Chapter 10 Notes - Cell Growth and Division

}}10-1 The Cell Cycle}}

Cell Cycle

→ the regular pattern of growth, DNA duplication, and cell division that occurs in eukaryotic cells

Interphase

→ G1, synthesis, and G2 together make up the interphase

%%Gap 1 (G1)%%

• The cell carries out its normal functions
• Cells increase in size, and organelles increase in number
• A cell spends most of its time in G1
• Before it can proceed to synthesis, it must have enough nutrition, adequate size, and relatively undamaged DNA (checkpoints)

%%Synthesis (S)%%

• The combining of parts to make a whole
• Cell makes a copy of its nuclear DNA
• End of S stage, cell nucleus contains two complete sets of DNA

%%Gap 2 (G2)%%

• Cells continue to carry out normal functions and additional growth occurs
• Everything must be in order before cell goes through mitosis and division (adequate cell size, undamaged DNA)

%%Rate of Cell Division%%

• Prokaryotic cells typically divide much faster than eukaryotic cells
• The rate of which cells divide is linked to the body's needs for those cells
• Rate of cell division is greater in embryos and children than adults
• Cells that only rarely divide enter a stage called G0

%%Cell Size%%

Cells have upper and lower limits

• cells that are too small cannot contain all the necessary organelles and molecules
  • a cell with too few mitochondria would not have enough energy to live
• the upper limit on a cell is due to the ratio of cell surface area-to-volume ratio
  • a further increase in size could result in a surface area to small for the adequate exchange of materials
• when cells grow too large, there is a higher demand on the finite amount of DNA in the cell
  • cells cannot just create more DNA to meet the demands of a larger cell

}}10-2 Mitosis and Cytokinesis}}

%%Mitosis (M)%%

• %%mitosis%% - the division of the nucleus and its contents

  • prophase - (longest phase) chromatin condenses into tightly coiled chromosomes, each consists of two identical sister chromatids → nuclear envelope breaks down, the nucleolus disappears → centrioles replicate and begin to migrate to opposite poles of the cell → spindle fibers (organized microtubules) grow from centrioles and radiate toward the center of the cell
  • metaphase - (shortest phase) spindle fibers attach to a protein structure on the centromere of each chromosome and align the chromosomes along the cell equator, the middle of the cell
  • anaphase - the centromeres divide, the spindle fibers pull the sister chromatids away from each other and to opposite sides of the cell
  • telophase - a complete set of identical chromosomes is positioned at each pole of the cell; nuclear membrane and nucleolus start to form → chromosomes begin to uncoil → spindle fibers break apart

• cytokinesis - the process that divides the cell membrane and the cytoplasm and its contents; the result is two daughter cells that are genetically identical to the original cell

  • animal cells: the membrane forms a furrow (trench) that is pulled inward by tiny filaments (drawstring)
  • plant cells: membrane cannot pinch because of the cell wall; a cell plate forms between the two nuclei → a new wall then grows as cellulose and other materials are laid down

• Body cells have 46 chromosomes each

  → DNA organization

  • DNA wraps around proteins that help organize and condense it
  • During Interphase - DNA is loosely organized (not condensed, looks like spaghetti)
  • Proteins must access specific genes for a cell to make specific proteins or to copy the entire DNA sequence
  • During Mitosis - Chromosomes are tightly condensed
  • Duplicated chromosomes must condense to be divided between two nuclei → so that it doesn't become entangled or get messed up (a cell might get two copies of one chromosome and none of a different one)
  • These changes allow a cell to carry out necessary functions
  • 

%%Structure%%

• chromosome - one long continuous thread of DNA that consists of numerous genes along with regulatory information
• histones - a group of proteins each of your chromosomes is associated at almost all times during the cell cycle
• chromatin - the complex of protein and DNA that makes up the chromosome chromatid - one-half of a duplicated chromosome
• sister chromatids - the two identical chromatids
• centromere - a region of the condensed chromosome that looks pinched, where sister chromatids are held together
• telomeres - the ends of DNA molecules form these structures; make out of repeating nucleotides that do not form genes → prevent ends of chromosomes from accidentally attaching to each other and the loss of genes
• the structure of the short, rod-like chromosome makes it possible to separate DNA precisely during cell division

}}%%10-3 Regulation of the Cell Cycle%%}}

%%External Factors%% - come from outside the cell; include messages from nearby cells and other parts of the organism

• help regulate the cell cycle
• include physical and chemical signals
• e.g. cell to cell contact
  • most mammal cells grown in the lab form a single layer on the bottom of a culture dish → once a cell touches other cells, it stops dividing
• many cells release chemical signals that tell other cells to grow
• e.g. growth factors Internal Factors kinase Apoptosis
  • a broad group of proteins that stimulate cell division → bind to receptors that activate specific genes to trigger cell growth
• various hormones may also stimulate growth of certain cell types

%%Internal Factors%% - can be triggered when external factors bind to their receptors

• kinase (help do) - an enzyme that transfers a phosphate group from one molecule to another when activated
  • typically increases the energy of the target molecule or changes its shape
  • cells have many types of kinases (almost always present in the cell)
  • help control the cell cycle are activated by cyclins
• cyclin (traffic lights) - a group of proteins that are rapidly made and destroyed at certain points in the cell cycle
• these 2 factors help a cell advance to different stages of the cell cycle

Apoptosis - programmed cell death

• occurs when internal or external signals activate genes that produce self-destructive enzymes
• the nucleus tends to shrink and break apart
• e.g. in the early stages of development, human embryos have webbing between fingers & toes → those cells go through apoptosis

