1. In the beads on a string model, the bead is made up of __________
a) 6 histone proteins
b) 8 histone proteins
c) 6 histone proteins and DNA
d) 8 histone proteins and DNA
Explanation: The “beads on a string” model is for the nucleosome. It consists of the 8 histone protein core or the bead and the DNA wound around imitating a string.
2. The unpacked stretches of DNA are the extra chromosomal load found in the eukaryotic genome.?
a) True
b) False
Explanation: Linker DNA is the stretches of DNA that are not packed into a nucleosome. Typically these are the regions engaged in gene expression, replication and recombination and are generally associated with non – histone proteins.
3. How many types of histone molecules are found in nature?
a) 3
b) 4
c) 5
d) 6
Explanation: Eukaryotic cells commonly contain five abundant histone molecules. They are named as H1, H2A, H2B, H3 and H4.
4. Nucleosome is made up of __________
a) DNA, histone core protein
b) DNA, histone core protein, linker H1
c) RNA, histone core protein
d) RNA, histone core protein, linker H1
Explanation: The core histone proteins are H2A, H2B, H3 and H4, over which the DNA is wrapped. Histone H1 is not a part of nucleosome core particle, instead it binds to the linker DNA and thus is referred to as linker histone. Thus, the histone core, linker histone and DNA are the components of the nucleosome.
5. Histones have a high content of negatively charged amino acids.
a) True
b) False
Explanation: As histones maintain a constant association with negatively charged DNA thus histone molecules are made up of high content of positively charged amino acid. Greater than 20% of the residues in each histone molecules are either lysine or arginine.
6. With respect to assembly of every core histone which of the following is wrong?
a) A conserved region
b) Histone fold domain
c) Disc shaped structure
d) 2 α helices and an unstructured loop
Explanation: The histone fold is composed of 3 α helices and two unstructured loops. In each of these cases the histone fold mediates the formation of head to tail heterodimers of specific pairs of histone.
7. Which of the following histone pairs forms tetramers in solution?
a) H1, H2A
b) H2A, H2B
c) H2B, H3
d) H3, H4
Explanation: H3 and H4 histone first forms heterodimers then they come together to form a tetramer with two molecules of each. In contrast, H2A and H2B form heterodimers only in the solution and histone H1 only acts as the linker histone.
8. With respect to the “tails” of the histone molecules which of the following is not true?
a) N – terminal extension
b) Lacks defined structure
c) Required for the association of nucleosome
d) Sites for extensive modifications
Explanation: The “tail” of the histone is not required for the association for the DNA with the histone octamer into a nucleosome. This is proved when the nucleosome is treated with the protease, trypsin. Trypsin is known to cleave proteins after positively charged amino acid thus, when the N – terminal tail is removed no structural variation is observable in the nucleosome.
9. How many contacts are observed between the DNA and the histone core protein?
a) 14
b) 21
c) 54
d) 17
Explanation: 14 distinct sites of contact are observed, one for each time the minor grove of the DNA faces the histone octamer. 147 base pairs of DNA is wound around the histone octamer and each minor grove occurs after 10 base pairs thus, 14 contacts are observable
10. Association of DNA and histone is mediated by _________
a) Covalent bonding
b) Hydrogen bonding
c) Hydrophobic bonding
d) Vander Waals interactions
Explanation: Association of DNA and histone is mediated by a large number of hydrogen bonds, that is, ≈140 bonds. The majority forms between the protein and the oxygen of the phosphodiester backbone near the minor grove. Only 7 hydrogen bonds are made between the protein side chains and the bases in the minor groves of the DNA.