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AP Biology - Cell Division

Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet. Provided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. By David Knuffke. Modified by Eric Friberg
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Transcript of AP Biology - Cell Division

Cell Division
Big Questions:
Make Sure You Can:
Why Divide?
The Cell Cycle
Mitosis
The "continuity of life"
In order to survive, the individual must replace damaged cells.
In order to grow, cell production must be greater than cell death.
In order to survive, the species must replace individuals.
Cell division accomplishes these purposes.
A dividing amoeba
A dividing bone marrow cell
A dividing sea urchin embryo
The phases of a cell's life
G1- growth
S- DNA replication
G2- preparation for division
M - Mitosis

G0- Non-dividing state (most cells in you)
Chromosomes!
Very tightly controlled (why?)
Tightly coiled pieces of DNA that condense prior to division
Remember!
Prokaryotes only have one, circular chromosome.

Eukaryotes have many, linear chromosomes
Most eukaryotic cells have 2 copies of every chromosome.

They form in attached, identical pairs.

Chromatid: 1 member of the pair
Centromere: region where they are joined
Chromatids
Centromere
S phase
Mitosis
Make sure you understand the chromosome, chromatid relationship



It can be confusing...
Haploid vs. Diploid
1 individual of every homologous chromosome
pair (n)
2 individuals of every homologous chromosome (2n)
Human cells have 23 pairs of homologous chromosomes

How many chromatids are present during:
G1?
G2?
Interphase
Chromosomes Condense
Nuclear envelope breaks down
mitotic spindle begins to form
Animal cells: centrioles divide.
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
Chromosomes begin to migrate to cell equator.
2 complete spindles at cell poles.
Chromosomes are at metaphase plate.
Spindle attaches to "kinetochore" of chromosomes at centromere
Chromatids split apart at centromere.
Migration of chromatids to cell poles mediated by the kinetochore.
Chromosomes decondense
Nuclear envelope reforms
Cytokinesis: cell membrane divides
Growth
Replication of DNA
Preparation for division

Most of a cells life cycle
Newt, Whitefish, Onion
Differences between plant-like and animal-like cells (Why?)
Plants
Animals
A "contractile ring" of microfillaments pinches the cell in 2
Vesicles from both cells deposit a new cell wall partition ("cell plate") in the middle of the cell.
Organelle apportionment is essentially random.
Mitosis at a Glance
The Evolution of Mitosis
There are more similarities between eukaryotic and prokaryotic cell division than might be apparent at first glance.

Proteins involved in binary fission
& eukaryotic cell division have a
large degree of homology.
Some unicellular eukaryotes demonstrate "intermediate" modes of division.
Binary fission in bacteria
(aka "rolling circle replication")
Let's play "spot the phases"
"Non-Reductive" Eukaryotic Cell Division
Describe the roles that mitosis plays in eukaryotic organisms.

Explain how mitosis produces two genetically identical cells

Explain how interphase prepares a cell for mitosis.

Explain why many cells never divide.

Explain the function of each stage of mitosis

Compare the events of mitosis in plant-like and animal-like cells
Why do cells need to divide?

How does cell division provide for continuity of life processes in an individual and in a species?
"Binary Fission"
The splitting of the cell into two.
Watch it Happen!
Cell Cycle Control
Why?
How?
Cancer:
Cells should only divide when they need to.
The cell cycle is under both internal and external control
It's best to think of cell cycle as consisting of a series of "checkpoints" that the cell must pass through in order to be able to divide.

What happens if cells don't pass a checkpoint?
The G1 checkpoint:
A breast cancer cell: It's become a "selfish" cell
Internal Controls:
External Controls:
Protein molecules that are present in varying concentrations during the cell cycle
Proteins and other environmental signals generated by other cells that stimulate cell division.
Evidence suggesting that there are molecules present during the cell cycle that cause cells to progress
Determines if a cell should replicate its DNA.

