Cell cycle - class-VIII

Howard and Pelc divided cell cycle in four stage. The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. The stages  are G$ _1$-S-G$ _2$-M. The G$ _1$ stage stands for "GAP 1". The S stage stands for "Synthesis". This is the stage when DNA replication occurs. The G$ _2$ stage stands for "GAP 2". The M stage stands for "mitosis", and is when and cytoplasmic division occur.

In mitosis DNA content is doubled during S phase resulting in 2n chromosomes having two chromatids. These 2 chromatids of 2n get arranged at metaphase plate in metaphase. And in anaphase, the 2 chromatids of 2n chromosomes are separated from each other . so the cell during this phase becomes 4n.

Early in the history of biology, scientists believed cells arose spontaneously. With the development of cell theory, people finally realized that only cells can beget other cells. In fact, two categories that define something as living or not are growth and reproduction, both of which cell division accomplishes. Cell division, also called mitosis, occurs in all living things. As living things grow, some cells die or become damaged and need replacements. Some single-celled organisms use a type of mitosis as their only form of reproduction. In multicellular organisms, cell division allows individuals to grow and change by expanding the number of total cells. The division of the nucleus is equal during cell division. This ensures the characteristic of both parent cell in daughter cells and restores the size of the nucleus.

Mitosis is equational division and meiosis is reductional division. Both consists of 4 phases, namely prophase, metaphase, anaphase and telophase. Prophase is the first phase of both mitosis and meiosis I. During this phase, the chromatin condenses and forms visible chromosomes. The mitotic spindle begins to form, nucleolus disappears, the nuclear envelope disappears. 

In sexual reproduction, an egg cell from female and a sperm from a male fuse to form a zygote, precursor to an embryo offspring. The offspring is generally distinct from parents. Both the gametes contain half the number of required chromosomes, that is they are haploid in nature. The zygote contains the exact number of chromosomes required. This is how chromosome number is conserved from generation to generation. The zygote undergoes further cell division to form an embryo. 

1. In G$ _1$  phase the amount of DNA is the same as 40 because the cell is metabolically active but no division of DNA is observed.
   
2. In G$ _2$ phase the DNA gets doubled as it has gone through the synthesis phase which is characterized by the replication of DNA.
3. In mitosis since equational division occurs the DNA content remain 4C through the process before the transfer of DNA into daughter cells.

G$ _1$ , S , G$ _2$, M , cytokinesis. The correct sequence has G$ _1$ as a preparation for S and G$ _2$ as the time between the completion of S and entry into M. Cytokinesis occurs after the other stages to create two daughter cells.

In the maturation phase, the enlarged cells eventually acquire a specific size and form according to their location and role following biochemical, physiological and morphological changes, i.e., the cells undergo specialization or transformation. As a result, various kinds of cells get differentiated. These differentiated cells form different kinds of simple and complex tissues that perform different functions. This includes cell wall growth and protoplasm modification.

The cell cycle mainly consists of 4 stages G$ _1$, S, G$ _2$, and M. The G$ _{0}$ phase also known as the extended G1 phase is a period in which the cell exits from the cell cycle. In this stage, the cell exists in a quiescent state and does not divide. Thus the correct answer is option A.

During the cell cycle, RNA and protein synthesis takes place during G$ _1$ and G$ _2$ phase. The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. The stages are G$ _1$-S-G$ _2$-M.

G1 phase and the other subphases of the cell cycle may be affected by limiting growth factors such as nutrient supply, temperature, and room for growth. Sufficient nucleotides  and amino acids must be present in order to synthesize mRNA and proteins. Physiological temperatures are optimal for cell growth. In humans, the normal physiological temperature is around 37 C. Therefore, the correct answer is option D.

Generation time is defined as the total time taken by a cell to complete one full cell cycle, which includes interphase and the M-phase.

The eukaryotic cell cycle follows a specific sequence: G1 phase, followed by the S phase (DNA synthesis), then the G2 phase, and finally the M phase (mitosis), which concludes with cytokinesis.

The cell cycle includes a series of events, that take place  in a cell which leads to the duplication of DNA and division of cells to form two daughter cells. This takes place in three different  periods and can be referred to as Interphase , Mitotic phase(M) and cytokinesis. While during the interphase or the initial phase, the cells grow in size and is ready for division, during the mitosis, the DNA starts duplicating, the nuclear membrane dissolves and the cells starts its division process to give rise to  two distinct daughter
cells. Cytokinesis is the stage during which the two daughter cells are formed and separated.
So, the correct answer is option D.

One cell cycle of yeast is almost 90 minutes and a human cell takes almost 24 hours to divide by mitosis therefore by calculations it seems that a yeast can perform mitosis almost 16 times in the time of 1 division of a human cell.

At the start, the culture of E. coli has cell population of 1.5 X 10$^6$ cells/ml. After first generation, it will be doubled, that is 3 X 10$^6$ cells/ml. After second generation, it will be 6 X 10$^6$ cells/ml. After third generation, it will be 12 X 10$^6$ cells/ ml. It is given that for a cell population to increase from 1. 5 X 10$^6$ cells/ml to 12 X 10$^6$ cells/ml, it takes 36 minutes. It means, for 3 generation, it takes 36 minutes. So, its generation time is 36/3 = 12.

