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 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 mm 2 Volume = 1 mm 3 Ratio = 6 : 1 Surface Area = 24 mm 2 Volume = 8 mm 3 Ratio = 3 : 1 Surface Area = 54 mm 2 Volume = 27 mm 3 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 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 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 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 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