Cellular Reproductionand 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

– DNA is replicated

– cell divides into two identical daughter cells

Essential Feature of Cell Division

1. Must ensure a complete copy of the genetic information is transferred to both daughter cells.

Two Fundamental Types of Cells(organisms):

• Prokaryotic

• Eukaryotic

Prokaryotic Cell

• no nucleus – genetic material (DNA) in cytoplasm

• no membrane-bound organelles

• example: bacteria

• cell division is called binary fission

Eukaryotic Cell

• membrane-bound organelles, including a nucleus • genetic material (DNA) contained within the nucleus

• examples: fungi, protists, plants, animals

• cell division of somatic cells called mitotic cell division

During non-division phase of cell cycle

• DNA molecules in extended, uncondensed form = chromatin

– cell can only use DNA to produce molecules when in extended state

– easier to sort and organize DNA into daughter cells

During division phase of cell cycle

• DNA molecules condense to form chromosomes prior to division

– each chromosome is a single molecule of DNA

Prokaryotic Cell Cycle

• Prokaryotic chromosome a circular loop

1. chromosome attaches to one point on plasma membrane

2. chromosome is replicated – replicated chromosome attached to plasma membrane at a different nearby point

3. cell elongates – new plasma membrane is added between between chromosomes, pushing them towards opposite ends of cell

4. plasma membrane grows inward at middle of cell

5. parent cell is divided into two identical daughter cells

Cell wall

Plasma membraneChromosome

• Division of somatic cells(non reproductive cells) ineukaryotic organisms

What is Mitotic Cell Division?

• 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 characteristicnumber of chromosomes:

Prokaryotes – one chromosome

Crayfish – 200 chromosomes

Human – 46 chromosomes => 23 pairs of chromosomes

Diploid organisms receive onechromosome from female parent(= maternal) and one chromosomefrom male parent (= paternal)

A “matched” pair of maternal andpaternal chromosomes are calledhomologues

Structure of a eukaryotic chromosome

• unreplicated chromosome

Chromatid Chromatidcentromere

Prior to cell division:• chromosomes (DNA) are replicated (duplicated)

duplicatedchromosome

• duplicated chromosome – attached at their centromeres – as long as attached, known as sister chromatids

daughterchromosomes

sisterchromatids

Eukaryotic Cell Cycle

2 major phases:

• Interphase (3 stages) – DNA uncondensed (= chromatin)

• Mitotic cell division (4 stages) – DNA condensed (= chromosomes)

Interphase

• non-dividing state• 3 stages:

G1 – cell grows in size – organelles replicated - majority of cell life span (90%) in

this stageS – replication of DNA – synthesis of proteins associated with DNA

G2 – synthesis of proteins associated with mitosis

G1

G2

SMitoticCell

Division

Simplified cell cycle

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

i) chromosomes condense

• 3 major events

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 (thin tubes)

• 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

sistersisterchromatidchromatid

sistersisterchromatidchromatid

centromerecentromere

spindle fiberspindle fiber kinetochoreskinetochores

centriole

asterfibers

• 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 organise division of sister chromatids into daughter cells

chromatin

nucleolus

nucleus

centrioles

condensingchromosomes

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

free spindle fibers

V-shaped chromatid

Telophase

• spindle fibers disintegrate

• nuclear envelopes form around both groups of chromosomes

• chromosomes revert to their extended state

• nucleoli reappear

• cytokinesis occurs, enclosing each daughter nucleus into a separate cell

chromosomesdecondensing

nuclear envelopereforming

nucleolus reappears

pinching of cellmembrane at equator

cytokinesis

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

Summary of Mitosis

• Prophase: • Chromosomes condense• Nuclear envelope disappears• Spindles move to opposite sides of the cell• Spindle fibers attaches to centromeres on the chromosomes

• Metaphase• Chromosomes lined up on equator of the cell• Spindles at opposite ends of cell

• Anaphase• Centromeres snap• 1 copy of each chromosome is pulled to opposite poles by the

spindle

• Telophase• Chromosomes de-condense• Nuclear envelope reappears• Cytoplasm divided into 2 cells

Cancer

• Cancer is a disease of uncontrolled cell division. It starts with a single cell that loses its control mechanisms due to a genetic mutation. That cell starts dividing without limit, and eventually kills the host.

• Normal cells are controlled by several factors. They stay in the G1 stage of the cell cycle until they are given a specific signal to enter the S phase, in which the DNA replicates and the cell prepares for division. Cancer cells enter the S phase without waiting for a signal.

• Another control: normal cells are mortal. This means that they can divide about 50 times and then they lose the ability to die. This “clock” gets re-set during the formation of the gametes. Cancer cells escape this process of mortality: they are immortal and can divide endlessly.

• A third control: cells that suffer significant chromosome damage destroy themselves due to the action of a gene called “p53”. Cancer cells either lose the p53 gene or ignore its message and fail to kill themselves.

Cancer Progression• There are many different forms of cancer, affecting

different cell types and working in different ways. All start out with mutations in specific genes called “oncogenes”. The normal, unmutated versions of the oncogenes provide the control mechanisms for the cell. The mutations are caused by radiation, certain chemicals (carcinogens), and various random events during DNA replication.

• Once a single cell starts growing uncontrollably, it forms a tumor, a small mass of cells. No further progress can occur unless the cancerous mass gets its own blood supply. “Angiogenesis” is the process of developing a system of small arteries and veins to supply the tumor. Most tumors don’t reach this stage.

• A tumor with a blood supply will grow into a large mass. Eventually some of the cancer cells will break loose and move through the blood supply to other parts of the body, where they start to multiply. This process is called metastasis. It occurs because the tumor cells lose the proteins on their surface that hold them to other cells.

G2 Phase

Production of mitosis proteins

The Cell Cycle

Mitosis animationhttp://www.sci.sdsu.edu/multimedia/mitosis/mitosis_qt1.html