Which one of the following statements about distribution of chromosomes within the interphase nucleus of a mammalian cell is correct?
The gene-poor chromosomes tend to locate towards the nuclear envelope.
Chromosome Distribution in Interphase Nuclei
During the interphase stage of a mammalian cell cycle, the nucleus is not a disorganized sack where chromosomes are scattered randomly. Instead, the chromosomes occupy specific regions within the nuclear volume, known as chromosome territories. The organization of these territories and the positioning of different chromosomal regions within the nucleus are non-random and influenced by factors like gene density, transcriptional activity, and chromatin state.
Analyzing Statements on Chromosome Distribution
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Statement 1:
Chromosomes are randomly distributed within the nuclear volume.
This statement is incorrect. Research shows that chromosomes occupy distinct, non-overlapping territories. Their positions relative to each other and to the nuclear envelope or nucleolus are not random.
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Statement 2:
The gene-poor chromosomes tend to locate towards the nuclear envelope.
This statement is consistent with observations about nuclear architecture. Chromosomal regions and even entire chromosomes that are relatively gene-poor, containing large amounts of heterochromatin (densely packed, transcriptionally less active chromatin), often tend to be positioned towards the periphery of the nucleus, near the nuclear envelope. Conversely, gene-rich regions, which are typically more actively transcribed (euchromatin), are often found more towards the interior of the nucleus.
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Statement 3:
The larger chromosomes tend to locate at the nuclear periphery.
Chromosome size is not the primary determinant of nuclear position. While there might be some correlation in specific cases, the positioning is more strongly linked to factors like gene density and transcriptional activity rather than physical size.
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Statement 4:
Centromeric region of all the chromosomes tend to concentrate at the center of the nucleus.
Centromeres, the constricted region of a chromosome, show varied positioning depending on the organism and cell type. In some organisms, centromeres might cluster, but in mammalian cells, while they might have preferred radial positions, they do not universally concentrate all together at the exact center of the nucleus during interphase.
Gene-Poor Regions and Nuclear Location
The radial distribution of chromosomes within the interphase nucleus is a key aspect of nuclear architecture. There is a general correlation between gene density and nuclear position: gene-rich areas are often located more centrally, while gene-poor areas are frequently found at the periphery, associated with the nuclear lamina (a protein meshwork lining the inner nuclear envelope). This arrangement is thought to play a role in regulating gene expression by influencing accessibility to transcription machinery.
Therefore, the statement that gene-poor chromosomes tend to locate towards the nuclear envelope accurately describes a characteristic feature of chromosome distribution in the interphase nucleus of a mammalian cell.
Important Questions from Cell Division and Cell Cycle
During cell cycle, entry in the S-phase is tightly regulated. This is possible because:
A. APC/C promotes ubiquitination of S-phase cyclins and mitotic cyclins, marking them for proteolyses at the mitotic exit.
B. Cyclin B1 helps in the activation of S-phase CDKs only in late G1.
C. As mitotic CDK activity declines in late mitosis, cdc14 phosphatase activates APC/C by dephosphorylating Cdh1, thus promoting formation of APC/CCdh1
D. Securin keeps S-phase cyclins in inactive state till late G1.
Which one of the options represents all correct statements?
Following statements were made about cell cycle regulation:
A. De novo synthesis and destruction of Cyclin B are essential for cell cycle progression in yeast.
B. De novo synthesis and destruction of Cyclin B and the related Cyclin dependent Kinase (CDK) are essential for cell cycle progression.
C. CDK activity is regulated by both activating and inhibitory phosphorylation.
D. Retinoblastoma (Rb) functions as an inhibitor of G2 to M transition.
E. Inactivation of Sic 1 is essential for transition into S phase.
Which one of the following represents the combination of the correct statements?
The table below lists cell cycle regulatory proteins and their known functions
Which one of the following options represents the correct match between cell cycle regulatory proteins with their known functions?Cell Cycle regu latory proteins Function A Cdk-activating kinase (CAK) (i) Suppresses G1/S-Cdk and S-Cdk activation in G1; helps cells withdraw from cell cycle when they terminally differentiate; phosphorylation by Cdk2 triggers its ubiquitylation by SCF. B Wee1 kinase (ii) Suppresses G1/S-Cdk and S-Cdk activities following DNA damage C p27 (mammals) (iii) Phosphorylates inhibitory sites in Cdks: primarily involved in suppressing Cdk1 activity before mitosis D p21 (mammals) (iv) Phosphorylates an activating site in Cdks Following statements were made about the characteristics of cyclin proteins:
A. Synthesis of M-cyclin is dependent on the cyclin mRNA that is newly transcribed after every cycle.
B. Destruction of M-cyclin toward the end of mitosis is driven by ubiquitin independent proteolytic system.
C. G1 cyclins can be activated by mitogenic factors.
D. Retinoblastoma (Rb) is a key target of the activated cyclin D - Cdk 4/6 complex.
E. While cyclin A1 expression is ubiquitous, cyclin A2 expression is restricted to the germ cell lineages.
Which one of the following options contains a combination of all correct statements?
To test the impact of cAMP on protein kinase A conformation in cells, an investigator made FRET biosensor by fusing two fluorescent proteins at the N-and C-terminus of protein kinase A. In the absence of cAMP in the cellular milieu, no FRET signal was detected. However, upon cAMP addition, a strong emission at 530 nm was observed. What could be the best configuration of fluorophores that were used by the investigator?
