Chapter 12: The Cell Cycle & Mitosis

http://www.gocomics.com/inthebleachers/2006/10/18/

Purpose: to understand the cell cycle, cell division, and to realize that mitosis is just a small part of the cell’s life cycle. Know the stages of mitosis. Understand how the cell cycle is regulated. (AP Biology Major themes: IV- Continuity and Change, V-Structure & Function, VI-Regulation)

What you MUST know in Ch 12:Phases of the cell cycleEvents in mitosisCytokinesis in plants vs. animalsBinary fission vs. mitosisCheckpointsCyclins

This chapter should be review for you; although as usual there are some new details to know.

Cell Division “Every cell from a cell.” One of the defining

characteristics of living organisms, the ability to reproduce, is due to the ability of cells to divide

Growth of multicellular organisms from a single cell (zygote)

Repair and replacement of cells that are damaged or dead, by making a genetically identical cell

= mitosis (“t” in mitosis = “twin” cells)

Why do cells divide? o To maintain a high surface area-to-volume ratio

A large surface area (plasma membrane) relative to the cell volume (high surface area to volume ratio) allows the cell to efficiently exchange its contents (wastes) with the environment, and to take in environmental molecules (nutrients, oxygen) by diffusion or active transport.

If the surface area is too small to accomplish this, exchange will not occur efficiently enough to sustain the cell.

o To maintain an adequate genome-to-volume ratio genome = all of a cell’s DNA a finite (limited) amount of DNA is available to

code for all the proteins a cell needs (which directs virtually all cell activities)

If there is too little DNA for too large a cell, the DNA will not have the capacity to regulate all of the cell’s activities.

DNA Chromatin = DNA-protein complex, exists in a long,

thin fiber in interphase.

o Chromatin is contained in the nucleus Chromatin condenses (becomes more tightly coiled)

into chromosomes in prophase Each chromosome is made of 2 identical halves

(replicated during the S phase of interphase), called sister chromatids, which are connected to each other at the centromere. Each chromatid = 1 single molecule of DNA

Each sister chromatid has a kinetochore, which is attached to spindle fibers in prophase:

http://www.lbl.gov/Science-Articles/Archive/assets/images/2001/Jan-02-2002/kinetochore.jpghttp://academy.d20.co.edu/kadets/lundberg/images/chromosome.gif

Somatic cells are body cells (not reproductive cells), which are diploid. o Diploid cells contain 2 copies of every

chromosome (= 2n).o Diploid cells are made in mitosis o Human somatic cells have 46 chromosomes

Gametes are reproductive cells (sperm and egg cells), which are haploid.

o Haploid cells contain 1 copy of every chromosome (haploid = half as many chromosomes as diploid cells = n)

o Haploid cells are made in meiosiso Human gametes have 23 chromosomes

The Cell CycleThe cell cycle is how a cell spends its very interesting life:

Interphase Cell division: mitosis (division of the nucleus),

followed by cytokinesis (division of the cytoplasm) See Fig. 12.4, pg 209 “IPMAT-C” or however you can remember it….

http://www.daviddarling.info/images/cell_cycle.jpg

Interphase up to 90% of the cell cycle is spent in interphase

(variable, but interphase always takes up the majority of the cell’s life)

Cell grows, copies its chromosomes (DNA replication) in preparation for cell division

3 phases:

o G 1 phase (1st gap phase)—cell growso S phase (DNA synthesis=replication)—DNA is

copied; cell continues to growo G 2 phase (2nd gap phase)—cell growth and

preparation for cell division

DNA is in the form of chromatin during interphase. If you see chromosomes, it’s not interphase!

Other features present during interphase:o Nucleoli o Microtubule organizing centers (MTOC’s or

centrosomes in animals in animals, each MTOC contains a pair of

centrioles (spindle fibers are made by them) 1 centrosome replicates to form 2

Mitosis Yes!!! This is indeed the zillionth time you will have

reviewed mitosis! And believe it or not, you’ll revisit this EVEN IN COLLEGE! Again!

