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A Powerpoint lecture covering the entire cell cycle, including mitosis, cell growth, and mechanisms of control.
The Cell Cycle Introduction to Biology The Key Roles of Cell Division • The ability to reproduce is one of the key features that separates life from non-life. • All cells have the ability to reproduce, by making exact copies of themselves. • In unicellular organisms, division of one cell reproduces the entire organism • In multicellular organisms, cell division is needed for: o Development of an embryo from a sperm/egg o Growth o Repair LE 12-2 100 µm 200 µm 20 µm Reproduction Growth and development Tissue renewal Asexual Reproduction • Asexual reproduction is reproduction that involves a single parent producing an offspring. o The offspring produced are, in most cases, genetically identical to the single cell that produced them. o Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring. o Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges) Sexual Reproduction • In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow. o The offspring produced inherit some genetic information from both parents. o Most animals and plants, and many single-celled organisms, reproduce sexually. Contrasting Reproduction Types Cell Division • Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA. • A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells. Cellular Organization of the Genetic Material • A cell’s endowment of DNA (its genetic information) is called its genome. • DNA molecules in a cell are packaged into chromosomes. Chromosomes • The genetic information that is passed on from one generation of cells to the next is carried by chromosomes. • Every cell must copy its genetic information before cell division begins. • Each daughter cell gets its own copy of that genetic information. • Cells of every organism have a specific number of chromosomes. Prokaryotic Chromosomes • Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm. • Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information. Eukaryotic Chromosomes • In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin. • Chromatin is composed of DNA and histone proteins. • DNA coils around histone proteins to form nucleosomes. • The nucleosomes interact with one another to form coils and supercoils that make up chromosomes. Chromosomes During Cell Division • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached LE 12-4 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromeres Sister chromatids Phases of the Cell Cycle • The cell cycle consists of o Mitotic (M) phase (mitosis and cytokinesis) o Interphase (cell growth and copying of chromosomes in preparation for cell division) • Interphase (about 90% of the cell cycle) can be divided into subphases: o G1 phase (“first gap”) o S phase (“synthesis”) o G2 phase (“second gap”) LE 12-5 INTERPHASE S G1 (DNA synthesis) G2 G1 Phase: Cell Growth • In the G1 phase, cells increase in size and synthesize new proteins and organelles. S Phase: DNA Replication • In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated. G2 Phase: Preparing for Cell Division • In the G2 phase, many of the organelles and molecules required for cell division are produced. M Phase: Cell Division • In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. o Mitosis is the division of the cell nucleus. o Cytokinesis is the division of the cytoplasm. Important Cell Structures Involved in Mitosis • Chromatid – each strand of a duplicated chromosome • Centromere – the area where each pair of chromatids is joined • Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle • Spindle – long proteins (part of the cytoskeleton) that the centrioles produce o Helps move the chromosomes into place. Prophase • During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible. Prophase • The centrioles move to opposite sides of nucleus and help organize the spindle. Prophase • The spindle forms and DNA strands attach at a point called their centromere. Prophase • The nucleolus disappears and nuclear envelope breaks down. Metaphase • During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell. Metaphase • The spindle fibers connect the centromere of each chromosome to the two poles of the spindle. Anaphase • During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes. Anaphase • The chromosomes separate into two groups near the poles of the spindle. Telophase • During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin. Telophase • A nuclear envelope re- forms around each cluster of chromosomes. Telophase • The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus. Cytokinesis • Cytokinesis is the division of the cytoplasm. • The process of cytokinesis is different in animal and plant cells. Cytokinesis in Animal Cells • The cell membrane is drawn in until the cytoplasm is pinched into two equal parts. • Each part contains its own nucleus and organelles. LE 12-9a 100 µm Cleavage furrow Contractile ring of Daughter cells microfilaments Cleavage of an animal cell (SEM) Cytokinesis in Animal Cells • In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall. • Instead, a cell plate forms between the divided nuclei that develops into cell membranes. • A cell wall then forms in between the two new membranes. LE 12-9b Vesicles Wall of 1 µm forming parent cell cell plate Cell plate New cell wall Daughter cells Cell plate formation in a plant cell (TEM) LE 12-10 Nucleus Chromatin condensing Chromosomes Cell plate 10 µm Nucleolus Prophase. The Prometaphase. We Metaphase. The spindle is Anaphase. The Telophase. Daughter chromatin is condensing. now see discrete complete, and the chromatids of each nuclei are forming. The nucleolus is chromosomes; each chromosomes, attached chromosome have Meanwhile, cytokinesis beginning to disappear. consists of two identical to microtubules at their separated, and the has started: The cell Although not yet visible sister chromatids. Later kinetochores, are all at daughter chromosomes plate, which will divide in the micrograph, the in prometaphase, the the metaphase plate. are moving to the ends of the cytoplasm in two, is mitotic spindle is starting nuclear envelope will the cell as their growing toward the to form. fragment. kinetochore micro- perimeter of the parent tubules shorten. cell. LE 12-6ca INTERPHASE PROPHASE PROMETAPHASE LE 12-6ca INTERPHASE PROPHASE PROMETAPHASE LE 12-6ca INTERPHASE PROPHASE PROMETAPHASE LE 12-6da METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS LE 12-6da METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS LE 12-6da METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Virtual Onion Root Tip Mitosis Lab Click here to start Binary Fission • Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission • In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart LE 12-11_1 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Chromosome Two copies replication begins. of origin Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. LE 12-11_2 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Chromosome Two copies replication begins. of origin Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Origin Origin Replication continues. One copy of the origin is now at each end of the cell. LE 12-11_3 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Chromosome Two copies replication begins. of origin Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Origin Origin Replication continues. One copy of the origin is now at each end of the cell. Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. The Evolution of Mitosis • Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission • Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis LE 12-12 Bacterial chromosome Prokaryotes Chromosomes Microtubules Intact nuclear envelope Dinoflagellates (Type of plankton) Kinetochore microtubules Intact nuclear envelope Diatoms (Type of Algae) Kinetochore microtubules Centrosome Fragments of nuclear envelope Most eukaryotes The Cell Cycle Control System • The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock • The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received LE 12-14 G1 checkpoint Control system S G1 M G2 M checkpoint G2 checkpoint • For many cells, the G1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase LE 12-15 G0 G1 checkpoint G1 G1 If a cell receives a go-ahead If a cell does not receive a signal at the G1 checkpoint, go-ahead signal at the G1 the cell continues on in the checkpoint, the cell exits the cell cycle. cell cycle and goes into G0, a nondividing state. • An example of external signals is density- dependent inhibition, in which crowded cells stop dividing • Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum (connective tissue) in order to divide LE 12-18a Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). 25 µm Normal mammalian cells LE 12-18b Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. 25 µm Cancer cells Loss of Cell Cycle Controls in Cancer Cells • Cancer cells do not respond normally to the body’s control mechanisms • Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue • If abnormal cells remain at the original site, the lump is called a benign tumor • Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors LE 12-19 Lymph Tumor vessel Blood Glandular vessel tissue Metastatic Cancer cell tumor A tumor grows from a Cancer cells invade Cancer cells spread A small percentage single cancer cell. neighboring tissue. through lymph and of cancer cells may blood vessels to survive and establish other parts of the a new tumor in another body. part of the body.
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