CHAPTER 11CHAPTER 11

THE REPRODUCTIONTHE REPRODUCTION

OFOFCELLSCELLS

MitosisMitosis Cells divide to make more cells. Cells divide to make more cells.

While all the other organelles can be While all the other organelles can be randomly separated into the daughter randomly separated into the daughter cells, the chromosomes must be cells, the chromosomes must be precisely divided so that each precisely divided so that each daughter cell gets exactly the same daughter cell gets exactly the same DNA.DNA.

MitosisMitosis is normal cell division, which is normal cell division, which goes on throughout life in all parts of goes on throughout life in all parts of the body. the body. MeiosisMeiosis is the special cell is the special cell division that creates the sperm and division that creates the sperm and eggs, the gametes.eggs, the gametes.

Mitosis and meiosis occur in Mitosis and meiosis occur in eukaryotes. Prokaryotes use a eukaryotes. Prokaryotes use a different method—”fission” to divide.different method—”fission” to divide.

Humans have 46 chromosomes, 23 Humans have 46 chromosomes, 23 from each parent. Every cell has the from each parent. Every cell has the same 46 chromosomes Each same 46 chromosomes Each species has a characteristic number species has a characteristic number of chromosomes: corn ahs 20, house of chromosomes: corn ahs 20, house flies have 10, chimpanzees have 48.flies have 10, chimpanzees have 48.

ChromosomesChromosomes The essential part of a The essential part of a

chromosome is a single very long chromosome is a single very long strand of strand of DNADNA. This DNA . This DNA contains all the genetic information contains all the genetic information for creating and running the for creating and running the organism. organism.

The DNA is packaged by proteins The DNA is packaged by proteins bound to it. At different times, bound to it. At different times, these proteins cause the DNA to these proteins cause the DNA to be spread out like spaghetti in a be spread out like spaghetti in a bowl, or tightly condensed into the bowl, or tightly condensed into the X-shaped chromosomes we can X-shaped chromosomes we can see in the microscope.see in the microscope.

Chromosomes haveChromosomes have ChromatidsChromatids CentromereCentromere

More ChromosomesMore Chromosomes Before replication, Before replication,

chromosomes have one chromosomes have one chromatid. chromatid.

After replication, chromosomes After replication, chromosomes have 2 sister chromatids, held have 2 sister chromatids, held together at the centromeretogether at the centromere

In mitosis, the two chromatids In mitosis, the two chromatids of each chromosome separate, of each chromosome separate, with each chromatid going into with each chromatid going into a daughter cell.a daughter cell.

Remember that diploid cells Remember that diploid cells have two copies of each have two copies of each chromosome, one from each chromosome, one from each parent. These pairs of parent. These pairs of chromosomes are NOT chromosomes are NOT attached together.attached together.

Cell CycleCell Cycle

Some cells divide constantly: (cells in the embryo, skin cells, gut lining cells, etc.) Other cells divide rarely or never: only to Some cells divide constantly: (cells in the embryo, skin cells, gut lining cells, etc.) Other cells divide rarely or never: only to replace themselves. replace themselves.

Actively dividing cells go through a cycle of events that results in mitosis. Most of the cycle was called “interphase” (the cell Actively dividing cells go through a cycle of events that results in mitosis. Most of the cycle was called “interphase” (the cell increases in size, but the chromosomes are invisible.)increases in size, but the chromosomes are invisible.)

The 3 stages:The 3 stages:**G1 G1 (“Gap”) is the period between mitosis and S, when each chromosome has 1 chromatid. It is the time when the cell (“Gap”) is the period between mitosis and S, when each chromosome has 1 chromatid. It is the time when the cell grows and performs its normal function. The *grows and performs its normal function. The *S phaseS phase (“Synthesis”) is the time when the DNA is replicated, when the (“Synthesis”) is the time when the DNA is replicated, when the chromosome goes from having one chromatid to having 2 chromatids held together at the centromere.chromosome goes from having one chromatid to having 2 chromatids held together at the centromere.**G2G2 is the period between S and mitosis. The chromosome have 2 chromatids, and the cell is getting ready to divide. is the period between S and mitosis. The chromosome have 2 chromatids, and the cell is getting ready to divide.

Machinery of MitosisMachinery of Mitosis The chromosomes are pulled The chromosomes are pulled

apart by the apart by the spindlespindle, which is , which is made of made of microtubulesmicrotubules. They . They attach to each centromere, and attach to each centromere, and anchored on the other end to anchored on the other end to centrioles.centrioles.

There are 2 centrioles, one at There are 2 centrioles, one at each end of the spindle. The each end of the spindle. The chromosomes are lined up chromosomes are lined up between the poles of the spindle.between the poles of the spindle.

When they contract, the When they contract, the chromosomes are pulled to the chromosomes are pulled to the opposing poles.opposing poles.

