Biology
Class Notes
5-1
Topic: Cell Cycle
Aim: How do cells grow?
(A) LIFE OF A CELL
 Reproduction: the ability of individuals within a species to produce more
of their own kind
 All living things reproduce
 All cells come from preexisting cells. (cell theory)
 Each cell goes through a cell cycle
o 1—cell grows in size by adding organic and inorganic compounds
o 2—DNA replicates
o 3—cell divides
 Although each cell does increase in size to some degree, in almost all
cases, living things grow by producing more cells.
 Why must a cell reproduce instead of continuing to get larger?
o
the nucleus must be able to control all activities in the cell
o
materials must be able to be transported throughout the cell
 Cells go through cell division for the following reasons:
o cell growth
o repair and replacement of damaged cell parts
o reproduction of the species
(B) DNA REPLICATION
 Before the cell divides, the DNA must replicate itself
 DNA is “packaged” in condensed rod-like structures called chromosomes
 each chromosome is made up of 2 sister chromatids
 the 2 sister chromatids are held together by a centromere
 Each species has a certain number of chromosomes in each cell.
o Humans—46
o Fruit fly—8
o Cabbage—18
 All the cells of the body are called somatic cells. These cells have two
sets of chromosomes (one from each parent.) This is said to be
diploid. (represented by 2n)
 Sex cells, or gametes, only have one set of chromosomes. This is said
to be haploid.
 The human diploid number is 46, while the haploid number is 23.
(C) THE CELL CYCLE
 Cells go through phases or a cell cycle during their life before they divide
to form new cells
 The cell cycle includes two main parts—interpahse and cell division
 interphase includes cell growth and DNA replication
 cell division includes mitosis and cytokinesis
1) INTERPHASE
 cells spend most of their time in interphase
 growth phase of the cell cycle
 cells mature and increase in size by making more
cytoplasm and organelles
 cells also carry out most of their metabolic processes
during interphase
 once the cell has reached its maximum size, DNA
replication will occur
2) MITOSIS
 mitosis is the process during cell division in which the
nucleus divides.
 Each new nucleus ends up with the same exact number
and type of chromosomes.
 Mitosis consists of 4 phases:
Phase 1: Prophase
o chromosomes coil up and become visible
o the nuclear envelope dissolves and a spindle forms
Phase 2: Metaphase
o chromosomes line up along the middle of the cell
Phase 3: Anaphase
o Centromeres divide
o Chromatids (now called chromosomes) moves towards opposite
ends of the cell
Phase 4: Telophase
o nuclear envelope forms at each pole
o chromosomes uncoil
o spindle dissolves
o cytokinesis begins
3) CYTOKINESIS
 cytokinesis is the division of the cytoplasm of the cell
 the end result of mitosis and cytokinesis is TWO
GENETICALLY IDENTICAL CELLS.
 In animal cells, the cytoplasm pinches inward until two
separate cells are formed.
 In plant cells, a cell plate forms across the middle and
eventually separates the plant cell into two new cells.
SUMMARY OF MITOSIS