Cell Reproduction Chapter 9

Ms. Tetrev

• For the best game, the sports courts or fields must be kept at regulation size. WHY?

• Do large cells perform tasks more efficiently than smaller cells?

Answer

• NO! Bigger is not better at a cellular level. Small cells transport substances more efficiently than large cells. Diffusion is a slow process; the larger the cell, the less efficient transport within it becomes.

• The key factor that limits the size of a cell is the ratio of its surface area to its volume.– As the cell grows, its volume increases much more

rapidly than the surface area.

• The cell might have difficulty supplying nutrients and expelling enough waste products

• By, remaining small, cells have a higher ratio of surface area to volume and can sustain themselves easier.– Small cells maintain a more efficient transport

systems.– If a cell becomes too large, it becomes almost

impossible for cellular communications.

• Cells grow until they reach their size limit, then they either stop growing or divide.

• Cell division prevents a cell from growing too large and it is also the way a cell reproduces. It also is the way the cell reproduces so that you grow and heal certain injuries.

Reasons why cells divide

• Growth -- increase in size• Repair -- fix damaged tissue• Asexual Reproduction -- a bacteria cell dividing• Regeneration -- growing lost body parts, I.e.

starfish

Cell Cycle

Cells reproduce by a cycle of growing and dividing called the cell cycle. This is a method by which Eukaryotic cells reproduce.• Each time the cell goes through one complete cycle it

becomes two cells.

• Has three main stages– Interphase– Mitosis– Cytokinesis

Cell Cycle

• Interphase is the stage during which the cell grows, carries out cellular functions, and replicates.

• Mitosis is the stage of the cell cycle during which the cell’s nucleus and nuclear material divide.

• Cytokinesis is the method by which a cell’s cytoplasm divides, creating a new cell.

Interphase is divided into 3 stages

• The first stage of interphase, G1

– The cell is growing, increasing in size, carrying out normal cell functions, making proteins & organelles, and preparing to replicate DNA.

• The second stage of interphase, S or synthesis– The cell copies its DNA (in the form of

chromosomes) in preparation for cell division. Chromosomes in the nucleus replicate.• Chromosomes are the structures that contain the

genetic material that is passed from generation to generation of cells.• Chromatin is the relaxed form of DNA in the cell’s

nucleus.

• The third stage of interphase, G2 or Gap 2– The cell prepares for the division of its nucleus.– A protein that makes microtubules for cell division

is synthesized at this time.– Centrioles replicate in animal cells, to be used in

division.

Cell Division--Mitosis

Vocabulary

• This spot that holds the 2 chromatid copies together is called a Centromere

• These structures at the poles whichattach to the spindle fibers and pull the chromosomes are centrioles

Vocabulary

This network of fibersthat attach and pullthe chromosomesapart is called theMitotic spindle

Vocabulary

• DNA that is all spread out inan interphase nucleus is calledChromatin

• When chromatin scrunches together it is called a Chromosome

Vocabulary

• This is called a Cleavage Furrow• This cell is animal cell. Plants don’t have• The place in the cell membrane of an animal

cell that pinches in during cytokinesis

Mitosis

• It is a continuous process of replication of chromosomes and forms two new nuclei– Mitosis begins after interphase and ends before

cytokinesis

Prophase• In nucleus, chromosomes condense• Chromosomes become more visible• In the cytoplasm, proteins make microtubules• Later in prophase, mitotic spindle stretches out between

poles• Nuclear envelope and nucleolus break apart and

disappear• The centromere on each chromosome attaches to a

spindle fiber• Spindle fibers begin to move towards the poles of the

cell

Early Prophase

Nuclear Envelope and nucleolus disappear

Chromosomes condense

Late Prophase

Sister chromatids become more visible

Metaphase

• Chromosomes align in the center of the cell on the metaphasial/equatorial plate

• Towards the end of metaphase chromosomes are aligned an equal distance from the ends of the cell.

Metaphase

Chromosomes aligning in the center

Anaphase

• Centromeres divide• Spindle fibers pull one set of chromatids towards

one pole and the other towards the opposite pole

• Once separated the chromatids are two identical sets of daughter chromosomes

• Precise alignment is critical so that each daughter cell receives an identical set of chromosomes.