From here cells either enter S phase or G0

"Senescence": Cells that have stopped dividing
MPF: Mitosis Promotion Factor
Cdk: "Cyclin-dependent" kinase
Present in a constant amount
Cyclin: the Cdk "on switch"
made in an increasing amount as the cell moves through interphase
MPF: Cyclin + Cdk
turns on other proteins needed for mitosis (e.g. microtuble formation, breaking of nuclear membrane)
No Cyclin = No Mitosis

What kind of feedback is this?
PDGF: Platelet Derived Growth Factor
What are platelets? Why do they make a growth factor?
Positional Inhibition
Normal animal cells must be anchored and not too crowded ("density-dependent").
Cancerous cells don't care
Uncontrolled Cell Division
Mutations Happen!
Every second of every day, your DNA is beset by entropic forces.
You have a whole series of genes that make sure mutated cells dont divide.
...but what happens when these genes get mutaed?
"Who watches the watchmen?"
- Juvenal
Proto-oncogenes
stimulate cell division
"The accelerator"
Oncogenes: mutated versions. Always "on".
Tumor Suppresor Genes
inhibit cell division
"The brake pedal"
mutated versions always "off".
Cancer requires ~6* mutations in different genes (it's a "multi-step" pathway)
The Stages of Cancer
* The "Knudson hypothesis" Suggested by Carl Nordling, based on the fact that cancer occurs on average as a sixth function of an indivdual's age. Who says math is useless?
A multistep model of colon cancer development
Metastasis is what kills people.


How do we treat cancer?
Gleevac: A novel cancer treatment
Meiosis
Big Questions:
Make Sure You Can:
What Sex Is
Why Sex Is
Meiosis
Sexual Reproduction is highly varied
At least in terms of mechanics, & lifecycles.
Fundmentally all sexual reproduction involves the same cellular process ("fertilization"):

Sexually reproducing organisms need to make haploid cells ("gametes") from diploid cells...or there would be problems during fertilization
Most organisms do not sexually reproduce
"Reductive"
Eukaryotic cell division
Diploid Chromosomes
Stages of Meiosis:
Interphase
No DNA Replication
Meiosis at a Glance:
Prophase I:
Metaphase I:
The variety of sexual life cycles seen among organisms
Budding Hydra
Sprouting Redwood
Parthenogentic Rotifer
Haploid cell
Haploid cell
+
Diploid cell
Remember that every diploid cell has two copies of each chromosome.
During S phase each of these chromosomes is replicated.

"Sister Chromatids": The replicated copies of a particular chromosome

"Homologous Pairs": The set of 2 replicated copies of a particular chromosome (sometimes shortened to "homologues")
Is it making sense yet?
How about now?
The fate of chromosomes during meiosis
(n)
(n)
(2n)
n - the "haploid number"
2n - the "diploid number"
Different species have different numbers of chromosomes

Humans: n = 23 2n = 46
Fruit Flies: n = 4 2n = 8
Dogs: n = 39 2n = 78
With a focus on differences from mitosis
"Crossing Over"
When chromosomes condense during prophase 1, homologous pairs physically connect to eachother ("synapsis"), forming structures called "tetrads".

At each connection ("chiasma"), DNA is exchanged between the homologous pairs.







Every chromatid that is produced has a unique combination of DNA from both chromosomes in the pair.

This results in every gamete produced having a unique sequence of DNA in each chromosome.

"Recombination": Combining DNA from 2 different sources.
During metaphase 1, homologous pairs of chromosomes line up at the metaphase plate still attached to each other.

When they separate during anaphase 1, the homologous pairs will separate.

Sister chromatids will remain attached.








This is a major difference from mitosis, where chromosomes line up "single file" during metaphase.


In meiosis, that doesn't happen until metaphase II.
Fundamentally, meiosis serves two major purposes:
Produce haploid cells
Create cells with unique combinations of genetic information.

How does meiosis lead to these outcomes?

Why does meiosis look so similar to mitosis?
3 Ways to make clones:
Nobody knows why sex evolved.
Here's what we do know:
The "Reproductive handicap" of sex
In sexually reproducing organisms, one gender ("males") is not capable of producing an offspring.

And yet, sex has evolved many, many times in many different lineages...why?
Sex increases variation exponentially
Sexual reproduction leads to a tremendous amount of variation in a population.

