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