Test: Nucleus And Chromosomes - NEET MCQ

Nucleus of a cell then it is discovered by scottish botanist Robert brown in 1831. The presence of a nucleus in an atom was discovered and proved by Nobel Laureate Dr. Ernest Rutherford through his gold leaf scattering experiment.

Lampbrush chromosomes are a special form of chromosome found in the growing oocytes (immature eggs) of most animals, except mammals. They were first described by Walther Flemming in 1882. Lampbrush chromosomes of tailed and tailless amphibians, birds and insects are described best of all.

Theodor Boveri and Walter Sutton are the scientists credited with an extension of Mendel's laws, known as the chromosome theory of inheritance. In essence, this theory, which was developed in the early 1900s, explains what Mendel observed in his pea plants, otherwise known as the Boveri and Sutton chromosome theory.

Ecdysone stimulates molt and metamorphosis which induces the development of imaginal disks and apoptosis of larval tissue. Ecdysone is also known to stimulates balbiani ring formation in polytene chromosomes. Thus, the correct answer is option C.

Supernumerary chromosomes are also called B chromosomes. They are present in many wild populations like plants, fungi species. These chromosomes are not essential for the life of a species and are lacking in some of the individuals. B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies.

A genome is an organism's complete set of DNA, including all of its genes. It refer to a haploid set of chromosomes of any species. Each genome contains all of the information needed to build and maintain that organism.

The nucleolus organizer region (NOR) or nucleolar organizer is a chromosomal region around which the nucleolus forms. This region is the particular part of a chromosome that is associated with a nucleolus after the nucleus divides. Nucleolus organizer regions are head-to-tail arrays of genes encoding the precursor of the three largest ribosomal RNAs. NORs include active rRNA genes, which give rise to secondary constrictions of metaphase chromosomes, and silent rRNA genes, which are often highly compacted in dense heterochromatin. At metaphase, a proteinaceous remnant of the nucleolus often remains associated with the secondary constriction. So, the nucleolar organizer is a secondary construction and not primary or tertiary construction and centriole.

Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long (a length to width ratio of more than 10 million to 1 in human DNA). For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 micrometers (0.09 mm) of chromatin, which, when duplicated and condensed during mitosis, result in about 120 micrometers of chromosomes. In prokaryotic cells, no organized nucleus is found. Due to the absence of histone proteins, chromosomes are also not found. Instead, a fibrillar area called nucleoid is found attached to the inner side of the membrane. Nucleoid is occupied by an extremely involuted double stranded circular DNA. Nuclear membrane and nucleolus are absent in nucleoid.

Genome is the word to describe the complete set of chromosomes inherited as a single unit from one parent.

The nucleolus is the largest structure in the nucleus of eukaryotic cells. It primarily serves as the site of ribosome synthesis and assembly (packaging of rRNA and proteins). Nucleolus also has other important functions like the assembly of signal recognition particles and playing a role in the cell's response to stress. The nucleolus is made of proteins and RNA and forms around specific chromosomal regions. Malfunction of nucleolus can be the cause of several human diseases. Thus, option D is correct. Nucleolus in a eukaryotic cell is living. not visible in metaphase, only visible in prophase. It is not bound by a membrane.

Site of mRNA synthesis in lampbrush chromosomes is lateral loops. Lampbrush chromosomes are a special form of chromosome found in the growing oocytes. Chromosomes transform into the lampbrush form during the diplotene stage of meiotic prophase I due to an active transcription of many genes. They are highly extended meiotic half-bivalents, each consisting of 2 sister chromatids. A given loop always contains the same DNA sequence, and it remains extended in the same manner as the oocytes grows.These chromosomes are producing large amounts of RNA for the oocyte, and most of the genes present in the DNA loops are being actively expressed. Each lateral loop contains one or several transcription units with polarized RNP-matrix coating the DNA axis of the loop. The majority of the DNA, however, is not in loops but remains highly condensed in the chromomeres on the axis, where genes are generally not expressed.

Chromosomes are composed of DNA, RNA, histones, non-histones. A chromosome is a packaged and organized chromatin, a complex of macromolecules found in cells, consisting of DNA and protein. The functional segments of DNA are known as genes. The DNA-bound macromolecules are proteins, which serve to package the DNA and control its functions.

The role of nucleus in morphology of plant was proved by Hammerling's experiment on Acetabularia. Acetabularia has two different strains one is mediterranea and the other is crenulata. The nucleus is present in the rear part of the cell. Now when he grafted the two using rear part of one and cap part of other he obtained an intermediate form. When this intermediate form was decapitated and allow to grow the organism resemble the type of which the rear part was used in grafting . This showed that nucleus governs the morphology of the cell.

The most diagnostic character of the centromere is the kinetochore. The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore forms in eukaryotes assemble on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis.

