Molecular Text 4.4

·         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

·        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

§  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

·         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 __

·         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

·      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 __)

·         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?

·         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

·         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 __

·         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 __

·         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 __. 

·         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

·         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.

·         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

·         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 __. 

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?

·         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