THE LIFE CYCLE OF A CELL
Imagine the life cycle of a cell if it were to live for 24 hours….
Interphase
- the first 10 hours are spent in the first growth period (G1) where the cell differentiates into a specific kind or type that has a specific function
- the next 9 hours are spent in the synthesis period (S) where the DNA is produced and every chromosome is duplicated -- giving rise to carbon copies called sister chromatids
- the next 4 hours are spent in the second growth period (G2) where the cell undergoes final preparation for the mitosis stage
Mitosis
- the last hour is taken up the mitosis which is the most visibly obvious stage
Definition:
The process throgh which: (i) single cells are reproduced and (ii) multicellular organisms grow. The cells which arise from the original parent cells are called daughter cells.
- once the cell reaches a maximum size, it usually has two alternatives: to reproduce or to die
- mitosis ensures that vital cells are replaced by new ones once they wear out and die
- mitosis must be precise - the nuclear material must be duplicated precisely, since the daughter cells must have the genetic capacity to carry out all of the many cell processes of the parent
- mitosis is a continuous process that has very distinct events or phases
- these phases are analogous to frames or "snapshots" of a motion picture
Phases:
1. PROPHASE
- nucleolus disappears
- nuclear membrane begins to disappear
- chromosomes coil, shorten, and thicken, and are thus visible
- chromosomes are attached at a single site called a centromere
- centrioles migrate to opposite poles of the cell
- spindle fibres form (like clothes lines fixed at both ends)
2. METAPHASE
- sister chromatid pairs of each chromosome align themselves along the equator of the cell
3. ANAPHASE
- the centromere breaks in half and both chromosome pairs separate into daughter chromosomes
- each chromatid migrates (along the spindle fibres) to opposite ends of the cell
4. TELOPHASE - the reverse of all the events of phrophase take place
- centrioles replicate
- chromosomes uncoil
- nuclear membrane reforms
- nucleolus reappears
- cell begins to divide
C. Cytoplasmic Division
Cytokinesis: The division of the cytoplasm to ensure the equal distribution of organelles to each of the daughter cells. Cytokinesis is different in plant cells and animal cells.
- - plant cells produce a cell plate between the two nuclei of the daughter cells
- that effectively separates the cytoplasm of each cell
- - animal cells produce a cleavage furrow around the middle of the parent cell
- that effectively "pinches" it into two equal halves