Cell Cycle

Cell Cycle: cell cycle is serious of events when cell produces new cells in this process genetic material replicate and produce new cells with having same type of genome or different genome.

If cell produces  new cell with same genome means parent genome  duplicated produce same genome that process is called Mitotic division. We can see this type of reproduction in vegetative cells .

If cells produces new cell with combination of two genomes half from one parent and other half from other genome that process is called Meiosis cell division. We can observe this process in reproductive cells .

Both cell cycle are control by some proteins ,which activates  cell cycle. four groups—growth factors Growth factors, growth factors receptors, signal transducers, and nuclear regulatory proteins. First, a growth factor must bind to its receptors on the cell membrane. Second, the receptor must become temporarily activated by this binding event. Third, this activation must stimulate a signal to be transmitted, or transduced, from the receptor at the cell surface to the nucleus within the cell. Finally, transcription factors within the nucleus must initiate the transcription of genes involved in cell proliferation.

Chromosomes and chromatin structure Cell Division  Mitosis  Meiosis   Eukaryotic Gene and Gene Expression

Cell Cycles are regulate

Cell Cycles are regulate by two enzymes , cyclins  and cyclin dependent protein kinase. Cyclin is an enzyme which doesn’t need any initiating enzymes this enzyme activates itself. Cycle dependent kinases  are inactive in the absence of a partner cyclin. When activated by a bound cyclin, cycle dependent protein kinase perform phosphorylation. This complex phosphorylates enzymes which require for replication. Cyclin and cyclin dependent protein kinase complex signalling particularly in two phases ,G₁ and S phase .

Cyclins:

G₁ Cyclins ( D Cyclins)

S-phase Cyclins (Cyclins E and A)

Mitotic Cyclins (B Cyclins)

Cyclin –dependent Kinases :

G₁ Cyclin Dependent Kinase 4

S-Phase Cyclin Dependent Kinase2

M-Phase Cyclin dependent Kinase1

Regulation process:

1.G₁-cyclins D bind to their Cyclin dependent protein kinase4  and signal the cell to prepare the chromosomes for replication.

2.Cyclins A bound to Cyclin dependent protein kinase 2 — enters the nucleus and prepares the cell to duplicate its DNA.

3.DNA replication continues, cyclin E is destroyed, and the level of mitotic cyclins begins to rise (in G₂)

4. Mitotic Cyclins (B Cyclins) and M-Phase Cyclin dependent Kinase1 complex  initiates mitotic division.

5. Destroys B cyclins. This is also done by attaching them to ubiquitin.

Again this process repeats for another cell cycl

Different Phases in cell cycle:

Cell cycle consists of  G_{1 ,}S, G₂ and M phases:

       G₁

            } Gap Phase

       G₂

G_1, G₂ and S Phase together called as Interphase.

G1 Phase:
the first growth period of the cell cycle,, the cell grows in size and synthesizes mRNA and proteins that are required for DNA
synthesis.
S Phase:Synthesis phase ,replication takes place .S phase a number of events additional to chromosomee replication take place. Cell growth continues through S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis. Once DNA replication is complete the cell contains twice its normal number of chromosomes and becomes ready to enter the phase called G2. 

G2 Phase:
the second growth period of the cell cycle, following DNA replication and preceding prophase,It synthesizes the protein that constructs the microtubules, which are responsible for separating the chromosomes in the anaphase of mitosis.
Length of cell cyle( time for each phases):
The length of the cell cycle is important because it determines how quickly an organism can multiply. For single-celled organisms, this rate determines how quickly the organism can reproduce new, independent organisms. For higher-order species the length of the cell cycle determines how long it takes to replace damaged cells. The duration of the cell cycle varies from organism to organism and from cell to cell.
Time take for cell cycle differ from species to species , generally unicellular organisms DNA is very less ,so cell cycle is faster than multicellur organisms. Sothe lengths of G1 and G2 vary in cells based on the individual cell's level of preparedness for proceeding in the cell cycle.
G1 is typically the longest phase of the cell cycle it is around 10 hrs time.
S phase takes 5 to 6 hrs
G2 Phase takes 3 to 4hrs 
that means interphase  in between 18  to 20 hrs.