Chapter 12 The Cell Cycle Omnis cellula e cellula 1855- Rudolf Virchow German scientist all cells arise from a previous cell Every cell from a cell In order for this to be true, cells must have the ability to divide 1
Cell division Cell division is an important process: Reproduction - many unicellular organisms are able to reproduce simply by dividing their cells Other higher life forms form sexual cells (gametes) or other asexual reproductive cells (spores) by cell divisions Cell division is an important process: Growth and Development Adding cells to overall body mass Physical changes that occur in cells Cell division 2
Cell division Cell division is an important process: Maintenance Tissue renewal Replace damaged cells Cell Cycle The cell cycle is the life of a cell from the time it is produced by the division of another cell until its own division into two cells Not all cells complete the cell cycle Not all cells are capable of cell division Regulation of the cell cycle is very important uncontrolled cell division forms the basis of cancers 3
Genetic organization In order for a cell to divide and produce a viable copy, the genetic material must be passed from one cell to the next The entire complement of genetic material in a cell is called the genome A typical human cell has >2 m of DNA inside each nucleus Before the cell can divide, the entire genome must be copied and separated This duplication and separation is manageable because the DNA molecules are organized into either chromatin or chromosomes In humans, recall that there are 23 pairs of chromosomes 46 total in a somatic cell diploid (2n) Gametes sexual cells are haploid (n) 4
Chromatin DNA molecules and proteins Very long Allow for each gene to be expressed Chromosome Condensed DNA (chromatin) and proteins When cells divide, the genome must be copied DNA replication is the process in which the genome is copied The duplicated DNA is organized into chromosomes Each duplicated chromosome consists of a pair of sister chromatids joined by a centromere 5
For cell division to occur: Two processes must occur Division of the nucleus karyokinesis Division of the cytoplasm cytokinesis Two basic types of karyokinesis Mitosis used in somatic cells to produce two identical cells Meiosis used to produce gametes The cell cycle alternates between cell divisions and periods of normal activity between divisions interphase 6
Cell cycle of a somatic cell Interphase Gap 1 growth and normal function Synthesis DNA replication Gap 2 prepares chemically for division Mitotic phase Mitosis karyokinesis cytokinesis Mitotic phase karyokinesis by mitosis Prophase (your book refers to late prophase as prometaphase) Metaphase Anaphase Telophase compare and contrast animal and plant cell mitosis 7
Animal cell- Interphase Nuclear envelope complete Nucleolus present Chromatin Duplicated during S Centrosome present Prophase Chromatin condenses into chromosomes Nuclear envelope erodes Nucleolus disappear Centrosome divides and mitotic spindle formation begins 8
Chromosomes attached to mitotic spindle by centromeres Chromosomes line up along equator of cell Metaphase Anaphase Sister chromatids are pulled apart Sister chromatids become a chromosome Recall that each sister chromatid was a copy of the chromosome made during DNA replication 9
Telophase Daughter nuclei begin to form Chromosomes unwinds into chromatin At completion G1 of interphase Cytokinesis occurs Cleavage furrow Cytokinesis in animal cells cleavage furrow formed by constriction of proteins 10
Mitosis in plant cells No centrosomes!!! Cytokinesis in plant cells Formation of cell plate by vesicles from Golgi 11
Regulation of the cell cycle The timing and rate of cell division in different parts of a plant or animal are crucial to normal growth, development and maintenance. Human epidermal cells divide constantly Human liver cells rarely divide but can if needed Mature muscle and nerve cells do not have the ability to divide What causes this? Same genome??? 12
Current research points to chemical checkpoints Primarily using: Kinase enzymes Cyclins Much of this research is still in its early stages but is crucial to our understanding of cancer It is interesting that understanding the basic cycle of cell life seems so simple yet reminds us of how much we really have to learn! Cancer Cancer cells do not respond normally to these control mechanisms Cancer cell divide excessively and can invade other tissues normal cells divisions are controlled by cell division mechanisms Contact inhibition density dependent inhibition Cancer cells do not exhibit this regulation 13
Cancer begins when a single cell in a tissue undergoes transformation Process that converts a normal cell into a cancerous one Normally, the immune system recognizes a transformed cell and destroys it If the transformed cell evades the immune system it may form a tumor Mass of abnormal cells 14
Tumors Benign tumor abnormal cells remain at the original site and simply form a lump Most are not harmful and are easily removed Malignant tumor Invasive to other tissues Can impair function of tissue or organs May cause serous damage May also be capable of metastasis Able to spread to distant parts of the body through lymph or blood vessels 15