Cell ReproductionThe process by which new cells are

formed.Asexual reproduction: when one parent cell divides to produce offspring that are genetically identical to the parent = cell divisionSexual reproduction: genetic information from two cells is combined to form the genetic code for a new organism.

Why cells divide?

• To replace lost or damaged cells

• Cells divide when they grow too large to function properly.– When cells get too large, they have trouble

passing substances throughout the area of the cell

Diffusion• Our cells need certain molecules in order to

produce the energy they need to carry out their functions.

• These molecules come into our body when we digest our food or inhale oxygen.

• These molecules have to enter our cells through the cell membrane, in order to be used efficiently.

• This passage of materials occurs through the process of diffusion.

• Cell membranes are composed of molecules that allow it to act as a semi-permeable barrier.

Diffusion and the Cell Membrane

• Diffusion: the passive movement of molecules from an area of high

concentration to an area of low concentration

concentration gradient: the difference in concentration between the two regions

• Molecules that are small or uncharged, e.g. O2, CO2,H2O can move easily through the membrane due to the motion of the water molecules.

Osmosis• The Diffusion of water/solvent

across a semi-permeable membrane separating two solutions.

• Water molecules move from a region of high concentration [ ] to a region of low [ ].

• Direction depends on the relative [ ] of water molecules on either side of the cell membrane

[ ] = concentration

What does cell size have to do with it?

• Surface area and volume• A greater surface area relative to volume

allows diffused materials more area to move in and out.

• Diffusion works well over short distances. Once molecules enter the cell the rate of diffusion slows.– Hence cell size is limited

• This is one reason that cells must divide instead of just getting bigger.

Splitting cells• Cells can’t just divide down the middle b/c each

offspring cell must have the full complement of DNA from its parent cell.

• If it is missing chromosomes it will be missing information and will not be able to function appropriately.

• Therefore in order for a cell to divide, its nucleus must undergo a special division of its chromosomes called mitosis.

• The rest of the cytoplasm of the cell can then divide in a process called cytokinesis.

Mitosis• Mitosis: cell division

that results in duplicate copy of an original cell containing the exact amount of genetic information.

• Happens in all somatic cells

• A cell’s total inherited amount of DNA is called its genome.

The Cell Cycle• Interphase Mitosis Cytokinesis

• Interphase: the growth phase in which cells make new molecules to increase its volume and mass

• Further subdivided into the G1, S, and G2 phases– G1 phase: Gap 1 Rapid

growth and metabolic activity– S phase: Synthesis phase

DNA synthesis and replication

– G2 phase: Gap 2 Centrioles replicate, cell prepares for cell division

Mitosis• Division of the cell’s nucleus where the daughter

cells receive the exact number of chromosomes and genetic makeup as the parent cell

• Consists of the four phases: prophase, metaphase, anaphase, telophase

• N.B. The cell cycle has checkpoints which determine whether cell division should or should not occur; for example:– There are not enough nutrients to support cell growth– The DNA has not replicated– The DNA is damaged

Stages of Mitosis

1. (Interphase)

2. Prophase

3. Metaphase

4. Anaphase

5. Telophase

These 5 stages can also be subdivided into early and late stages.

ex: Early telophase, late telophase

http://www.cellsalive.com/cell_cycle.htm

Interphase• During this time, the

nucleus is NOT actively dividing

• The growth phase in which cells make new molecules to increase its volume and mass

• Chromosomes are spread out in a delicate network called chromatin

• Replication of chromosomes and centrioles

Prophase• Centrioles move to

opposite sides of the nucleus

• Chromosomes start to shorten and thicken

• Nuclear membrane begins to breakdown

• Spindle fibres form (pathway for chromatids)

Metaphase• Paired chromatids line

up at the EQUATOR of the cell

• The centromere of each chromosome attaches to its respective spindle fibre

• Nuclear membrane no longer apparent

Early Anaphase• Spindle fibres

contract (shorten)• Centromere splits

between chromatids

Mid Anaphase• Chromatids separate

and move to opposite poles (centrioles)

• Chromatids arrive at opposite sides of cell

Late Anaphase

Early Telophase

• Nuclear membrane reappears

• Chromosomes look less thick and appear as thin threads of chromatin again

• Spindle fibres disappear• Nuclear division complete:

daughter cellLate Telophase• Cytokinesis occurs

and cytoplasm divides• Cell membrane

pinches the 2 cells apart at the equator

•

http://www.sumanasinc.com/webcontent/animations/content/mitosis.html

Cytokinesis• Cell division• The separation of the

cytoplasm by a new cell membrane or cell wall to form two new daughter cells

• In plant cells, a cell plate is formed before cytokinesis is complete

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120073/bio14.swf::Mitosis and Cytokinesis

Cdck- cell division activity

Mitosis Animation

Errors in Mitosis

• Mutations– Cause a permanent error, or change, in the

normal DNA molecules– Can be caused by various mutagens such as

toxic compounds, radiation, or viruses– Mutations are passed on during anaphase

and therefore are found in a localized group of cells

Errors in Replication• Mutations are heritable changes in DNA, which

can be passed on to future generations. They can occur in any gene and randomly.– Nondisjunction

• Any one gene has a one in a million chance to be mutated. We have so many genes that mutations are fairly common. Each of us carries several mutations in our bodies.– Causes may include radiation and exposure to

chemicals, viruses…• Mutations can be:

1. Useful (positive)2. Harmful (negative)3. No effect (neutral)

Cells do not divide forever

• After cells have become damaged beyond repair or because they are too old they leave the cell cycle.

• Usually this occurs after 20 to 30 rounds of cell division

• Cells can break themselves down using genes that code for proteins that kill the cell.

Apoptosis• Programmed cell

death carried out by a series of enzyme reactions

• Failure of apoptosis can result in some forms of cancer

Abnormal blood vessels Abnormal blood vessels grow where they don't grow where they don't belong or are missing belong or are missing where they should be.  where they should be.  Usually the body part near Usually the body part near the "bad veins" grows way the "bad veins" grows way too large (hypertrophy)too large (hypertrophy)

• Too much apoptosis can result in degenerative diseases such as muscular dystrophy

Human Embryonic Stem Cells

Cell division out of control

• Some cells can be altered in such a way that they ignore the stop signs in the cell cycle and divide excessively leading to increased chances of mutations occurring

• This increasing mass of cells forms a tumour

• If these tumour cells become further mutated and continue to divide, cancer can develop