Chapter 9 - Mitosis

Chapter 9: Mitosis

Cell Cycle: the life of a cell from origin to division into 2 new daughter cells

Cell Divison: process by which a cell divides into 2 new cells

1. Living things grow by producing more cells, not because each cell increases in size

2. Repair of damaged tissue

3. If cell gets too big, it cannot get enough nutrients into the cell and wastes out of the cell

• Original cell is called the parent cell; 2 new cells are daughter cells

• Daughter cells exactly the same as the parent

• Cell replicates all its DNA so each daughter cell gets complete set of genetic info from parent cell - same kind of and number of chromosomes as original cell

• Many organisms, especially unicellular ones, reproduce by mitosis. (Asexual reproduction)

Prokaryotic cell division

• Prokaryotes have a ring of circular DNA and a few associated proteins, folded dup in area called nucleoid

• Uses asexual reproduction (Binary fission)

• Binary fission: - splitting in 2 

                                  - two replicate chromosomes are distributed to 2 daughter cells

                                  - produces 2 daughter cells identical to original cell

Eukaryotes: In multicellular fungi, plants, and animals, cell division is important for growth, renewal, and repair

Making new cells:

• nucleus

• chromosomes

• cytoskeleton

• centrioles: in animals, plants don’t use

• microtubules spindle fibers

Division of organelles and cytoplasm = cytokinesis

Overview of Mitosis

1. Interphase

2. Prophase

3. (Pro-metaphase)

4. Metaphase

5. Anaphase

6. Telophase / cytokinesis

Interphase

• 90% of cell life cycle

• doing its “everyday job”

• produce RNA, synthesize proteins/enzymes

• prepares for duplication if triggered

• the nucleus is well defined, DNA loosely packed

• preparing for mitosis

• replicates chromosomes

Duplicated chromosome:

• 2 sister chromatids(attached in the middle)

• narrow at centromeres

• contain identical shape

DNA

• located in nucleus and controls all cell activities including cell division

• long, thread-like DNA in a non-dividing cell is called a chromatin

• doubled, coiled, short DNA in a dividing cell is a chromosome

Every organism has its own number of chromosomes

• Humans = 46 chromosomes, 23 pair

• Dogs = 78, 39 pairs

• Goldfish = 94 chromosomes, 47 pairs

• Lettuce = 18 chromosomes, 9 pairs

• All somatic (body) cells in an organism have the same kind and number of chromosomes

Humans: 46 chromosomes (diploid number)

Human skin cell: 46

Human heart cell: 46

Human muscle cell: 46

Prophase

• chromatin condenses, visible chromosomes

• Centrioles move to opposite poles of a cell

• Protein fibers cross cell to form mitotic spindle

• microtubules; actin, myosin

• coordinates movement of chromosomes

• nucleolus disappears 

Pro-metaphase

• spindle fibers attach to centromeres, creating kinetochores

• microtubules attach at kinetochores

• connect centromeres to centrioles

• chromosomes begin moving

Metaphase

• chromosomes align along middle of cell 

• spindle fibers coordinate movement

• helps ensure chromosomes separate properly so each new nucleus receives only 1 copy of each chromosome

Anaphase

• sister chromatids separate at kinetochores

• move to opposite poles

• pulled at centromeres

• pulled by motor proteins “walking” along microtubules 

• poles move further apart

• polar microtubules lengthen 

• once separated from each other, they are individual chromosomes 

Telophase

• chromosomes arrive at opposite poles

• daughter nuclei form

• nucleoli form

• chromosomes uncoil

• spindle fibers disperse

• Cytokinesis begins (??)

Cytokinesis

• Animals

• constriction belt of actin microfilaments around equator of cell

• cleavage furrow forms

• splits cell in 2

Cytokinesis- the division of the rest of the cell (cytoplasm and organelles) after the nucleus divides

In animal cells, the cytoplasm pinches in

In plant cells, a cell plate forms

Coordination of cell division

• A multiceullar organism needs to coordinate cell division across different tissues and organs

• critical for normal growth, development and maintenance

• coordinate timing of cell division

• coordinate rates of cell division

• not all cells have the same cell cycle

Frequency of division varies:

• embryo cell cycle < 20 minutes

• skin cells 12-24 hour cycle

• liver cells divide every year or two

• muscle and nerve cells do not divide at all after maturity

Two irreversible points in cell cycle

• replication of genetic material

• separation of sister chromatids

Checkpoints

• process is assessed and possibly halted

G1/S: 

• can DNA synthesis begin?

G2/M:

• has DNA synthesis been completed correctly? 

• commitment to mitosis

Spindle checkpoint: 

• are all chromosomes attached to spindle? 

• can sister chromatids separate correctly?

G1/S checkpoint is most critical

• primary decision point, “Restriction point”

• if cell receives “GO” signal, it divides

• internal signs: cell growth, cell nutrition

• external signs: “growth factors”

if cell does not receive signal, it exits cycle and switches to G0 phase

G0 Phase

• nondividing, differentiated state

• most human cells in G0 phase

• liver cells in G0 but can be called back to cell cycle

• muscle and nerve cells cannot leave G0, they are done growing

How do cells know when to divide?

• cell communication signals

• chemical signals in cytoplasm give cues

• signals usually mean proteins

• activators and inhibitors

G1 Checkpoint - decides whether or not cell will divide

S (G2) Checkpoint - determines if DNA has been properly replicated

Mitotic spindle checkpoint - ensures chromosomes are aligned at mitotic plate

Apoptosis - programmed cell death

Density Dependent Inhibition

• cells grown in culture will rapidly divide until a single layer or cells is spread over the area of the petri dish after which they will stop dividing

• if cells are removed, those bordering the open space will begin dividing again and will keep dividing until space is full

Characteristics of Cancer Cells

1. Form tumors

• cancer cells have lost contact inhibition

2. Undergo angiogenesis

• bring nutrients and oxygen to tumor

3. Lack differentiation

• non specialized, immortal (enter cell cycle repeatedly)

4. Have abnormal nuclei

• enlarged, extra copies of genes

Normal cell growth is closely regulated

• cell size, division, and death are carefully controlled to ensure that the organism and its organs achieve and maintain appropriate size

1. Proto-oncogenes (Ras) = stimulates cell division

2. Tumor-suppressor genes (p53, Rb, BRCA1) = inhibits cell division

• Mutations in these genes can lead to cancer

Ras gene (proto-oncogene) - mutation in ras leads to 30% of cancers

p53 gene (tumor-suppressor gene) - 50%+ of cancers, halt cell cyle for DNA repair and activate apoptosis

• Cancer results when mutations accumulate (5-7 changes in DNA)

• Activate onocgenes + loss of tumor suppressor genes

• Longer we live, more likely for cancer to develope

Proto-oncogenes -> Oncogenes

1. Cancer starts from a single mutated cell

2. Accumulates mutations and has ability to start tumor

Cancer:

• uncontrolled cell growth

• cells grow and divides in absence of growth signals

• refuses to respond to death signal

• other signals allow cancerous cells to spread to other parts of the body