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CELL GROWTH AND DIVISION
FORMATION OF NEW CELLS: CELL DIVISION
• 2 trillion cells are produced by an adult human body every day!
• 25 million new cells per second!
• New cells form when older cells divide
• Cell division = cell reproduction
LIMITS TO CELL GROWTH
• Why do cells divide instead of continuously grow?
• DNA overload – The larger the cell becomes, the more demands
the cell places on its DNA
• Exchanging Material – The larger the cell, the more trouble the cell
has moving enough nutrients and wastes across the cell membrane
CELL DIVISION
• Daughter cells:
• Before a cell becomes too large it divides into two
TYPES OF CELL DIVISION
• Prokaryotic
• Eukaryotic
• Growth
• Development
• Repair
• Asexual reproduction
• Sexual reproduction: Formation of gametes (i.e. sperm and egg)
THE CELL CYCLE
• A repeating sequence of cellular growth and division during the life of an organism
• A cell spends 90% of its time in the first 3 stages of the cycle called interphase
• A cell will enter the last two phases of the cell cycle only if it is about to divide
CELL DIVISION:
DNA must be passed on
THE CELL CYCLE: 4 MAIN STAGES
• Interphase – Gap 1 (G1)
– Synthesis (S)
– Gap 2 (G2)
• Mitosis
INTERPHASE
• Important role in preparing the cell to divide
• Provides critical time for the duplication of organelles and for DNA replication
• By the end of interphase, an individual cell has two full sets of DNA, or chromosomes, and is large enough to divide
GAP 1 (G1) PHASE
• The main phase of a cell’s life
• Rapid cell growth
• Routine cell functions
• Checkpoint before
heading to the Synthesis (S) stage
• A cell that is not dividing remains in the G1 phase, sometimes called G0
• Most muscle and nerve
cells never divide (they cannot be replaced)
SYNTHESIS (S)
• Synthesis means “the combining of parts to make a whole”
• Cell copies its DNA
GAP 2 (G2)
• Cells continue to carry out their normal functions
• Additional growth occurs
• The cell prepares to divide
– Hollow protein fibers (microtubules) are
rearranged to prepare for mitosis
• Critical checkpoint before entering mitosis
MITOSIS (M)
• Includes mitosis and cytokinesis
• Mitosis: division of the cell nucleus and its contents
• Each nucleus ends up with the same number of chromosomes as the original cell
• Cytokinesis: process that divides the cytoplasm
CHROMOSOMES CONDENSE AT THE START OF MITOSIS
DNA (DEOXYRIBONUCLEIC ACID)
• Stores hereditary information & directs cell activities
• Must be present in the new cells
• Copied during cell division
• Each cell ends up with a complete set (copy)
GENE
• Units of DNA that code for proteins and RNA
• A single molecule of DNA has thousands of genes
• Genes are located on chromosomes
CHROMOSOMES
• Prokaryotic cell – The main ring of DNA
• Eukaryotic cell – One of the structures in
the nucleus that are made up of DNA and protein
CHROMOSOME PARTS
• Chromatids: The two exact copies of DNA that make up a chromosome
• Centromere: The region of the chromosome that holds the two sister chromatids together during mitosis (cell division)
CHROMOSOME
• Passes on the genetic information in eukaryotic cells from one generation of cells to the next
• The cells of every organism had a specific number of chromosomes
TELOMERE
• The ends of DNA
• Repeating nucleotides that do not form genes
• Prevent the end of chromosomes from accidentally attaching to each other
• Prevent the loss of genes
• A short section of nucleotides is lost from a new DNA molecule each time it is copied
• It is important that the nucleotides are part of the telomeres, and not the genes themselves!