ProphaseProphase In prophase, the cell begins In prophase, the cell begins

the process of division. the process of division. 1. The chromosomes 1. The chromosomes

condense (long & thin to condense (long & thin to short & fat).short & fat).

2. The nuclear envelope 2. The nuclear envelope disappears. disappears.

3. The centrioles move to 3. The centrioles move to opposite poles. During opposite poles. During interphase, the pair of interphase, the pair of centrioles were together just centrioles were together just outside the nucleus. outside the nucleus.

4. The spindle starts to 4. The spindle starts to form, growing out of the form, growing out of the centrioles towards the centrioles towards the chromosomes.chromosomes.

Kinetochore vs NonkinetochoreKinetochore vs Nonkinetochore

KinetochoreKinetochore = protein = protein structure located at structure located at centromere regionscentromere regions

KinetochoreKinetochore microtubules microtubules = attach to the = attach to the kinetochores!! (PULL kinetochores!! (PULL CHROMOSOMES)CHROMOSOMES)

Nonkinetochore Nonkinetochore microtubulesmicrotubules = overlap = overlap with nonkinetochore with nonkinetochore microtubules from the microtubules from the opposite pole. (ELONGATE opposite pole. (ELONGATE CELL)CELL)

MetaphaseMetaphase

chromosomes line up on chromosomes line up on the equator the equator (metaphase (metaphase plate)plate) of the cell, with the of the cell, with the centrioles at opposite centrioles at opposite ends and the spindle ends and the spindle fibers attached to the fibers attached to the centromeres. centromeres.

Entire structure – Entire structure – nonkinetochore nonkinetochore microtubules + microtubules + kinetochore microtubules kinetochore microtubules = spindle = spindle

AnaphaseAnaphase

In anaphase, the In anaphase, the centromeres divide. centromeres divide.

At this point, each At this point, each chromosome goes from chromosome goes from having 2 chromatids to having 2 chromatids to being 2 chromosomes, being 2 chromosomes, each with a single each with a single chromatid.chromatid.

Then the spindle fibers Then the spindle fibers contract, and the contract, and the chromosomes are pulled chromosomes are pulled to opposite poles.to opposite poles.

TelophaseTelophase

Nonkinetochore Nonkinetochore microtubules elongate the microtubules elongate the cellcell

The spindle disintegratesThe spindle disintegrates The nuclear envelope re-The nuclear envelope re-

forms around the two sets forms around the two sets of chromosomes.of chromosomes.

Daughter nuclei begin to Daughter nuclei begin to reform reform

The cytoplasm divides The cytoplasm divides into 2 separate cells.into 2 separate cells.

Cytoplasmic DivisionCytoplasmic Division The organelles (other than the The organelles (other than the

chromosomes) get divided up into the chromosomes) get divided up into the 2 daughter cells passively: they go 2 daughter cells passively: they go with whichever cell they find with whichever cell they find themselves in.themselves in.

Plant and animal cells divide the Plant and animal cells divide the cytoplasm in different ways. cytoplasm in different ways.

In plant cells, a new cell wall made of In plant cells, a new cell wall made of cellulose forms between the 2 new cellulose forms between the 2 new nuclei, about where the chromosomes nuclei, about where the chromosomes lined up in metaphase. Cell lined up in metaphase. Cell membranes form along the surfaces of membranes form along the surfaces of this wall. When the new wall joins with this wall. When the new wall joins with the existing side wall, the 2 cells have the existing side wall, the 2 cells have become separate.become separate.

In animal cells, a ring of actin fibers In animal cells, a ring of actin fibers (microfilaments are composed of actin) (microfilaments are composed of actin) forms around the cell equator and forms around the cell equator and contacts, pinching the cell in half. contacts, pinching the cell in half.

Summary of MitosisSummary of Mitosis Prophase: Prophase:

• Chromosomes condenseChromosomes condense• Nuclear envelope disappearsNuclear envelope disappears• Centrioles move to opposite sides of the cellCentrioles move to opposite sides of the cell• Spindle forms and attaches to centromeres on the chromosomesSpindle forms and attaches to centromeres on the chromosomes

MetaphaseMetaphase• Chromosomes lined up on equator of spindleChromosomes lined up on equator of spindle• Centrioles at opposite ends of cellCentrioles at opposite ends of cell

AnaphaseAnaphase• Centromeres divide: each 2-chromatid chromosome becomes Centromeres divide: each 2-chromatid chromosome becomes

two 1-chromatid chromosomestwo 1-chromatid chromosomes• Chromosomes pulled to opposite poles by the spindleChromosomes pulled to opposite poles by the spindle

TelophaseTelophase• Chromosomes de-condenseChromosomes de-condense• Nuclear envelope reappearsNuclear envelope reappears• Cytoplasm divided into 2 cellsCytoplasm divided into 2 cells

