cell cycle

what is the cell cycle

its the period from the formation of the cell until the cell divides its cell again ( life cycle of an infidel cell) length of a cell cycle is variable. it consists of two periods of relatively short dividing and non dividing periods.

do cells divide after they have grown

once they have grow and become specialized most cells do not divide again however some cells retain the ability to divide.

first phase : interphase

the period between mitotic divisions but its nit a resting period, there are still intense metabolic activities taking place in the cell. during interphase the nucleus is involved in protein synthesis and chromatin is copied.

then we have cell division

there is first nuclear division followed by cytoplasmic division. there are two types of nuclear division depending on the number of chromosomes present in each of the daughter cells is the same as the number of chromosomes in the parent cell or half. Half would be meiosis and same amount would be mitosis.

interphase includes

G1 : the longest phase sometimes lasting several days (its variable) and it when the cell grows and performs its specialized function.

S: DNA synthesis (DNA is copied) occurs in each chromosome so that each daughter cell receives a complete set. also histones for each daughter chromatids is produced.

G2: short phase in which the cell prepares for mitotic division

not interphase G0: the cell sops dividing and this can last the entire lifespan of the organism.

duration of s g2 and m (not interphase) are all the same around several hours.

when does mitosis take place

it is the division of the nucleus to produce two nuclei contains identical sets of chromosomes and it takes place when an organism grows and when tissue are repaired also occurs in asexual reproduction.

stages of mitosis

prophase: chromosomes condenses into visible chromosomes, nuclear envelope disappears and spindle fibers forms, basically the cell is getting ready to divide

metaphase: chromosomes line up in the middle of the cell and spindle fibers attach to them.

anaphase : sister chromatids separate and go to opposite sides of the cell

telophase: chromosomes reach opposite side and begin to uncoil and two new nucleus envelope form and two new nuclei are formed

cytokinesis

division of the cytoplasm or cytokinesis usually follows nuclear division. in cytokinesis the cell organelles become evenly distributed between the two daughter cells. sometimes the cytoplasm isn’t equal between the two cells and sometimes after mitosis cytokinesis does not occur leaving binucleate or multinucleate cells. in animal cells there is an invagination at the equator of the cell membrane and this constriction causes a furrow in the cytoplasm which extends and then divides the cytoplasm into two cells. there is a ring of microfilament present beneath the surface of the cleavage furrow and cytokinesis occurs as a result of that constriction.

what is meiosis

nuclear division to produce four daughter nuclei. each containing half the number of chromosomes of the original nucleus or parent cells. meiosis precedes the formation of gametes. chromosome number is 23 for sperm and egg and they stay constant from generation to generation. in sexual reproduction the cells fuse and form a zygote, this new cell has the normal number of chromosomes now since it was doubled by the amalgamation of two nuclei. meiosis also has two successive divisions meiosis 1 and meiosis 2 both divisions superficially resemble mitosis.

prophase 1

the first feature is that the chromosomes appear to thin long and slender and the homologous chromosomes line up and pair point for point except for the XY chromosomes. the second feature is crossover and consists of the interchange of chromatid segment between 2 paired homologous chromosomes.

what is interkenisis

it is the phase before meiosis 2 begins and it has no DNA synthesis.

meiosis 2

the 23 double structured chromosomes divides at the centromere and new daughter cells get 23 chromatids and the amount of DNA in these cells is held normal somatic cells.

purpose of two meiotic divisions

1. to provide for genetic variability through the processes of crossover, and random distribution of homologous chromosomes to the daughter cells

2. to provide each germ cell with both a haploid number of chromosomes and half the amount of DNA of a normal somatic cell.