CHROMOSOMES
• Not visible in the nucleus of most cells except during cell division
• When a eukaryotic cell prepares to divide, the chromosomes become visible
HOMOLOGOUS CHROMOSOMES
• Each human somatic cell
– 2 copies of 23 different chromosomes
– 46 total chromosomes
• The 23 chromosomes
have different sizes, shapes and number of genes
HAPLOID AND DIPLOID
• Diploid: A cell that has a set of paired chromosomes; one from each parent
• A cell that contains two haploid sets of chromosomes
• Found in somatic cells
• All of the cells in the body, except the gametes
MITOSIS AND CYTOKINESIS PRODUCE TWO GENETICALLY IDENTICAL DAUGHTER CELLS
STAGES OF MITOSIS
ProphaseMetaphaseAnaphaseTelophase
PROPHASE • The longest phase of mitosis
• Chromatin condense into tightly coiled
chromosomes
• Chromosomes become visible
• The nuclear envelope dissolves
• The centrioles separate to the opposite sides of the nucleus
• The spindle forms to help separate the chromosomes
METAPHASE
• The chromosomes move to the center of the cell and line up on the equator
• Spindle fibers link the chromatids of each chromosome to opposite poles
ANAPHASE
• Centromeres divide
• The sister chromatids move to opposite poles
• The spindle fibers shorten
TELOPHASE
• A nuclear envelope forms around the chromosomes at each pole
• Chromosomes uncoil
• The spindle dissolves
• Spindle fibers break down and disappear
CYTOKINESIS
• The cytoplasm divides to form the two cells
CYTOKINESIS: ANIMAL CELLS
• The cytoplasm of the cell is divided in half
• The cell membrane grows to enclose each cell
• The result is two genetically identical cells
CYTOKINESIS: PLANT CELLS
• The rigid cell wall causes the cytoplasm to divide in a different way
• A cell plate forms between the two new cells
• A cell wall forms on both sides of the plate
CELLS DIVIDE AT DIFFERENT RATES
• Prokaryotic cells divide quickly! – do not have organelles or a cytoskeleton
CELL SIZE IS LIMITED
• To maintain a suitable cell size, growth and division must be coordinated
• Cells do not want to become larger or smaller with each division, but stay the same size
INTERNAL AND EXTERNAL FACTORS REGULATE CELL
DIVISION
EXTERNAL FACTORS
• Physical signals
– Example: cell-cell contact stops cell division
• Chemical signals
– Example: growth factors are proteins that
stimulate cell division
INTERNAL FACTORS
• Kinases – Enzymes that transfers a phosphate group from one molecule
to a specific target molecule
– Increase the energy of the target molecule or change its shape
• Cyclins – Group of proteins that are rapidly made and destroyed at
certain points in the cell cycle
– Checkpoints: • G1
• G2
• Mitosis
CELL GROWTH (G1) CHECKPOINT
• Decides whether the cell will divide or not
• If the conditions are not favorable, the cell will not divide
DNA SYNTHESIS (G2) CHECKPOINT
• DNA repair enzymes check DNA replication
• If acceptable, proteins trigger mitosis
MITOSIS CHECKPOINT
• Triggers the exit from mitosis
• Signals the G1 phase, the major growth period
APOPTOSIS
• Greek word meaning “A falling off”
• Programmed cell death
• Occurs when internal or external signals activate genes that help produce self-destructive enzymes
• Nucleus shrinks, cell collapses inward
CELL DIVISION IS UNCONTROLLED IN CANCER
• Cancer: common name for a class of diseases characterized by uncontrolled cell division
• Regulation in the cell cycle breaks down
• Tumors: disorganized clumps formed from cancer cells
CANCER
• Benign tumor: cancer cells remain clustered together – 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 – Breakaway cells can be carried in the bloodstream
or lymph system to other parts of the body where they can form more tumors
CANCER
• Cancer cells come from normal cells that have suffered damage to the genes that help make proteins involved in cell-cycle regulation
• Inherited genes
• Environmental carcinogens
CANCER TREATMENT
• Radiation – Kill cancer cells and shrink tumors – Localized
• Chemotherapy
– Drugs that kill actively dividing cells – Systemic
• HeLa cells
– 1951 Henrietta Lacks – Cervical tumor cells
