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The G2 checkpoint is often referred to as the  G2-M checkpoint  because it is located at the boundary between the G2 phase and the M phase (mitosis) of the cell cycle. This checkpoint plays a key role in deciding whether the cell is ready to move from G2 phase into mitosis. The name "G2-M" itself reflects its position and function, it lies at the end of the G2 phase but directly regulates the cell's entry into the M phase. During the G2 phase, the cell completes the synthesis of proteins and prepares all necessary components for mitosis. However, before entering mitosis, the cell must ensure that the DNA, which was replicated during the S phase, is completely and correctly duplicated and that there is no DNA damage. At this critical point, the  G2-M checkpoint becomes active.  It functions like a gatekeeper or quality control system. It checks: Whether DNA  replication has been completed without any errors Whether the DNA is  free of any damage Whether the ...

Cyclins are essential regulatory proteins that control the progression of the cell cycle. Their activity is highly regulated in a dynamic manner to ensure the cell cycle proceeds at the appropriate time and under the right conditions. This regulation involves the synthesis, activation and degradation of cyclins at specific points during the cycle. The precise control of cyclin levels and activity is crucial for maintaining normal cell function and preventing diseases like cancer. The dynamic regulation of cyclins helps coordinate various processes such as DNA replication, cell division and checkpoint control, ensuring the cell divides only when it is ready and the conditions are appropriate. Importance of Dynamic Regulation of Cyclins: 1. Controlled Progression through the Cell Cycle: Cyclins regulate the transition of cells from one phase of the cell cycle to the next by activating Cyclin-Dependent Kinases (CDKs). The dynamic regulation of cyclins ensures that the cell proceeds from o...

The G2 phase (also known as Gap 2 phase) is the third and final phase of  interphase  in the cell cycle. It occurs after DNA replication in the S phase and before the cell enters mitosis (M phase). While the G2 phase may seem like a resting phase, it is actually a highly active period during which the cell prepares for division. The G2 phase ensures that the cell has successfully completed DNA replication and is ready to divide. It also serves as a  checkpoint  for the cell to correct any errors that may have occurred during DNA replication, ensuring that only healthy, complete genetic material is passed on to daughter cells. 1. DNA Damage Check and Repair through the G2-M Checkpoint One of the major significances of the G2 phase is to check for DNA damage that may have occurred during the  S phase , where DNA replication takes place. If there is any damage in the DNA, the cell does not immediately move into mitosis. Instead, a special control system known as th...

SAQ i) Match the items in column I with column II: Answer: a) → iv;    b) → iii;    c) → vii;    d) → viii;    e) → vi;    f) → i;    g) → ii;    h) → x;    i) → v;    j) → xi;    k) → ix ii) Fill in the blanks: a) Autophagy includes the formation of three different vesicles including ....................., which circularizes to form ......................, which then fuses with a lysosome to form the ..................... . Answer: phagophore, autophagosome, autolysosome b) Apoptosis can be mediated by two modes or pathways, ....................... or ....................... . Answer: intrinsic, extrinsic c) Pharmacological inhibitors such as ....................... block the lysosomal proton transport and thus autophagy. Answer: Bafilomycin A1 d) ...................... is released from the mitochondria upon MOMP and binds to the adapter protein Apaf1 to form the complex known as ..........

SAQ 1 Mark the correct option: a) The cellular response to a particular signaling molecule depends on the number of     i) receptor     ii) ligand     iii) receptor-ligand complexes Answer: iii) receptor-ligand complexes b) The binding of receptor to ligand binding is mediated through weak     i) non-covalent forces     ii) covalent forces Answer: i) non-covalent forces c) Different receptors of the same class that bind different ligands often induce the     i) same cellular responses in a cell     ii) different cellular responses in a cell Answer: i) same cellular responses in a cell d) Many signaling molecules bind to multiple types of receptors, each of which can activate different intracellular signaling pathways and thus induce     i) same cellular responses     ii) different cellular responses Answer: ii) different cellular responses SAQ 2 Fill in the blanks: a) The binding of ligands to many...

