This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Chromatin Centromere and Telomere”.
1. Nucleosome was first described in 1974 by ___________
a) William Asbury
b) Rosalind Franklin
c) Roger Kornberg
d) John Crick
Explanation: The basic structural unit of chromatin, the nucleosome, was described by Roger Kornberg in 1974.Two types of experiments led to Kornberg’s proposal of the nucleosome model. First, partial digestion of chromatin with micrococcal nuclease was found to yield DNA fragments approximately 200 bp long. In contrast, a similar digestion of naked DNA yielded a continuous smear of randomly sized fragments.
2. The extent of chromosome coiling in non – dividing cells is _________
Explanation: In non – dividing or interphase cells most of the chromatin is decondensed and distributed throughout the nucleus. This form of chromatin is known as Euchromatin. During this period of the cell cycle, genes are transcribed and the DNA is replicated in preparation for cell division.
3. The whole length of DNA is transcriptionally active ___________
Explanation: About 10% of interphase chromatin is in a very highly condensed state that resembles the chromatin of cells undergo mitosis. This type of condensed chromatin is known as the heterochromatin. Heterochromatin is transcriptionally inactive and contains highly repeated DNA sequences, such as those present at centromeres and telomeres.
4. Why are chromosomes condensed?
a) To facilitate accommodation
b) Always condensed
c) To facilitate cell division
d) To facilitate distribution in daughter cells
Explanation: As the cells enter mitosis, there chromosomes become highly condensed so that they can be distributed to daughter cells. The chromatin in interphase nuclei are organized in loops, which are thought to fold upon themselves to form the compact metaphase chromosomes of mitotic cells in which the DNA has been condensed nearly ten – thousand folds.
5. At which phase do transcription ceases?
c) S phase
d) G phase
Explanation: The chromatin in interphase nuclei are organized in loops that fold upon them to form the compact metaphase chromosomes of mitotic cells which are condensed nearly upto ten – thousand folds. Such condensed chromatin can no longer be used as a template for RNA synthesis and thus transcription ceases during mitosis. As prophase marks the beginning of mitosis thus it is the right option.
6. The part that plays a critical role in even distribution of parental DNA during division is ___________
c) Spindle fibre
Explanation: The centromere plays a critical role in even distribution of parental DNA during cell division. It is a specialized region on the chromosome known as the constricted chromosomal region that holds the sister chromatids together and attaches the chromosomes to the spindle fibres during metaphase of division.
7. With respect to centromere which of the following is wrong?
a) Constricted chromosomal region
b) Holds the sister chromatids together
c) Attaches to spindle fibres
d) Facilitates even distribution
Explanation: As the cell enters mitosis, chromatin condensation leads to the formation of metaphase chromosome consisting of two identical sister chromatids. These sister chromatids are held together at the centromere, which is known as the constricted chromosomal region. As mitosis precedes the microtubules of mitotic spindles attaches to the centromere, and the two sister chromatids separate and move to the opposite poles of the spindle.
8. The protein that binds to the spindle fibres are known as centromeric proteins _________
Explanation: The proteins associated with centromeres form a specialized structure called the kinetochore. The spindle fibres thus formed attaches to these proteins which finally lead to the segregation of the chromosomes to the daughter cells.
9. Centromeric DNA was initially defined in ___________
Explanation: Centromeric DNA sequences were initially identified in yeasts, where their functions were assayed by following the segregation of plasmids at mitosis. Plasmids that contain functional centromeres segregate like chromosomes and are equally distributed to daughter cells following mitosis.
10. How many satellite sequence elements are present in yeast centromere?
Explanation: The centromere sequences of the well studied yeast are contained in approximately 125 base pairs. It has been found to consist of three sequence elements: two short sequences of 8 to 25 bp separated by 78 to 86 bp of very A/T rich DNA.
11. The centromere is A/T rich region ____
Explanation: Yes the centromere is A/T rich region. For example, the centromere of Drosophila is about 420 kb consisting of two highly repetitive satellite DNAs, AATAT and AAGAG, in addition to a non-repetitive region of A/T rich DNA. Again in Arabidopsis, centromeres consist of 3 million bp of an A/T rich 178 bp satellite DNA. In humans also, this feature is observable which is a 171 bp A/T sequence arranged in tandem repeats spanning around 1–5 million base pairs.
12. Which alternate form of histone is seen in centromeric histones of humans?
b) SMC protein
Explanation: It has been observed that the human chromatin near the centromere has a unique structure. In particular, histone H3 is replaced by its variant CENP-A. CENP-A is uniformly present in centromere of all organisms that have been studied and thus are thought to be of prime requisite for the assembly of the kinetochore proteins needed for centromere function.
13. Telomere is not related to __________
b) Chromosome degradation
Explanation: The sequences at the ends of eukaryotic chromosomes are called the telomeres. They play a critical role in maintenance and replication of the DNA and also play a part in chromosomal degradation. Degradation of the telomeres leads to ageing.
14. Which of the following nucleotides is rich I telomere of an organism?
a) A, T
b) T, G
c) G, C
d) C, A
Explanation: The telomere DNA sequences of a variety of eukaryotes are similar, consisting of repeats of a simple sequence DNA containing a cluster of G residues on one strand. For example the repeat sequence of telomere in humans and other mammals is TTAGGG, and in Tetrahymena is TTGGGG. This sequence is repeated over hundreds and thousands of times and is terminated with a 3’ overhanging DNA.
15. How does telomerase activity depend on age?
c) Remains the same
d) Does not occur
Explanation: Telomerase activity does not occur in normal somatic cell as it is present in inactive form. Thus the ends of the DNA does not get replicated every time a cell divides which leads to aging.
16. In cancer telomerase activity__________
c) Remains constant
d) Plays no role
Explanation: Cancer cells have high levels of telomerase activity allowing them to maintain the ends of their chromosomes through indefinite divisions. Since normal somatic cells lacks active telomerase activity they do not divide indefinitely.
Sanfoundry Global Education & Learning Series – Molecular Biology.
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