Asexually reproducing organisms only generate variation through mutations and horizontal genetic transfer.
Sexually reproducing organisms generate variation through the events of meiosis. Let's consider humans (n = 23, 2n = 46):
"indendent assortment" of homologues during metaphase I
The easiest to understand mathematically.
Each homologous pair has a 50/50 chance of lining up so that "mom's" pair or "dad's" pair winds up in either of the cells produced.
Mathematically, this means that there are 2 possibilities for each of the 23 tetrads.
2
23
=
8,388,608 possible combinations per gamete
The random nature of fertilization
One male gamete will combine with one female gamete
If there are 8,388,608 possible combinations in a gamete, and each gamete has an equal chance of combining with a gamete from the opposite gender, then our possible genetic combinations for an offspring is equal to the possible number of combinations for each gamete, multiplied by each other
(2 )
23
(2 )
23
=
70,368,744,177,664 possible combinations per offspring
Crossing Over during prophase I
Crossing over produces genetically unique chromatids. It is a random process, occuring an unpredictable number of times per meiotic cycle.
Due to this, it is not easy to mathematically model, but it is easy to draw a fundamental conclusion about the number of possible variants produced.
Functionally
Infinite
genetically unique offspring produced in a sexually reproductive species
A bit more on chromosomes:
Analysis of chromosomes can tell us a lot about an individual.
"Karyotype": A picture of an individuals chromosomes.
A technician making a karyotype
Pre-sorted:
Post-sorted:
Autosomes: Chromosomes that both genders have in equal numbers (humans: 1-22)

Sex Chromosomes: Chromosomes that determine gender (humans: X & Y)
Normal human male karyotype:
Normal human female karyotype:
How is sex possible?

Why does sex exist?

Where does variation in a population come from?
SEX! SEX! SEX!
Identify similarities and differences in sexual life cycles among various groups of organisms.

Explain how meiosis leads to the transmission of genetic information from parent to offspring.

Compare the events and outcome of meiosis with mitosis.

Explain the process and function of crossing over.

Explain how various aspects of meiosis and sexual reproduction increases variation in a species.
If only there were an awesomely cheesy/bizarre cartoon...
Chromosomal Abnormalities
How it happens
Some Examples
Single Chromosome
Whole Sets
Non-disjunction
Large-scale rearrangement
In a non-disjunction event, chromosomes do not separate during anaphase.

This leads to gametes with irregular numbers of chromosomes.
This must happen in a germ-line cell to have an effect (why?)
Segments of chromosomes can also get altered during DNA replication.

There are four major types of chromosomal rearrangements
How do these alterations effect an organism?
"-somy" mutations
involve individual extra/missing chromosomes
Usually result in early miscarriage
Down's Syndrome: Trisomy 21
Sex Chromosome "-somy" mutations
Three copies of chromosome 21
characteristic facial appearance.
varying levels of retardation
other varying effects
Are generally more tolerated by the organism. Don't result in miscarriage
Why would an extra 21 be tolerated by the organism?
Turner's Syndrome: Monosomy X
Phenotypically female.
Sterile
Associated with heart defects
Kleinfelter's Syndrome: XXY
Phenotypically male.
Reduced fertility
Some "female" secondary sexual characteristics (e.g. over-developed breast tissue)
Multiple Y Chromosomes/Multiple X Chromosomes
Not a problem. Why?
Multiple Y:
The Y chromosome is the smallest human chromosome.

It contains very few genes.

It's major function is to direct testes formation early in development.
Multiple X:
The X chromosome is pretty large.

It contains many genes.

BUT: Both males and females have to function with it (and males only get 1 copy).

X-inactivation: Early in development every X chromosome in a cell except for one is turned into an inactive "Barr body".

So multiple X chromosomes are not a problem
"-ploidy" mutations
involve extra homologous sets of chromosomes.
Not usually tolerated in animals.
Rearrangements:
No rule about how a chromosomal rearrangement will affect an organism

sometimes detrimental, sometimes silent.
A translocation in blood stem cells leads to CML leukemia
Map showing diseases associated with various translocations in the human genome:
Calico cats are always female!
Gets cell through G2 Checkpoint
Getting through M Checkpoint
-The kinetochores are continually releasing a inhibitory protein.
-When the kinetochores is attached to a microtubule the
production of that protein stops.
-Once all the kinetochores are attached, the inhibition signal is now absent,
an enzyme called separase can break the bond between the sister chromatids and anaphase can begin.

Growth Factors
VEGF: Vascular Endothelial Growth Factor
How do they know?
Dr. Saltzman!
projectstealth.org/
Full transcript