SAQ 1 a) What would happen if there were substances that could bind to Ga subunits just like GTP does, but could not be hydrolysed by the intrinsic GTPase? If substances were present that could bind to the Gα subunits just like GTP does, but could not be hydrolyzed by the intrinsic GTPase activity, it would lead to a persistent activation of the G-protein signaling pathway. This is because the normal process of signal termination in G-protein signaling involves GTP hydrolysis by the intrinsic GTPase activity of the Gα subunit. When GTP is hydrolyzed to GDP, the Gα subunit becomes inactive, effectively terminating the signaling cascade. However, if a substance mimics GTP binding but does not undergo hydrolysis, the Gα subunit would remain in its active GTP-bound form indefinitely. This leads to prolonged activation of downstream signaling pathways, such as the activation of second messengers like cAMP, DAG and IP3, and the continuous activation of effector proteins. Such persistent sig...

Cyclins are  regulatory proteins  that play a very important role in the regulation of the cell cycle. They are a special group of regulatory proteins that control the proper timing and order of different phases of the cell cycle, such as G1, S, G2 and M phase. Cyclins do not work alone. They bind with specific enzymes called  Cyclin-Dependent Kinases (CDKs)  to form an active  cyclin-CDK complex.  This complex helps in starting and controlling various steps of the cell cycle. The levels of cyclins are not constant. They rise and fall at specific times during the cycle. This timely appearance and disappearance of cyclins ensure that the cell cycle proceeds in a proper sequence. Cyclins Have the Following Roles in the Cell Cycle: 1. Regulation of Phase Transitions: Cyclins regulate the transition of the cell from one phase to another. Each cyclin activates a specific CDK complex that is required for the progression to the next phase. For example,  Cycli...

The S-phase (Synthesis phase) is a critical part of interphase in the cell cycle, occurring between the G1 (gap 1) phase and the G2 (gap 2) phase. During this phase, the cell duplicates its DNA to prepare for cell division, ensuring that the genetic material is accurately passed on to the daughter cells. This phase is essential for maintaining genomic integrity and stability. Key Events in the S-phase 1. DNA Replication: The main event of the S-phase is DNA replication. This process ensures that the entire genome is copied so that each daughter cell will have an identical set of chromosomes. The helicase enzyme unwinds the DNA double helix, creating two single strands. These single strands act as templates for the synthesis of new complementary strands. 2. Activation of DNA Polymerases: DNA polymerases are key enzymes that catalyze the addition of new nucleotides to the growing strand. On the  leading strand,  DNA polymerase synthesizes continuously in the 5' to 3' direction. ...

In eukaryotic cells, the transition from G1 to S phase of the cell cycle is one of the most important and carefully regulated steps. This phase decides whether a cell will proceed toward DNA replication or stay in a resting state (G₀ Phase) or even go for repair. Cells make this decision based on internal and external signals, and this entire decision making process is controlled by specific regulatory proteins. There are five main types of protein regulators that play a central role in G1 to S phase transition. These proteins work in coordination and any disturbance in their function can lead to abnormal cell division or diseases like cancer. 1. Cyclins (Cyclin D and Cyclin E) Cyclins are regulatory proteins that appear and disappear during different phases of the cell cycle. During the early G1 phase,  Cyclin D  is produced in response to external signals like growth hormones or mitogens. It binds to  CDK4  and  CDK6,  forming active complexes that start ...

Cyclins are a special group of regulatory proteins that control the proper timing and order of the cell cycle. They are called  "cyclins"  because their concentration inside the cell rises and falls in a  cyclical pattern  during different phases of the cell cycle. They are not enzymes by themselves but become functional when they bind with another group of proteins called  cyclin-dependent kinases (CDKs).  These CDKs are enzymes that phosphorylate target proteins and this phosphorylation triggers specific events in the cell cycle like DNA replication or mitosis. Cyclins are essential because they ensure that the cell cycle progresses in the correct sequence. Without them, cells would not know when to grow, when to copy DNA, or when to divide. Cyclins are produced at specific times during the cell cycle and once their function is complete, they are broken down by a system called the  ubiquitin-proteasome pathway.  This degradation prevents the cel...