2.5 Cell Division

IB Biology I

QuestionsWhy are you bigger now than you were born?Answer: Cells divide and take up more space

Why do cells divide instead of growing?Answer: Large cells have reduced SA:VOL ratio = less efficient than smaller cells

What needs to happen for you to grow?Answer: 1)More cells need to be produced2)Each cell needs a copy of the organism’s DNA3)Each cell needs copies of organelles to perform all

functions

Prokaryotes x Eukaryotes

How would cell division work? Think about each cell’s structure.

Prokaryotes x Eukaryotes

Prokaryotes divide by binary fission

Eukaryotes have chromosomes carrying their genes in the nucleus. A complete set is necessary for the cell to function properly

What is the purpose of cell division?

Growth (example: baby adult)

Reproduction (asexual reproduction in single celled organisms)

Replacement of dead/damaged/infected (example: skin/red blood cells/bone cells)

Gamete formation in multi-cellular organisms (meiosis)

Chromosome StructureIn eukaryotes: made up of DNA and proteins

At different times, proteins cause the DNA to: be spread out like spaghetti in a

bowl be tightly condensed into the X-

shaped (these we can see in the microscope)

Central constricted region called centromere that serves as an attachment point for the spindle fibers during mitosis.

Chromosome StructureChromosomes exist in 2 different states:before DNA replication, chromosomes have one chromatid.after DNA replication, chromosomes have 2 sister chromatids, held together at the centromere. Each chromatid is one piece of DNA with its supporting proteins.

Why is DNA replication necessary?

Chromosomes

How often do cells divide?

Some cells divide constantly (e.g. skin) while others rarely or never divide (e.g. neurons)

Cycle: Interphase – Mitosis – Interphase – Mitosis

Interphase = period between 2 cell divisions. Cell increases in size, but the chromosomes are invisible (long and thin)

3 stages: G1 , S, G2

MitosisMitosis is the division of the eukaryote nucleus, which goes on throughout life in all parts of the body.

Organelles can be randomly separated into the daughter cells but chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.

Every human cell has the same 46 chromosomes

Mitosis is usually divided into 4 phases:

Prophase (P) Metaphase (M) Anaphase (A) Telophase (T)PMAT

ProphaseImportant events:1.The chromosomes condense (the

proteins attached to the DNA cause the chromosomes to go from long thin structures to short fat one, which makes them easier to pull apart)

2.The nuclear envelope disappears (the double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division)

3.Pair of centrioles separate and move to opposite ends of the cell (except in plants)

4.The spindle fibers (microtubules) start to form, growing out of the centrioles towards the chromosomes.

Metaphase•Important events:

1)chromosomes line up on the equator of the cell.

2)the centrioles are at opposite ends and the spindle fibers attach to the centromeres.

Anaphase•Important events:

1)the centromeres divide (each 2-chromatid chromosome becomes two 1-chromatid chromosomes)

2)spindle fibers contract, and the chromosomes are pulled apart to opposite poles of the cell, towards the centrioles.

Telophase•Important events:

1)chromosomes are at the poles of the cell

2)spindle fibers and aster disintegrate

3)nuclear envelope re-forms around the two sets of chromosomes

4)cytoplasm is divided into 2 separate cells = cytokinesis.

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

Plant x Animal cells: Plants: a new cell wall made of cellulose forms between the 2 new nuclei (cell plate)

Animals: a ring of actin fibers (microfilaments) forms around the cell equator, pinching the cell in half.

Summary of MitosisProphase:

Chromosomes condense Nuclear envelope disappears Centrosomes move to opposite sides of the cell Spindle fibers form and attach to centromeres on the

chromosomes

Metaphase Chromosomes are lined up on equator of cell Centrioles are at opposite ends of cell

Anaphase Centromeres divide: each 2-chromatid chromosome

becomes two 1-chromatid chromosomes Chromatids pulled to opposite poles by the spindle

fibers

Telophase Chromosomes decondense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells

http://biology.uoregon.edu/reference/ort_mitosis/CellCycle.html

Draw labeled diagrams of the four stages of mitosis in an animal cell with four chromosomes

Important:The four diagrams must have the name of the phase, otherwise award [3 max].The four stages must be included to receive [5]. If correct number of chromosomes is not shown award [4 max].

How does mitosis ensure the daughter cells are genetically identical?

What happens when there is no control of cell division?

Tumors = result of uncontrolled cell divisionThe genetic checks that stop cells from reproducing fail to work and they grow out of controlOncogene = gene that turns a normal cell into a cancer cellTumors can occur in any organ or tissue, though are most common after exposure to carcinogens (e.g. tobacco smoke) or in particularly active tissues (e.g. breast, skin)Angiogenesis: tumor recruits blood vessels and grows largerMetastasis: part of the tumor invades the blood vessel, travels through the blood and starts to forma a tumor in another part of the body

Where is the lack of control?

Normal cells are controlled by several factors:

Normal cells stay in the G1 stage of the cell cycle until they are given a specific signal. Cancer cells enter the S phase without waiting for a signal.

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

Normal cells that suffer significant chromosome damage destroy themselves due to the action of a gene called “p53”. Cancer cells either lose the p53 gene or ignore its message and fail to kill themselves (process known as apoptosis)

http://www.e-learningforkids.org/Courses/Liquid_Animation/Conditions_Diseases/Cancer/index.html

Cancer TreatmentMost common treatments: surgery, radiation or chemotherapy

It is hard to remove all the tumor cells. Tumors often lack sharp boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body.

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

Chemotherapy also has the problem of natural selection within the tumor. If any of the tumor cells are resistant to the chemical, they 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 chemotherapy. Using multiple drugs can decrease the risk of relapse: it’s hard for a cell to develop resistance to several drugs at the same time.