I. A Histone:

   A. A type of protein that functions to package DNA into structures called nucleosomes. A nucleosome is the basic unit/building block of eukaryotic chromosome        structure because a more compact means of packaging DNA does not exist. It is a complex comprised of a structure called a histone octamer and a segment of        DNA that is about 147 nucleotides long which wraps around the octamer about 1.7 times. A histone octamer is a structure composed of 8 histones called core          histones because they make up the nucleosome core. A histone octamer consists of the following: 2 H2A histones, 2 H2B histones, 2 H3 histones, and 2 H4            histones. The packaging of DNA around histone octamers is so condensed because histones are positively charged, and their positively charged amino acids            attract the negatively charged DNA.

   B. Because they comprise nucleosomes and nucleosomes make up chromatin, histones are the main proteins found in chromatin.  

   C. There are 2 types of histones: linker histones (H1 and H5) core histones (H2A, H2B, H3, and H4). 

 

II. H1:

   A. A linker histone: a histone protein that holds the DNA wrapped around the 8 histones that make up a nucleosome core in place; it helps keep DNA                           wrapped around a single nucleosome. 

   B. Helps DNA link nucleosomes together, bringing nucleosomes together to form compact chains of nucleosomes that are about 30 nm in width (the 3rd level             of DNA packaging as will be discussed below). This is why H1 is called a “linker” histone. The DNA that links nucleosomes together is about 80 nucleotides         long.

 

III. H2A, H2B, H3, and H4: 

  A. Core histones; they make up the nucleosome core (the histone octamer). 

  B. A histone tail:

       1. A long, thin region of a histone core protein that sticks out from the region of the nucleosome wrapped with DNA. 

       2. At the end of some of these tails is a carboxyl group (COOH). These ends are called C-terminal ends. At the other end is an amine group (NH3). These                    ends are called N-terminal ends. 

       3. Enzymes called histone modifying enzymes can chemically modify histone tails. These modifications can promote the formation of heterochromatin, which            results in gene silencing, depending on the number, location, and combination of these modifications. Note: heterochromatin is the most highly condensed              form of interphase chromatin (the chromatin that exists during interphase). 

 

IV. DNA Packaging:

      A. There are 4 known levels of DNA packaging.

      B. Level 1: DNA in level 1 of DNA packaging is in its most basic form of packaging. It’s condensed into a structure that resembles beads on a string. The                    beads are nucleosomes, and the string is DNA (called linker DNA) that links the nucleosomes together. The string of beads is about 11 nm in width.

      C. Level 2: DNA in level 2 of DNA packaging is at its 2nd most basic form of packaging. It is condensed into a compact chain of nucleosomes that are about               30 nm in width.

      D. Level 3: DNA in level 3 of DNA packaging is at its 3rd most basic form of packaging. The 30 nm width nucleosome chains mentioned above are further                 condensed into loops (like a straightened shoe lace curling). 

      E. Level 4: DNA in level 4 of DNA packaging is at its 4th most basic form of packaging. The loops mentioned above are further condensed into the familiar               shape of a chromatid (like a single blade of a propeller).