Cellular Reproduction and the Cell Cycle

What do all cells require to survive? A complete set of genetic instructions – produce required molecules – direct life processes Genetic instructions are coded in the DNA of cells

Why do cells divide? Growth Repair Development

Cell Cycle Activities of a cell from one cell division to the next – Cell grows, adding more cytoplasmic constituents Why? – DNA is replicated Why? – cell divides into two identical daughter cells

Essential Features of Cell Division Transmit a complete copy of genetic information (DNA) Transmit materials necessary for cell to survive and use genetic information

Two Fundamental Types of Cells (organisms): Prokaryotic Eukaryotic

Prokaryotic Cell no nucleus – genetic material (DNA) in cytoplasm no membrane-bound organelles cell division is called binary fission example: bacteria

Prokaryotic Cell Cycle Prokaryotic chromosome a circular loop chromosome attaches to one point on plasma membrane chromosome is replicated – replicated chromosome attached to plasma membrane at a different nearby point

cell elongates – new plasma membrane is added between between chromosomes, pushing them towards opposite ends of cell plasma membrane grows inward at middle of cell parent cell is divided into two identical daughter cells

Cell wall Plasma membrane Chromosome

Eukaryotic Cell membrane-bound organelles, including a nucleus genetic material (DNA) contained within the nucleus cell division of somatic cells called mitotic cell division examples: fungi, protists, plants, animals

Chromosomes In Eukaryotic cells, the genetic information that is passed from generation to the next is carried by chromosomes. Chromosomes: threadlike structure within the nucleus containing the genetic information that is passed from one generation of cells to the next. Each cell has a specific number of chromosomes Fruit flies = 8 chromosomes Humans = 46 chromosomes

Chromosomes Chromosomes are not visible in cells until cell division Prior to cell division the DNA and protein molecules that make up the chromosomes are spread throughout the nucleus. During cell division the chromosomes condense into compact, visible structures. Chromosomes are copied before cell division and consist of identical “sister” chromatin.

Chromosomes A Centromere B Sister chromatids

Limitations on Cell Growth If a cell is larger then it demands more on the cell’s DNA. DNA Overload: When a cell is small the information stored in the DNA is able to meet the cells needs, but as a cell increases in size the DNA in not copied so there is an “information crisis”. It will also have trouble moving enough nutrients and wastes across the cell membrane. Exchanging Materials: The rate at which exchange occurs depends on the surface area of the cell, which is the total area of the cell. A cells waste production depends on the amount used by the cell. When a cell is larger it is producing a higher number of waste and exchanges waste for nutrients at a lower number.

Division of the Cell Before a cell gets too large, and begins undergoing problems, a cell will divide forming 2 daughter cells in cell division. Cell division solves the problem of increasing size by reducing cell volume. Each daughter cell has an increased ratio of surface area to volume, allowing efficient exchange of materials with the environment.

Before Cell Division Cells Replicate: Prior to cell division a cell will make copies of its DNA. This replication solves the problem of information storage because each daughter cell will have their own complete set of genetic information. Cell division also solves the problem of increasing size by reducing cell volume, which allows for efficient exchange of materials with the environment.

The Cell Cycle Cell Cycle: Series of events that cells go through as they grow and divide During the cell cycle, a cell…. grows prepares for division divides to form 2 daughter cells which begin their own cell cycle

The Cell Cycle 4 phases of the Cell Cycle M Phase: S Phase: Mitosis and cytokinesis S Phase: Chromosome replication or synthesis G1 and G2 Phase: “Gap” phases, occur between M and S Phase Period of intense growths and activity

The Cell Cycle A: G1 Phase B: S Phase C: G2 Phase D: M Phase

Events of the Cell Cycle Interphase: G1, S, and G2 Phase G1 Phase: Cells do most of their growing. The cells will increase in size and synthesis new proteins and organelles. S Phase: Chromosomes are replicated and the synthesis of new DNA molecules takes place. Key proteins associated with the chromosomes are also synthesized. G2 Phase: Shortest of the 3 phases. Organelles and molecules required for cell division are produced

Events of the Cell Cycle After the cell has gone through the 3 phases of Interphase the cell will be ready to go through M Phase (cell division).