Through phosphorylation, Cdks signal the cell that it is ready to pass into the next stage of the cell cycle. As their name suggests, Cyclin-Dependent ProteinKinases are dependent on cyclins, another class of regulatory proteins. Cyclins bind to Cdks, activating the Cdks to phosphorylate other molecules.

So, the correct option is ‘binding to cyclin, plus, phosphorylation by Cdk activating protein kinases’.

The death of cells which occurs as a normal and controlled part of an organism's growth or development is called apoptosis. In this endoplasmic reticulum, lysosome, mitochondria, Golgi complex are involved.

So, the correct option is ‘ER, lysosome, mitochondria’. 

The cell progresses through defined phases of the cell cycle during its division.

Interphase is the longest part of the cell cycle, occupying approximately 90 to 95 percent of the total time in most actively dividing cells.

A. Replication protein (RPA) - is the major protein that binds to single-stranded DNA (ssDNA) in eukaryotic cells. ... During DNA replication, RPA prevents single-stranded DNA (ssDNA) from winding back on itself or from forming secondary structures.

B. ATM (ataxia telangiectasia mutated ) protein

 It activates checkpoint signalling upon genotoxic stresses such as ionizing radiation (IR), ultraviolet light (UV), or DNA replication stalling, thereby acting as a DNA damage sensor.

C.Cyclin D - a member of the cyclin protein family that is involved in regulating cell cycle progression. 

D.PSS - an autoimmune disease of the connective tissue. It is characterized by thickening of the skin caused by the accumulation of collagen, and by injuries to small arteries.

So, the correct option is ‘Replication protein A (RPA)’.

• Cyclin-dependent kinases, through phosphorylation,  signal the cell that it is ready to pass into the next stage of the cell cycle. As their name suggests, Cyclin-Dependent Protein Kinases are dependent on cyclins, another class of regulatory proteins. Cyclins bind to Cdks, activating the Cdks to phosphorylate other molecules.

The cell cycle starts from interphase . Interphase is the phase in the cell cycle that prepare a cell and its nucleus for division. It has three stages- $G _1,S $ and $G _2$.M- phase is the final phase of cell cycle..It represent the phase of actual division. It consists of karyokinesis (division of nucleus ) and cytokinesis (division of cell protoplast).

The cell cycle starts from interphase . Interphase is the phase in the cell cycle that prepare a cell and its nucleus for division. It has three stages- $G _1,S $ and $G _2$.M- phase is the final phase of cell cycle..It represent the phase of actual division. It consists of karyokinesis (division of nucleus ) and cytokinesis (division of cell protoplast).

The cell cycle is a sequence of event which occurs during cell growth and cell division. It has two phases; interphase and the M phase. Interphase is the period between the end of 1 division and the beginning of next cell division. It is also known as resting phase or the non-dividing stage.

Cell division is the process by which a cell divides to form to nearly equal daughter cell which resembles the parent. The cell cycle is the sequence of events which occur during the cell growth and cell division. It is completed into two steps interphase and M-phase.

During the cellular division phase, metaphase is a stage where all the chromosome arranges themselves on the metaphase plate.

Cell cycle consists of two basic phases, long nondividing I phase and a short dividing M phase . In the average duration of 24 hours in the cell cycle of dividing human cell , M phase lasts for one hour (less than $5%$ of total) while I phase or interphase occupies $75-95%$ of total generation time.

Answer is option B i.e. "Two"

• Correct statements are statements C and E because-
  

• While, Nucleus was discovered by  Robert Brown in 1831 and published in 1833.

• During interphase chromatin is distributed throughout the nucleus.
• Every chromosome essentially has a primary constriction which is also known as kinetochores. Nucleolus organiser regions (NORs) are chromosomal regions crucial for the formation of the nucleolus.

All the different cells of our body usually live, grow and divide in harmony. Our body has many different signals that control how much, and how often our cells divide. If any of these signals are faulty or missing, the result can be cancer, where too many cells are produced.

The major events that take place in mitosis are the alignment of chromosomes at the center of the cell and the subsequent separation of sister chromatids to opposite mitotic spindle poles. Those occur in metaphase and anaphase respectively.

Interkinesis (often misspelled as intermitosis) is the short resting phase between meiosis I and meiosis II. It is distinct from the interphase that occurs before the start of meiosis I.

At G1 phase, there will be 14 chromosomes as no DNA replication has taken place. After S Phase, the DNA content doubles but chromosome number still remains same here 14. After completion of M phase, the number of chromosomes will be 28 with each daughter cell having same number of chromosomes as that of parent i.e 14

$G _2$ phase is the phase of cell cycle occurs immediately prior to mitosis.
sequence of cell cycle is -- $G _1$ to $S$ to $G _2$ to $M$ to cytokinesis.