Cell Reproduction The 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 division Sexual 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. O 2, CO 2,H 2 O 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

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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 cell Late Telophase Cytokinesis occurs and cytoplasm divides Cell membrane pinches the 2 cells apart at the equator

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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 hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::53 5::/sites/dl/free/ /120073/bio14.swf::Mi tosis and Cytokinesis CdckCdck- 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 grow where they don't belong or are missing where they should be. Usually the body part near the "bad veins" grows way too large (hypertrophy) Abnormal blood vessels grow where they don't belong or are missing where they should be. Usually the body part near the "bad veins" grows way 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