Cancer - the common name for a class of diseases characterized by uncontrolled cell division

• regulation of the cell cycle is disrupted
• cancer cells grown in a culture dish continue to divide, even when surrounded by neighboring cells
• divide much more often than healthy cells
• tumors - disorganized clumps that cancer cells form
• benign tumor - the cancer cells typically remain clustered together
  • may be relatively harmless and can probably be cured by removing it
• malignant tumor - some of the cancer cells can break away or metastasize from the tumor
  • these breakaway cells can be carried in the bloodstream or lymphatic system to other parts of the body, where they can form more tumors called %%metastases%%
  • once a tumor metastasizes, it is much more difficult to rid the body of tumors entirely
• tumors require lots of food and a hearty blood supply but contribute nothing to the body's function
• carcinogens - substances known to produce or promote development of cancer
  • e.g. tobacco smoke and air pollutants → associated with lung cancer
• Cancer Treatments:
  • surgery
  • radiation and chemotherapy
  • radiation therapy - use of radiation to kill cancer cells and shrink tumors
    • damages a cell's DNA so much that it cannot divide
    • usually localized - used to target a specific region (can also hurt healthy cells)
  • chemotherapy - use of certain drugs to kill actively dividing cells
    • kills both cancer and healthy cells
    • systematic - drugs travel throughout the entire body

}}10-4 Asexual Reproduction}}

Reproduction - a process that makes new organisms from one or more parent organisms; sexual and asexual

Sexual Reproduction - the joining of two specialized cells called gametes (egg and sperm cells)

• offspring that result are genetically unique

Asexual Reproduction - the production of offspring from a single parent, does not involve the joining of gametes

• offspring are genetically identical to each other + single parent

Binary Fission - the asexual reproduction of a single-celled organism by which the cell divides into two cells of the same size

• 

• similar results to mitosis, processes are different

• prokaryotes do not have nuclei or spindle fibers, less DNA than eukaryotic cells

• %%plasmid%% - a single circular chromosome where most of the DNA in bacteria is

• fission starts when the bacterial chromosome is copied → both chromosomes are attached to the cell membrane

• as cell grows, chromosomes move away from each other

• when the cell is twice its original size, it undergoes cytokinesis → membrane pinches inward, new cell wall forms, completing the separating into two daughter cells

Some eukaryotes also reproduce asexually, through mitosis

e.g. a new plant can emerge from a stem cutting; a new sea star can grow from the arm of another

• especially common in simpler plants and animals
• occurs in both multicellular and unicellular eukaryotes
  • budding - a small projection grows on the surface of the parent organism, forming a separate new individual
  • e.g. hydras and some types of yeast
  • 
  • fragmentation - a parent organism splits into two pieces, each of which can grow into a new organism
  • e.g. flatworms and sea stars
  • 
  • vegetative reproduction - involves the modification of a stem or underground structure of the parent organism; offspring often stay connected to original organism
  • e.g. strawberries and potatoes
  • 

}}10-5 Multicellular Life and Cell Differentiation}}

Zygote - a single fertilized egg your body begins as

• to form the different structures of your body, cells must specialize
• %%cell differentiation%% - the process by which a cell becomes specialized for a specific structure or function during multicellular development
• almost every cell has a full set of DNA, each type of cell expresses only specific genes it needs to function
• %%in plant cells, the first division of a fertilized egg is asymmetric%%
  • the apical cell forms most of the embryo, including the growth point for stems and leaves
  • basal cell - provides nutrients to the embryo and growth points for the roots
  • plant cells continue to differentiate based on their location
• %%in animals, an egg undergoes many rapid divisions after it is fertilized%%
  • resulting cells can migrate and quickly begin to differentiate
  • %%blastula%% - the early animal embryo generally takes the shape of a hollow ball
  • %%gastrula%% - as it develops, part of the ball folds inward, forming an inner layer and creating an opening in the outer cell layer

%%stem cells%% - a unique type of body cell that can divide and renew themselves for long periods of time, remain undifferentiated in form, and differentiate into a variety of specialized cell types

• when a stem cell divides, it forms either 2 stem cells or 1 stem and 1 specialized cell
• classified by their ability to develop into the differentiated cells types of different tissues
• the more differentiated a stem cell already is, the fewer types of cells it can form
  • totipotent - "can do everything" can grow into any other cell types; these consist of only a fertilized egg and the cells produced by the first few divisions of an embryo
  • pluripotent - "can do most things" can grow into any cell type except for totipotent stem cells
  • multipotent - "can do many things" can grow only into cells of a closely related cell family
  • embryonic stem cells have potential to form almost any cell type

%%adult stem cells%% - partially undifferentiated cells located among the specialized cells of many organs and tissues

• found all over the body: brain, liver, bone marrow, skeletal muscle, dental pulp, fat
• found in children and in umbilical cord blood
• somatic stem cell is more accurate
• advantage: adult stem cells can be taken and put back into a patient
• disadvantages: few in number, difficult to isolate, sometimes tricky to grow
  • may contain more DNA abnormalities than embryonic stem cells

Embryonic Stem Cells:

• Most embryonic stem cells come from donated embryos grown in a clinic
• Inner cell mass - the stem cells are taken from a cluster of undifferentiated cells in the 3-5 day-old embryo
• Embryonic stem cells are pluripotent so they can form any of the 200 cell types of the body
• Used to treat patients w leukemia and lymphoma

Downside:

• patient's body might reject them
• cells could grow unchecked and form a tumor
• raises many ethical questions - involves destruction of embryo