Ch. 8: The Cell Cycle

Ch. 8: The Cell Cycle

Why does a cell divide?

-As a cell absorbs nutrients and gets larger, the volume of the cell increases faster than the surface area.

-Therefore, the demands of the cell (the volume) exceed the ability of the cell to bring in nutrients and export wastes. Solution? Divide into two smaller cells

When is cell division occurring?

GROWTH -increase number of cellsREPAIR -replace lost cells due to injury, diseaseCANCER – Abnormally high rates of cell division due to mutation

Different kinds of cells divide at different rates:E. coli – 20 minutes (What domain?)

Yeast cell – 2 hours (What domain? What kingdom?)

Amoeba – a few days (What domain? What kingdom?)

Human embryo cell – 15-20 minutesHuman adult cell – 8 hours to 100 days

AgingAll cells die after a certain number of divisions (programmed cell death-”apoptosis”). At any given time some cells are dividing and some cells are dying.

Childhood Cell division > cell death

Adulthood Cell division = cell death

Aging Cell division < cell death

Control of the Cell Cycle Cell proliferation

Interphase

Interphase ~ 90% of the time.

   G1: Little new cell absorbs nutrients and grows larger. Does protein synthesis, its job.

   S phase: Synthesis of new DNA (DNA replication) for daughter cells in preparation for mitosis.

   G2: Cell continues to grow, do protein synthesis, do its job. Gets too large, needs to divide.

Chromosomes exist in 2 different states, before and after they replicate their DNA.

Before replication chromosomes = 1 chromatid.

After replicationchromosomes = 2 sister

chromatids, held together at the centromere. Each chromatid is one piece of DNA with its supporting proteins.

Structure of a eukaryotic chromosome

• unreplicated chromosome

arm armcentromere

Prior to cell division:• chromosomes (DNA) are replicated (duplicated)

duplicatedchromosome

• duplicated chromosome – attached at their centromeres – as long as attached, known as sister chromatids

daughterchromosomes

sisterchromatids

• Division of somatic cells (body cells)(non reproductive cells) ineukaryotic organisms

What is Mitotic Cell Division?

• A single cell divides into two identical daughter cells (cellular reproduction)

=> Maintains chromosome ploidy of cell

MITOSISEqual distribution of the 2 sets of DNA amongst the 2 daughter cells.

4 Stages: “PMAT”

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

How the Cell Cycle Works

Mitosis Animation

Cell Cycle

As a cell enters mitosis from interphase it has 2 complete sets of chromosomes because of replication in the S phase.

Each set must be re-arranged and distributed into the 2 new daughter nuclei. This is mitosis.

• Chromatin condenses (coils) into chromosomes. (Sister chromatids joined by centromere)

• Nuclear membrane dissolves.

• Nucleolus disappears.• Centrioles divide and

move to opposite poles forming spindle b/w them.

Prophase…

chromatin

nucleolus

nucleus

centrioles

condensingchromosomes

Metaphase

-Sister chromatids line up on metaphase plate.

-Centromeres lock on to spindle fibers

Anaphase• Centromeres divide.• Spindle fibers

contract pulling sister chromatids apart to poles.

•Telophase:New nuclear membranes form around new nuclei

Mitosis Movie

CYTOKINESIS – Cytoplasm splits into 2 cells.Animal cells: Cleavage furrow forms from outside in.

Plant cells: Division/cell plate forms from inside out.

Cell now returns to interphase . The chromosomes uncoil back into chromatin. The whole cell cycle starts over again…..

http://www.cellsalive.com/mitosis.htm

At any point in time the cells in a tissue will be at different stages in the cell cycle.

Mitosis Stages

Put these in the correct order..

The Guarantee of Mitosis…The 2 daughter cells formed are identical to each other and identical to the mother cell. Why is this so important?

