Cell Size and the Cell Cycle Why do cells divide? 2 reasons The larger a cell becomes: the more demands are placed on its DNA. the more trouble a cell has moving nutrients and wastes across the cell membrane.

1. DNA Demand As cells grow larger they usually do not make extra copies of their DNA. Therefore, as a cell grows, there is greater demand placed on the information contained in its DNA. In time the cell would not be able to keep up with the demands for information from its DNA.

Surface area to volume ratio 2. Cell Transport Issues As cells grow larger it becomes more difficult to efficiently move materials in and out of the cell by diffusion. The smaller a cell, the larger the surface area to volume ratio; large surface area to volume ratios makes cells efficient. Cell Size 1x1x1 cm 2x2x2 3x3x3 Surface Area 6cm2 24cm2 54cm2 Volume 1cm3 8cm3 27cm3 Surface area to volume ratio 6:1 3:1 2:1

Cell Division when a cell becomes too large it divides, creating two new “daughter” cells through cell division DNA is replicated 1st (each new daughter cell will get a copy) cell size is reduced

Cell Cycle repeating sequence of cell growth and division during an organism’s lifetime cells grow, prepare for division, and divide to form two daughter cells, each of which then begin the cell cycle again. regulated by proteins that receive and send signals (checkpoints in the cell cycle)

Cell Cycle Phases G1 Growth S Synthesis   G2 Growth M Cell Division

Cell Cycle Phases G1 Growth S Synthesis G2 Growth Interphase   G2 Growth M Cell Division Interphase

Interphase every phase except ‘M’ Phase (cell division) cells spends most of their time in interphase

Interphase G1 First Growth Phase S Synthesis Phase increase in size and make new proteins and organelles S Synthesis Phase chromosomes (DNA) are replicated G2 Second Growth Phase preparation for mitosis shortest phase organelles and molecules required for cell division are produced

M Phase Cell Division divided into two parts Mitosis Cytokinesis division of the cell’s nucleus Cytokinesis division of the cell’s cytoplasm