Cell Cycle Biology 5(A). Learning Objectives Describe the stages of the cell cycle including DNA replication and mitosis Describe the importance of the.

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