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Cell CycleBiology 5(A)
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
Importance of cell cycle to growth of organisms•Cells limited in size
Cell Cycle and Growth
1 mm 2
mm 3 mm
Surface Area = 6 mm2
Volume = 1 mm3
Ratio = 6 : 1
Surface Area = 24 mm2
Volume = 8 mm3
Ratio = 3 : 1
Surface Area = 54 mm2
Volume = 27 mm3
Ratio = 2 : 1
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 and Growth
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 preparesfor division– G1– S– G2
Cell Cycle
Interphase – preparing for cell division•G1 phase – primarygrowth phase•S phase – DNA replication•G2 phase – second period of growth •Organelles replicatethroughout
Cell Cycle
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
Mitosis
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
Prophase
Image by Magnus Manske [GFDL]
centriole
chromosome
spindle fibers
Process of prophase•Centrioles move apart
•Spindle fibers form between the centrioles – Plant cells develop spindle fibers but lack
centrioles
Prophase
Process of prophase•Nuclear membrane breaks down and disappears•Centriole pairs at opposite ends of cell•Chromosomes attached to centrioles by spindle fibers
Prophase
centriole
chromosome
spindle fibers
Metaphase – second phase of mitosis•Chromosomes pushed and pulled by spindle fibers •Chromosomes arranged along spindle equator•Centromeres attached to separate spindle fibers
Metaphase
centriole
chromosome
spindle fibers
spindle equator
Anaphase – third phase of mitosis•Spindle fibers shorten, pulling chromatids apart at centromere•Chromatids are now chromosomes and move to opposite ends of cell
Anaphase
chromosome
Telophase – final phase of mitosis•Chromatids reach opposite poles•Spindle fibers disappear •Nuclear membrane forms•Chromosome uncoil and return to threadlike mass
Telophase
nuclear membrane
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
Cytokinesis
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 nucleotidesbond 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
DNA Replication
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
