-CENP-A binds to satalite DNA seqences at the centromere to form kinetochore
2nd Level of Packing
30nm Fiber Interaction:
- Histome core and drawn closer together and packing more tightly
- 30 nm fiber-interaction between histone proteins of neigbouring nucleosomes
- H1 (linker) histones are essential for formation of 30 nm fiber
- If you remove H1 protein it’ll fall apart back to the beads on the string which is why its called a linker
- Nucleosomes line up end to end in two stacks
- Alternating exons interact across helix via linker DNA
3rd Level of Packing
Chromatin Loops:
- the 30nm fibres form these larger loop structures.
- The proteins that tie these loops together (twist ties) are cohesion proteins.
4th Level of Packing
Mitotic Chromo
-final ultimate compact version of chromatin
2 Parts of Chromatin Structure
Euchromatin
Heterochromatin
Euchromatin
- DNA that becomes dispersed during interphase
- is trasciptionaly active (RNA Pol has access to this)
Heterochromatin
-DNA that remains compacted during interphase
- is trasciptionally inaction, its very very compacted. permanently compacted during interphase.
-Constitutive: Permanently compacted
Facultative: Transiently compacted (goes back and forth between euchromatin and heterchomatin state)
Example of X Chromosome Inactivation
-Inactivation through heterochromatization is what happens to the X chromo with organisms that have 2 X chromos. Cats have 2 X chromes .
-Calico cat it’s a pretty sure thing that its a female: genetic mosaic
-Pigment genes for cats are on X xhromo. Dosage compensation. the 2 X’s must be compensated, so one X chromo inactive due to heterochromatin called barbin
-Occurs in early devo (8 cell). Inactivation is completely random. That’s how you get mosaic
-Therefore its random if mom or dad is inactivated or not
Those become somatic cell lineage, therefore if theres pigment genes on one X one patch of fur comes from maternal genes and the other patch comes from paternal genes
-Reactivation of X chromosome occurs before meiosis
-XIST plays a role in the inactivation
Histone Code
- each bound proteins alter the structure and the eventualy the function (ie. Does it transcribe or not)
- Not indépendant events, they can interact with each other
- Acetylation = active chromosomes
- methylation = inactive chromo
Describe the pathway for heterochromatic formation as influenced by histone H3 modification, describe the modification what it is and the portions involved.
Step 1: RNAs transcribed from both strands of repeated DNA
Step 2: Form dsRNAs
Step 3: Dicer enzyme generates Single stranded siRNA guide
Step 4: Histone MTase guided to Euchromatic DNA
Step 5: Methylation of K9
Step 6: Binds HP1 protein
Step 7: Binds associated proteins with chromodomains
-These are RNA sequences. Make longer trascnips and base par them together, then base pair // is diced up by dicer in smaller sequences and they see as a guide for histone mehtylase. Affd s methyl to the K9 (Lys9) residue of H3 core histone (replacing the acetyl group. Acetyl groups are characterstic of euchromatin, and are recomved from lysine reside). All acetyls are methyls which serve as binding sites for HP1 proetion. binds together. pulls the DNA tiger together. Methylation of H3 makes it capable of binding proteins with chromodomain. Once HP1 is bound to histone it promotes binding of other proteins and interconnecting network of methylated nucleosomes.
Lys9
Lys 9 of Histone H3 is is methylated in heterochromatic DNA and unmethylated in euchromatic DNA.
Methylation is catalyzed by histone methyltransferase enzyme.
Enzyme is localized within heterochromatin.
Methylation of H3 makes it capable of binding proteins with chromodomain (e.g. Heterochromatic Protein I).Once HP1 is bound to histone it promotes binding of other proteins and interconnecting network of methylated nucleosomes.
Mtase = methylase
Mitotic Chromosome Preparation: How do you make a mitotic spred
Need a source of DNA, normally blood, isolate WBC, then extract the cells, culture the cells and then mitotically arrest the leukocyte [halt the cells in mitosis with colchicine which interferes with microtubule assembly and it can’t proceed past mitosis and halts in prophase after they’ve condensed] collect cells by centrifugation, wash and remove supernatant
then you fix them put them on a slide and stain them then you can create a karyotype.
2 Chromosomal Aberrations
Inversions
Translocations
Inversions
- Genetic information retained but can result in abnormal gametes
- (the pattern is invested in 2 homologies) this occurs during meiosis, one of the homologies is looped, one linear one loop and recombination results in an inversion.
- Recombination can result in duplications or deletions of genes
Translocations
- All or one piece of a chromosome becomes attached to another chromosome
- human chromo 2 is the amalgam of 2 chromes in apes
Telomeres
-Tellers are ends of chromes. -At the ends of chromo are repeating sequences
the repeating sequences is essential for stability and telomere function
-(TTAGGG) repeated up to 500 times
-Found in all vertebrate organisms
-RAP1 binds to this repeats structure and plays a role with chromosome integraity.
-RAP1 is part of protein complex that protects against degradation of telomere end. Protects against DNA repair machinery.
End-Replication Problem
- in replication, it needs a primer to prime the DNA sequence which comes in the form of RNA and the enzyme responsible for that is called primate. Primate creates small RNA primer which primes the rest of DNA synthesis. You have remement of RN which is removed and your left with a shorter replicated strand. This isn’t filled in.
- Through replicated cycles (after a certain amount of cells cycles they die) this is related to the shortening of the DNA
- Germ line cells don’t have shorting of their telomers (these are the cells for the production of games) the repeating sequences act as a buffer so that it can be chewed off and it will still be okay.
Telomere Structure
- 3’ overhang invades DNA duplex and creates loop structure
- Binds telomere specific proteins (protects telomere)
- Chromo loops around and tucks itself back into it, binds proteins that serve as protection.
- Somatic cells have shortening of telomeres, and germline cells do not.\
- Germline cells have telomerase which allows them to not degrade.
Telomerase Function
- Within the telomerase is RNA sequence, the stretch of RNA in the telomerase is complementary so it can base pair to the telomere repeats and serves as a template fore more DNA to extend it out, elongates it out. Once it’s elongated out enough, the gap can be filled with polymerase alpha primase. Once you intend it out long enough it can become a template that ca be filled in. Only in gremlin cells.
- RNA binds to telomere repeat
- Serves as template for DNA polymerization
- Gap filled by polymerase α-primase
Telomeres and Chromosome Integrity
- somatic cells go down until they reach crisis point, leads to ssenescence (can maybe exit the crisis point)
- cells that are able to reactivate telomerase can exit crisis and become immortal
Space Boy!
Telomers of fellow in space lengthen in space when back on earth they shorted to their original length