ASEXUAL REPRODUCTION
BINARY FISSION IS SIMILAR IN FUNCTION TO MITOSIS
BINARY FISSION
• Asexual reproduction that produces identical offspring
• A single parent passes exact copies of all its DNA to its offspring
BINARY FISSION
• The DNA is copied
• The cell divides
• A new cell membrane is added to a point on the membrane between the two DNA copies
• Each cell contains one of the circles of DNA and is a functioning prokaryote
ADVANTAGES TO ASEXUAL REPRODUCTION
• Simplest and most primitive method of reproduction
• Many offspring
• Short time period
• No energy required – make gametes – find a mate
DISADVANTAGES TO ASEXUAL REPRODUCTION
• The DNA varies little between individuals
• Little variation can be very bad in a changing environment
• The population may not be able to adapt
TYPES OF ASEXUAL REPRODUCTION
Fission
• The separation of a parent into two or more individuals of about equal size
• Example: amoeba
TYPES OF ASEXUAL REPRODUCTION
Fragmentation
• Body breaks into several pieces
• Example: planarians, sea stars (can also reproduce sexually)
TYPES OF ASEXUAL REPRODUCTION
Budding
• New individuals split off from existing ones
• Example: hydra
SOME INTERESTING TYPES OF ASEXUAL REPRODUCTION
• Parthenogenesis – Special form of cloning, seen in snakes, dandelions,
hawkweeds, some fishes, frogs and lizards
• Protogyny
– Female to male
• Protandry
– Male to female
MULTICELLULAR LIFE
DIVERSITY OF CELLULAR LIFE
• Unicellular: single cell performs all functions needed for survival
• Multicellular: cells in
multicellular organisms are specialized to perform particular functions within the organism
LEVELS OF ORGANIZATION
• Cells
• Tissues
• Organs
• Organ Systems
CELLS
• Collection of living matter enclosed by a barrier that separates the cell from its surroundings
• Basic unit of all forms of life
TISSUES
• Group of similar cells that perform a particular function
• Examples: muscle, epithelial, nervous, connective
ORGANS
• Groups of tissues that work together to form a particular function
• Examples: heart, liver, pancreas
ORGAN SYSTEMS
• Groups of organs that work together to perform a particular function
• Examples: muscular, skeletal, circulatory, and nervous
SPECIALIZED CELLS PERFORM SPECIALIZED FUNCTIONS
• Cell Differentiation: Process by which unspecialized cells develop into their mature forms and functions
STEM CELLS
• Divide and renew themselves for long periods of time
• Remain undifferentiated in form
• Develop into a variety of specialized cell types
STEM CELLS • Every cell in the human body comes from a single fertilized cell
• The cells that form after the first few divisions can potentially
become ANY TYPE of cell in the body
STEM CELLS
• Once a cell becomes a specific cell it cannot be changed into another cell type
• Nerve cells and muscle cells are not replaced by the body when damaged
• For example, spinal cord injuries that cause paralysis cannot be fixed if new nerve cells cannot be grown
• Implants of stem cells can reverse the effects of brain injuries in mice…there is hope that the same will hold true for humans
• Found in developing embryos – Moral and ethical issues to
consider
• Adults also have some stem cells (i.e. in the bone marrow) – Possibly bone marrow stem
cells could develop into not just blood cells, but possibly nerve cells as well
ADULT STEM CELLS
• Partially undifferentiated cells
• Located among the specialized cells of many organs and tissues
• In the brain, bone marrow, skeletal muscle, dental pulp, fat, children, and umbilical cord blood
• Yay! Can be grown in culture and replanted in patient
• Nay! Fewer in number, difficult to isolate, tricky to grow
EMBRYONIC STEM CELLS
• Come from donated embryos in a clinic
• Result of in vitro fertilization
• Taken from a cluster of undifferentiated cells in the 3-5 day old embryo
• Yay! Can form any of the 200 cell types in the body; Can be grown indefinitely in culture
• Nay! A patient’s body may reject them as foreign; Destruction of the embryo is considered ethically unacceptable by some people
RESEARCH AND TREATMENT HOPE