PLANT vs ANIMALPLANT vs ANIMAL

PLANT CELLPLANT CELL ANIMAL CELLANIMAL CELL

* Cell plate* Cell plate * Cleavage furrow* Cleavage furrow

REGULATION OF THE CELL REGULATION OF THE CELL CYCLECYCLE

GROWTH FACTORS: occur in the presence of GROWTH FACTORS: occur in the presence of a wound – cells respond and grow a wound – cells respond and grow

DENSITY-DEPENDENT INHIBITION: Cells grow DENSITY-DEPENDENT INHIBITION: Cells grow depending on the density of cells. depending on the density of cells. (nutrients/space/adhesion)(nutrients/space/adhesion)

RESTRICTION POINT: go/no go decision. If all RESTRICTION POINT: go/no go decision. If all systems are “go” (all external and internal systems are “go” (all external and internal conditions are favorable), the step proceeds. conditions are favorable), the step proceeds.

G0 phase = nondividing state. G0 phase = nondividing state.

Cyclins – concentrations fluctuate Cyclins – concentrations fluctuate cyclicallycyclically

Cdk complex called MPF (maturation Cdk complex called MPF (maturation promotion factor (M-phase promoting promotion factor (M-phase promoting factor)factor)

Near end of M-phase, MPF switches Near end of M-phase, MPF switches off by activating an enzyme thatoff by activating an enzyme that destroys cyclindestroys cyclin. .

1. cyclin is synthesized through the 1. cyclin is synthesized through the cycle and accumulates during cycle and accumulates during interphaseinterphase

2. cyclin attaches to Cdk and the 2. cyclin attaches to Cdk and the protein complex is activated at the protein complex is activated at the end of interphaseend of interphase

3. MPF coordinates mitosis by 3. MPF coordinates mitosis by phosphorylating varous proteins, phosphorylating varous proteins, including other protein kinasesincluding other protein kinases

4. MPF is a cyclin-dependent kinase 4. MPF is a cyclin-dependent kinase enzyme that destroys MPF activity.enzyme that destroys MPF activity.

The Cdk component of MPF is The Cdk component of MPF is recycled, its kinase activity restored recycled, its kinase activity restored by association with new cyclin that by association with new cyclin that accumulates during interphase. accumulates during interphase.

Prokaryotic Cells: Binary FissionProkaryotic Cells: Binary Fission

““Division in half”Division in half” Most bacterial genes are carried on a Most bacterial genes are carried on a

single, circular chromosome & single, circular chromosome & associated proteins. associated proteins.

1.1. ReplicatesReplicates2.2. Attaches to different parts of the cell Attaches to different parts of the cell

membranemembrane3.3. Cell pulls apart and the replicate and Cell pulls apart and the replicate and

original chromosomes are separated. original chromosomes are separated.

CancerCancer Cancer is a disease of uncontrolled cell division. It starts with a Cancer is a disease of uncontrolled cell division. It starts with a

single cell that loses its control mechanisms due to a genetic single cell that loses its control mechanisms due to a genetic mutation. That cell starts dividing without limit, and eventually kills mutation. That cell starts dividing without limit, and eventually kills the host.the host.

Normal cells are controlled by several factors. They stay in the G1 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 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 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.division. Cancer cells enter the S phase without waiting for a signal.

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

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

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

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

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

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

Cancer TreatmentCancer Treatment Two basic treatments: surgery to remove the tumor, and radiation or Two basic treatments: surgery to remove the tumor, and radiation or

chemicals to kill actively dividing cells.chemicals to kill actively dividing cells. It is hard to remove all the tumor cells. Tumors often lack sharp It is hard to remove all the tumor cells. Tumors often lack sharp

boundaries for easy removal, and metastatic tumors can be very boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body.small and anywhere in the body.

Radiation and chemotherapy are aimed at killing actively dividing Radiation and chemotherapy are aimed at killing actively dividing cells, but killing all dividing cells is lethal: you must make new blood cells, but killing all dividing cells is lethal: you must make new blood cells, skin cells, etc. So treatment must be carefully balanced to cells, skin cells, etc. So treatment must be carefully balanced to avoid killing the patient.avoid killing the patient.

Chemotherapy also has the problem of natural selection within the Chemotherapy also has the problem of natural selection within the tumor. If any of the tumor cells are resistant to the chemical, they tumor. If any of the tumor cells are resistant to the chemical, they will survive and multiply. The cancer seems to have disappeared, will survive and multiply. The cancer seems to have disappeared, but it comes back a few years later in a form that is resistant to but it comes back a few years later in a form that is resistant to chemotherapy. Using multiple drugs can decrease the risk of chemotherapy. Using multiple drugs can decrease the risk of relapse. relapse.

MITOSIS LABMITOSIS LAB