Early Anaphase

Sister chromatids being pulled towards the poles of the cell

Late Anaphase

Telophase

• Two daughter nuclei are formed• Opposite of what happens during prophase• Nuclear envelopes form again• Chromosomes start to uncoil and form the

loose mass of DNA (chromatin)• Mitotic Spindle disappears• In Animal cells, e ach new nucleus has a pair

of centrioles outside of the nuclear envelope

Telephase

Cytokinesis

• Begins during telophase as new nuclei are formed

• In animal cells, the parental cell membrane folds inward to form a cleavage furrow (fold)

• As the furrow deepens, the cell is pinched in half until the membrane meets and divides in the middle

• Results in two identical cells with complete cell membranes formed.

Cytokinesis

• In plants, membrane bound fragments accumulate along the metaphase plate during late anaphase

• Fragments fuse to form a double membrane• Cell Wall forms between the double

membrane

Two Daughter Cells

Cell Cycle Regulation

Cells

• The rate of cell division varies depending on the type of cell.– Some cells such as skin, hair, nail cells have a

shorter cell cycle, and others such as bone tissue cells and nerve cells have longer cell cycles.

– The average human cell has a cell cycle of about 20 h

• Cyclins bind to enzymes called cyclin-dependent kinases (CDKs) –this is what signals the cellular reproduction process.

• The cell cycle has built-in checkpoints that monitor the cycle and can stop it if something goes wrong.

What is Cancer ?

Abnormal Cell Cycle: Cancer

• Cancer is the uncontrolled growth and division of cells. It happens when cells do not respond to normal cell cycle control mechanisms.

• Cancer cells can kill an organism by crowding out normal cells, resulting in the loss of tissue function.

Cancer

• Cancer arises from a loss of normal growth control. In normal tissues, the rates of new cell growth and old cell death are kept in balance. In cancer, this balance is disrupted. This disruption can result from uncontrolled cell growth or loss of a cell’s ability to undergo cell suicide by a process called“apoptosis.” Apoptosis, or “cell suicide,” is the mechanism by which old or damaged cells normally self-destruct.

Fourth orlater mutation

Third mutation

Second mutation

First mutation

Uncontrolled growth

Cell Suicide or Apoptosis

Cell damage—no repair

Cancer cell

division

Normal cell division

• Cancers are capable of spreading throughout the body by two mechanisms: invasion and metastasis. Invasion refers to the direct migration and diffusion by cancer cells into neighboring tissues. Metastasis refers to the ability of cancer cells to penetrate into lymphatic and blood vessels, circulate through the bloodstream, and then invade normal tissues elsewhere in the body.

Causes of Cancer

• The changes that occur in the regulation of cell growth and division of cancer cells are due to mutations or changes in the segments of DNA that control the production of proteins.

• Often the genetic change or damage is repaired by various repair systems, but if the repair systems fail cancer can result.

Causes of Cancer

• Various environmental factors can affect the occurrence of cancer cells.

• Substances and agents that are known to cause cancer are called carcinogens.

• What are some carcinogens to which you are regularly exposed to?

Apoptosis

• Programmed cell death• Cells going through apoptosis actually shrink

and shrivel in a controlled process.• This process can help to protect organisms

from developing cancerous growths.• Example : Early Hands and Feet development

Stem Cells

• Unspecialized cells that can develop into specialized cells when under the right conditions

Embryonic Stem Cells

• After fertilization, the resulting mass of cells divides repeatedly until there are about 100–150 cells. These cells have not become specialized.

• Extraction of stem cells from this type of an embryo requires its destruction

• Embryonic stem cells are extracted directly from an embryo before the embryo's cells begin to differentiate. At this stage the embryo is referred to as a "blastocyst."

Adult Stem Cells

• Found in various tissues in the body and might be used to maintain and repair the same kind of tissue

• Less controversial because the adult stem cells can be obtained with the consent of their donor

• Adult stem cells can be extracted either from bone marrow or from the peripheral system