}}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
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- 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
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