Question

10.) In complex eukaryotes, what relationships exist between these DNA elements?

a. Centromeres, tandemly repeated DNA and constitutive heterochromatin

b.Interphase chromosomes, mitotic chromosomes, nucleosomes, histones.

c. Supercoiling and chromatin

11.) Which of the following statements is true?

a. Histones are an example of a sequence-specific DNA-binding protein.

b. The majority of human DNA is not known to be useful to our cells.

c. Human females have 23 different chromosomes, whereas human males have 24.

d. Because nucleosomes play a key role in packaging DNA into chromosomes, they bind to the DNA molecule very tightly and, once they are assembled, they cannot be moved.

e. A nucleosome consists of DNA wrapped around eight histone proteins, plus a short segment of linker DNA.

f. Nucleosomes are found only in mitotic chromosomes.

g. Histone proteins have a high proportion of positively charged amino acids, which bind tightly to the negatively charged DNA backbone.

12.) Micrococcal nuclease (MNase) is an enzyme that non-specifically cuts exposed DNA in its phosphodiester backbone. Brief digestion of eukaryotic chromatin with micrococcal nuclease gives DNA fragments ~200 bp long. You repeat the experiment, but incubate the samples for a longer period of time. This longer digestion yields 146 bp fragments. Why? Explain, using a diagram of the packaging of DNA by histone proteins into the basic unit of chromatin.

Answer 1

1. A specific DNA region with a specific sequence of chromosome necessary for pair of sister chromatids (a dyad). The function of centromere is to provide a site for the   kinetochores – a structure which plays main role chromosome segregation. In eukaryotic cells, the DNA sequence centromere's that are present in this region consist of repetitive DNA which is also known as satellite DNA, the sequences in this region are similar but not identical. The DNA in the centromere is usually in the form of heterochromatin where cohesions are added necessary for sister chromatid cohesion and separation of coordinated sister chromatids.
2. Interphase chromosomes are usually indistinct and loose in the form of threads while as the mitotic chromosomes are distinct and thick and usually more visible under microscope. The mitotic chromosomes are linked to centrosomes with mitotic spindle. Histones are the basic protein having positive charge used to wrap or coiling of negatively charged DNA to form electrostatic interactions. These proteins are having basic amino acids like histidine, arginine and lysine. Nucleosomes are the particle made of DNA and the histones. These are more concentrated during the mitotic phase to form thick structures and loosely organized during interphase where DNA is loose and indistinct.
3. Chromatin consists of the DNA, histone proteins, non-histone proteins and RNA wound around to form chromatin. There are two types known as euchromation and heterochromatin. The DNA is wound around the histones by supercoiling and during this process high length DNA is wound to occupy lesser space. This is to the extent that 2 meters of DNA in a single cell occupies less than picmeter place. The supercoiling may be positive or negative depending on clockwise or anticlockwise coiling respectively. This supercoiling taking place around histone protein. Topoisomerases play important role in supercoiling.