Molecular Text 4.4

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  1. ·         Although enormously long strings of nucleosome form on the __, chromatin in a living cell rarely adopts the __
    o   Instead, the nucleosomes are arranged how, generating regular arrays in which the DNA is even more __
    ·         The structure of a __supports the __model for the stacking of nucleosomes in the 30-nm fiber
    • chromosomal DNA
    • extended “beads on a string” form
    • packed on top of one another
    • highly condensed
    • tetranucleosome 
    • zigzag
  2. ·        What forms nucleosomes to stack of tightly on each other in a 30-nm fiber are the __ formed by __, most notably the __, as well as the additional __that is often present in a 1-to-1 ratio with __, known as __.
    o   This __ is larger than the individual core histones and it has been considerably less well conserved during evolution
    • nucleosome to nucleosome linkages
    • histone tail
    • H4 tail
    • histone 
    • nucleosome cores
    • histone H1
    • linker histone
  3. §  A single __ molecule binds to each __, contacting both __, and changing the path of the DNA as it exits from the __
    ·         A change in the exit path in DNA seems crucial for __ so that it interlocks to form the 30-nm fiber
    • histone H1
    • nucleosome
    • DNA and protein
    • nucleosome
    • compacting nucleosomal DNA
  4. ·         It is possible that the __ found in chromosomes is a __ of several different variations
    o   These differences in __ probably introduce __ into the structure; and, the presence of many other __, as well as proteins that bind directly to histones, will add important additional features to any array of __
    • 30-nm structure
    • fluid mosaic
    • linker length
    • local perturbations
    • DNA-binding proteins
    • nucleosomes
  5. ·         Certain types of chromatin structure can be __—passed down from a cell to its descendants
    o   Because the __ that reults is based on an __ rather than on a __, this is a form of __
    ·         Chromatin structure plays a central role in the __, __, and __, including ourselves
    • inherited
    • cell memory
    • inherited protein structure
    • change in DNA sequence
    • epigenetic inheritance
    • development, growth, and maintenance of eukaryotic organisms
  6. ·      Biochemists had determined that mammalian chromatin consist of an approximately equal mass of __ and __, meaning that, on average, every 200 nucleotide pairs of DNA in our cells is associated with more than __ (that is, a mass of protein equivalent to the total mass of the __)
    • histone and non-histone proteins
    • 1000 amino acids of non-histone protein
    • histone octamer plus histone H1
  7. ·         There is an amazingly slow rate of evolutionary change in the sequence of the __

    o   A change in any one of the __ in __must be deleterious to these organisms
    ·         A combination of genetics and cytology had revealed that a particular form of chromatin does what?
    • four core histones
    • 102 amino acids in H4 
    • silences the genes that it packages without regard to nucleotide sequence—and doe so ina manner that is directly inherited by both daughter cells when a cell divides
  8. ·         There are two types of chromatin: a __ and a __
    o   __is a compact form and highly concentrated in specific regions; more than __ of the genome is packaged in this way
    • highly condensed heterochromatin and a less condensed euchromatin
    • Heterochromatin 
    • 10%
  9. ·         The DNA in heterochromatin contains very few __. And those __ that become packaged into __are __ by this type of packaging
    o   Heterochromatin encompasses several distinct types of chromatin structure whose common feature is an __; and, it creates __
    • genes
    • euchromatic genes
    • heterochromatin 
    • turned off
    • especially high degree of compaction
    • different types of compact chromatin with distinct feature that make it highly resistant to gene expression for the vast majority of genes
  10. ·         When a gene that is normally expressed in __is relocated into a region of __, it ceases to be expressed, and the gene is said to be __.
    o   These differences in gene expression are examples of __, in which the activity of a gene depends on its position relative to a nearby region of __ on a chromosome
    §  The __ associated with heterochromatin exhibit a feature called __, which in retrospect provided critical clues concerning __
    • euchromatin 
    • heterochromatin
    • silenced
    • position effects
    • heterochromatin
    • position effects
    • position effect variegation
    • chromatin function
  11. ·         In drosophila, __ that directly connect a region of __to a region of __tend to __the nearby __

    o   The ___ spreads a different distance in different early cells in the fly embryo, but once the __condition is established on a gene, it tends to be stably __

    o   Excessive genetic screens have been carried out in Drosophila in search for gene products that either __ or __ and __—that is, for genes that when mutated serve as either __ or __ of __. 
    • chromosome breakage events
    • heterochromatin 
    • euchromatin 
    • inactivate 
    • euchromatic genes

    • zone of inactivation
    • heterochromatic 
    • inherited by all of the cell’ progeny

    • enhance or suppress the spread of heterochromatin and its stable inheritance
    • enhancers or suppressors of position effect variegation
  12. ·         The detailed characterization of the proteins produced by these genes has revealed that many are __ that underlie a remarkable mechanism for __, one that requires the __, explaining the slow change in histones over time
    • nonhistone chromosomal proteins
    • eukaryotic gene control
    • precise amino acid sequences of the core histones
  13. ·         The amino acid side chains of the __ in the __ are subjected to a remarkable variety of __, including __, __, and __.
    o   A large number of these side-chain modifications occur on the __that protrude from the __. However, there are also specific side-chain modifications on the nucleosome’s globular core.
    • four histones
    • nucleosome core
    • covalent modifications
    • acetylation of lysines, the mono, di, and tri-methylation of lysines, and the phosphorylation of serines
    • eight relatively unstructured N-terminal “histone tails” 
    • nucleosome
  14. ·         All of those modifications are __. Modifications of side chains in a nucleosome are created by __, which usually act on __. A different enzyme ___.
    o   Ex: __add acetyl groups; histone __ remove
    o   __ add methyl groups and __remove

    • reversible
    • specific enzymes
    • one or a few sites
    • removes each modification
    • acetyl transferases (HATs) 
    • deacetylase complexes (HDACs)
    • Methyl transferases
    • demethylases
  15. ·         The initial recruitment of these enzymes depends on __ that bind to __, and these are produced at different times in the lfie of an organism
    ·         In some cases, the __ on nucleosomes can persist long after the __ that first induced them have disappeared, thereby carrying a memory in the cell of its developmental history. Different patterns of __ are found on different groups of __. 
    • gene regulatory proteins
    • specific DNA sequences along chromosomes
    • covalent modifications
    • gene regulatory proteins
    • covalent modification
    • nucleosomes
  16. o   Modification of histones are carefully controlled with important consequences.
    §  The acetylation of lysines on the N-terminal tails tend to __, in part because adding an acetyl group to lysine does what?
    • loosen chromatin structure
    • removes its positive charge, thereby reducing the affinity of the tails for adjacent nucleosomes.
  17. ·         The most profound effect of the histone modifications is their ability to __. These new proteins determine what?
    o   In this way, the precise structure of a domain of chromatin determines the __, and thereby the structure and function of the eukaryotic cell
    • attract specific proteins to a stretch of chromatin that has been appropariteyl modified
    • how and when genes will be expressed, as well as other biological functions

    expression of the genes packaged in it
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Molecular Text 4.4
Test One
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