Mitosis

Whitefish Blastula

Mitosis in Plant Cells

Onion Root Tip

Ch. 8: The Cell Cycle

Mitosis

Whitefish Blastula

Mitosis in Plant Cells

Onion Root Tip

Controls on the Cell Cycle:1. Cell Type: Some cells stop dividing

when mature (stay in G0) Ex: neurons & muscle cells2.Growth factors – protein that stimulates cell division3.Density-dependent inhibition (cell to cell contact) Cells stop growing when they touch other cells4.Anchorage (cells may required contact with a surface for cell division)

Cell Cycle Regulators:Intracellular factors:

1. Checkpoints & Cyclins: All chromosomes are replicated? All spindles are attached? Are there enough mitochondria, etc.. If cells “pass” these checkpoints, the next stage of cell cycle begins.

Cyclins = that start/stop cell cycle progression based on checkpoints

Extra-cellular factors: - Binding of growth factors- Cell to cell contact

Cell Cycle Regulators: Cyclins operate at

checkpoints

Cell Cycle Controls: External to Cell

Growing Out of Control: Cancer

• Cancer is a disease of the cell cycle• Cells do not respond normally to controls of the

cell cycle system• Tumor: abnormally growing mass of body

cells– Benign: abnormal cells remain at original sight

forming a lump; although can cause problems, normally can be fully removed by surgery

– Malignant: “cancerous” spread, displacing normal tissue and disrupting organ function

– Metastasis: spread of cancer cells via circulatory system transport (traveling cancer cells)

http://www.cancer.gov/cancertopics/understandingcancer/cancer/page2

Molecular Basis of Cancer:

• Tumor Suppressor Genes: - genes that normally halt the cell cycle

- problem if stuck “OFF” (cells keep dividing)

• Oncogenes: - genes that tell cells to divide

- problem if stuck “ON” (cells keep dividing)

Cancer categorized based on site of origin: (Not on

quiz)• Carcinoma: originate in internal or external

coverings of the body (skin or lining of intestine)

• Sarcoma: Arise in support tissue (bone or muscle)

• Leukemia: Arise in blood or bone marrow (abnormal number of white blood cells)

• Lymphoma: Arise in lymphatic cells of the immune system (lymph nodes or spleen)

Treating Cancer• Surgical removal of tumor

• Radiation: – Damages DNA in cancer cells more than normal

cell– Cancer cells do not have the ability to repair– Cancer cells die without dividing/reproducing

• Chemotherapy: (For widespread or metastatic tumors) – Chemicals that disrupt specific steps in the cell

cycle• freezes the mitotic spindle• prevents formation of spindle• Damage DNA or RNA involved in making growth factors

Controls on the Cell Cycle:

Some cells stop dividing when mature (stay in G0)

ex) many neurons (nerve) & muscle cells Others divide for growth & repair Influences of cell division:

Growth factors – protein that stimulates cell division Density-dependent inhibition (cell to cell contact)

Cell Cycle Regulators:• Proteins called cyclins start/stop the

cell cycle.• The cycle is regulate by various factors:

1. Intracellular factors (Checkpoints)- All chromosomes are replicated- All spindles are attached

2. Extra-cellular factors: - Binding of growth factors- Cell to cell contact

Growing Out of Control: Cancer

• a disease of the cell cycle• Cells do not respond normally to controls of the

cell cycle system• Tumor: abnormally growing mass of body

cells– Benign: abnormal cells remain at original sight

forming a lump; although can cause problems, normally can be fully removed by surgery

– Malignant: spread, displacing normal tissue and disrupting organ function

– Metastasis: spread of cancer cells via circulatory system transport

See slide show http://www.cancer.gov/cancertopics/understandingcancer/cancer/page2

Molecular Basis of Cancer:

• Tumor Suppressor Genes: - genes that normally halt the cell cycle

- problem if stuck “OFF” (cells keep dividing)

• Oncogenes: - genes that tell cells to divide

- problem if stuck “ON” (cells keep dividing)

Treating Cancer• Surgical removal of tumor

• Radiation: – Damages DNA in cancer cells more than normal

cell– Cancer cells do not have the ability to repair– Cancer cells die without dividing/reproducing

• Chemotherapy: (For widespread or metastatic tumors) – Chemicals that disrupt specific steps in the cell

cycle• freezes the mitotic spindle• prevents formation of spindle• Damage DNA or RNA involved in making growth factors