CELL CYCLE

 the orderly sequence of events that coordinates and regulates the cell proliferation ( when it will divide and how and so will not become malignant). there are certain cells that have lost the capacity for division 

• RBC
• muscle cell 
• neurons

certain cells that have the capacity of division but do not divide until getting stimulation:

• fibroblast cells
• lymphocytes
• hepatocytes

POTENCY

the body of a multicellular organism develops, cell specialise. this process of specialization is called DIFFERENTIATION. the specialised cells from tissue that have a specific function. these specialized cells lose the ability to divide.

cells that retain the ability to divide by mitosis are potent.

cell potency: the ability of cells to divide by mitosis giving rise to a further type of cells.

TOTIPOTENT CELLS

ability to make all cell types in the body and cell types that are important for the development of embryos for eg. zygotes in humans.

PLURIPOTENT CELLS

ability to make all cell types in the body for eg. embryonic cell mass.

MULTIPOTENT CELLS

ability to make members of the group of cell types for eg. hematopoietic cells.

UNIPOTENT CELLS

ability to make only a single type of cell for eg. the germinal layer of a cell

IMPOTENT CELLS

no capability of division for eg. mature RBC

DIVISION

it can be of two types

• asymmetric division: certain cells during division provide daughter cells that are different on the basis of size and function referred to as asymmetric cell division. for. eg. stem cell in hematopoietic organ, organogenesis in human female.
• symmetric division: daughter cells acquire a similar fate, leads to proliferation.

CHROMOSOME DENSITY

• heterochromatin: functionally distinct genomic compartment that is characterized by its relatively high gene density.
• euchromatin: characterized by low gene density

chromosome density is high in the middle than at the periphery.

CELL CYCLE

proper control of cell division is vital to all organisms. in unicellular organisms cell division must be balanced with growth so that cell size is properly maintained. it is an orderly series of events that leads to cell division and the production of two daughter cells, each containing chromosomes identical to those of the parent cell.

two main molecular processes are

• S-phase (each parent is duplicated to form two sister chromatids)
• M-phase ( sister chromatids are distributed in daughter cells)

the cell cycle in eukaryotes is a highly conserved, well-ordered series of events. each cycle consists of DNA replication leads to the creation of DNA molecules and is structured for segregation into daughter cells.

the cell cycle is divided into four major parts:

• cycling mammalian somatic cells grow in size and synthesize the RNAs and protein required for DNA synthesis during the G1 phase
• the period in which cells actively replicate their chromosome S- pase
• after processing through the second gap phase G2 phase
• cells begin the complicated process of mitosis also called the M- phase. which is further divided into several stages.

IN PROLIFERATING CELL

G1 is the period between the birth of a cell followed by mitosis and initiation of DNA synthesis which marks the beginning of the S-phase. at the end of the S-phase, cells enter the G2 phase containing twice the number of chromosomes they had in the G1 phase (4n in the diploid organisms and 2n in haploids). the end of G2 is marked by the onset of mitosis, during which numerous events leading to cell division occur. the G1, S and G2phases are collectively referred to as interphase, the period between one mitosis and the next. although chromosomes condense only during mitosis.

CELL CYCLE

during the cell cycle, the preparation of cell division, the cell follows the basic principle of replication and the equal distribution of chromosome in the daughter cell, that's why different phases are observed during cell cycle.

• interphase
• M-phase

INTERPHASE

preparatory phase for division

it includes G1, S, and G2 phases.

G1 PHASE

increase its size. lasts for 11 hrs. most metabolically active phase of the cell cycle. cell organelle duplication. the phase of extensive transcription and translation. helicase loaded. genome condensed and intact. cell cytosol filled with mRNA. addition of radioactive thymidine is used to check the phase of the cell cycle.

S-PHASE

DNA replication occurs. the phase of histone synthesis. centriole duplication initiation. lasts for 8 hours. helicase activation. euchromatin synthesized in the early S-phase, and heterochromatin synthesized in the late S-phase. addition of radioactive thymidine is used to check the phase of the cell cycle, genome check

G2 PHASE

complete centriole duplication. tubulin protein synthesis, the cell prepared itself for division (protein synthesis occurs for proteins to be utilized in M-phase), the phase of final check where the DNA damage is checked and if it is there then it could perform the apoptosis, lasts for 4 hours. double genome, sister chromatin present, initiation of the spindle.