“IPMAT-C”… except that interphase is NOT a part of mitosis

mitosis is the division of the nucleus, including DNA Mitosis makes twin GENETICALLY IDENTICAL cells here are some illustrations; Campbell’s pgs 210-211

http://www.accessexcellence.org/RC/VL/GG/images/MITOSIS.gif

http://www.bio.miami.edu/dana/250/mitosis.jpg

Prophase (“prepare”) nucleoli disappear chromatin condenses into chromosomes nuclear envelope (membrane) breaks down mitotic spindle forms:

o MTOC’s go to opposite poles of the nucleus and send out microtubules, which connect to the kinetochores of the just formed chromosomes, pulling them

o Mitotic spindle = all the microtubules involved in pulling the chromosomes apart

Metaphase (“meet in the middle”) Chromosomes line up on the metaphase plate, in

between the 2 poles of the mitotic spindle Microtubules pull each chromosome apart into

chromatids

Anaphase (“pull apart”) Microtubules pull the chromatids apart to opposite

poles, and the poles move farther apart At the end of anaphase, each pole has a complete set

of chromosomes (same # of chromosomes as the original cell)

Telophase (“two cells”) Nuclear envelope develops, forming 2 nuclei Chromosomes unwind back into chromatin Nucleoli reappear Simultaneously…

Cytokinesis occurs, dividing the cytoplasm into 2 cells Animal cells form a cleavage furrow, which pinches

the cell from the edges in, into 2 cells (actin microfilaments are involved)

Plant cells form a cell plate, which which forms in the middle, and extends to the edges, and then becomes the plasma membranes for the 2 daughter cells (vesicles from the Golgi apparatus are involved); cell walls develop between the new membranes.

Current thought is that mitosis evolved from prokaryotic binary fission; the way that bacteria divide (fig. 12.10, pg 215). Difference = duplicated chromosomes are attached to the plasma membrane of the dividing cell (no MTOC/mitotic spindle) in prokaryotes.

And now for the stuff that may be new to you….

Regulation of the Cell Cycle

Regulation is GOOD! Lack of regulation/control mechanisms is NOT!

The cell cycle control system involves various checkpoints in the cell cycle; these are critical control points where “stop” and “go ahead” signals can regulate the cycle.

Animals usually have “stop” signals that stop the cell cycle until they are overridden by “go” signals

3 checkpoints: G1, G2, M (Fig.12.13, pg 217) G 1 checkpoint = restriction point

o If it does not get the “go ahead” signal, the cell will be in the G0 or nondividing phase

o If it gets the “go ahead” signal, it will complete the cycle and divide

G 2 checkpoint triggers mitosis (see fig 12.14) M checkpoint ensures that anaphase does not begin

until all chromosomes are lined up and attached to the spindle at the metaphase plateo Stops daughter cells from having missing or extra

chromosomes

Cyclins are proteins that exist in the cell in concentrations that increase and decrease in cycles (thus “cyclins”) Cyclin-dependent kinases (Cdk) are protein kinases

that are attached to a cyclin, and are able to give “go ahead” signals at the G1 and G2 checkpoints

More than one Cdk may act upon a checkpoint Signals that may affect Cdk activity include:

o Internal signals , such as proteins that originate at the kinetochores (act on the M checkpoint)

o External signals , including growth factors, which are proteins that stimulate cell division

Normal cells have density-dependent inhibition of cell division, in which crowded cells stop dividing (or they

will use up nutrients….same concept as density-dependent limiting factors and environmental carrying capacity) and normal cells have anchorage dependence, in which they must be attached to something else in order to divide.

Here’s why lack of regulatory control is so bad…. CANCER!

CANCER cells do not respond to the normal cell cycle controls. THIS IS WHY YOU NEED TO UNDERSTAND MITOSIS AND THE CELL CYCLE! Can we cure every kind of cancer that we know of? Not even close…cancer treatment remains a major medical challenge today and into YOUR future. Cancer cells are, by definition, in a state of

excessive, uncontrolled division…they just keep dividing.

They do not show density-dependent inhibition, and the usual checkpoints are unresponsive to “stop” signals.

The result is a tumor (“malignant” = cancer) that can invade and destroy normal organs and tissues, and may spread (“metastasis”) throughout the body to other organs and tissues.

Normal cells (can be just one cell) undergo transformation into a cancer cell due to gene mutations (changes in the normal DNA) in genes that regulate the cell cycle control system. That cell then divides abnormally = cancer.

A more sophisticated understanding of cell cycle regulation, and developing treatment that re-establishes normal control of the cell cycle remains a challenge for medical researchers.