What is Mitotic Cell Division? Division of somatic cells (non reproductive cells) in eukaryotic organisms A single cell divides into two identical daughter cells (cellular reproduction) => Maintains chromosome ploidy of cell

Ploidy – refers to the number of pairs of chromosomes in cells haploid – one copy of each chromosome – designated as “n” diploid – two copies (= pair) of each chromosome – designated as “2n”

Each species has a characteristic number of chromosomes: Prokaryotes – one chromosome Crayfish – 200 chromosomes Human – 46 chromosomes => 23 pairs of chromosomes

Diploid organisms receive one chromosome from female parent (= maternal) and one chromosome from male parent (= paternal) A “matched” pair of maternal and paternal chromosomes are called homologues

Structure of a eukaryotic chromosome unreplicated chromosome arm centromere

Prior to cell division: chromosomes (DNA) are replicated (duplicated) duplicated chromosome – attached at their centromeres – as long as attached, known as sister chromatids duplicated chromosome

daughter chromosomes sister chromatids

Eukaryotic Cell Cycle 2 major phases: Interphase (3 stages) – DNA uncondensed (= chromatin) Mitotic cell division (4 stages) – DNA condensed (= chromosomes)

Mitotic Cell Division 2 major processes: mitosis – nuclear division => preserves diploid number of chromosomes cytokinesis – cytoplasmic division => cell divides into two daughter cells

Mitosis 4 phases: 1st – Prophase (3 major events) 2nd – Metaphase 3rd – Anaphase 4th – Telophase and Cytokinesis

Prophase 3 major events i) chromosomes condense ii) spindle fibers form iii) chromosomes are captured by spindle

Chromosomes Condense Recall that chromosomes were duplicated during interphase => each chromosome consists of 2 sister chromatids attached to each other at the centromere

Mitotic Spindle Forms spindle fibers are specialized microtubules spindle fibers radiate out from centrioles, forming the “aster” centrioles occur in pairs, and are duplicated during interphase

one pair of centrioles migrates to one pole of cell, the other pair migrates to opposite pole of cell

Spindle Captures Chromosomes When spindle fibers are fully formed nuclear envelope disintegrates and nucleolus disappears Spindle fibers attach to chromosomes at the kinetochore, a structure located at the centromere

other spindle fibers do NOT attach to chromosomes, but retain free ends that overlap at cell’s equator => “free spindle fibers” function of spindle fibers is to organize division of sister chromatids into daughter cells

chromatin nucleolus nucleus centrioles condensing chromosomes

chromosomes align along equator of the cell, with one Metaphase chromosomes align along equator of the cell, with one kinetochore facing each pole centrioles spindle fibers chromosomes

Anaphase sister chromatids separate spindle fibers attached to kinetochores shorten and pull chromatids poleward free spindle fibers lengthen and push poles of cell apart

V-shaped chromatid free spindle fibers

Telophase spindle fibers disintegrate nuclear envelopes form around both groups of chromosomes chromosomes revert to their extended state nucleoli reappear

Telophase chromosomes decondensing pinching of cell nuclear envelope reforming nucleolus reappears pinching of cell membrane at equator

Cytokinesis occurs, enclosing each daughter nucleus into a separate cell

Cytokinesis Animal cells: – microfilaments attached to plasma membrane form a ring around equator of cell – ring contracts, like a drawstring, dividing the cytoplasm

Plant cells: – stiff cell wall makes pinching impossible – Golgi complex buds off vesicles filled with carbohydrate – vesicles line up at equator and fuse, producing a structure called the cell plate – cell plate becomes new cell wall between the two cells

Mitotic Cell Division Functions: Growth, maintenance, repair of body tissues Forms the basis of Asexual Reproduction

Cell Cycle Regulators Internal Regulators: proteins that respond to events inside the cell Cyclin: protein that regulates the timing of the cell cycle in eukaryotic cells Many other proteins have been discovered that also assist in the cell cycle timing. There are also proteins that check the cell before it can enter into the next part of the cell cycle. Ex: A cell cannot enter mitosis until the chromosomes are replicated.

Cell Cycle Regulators External Regulators: proteins that respond to events outside the cell External regulators direct cells to speed up or slow down the cell cycle. Growth regulators: stimulate the growth and division of the cell.

Uncontrolled Cell Growth Cell growth is constantly regulated to prevent uncontrolled cell growth. Cancer: disorder in which some of the body’s own cells lose the ability to control growth. Cancer cells do not respond to the signals that regulate the growth in most cells. This constant growth causes masses call tumors.

Uncontrolled Cell Growth What causes the loss of growth control? Cancer cells have a defect in a gene called p53, which halts the cell cycle until chromosomes have been properly replicated. Damage to p53 causes the cell to lose the information needed to respond to signals that ensure orderly growth.