M PHASE

the phase of karyokinesis and cytokinesis lasts for 1 hour. absence of nuclear membrane, the spindle fibre present

 time of cycle in mammals =24 hours 

time of cell cycle in amphibians = 18-24 hours

MITOSIS

it is also referred to as equational division because the chromosome number in the daughter cell is similar to the parent cell. occurs in both somatic and reproductive cells. normally in higher animals, the nuclear membrane disappears referred to as open mitosis. however, in lower animals, the nuclear membrane persists referred to as closed mitosis. it occurs in all cells

STAGES OF MITOSIS

1. PROPHASE

the replicating chromosomes, each consisting of two closely associated sister chromatids, condense. outside the nucleus, the mitotic spindle assembles between two centromeres, which have replicated and moved apart. in diploid cells, there would be two copies of each chromosome present.

nuclear membrane still disintegrating

shortening and thickening of chromosomes continues.

the centrosome is moving in opposite directions while the spindle formation continues but the microtubule is not attached to the chromosome.

2. METAPHASE

nuclear membrane completely disappear 

centrosome forms the opposite poles

spindle formation is completed 

microtubules are attached to chromosomes that's why chromosomal movement begins.

owing to the metaphase all the chromosomes are arranged at the mid of the spindle referred to as the equator to form the equatorial plate or metaphasic plate.

a metaphasic plate is a place where during cytokinesis the equal daughter cells are formed.

3. ANAPHASE

separation of sister chromatids

moves to opposite poles

chromosome number: 4n (for a very short period of time)

amount of DNA: 2C ( for a very short period of time)

4. TELOPHASE

chromosome reach opposite poles and cytokinesis begins.

4. CYTOKINESIS

contractile ring is formed by actin filament at the equator site. 

cytokinesis inhibitor cytochalasin B- blocks the formation of the contractile rings.

MEIOSIS 

always occurs in all diploid cells

meiosis I: reductional division (half the chromosome)

meiosis II: equational division (half the DNA amount)

during meiosis, the pairing of homologous chromosomes is observed and during meiosis I the pairing is dissolved and chromosomes from the pair move to the cell. to provide haploid stage  (half the number of chromosomes of parent cell)

meiosis is subdivided into

• meiosis I
• meiosis II

MEIOSIS I

PROPHASE I

longest sub-stage of meiosis I. the prophase I is further divided as:

• LEPTOTENE: nuclear membrane intact. shortening and thickening of chromosomes. in some species during leptotene all the chromosomes are polarized at one pole referred to as the BOUQUET stage. the chromosome looks like a thick cylindrical shape hence, leptotene is also referred to as the cylindrical thread stage.
• ZYGOTENE: pairing between the homologous chromosomes is occurred because of the formation of the synaptonemal complex. bivalent structure: two homologous chromosome pairing. tetrad structure: 4 sister chromatids
• PACHYTENE: the stage where the crossing over takes place. the physical exchange of genetic material between non-sister chromatids of bivalent structure. the longest phase in the case of spermatogenesis.
• DIPLOTENE: the stage of the de- synapse (the dissolution occurs between the homologous pairing) but the non-sister chromatid still attaches to the point where the crossing over took place, referred to as charismata. the longest phase in the case of oogenesis. primary oocyte arrested at this stage for years. the resting phase of the primary oocyte.
• DIAKINESIS: last stage of prophase I. the spindle formation begins. the nuclear membrane disappearing. shortening and thickening of chromosomes continue. duration of completion of oogenesis- years. duration of completion of spermatogenesis -74-75 days

METAPHASE I

independent assortment of genes occur. independent assortment: some of the genes from the father. the pairing of chromosome 2^n. most important phase as independent assortment occurs. random management of homologous pairs to form metaphasic plates and therefore all the progeny from the parent are not alike. the microtubules or spindle fibres from the polar arranged to both of the chromosomes (not with both sister chromatids). difference between all children by the same parent by independent assortment.

ANAPHASE I

separation of homologous chromosomes.

TELOPHASE

chromosomes are arranged at opposite poles followed by cytokinesis 

MEIOSIS II

the stage of meiosis II is just similar to mitosis. significance: the DNA content becomes half.