The Cell Cycle Chapter 12

Objectives Understand the need for cell division Understand the process of mitosis and cytokinesis Know the parts of each phase of mitosis as discussed in class Understand what regulates the cell cycle and at what phases regulation may occur

Cell Cycle The Cell’s Time Clock Smallest unit of life –all living things must reproduce Cells replicate for growth, replacement, and repair Cell division requires Mitosis & Cytokinesis Cytokinesis

Parts of the Cell Involved Chromosome –Chromatid: Strand of DNA –Kinetochore: Region of chromatid where spindle fibers attach Nucleus –Nucleolus: Region of RNA –Nuclear membrane: Borders the nucleus Centrosomes –Centrioles in some organisms: produce microtubules (spindle fibers and asters) –Spindle fibers: microtubules that grow toward the chromatids

Interphase Cell performs normal function Three subphases: –G 1 : cell duplicates most organelles –S: quantity of DNA in the cell is doubled as chromosomes are replicated. Each chromosome has a pair of sister chromatids connected by a centromere that contains a kinetochore –G 2 : chemical components stockpiled Nucleolus present

Mitosis Nuclear division without a reduction in chromosome number Each new cell (daughter cell) will have the same quantity of DNA as the parental cell Why is this important? Mitotic events can be categorized into discrete stages based on what is happening to the structure of the cell Stage include: –Prophase Prometaphase –Metaphase –Anaphase –Telophase

Prophase (Including Prometaphase) Pro Three things visibly occur –Chromosomes condense (shorten) –Centrosomes migrate to the poles while producing spindle fibers –Nuclear membrane fragments

Metaphase Meta Chromosomes are moved to the equator of the cell (metaphase plate) by the growing spindle fibers Centrosomes are at the poles, nuclear membrane is gone Metaphase Plate

Anaphase Ana Centromere splits into two Spindle fibers shorten from kinetochore end separating sister chromatids Activated kinetochores “pull” chromatids along the spindle fibers and toward the poles

Telophase Telo Nuclear membrane reforms around each region of chromosomes Nucleolus reforms Cytokinesis (division of the cytoplasm) may occur

Cytokinesis May Vary Between Major Taxonomic Groups

Cell Cycle Regulation Cell cycle events are triggered by the cell- cycle control system; a set of molecules found in the cytoplasm affected by internal and external controls Checkpoints in G 1, G 2, and M phases of the cycle G 1 checkpoint is most critical. May throw cells out of cyclic phase into G 0, never to divide again

Cyclins & Cylin-Dependent Proteins Cyclin is a cytoplasmic protein that fluctuates in concentration Cyclin-Dependent Proteins: activate other proteins in the cell through phosphorylation (protein kinases)

What does MPF do?

Other Internal and External Factors Internal –M checkpoint does not proceed until signal is received that all kinetochores are attached to spindle microtubules External –Growth factors: cycle will not proceed if requirements are not met –Social signals Density-dependent inhibition: Under crowded conditions, chemical requirements are insufficient to allow cell growth Anchorage dependence: Cells must be attached to a substrate in order to replicate –DNA damage inhibits growth