Cell Cycle
Biology 5(A)
Cell Cycle
Learning Objectives
• Describe the stages of the cell cycle including DNA
replication and mitosis
• Describe the importance of the cell cycle to the growth of
organisms
Cell Cycle
Cell cycle – the
sequence of cell growth
and division that occurs
in a cell between the
beginning of one cell
division and the
beginning of next cell
division
Cell Cycle
Stages of the cell cycle
• Mitotic phase – cell
division
– Mitosis – process of
nuclear division
– Cytokinesis - division of
cytoplasm
• Interphase – cell prepares
for division
– G1
–S
– G2
Cell Cycle
Interphase – preparing for cell division
• G1 phase – primary
growth phase
• S phase – DNA
replication
• G2 phase – second
period of growth
• Organelles replicate
throughout
Mitosis
Somatic cell – body cell in multicellular, eukaryotic organism
• Each somatic cell contains same number of chromosomes
Mitosis - process by which each daughter cell receives an
exact copy of chromosomes present in parent cell
• Occurs in eukaryotic cells
• Results in formation of two identical daughter cells
Mitosis ensures that each new somatic cell receives the
correct number of chromosomes
Mitosis
Four phases of mitosis:
• Prophase
• Metaphase
• Anaphase
• Telophase
Prophase
Chromatin – hereditary material consisting of DNA and
protein
During prophase, chromatin begins coiling into chromatids
• Chromosome – unit of coiled chromatin
– Chromatid – each half of a chromosome
– Centromere – point at which chromatids
are held together
Image by Magnus Manske [GFDL]
Prophase
Process of prophase
• Centrioles move apart
• Spindle fibers form between the centrioles
– Plant cells develop spindle fibers but lack centrioles
spindle fibers
centriole
chromosome
Prophase
Process of prophase
• Nuclear membrane breaks down and disappears
• Centriole pairs at opposite ends of cell
• Chromosomes attached to centrioles by spindle fibers
spindle fibers
centriole
chromosome
Metaphase
Metaphase – second phase of mitosis
• Chromosomes pushed and pulled by spindle fibers
• Chromosomes arranged along spindle equator
• Centromeres attached to separate spindle fibers
spindle fibers
centriole
chromosome
spindle equator
Anaphase
Anaphase – third phase of mitosis
• Spindle fibers shorten, pulling chromatids apart at
centromere
• Chromatids are now chromosomes and move to opposite
ends of cell
chromosome
Telophase
Telophase – final phase of mitosis
• Chromatids reach opposite poles
• Spindle fibers disappear
• Nuclear membrane forms
• Chromosome uncoil and return to threadlike mass
nuclear membrane
Cytokinesis
Cytokinesis – division of cell’s cytoplasm
• Animal cell – cell membrane pinches together, furrow
forms along equator
– Groove deepens until cell membrane
separates, forming two daughter cells
• Plant cell – cell plate is formed in the middle of the
dividing cell
– Cell plate extends outward until it
separates the two daughter cells
DNA Replication
DNA Replication – process by which DNA code is copied
• Occurs during S phase of interphase
Cells need
• A complete set of
genetic material
• An exact copy of DNA
DNA Replication
Major Steps in DNA Replication
• Enzymes (helicases) unwind DNA and break hydrogen
bonds between complementary base pairs
– Separation of strands occurs at many different places
• Replication forks – sites at which separation and replication occur
DNA Replication
Major Steps in DNA Replication
• Free-floating nucleotides
bond to exposed bases
– A pairs with T
– C pairs with G
DNA Replication
Major Steps in DNA Replication
• Enzymes (DNA polymerases) bond nucleotides together
as the new DNA strand grows
– Each new strand grows from its 5’ end to its 3’ end
– Two parent strands of DNA molecule are antiparallel
– New strands oriented in opposite directions along parent
DNA Replication
Leading strand – elongates toward the replication fork
• Nucleotides added continuously to growing 3’ end
DNA Replication
Lagging strand – elongates away from the replication fork
• Synthesized discontinuously as Okazaki fragments
• Synthesized in 5’  3’ direction moving away from fork
• Enzyme (DNA ligase) connects Okazaki fragments
DNA Replication
• Separation and pairing of free nucleotides continues until
entire DNA molecule has been replicated
– Two original strands serve as mold for building complementary
strand
– Each new double helix of DNA contains one strand from
original molecule and one newly created strand
• “Semiconservative” replication
Cell Cycle and Growth
Importance of cell cycle to growth of organisms
• Cells limited in size
1 mm
2 mm
Surface Area = 6 mm2
Volume = 1 mm3
Ratio = 6 : 1
3 mm
Surface Area = 24 mm2
Volume = 8 mm3
Ratio = 3 : 1
Surface Area = 54 mm2
Volume = 27 mm3
Ratio = 2 : 1
Cell Cycle and Growth
Importance of cell cycle to growth of organisms
• Unicellular organisms – cell division is method of
reproduction
• Multicellular organisms – cell cycle aids in growth,
maintenance, and replacement
Cell Cycle
Learning Objectives
• Describe the stages of the cell cycle including DNA
replication and mitosis
• Describe the importance of the cell cycle to the growth of
organisms