Ploidy is the number of set of chromosomes in a cell. A typical human somatic cells contains 46 chromosomes ie., 23 homologous chromosome Paris. Similarly to humans, black molish and private hedge also has 46 chromosomes as a diploid number.

The nucleolus is not bound by limiting membrane. It has a dense fibrillar region in which portions called as secondary constriction or nucleolar organizing regions of different chromosomes meet. Here, rRNA and ribosomal proteins are packed into ribosomal units or pre-ribosomal particles or ribosome precursors or nuclear ribosomes. That is why nucleoli are called as ribosome producing machines. The ribosomal units so formed are joined together by thin filaments, forming a structure like a string of beads. It is called nucleonema. So, nucleonema is a part of the nucleolus and not part of chromosomes, nucleus and ribosomes. Thus, option B is correct and other options are wrong. 

Telomeres are an essential part of human cells that affect how our cells age. Telomeres are the caps at the end of each strand of DNA that protect our chromosomes, like the plastic tips at the end of shoelaces. Without the coating, shoelaces become frayed until they can no longer do their job, just as without telomeres, DNA strands become damaged and our cells can’t do their job.

A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. Karyotypes describe the number of chromosomes and what they look like under a light microscope. The factors considered while making karyotype is the length of the chromosomes, the length of its arms, the position of the centromere, banding pattern, any differences between sex chromosomes. The length of chromonemata is not taken into consideration while preparing a karyotype.
So, the correct answer is option D.

The aggregate of cytoplasmic or extranuclear genetic material in an organism is called "Plasmon".
So, the correct answer is 'Plasmon'.

The telomeres do not have any overhanging ends of nucleotides so it is known as non sticky chromosome ends called telomere. A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. 
So, the correct answer is option D.

Chromosomestransform into the lampbrush form during the diplotene stage of meiotic prophase I due to an active transcriptionof many genes. They are highly extended meiotic half-bivalents, each consisting of 2 sister chromatids. Lampbrush chromosomes are clearly visible even in the light microscope, where they are seen to be organized into a series of chromomeres with large chromatin loops extended laterally.

Highest arm ratio occurs in acrocentric chromosomes. If the p or short arm is so short that it is hard to observe but still present, then the chromosome is acrocentric. The human genome includes five acrocentric chromosomes. In an acrocentric chromosome, the p arm contains genetic material including repeated sequences such as nucleolar organising regions and can be translocated without significant harm. So, the correct answer is option D.

A chromosome in which the centromere is located quite near one end of the chromosome is called the acrocentric chromosome. Humans normally have five pairs of acrocentric chromosomes. Down syndrome is caused by an extra acrocentric chromosome (chromosome21). The acrocentric chromosome does not provide the attachment site for spindle fibers because centromere is located quite near one end of the chromosome.
So, the correct answer is 'Acrocentric'.

Nucleoli are the sites of most steps in ribosome biogenesis, from the transcription and processing of r-RNA to the initial assembly of ribosomal subunits.

The heterochromatin is condensed in the secondary constrlction I and II of the chromosome. Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin. Both play a role in the expression of genes. Chromatin is found in two varieties: euchromatin and heterochromatin. Heterochromatin has been associated with several functions, from gene regulation to the protection of the integrity of chromosomes; some of these roles can be attributed to the dense packing of DNA, which makes it less accessible to protein factors that usually bind DNA or its associated factors.

One solenoid composed of 6 nucleosomes. The solenoid defines the packing of DNA as a 30nm fibre of chromatin and results from the helical winding of at least five nucleosome strands. The nucleosomes resemble beads on a string, are further compacted into a helical shape via the NH₂ terminal protein interactions of the octameric histones, called a solenoid. DNA packed into solenoids, unlike DNA in nucleosome form, is not transcriptionally active. With more packing, solenoids are able to become increasingly more packed, forming chromosomes. At this point, solenoids coil around each other to form a loop, followed by a rosette, then a coil, and at last, two chromatids. The end result is the metaphase chromosome. The completely condensed chromatin has a diameter of up to 700 nm.

All chromosomes mentioned are large chromosomes. These chromosomes are producing large amounts of RNA for the oocyte, and most of the genes present in the DNA loops are being actively expressed. Each lateral loop contains one or several transcription units with polarized RNP-matrix coating the DNA axis of the loop.The majority of the DNA, however, is not in loops but remains highly condensed in the chromomeres on the axis, where genes are generally not expressed. This is about lampbrush chromosomes, the polytene and mega chromosomes are also larger than the other normal chromosomes.

Prophase I is the longest stage of meiosis I. The chromosomes appear as beaded structures at leptotene stage. It is the first stage of meiosis.
So, the correct answer is option A.