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