The Cell

Topic 2.5

Cell Division

Mitosis• Cellular division in eukaryotic cells.• Chromatin is arranged into

chromosomes.• Chromosomes double.• Cell grows in size.• Cells divide.• Is cellular cloning.

Phases of the Cell Cyclethe ‘life cycle’ of a cell.

There are 2 phases:1. Interphase2. M phase (mitotic phase)

a. Prophaseb. Metaphasec. Anaphased. Telophase & cytokinesis

Figure 12.4 The cell cycle

1. Interphase• The non-dividing phase in a cell• Lasts about ~ 90% of the cell cycle.• The cell grows and replicates DNA

preparing for Mitosis.• There are three periods:

3 periods of Interphase

1. Go – a cell functioning as normal

2. G1 phase – first growth phase3. S phase- synthesis of DNA4. G2 phase- 2nd growth phase

Mitosis is a reliable process. Only one error occurs

Per 100,000 cell divisions.

2. Mitosis: Prophase• The nucleolus

disappears.• Chromatin

condenses into visible chromosomes.

• There are two sister chromatids held together by a centromere.

• The mitotic spindle forms in the cytoplasm.

Figure 12.3 Chromosome duplication and distribution during mitosis

“Pro”metaphase• The nuclear

envelope disappears.

• Spindle fibers extend from each pole to the cell’s equator.

• Spindle fibers attach to the centromeres.

Figure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphase

Metaphase• Chromosomes

are lined up in the equator (middle) of the cell.

• This is called the metaphase plate.

Figure 12.6 The mitotic spindle at metaphase

Anaphase• Characterized by

movement. It begins when pairs of sister chromatids pull apart.

• Sister chromatids move to opposite poles of the cell.

• Chromosomes look like a “V” as they are pulled.

• At the end of anaphase, the two poles have identical number and types of chromosomes.

Figure 12.5 The stages of mitotic cell division in an animal cell: metaphase; anaphase; telophase and cytokinesis.

Telophase and Cytokinesis

• Microtubules elongate the cell.• Daughter nuclei begin to form at the two

poles.• Nuclear envelopes re-form.• Nucleolus reappears.• Chromatin uncoils.• Cells split their cytoplasm.• It is basically the opposite of prophase.

Figure 12.5x Mitosis

Figure 12.8 Cytokinesis in animal and plant cells

Figure 12-09x Mitosis in an onion root

Role of Mitosis•Growth: Multicellular organisms increase their size

through growth. This growth involves increasing the number of cells through mitosis. These cells will differentiate and specialize their function. 

•Tissue Repair: As tissues are damaged they can recover through replacing damaged or dead cells. This is easily observed in a skin wound. More complex organ regeneration can occur in some species of amphibian. 

•Asexual Reproduction: This the production of offspring from a single parent using mitosis. The offspring are therefore genetically identical to each other and to their “parent”- in other words they are clones. Asexual reproduction is very common in nature, and in addition we humans have developed some new, artificial methods

TumorsThe cancer cells are a mass of cells produced from uncontrolled cell

division and can occur in an tissue. These cells disrupt biological order and

function. If left unchecked, to bring the whole complex, life sustaining edifice

that is thehuman body crashing down' This mass is called a tumor. There are two major types of tumor:1.Benign Tumors this is a mass of cancerous cells that do

not invade other areas of the body. These are not as dangerous to health but may still require removing to prevent effects on neighboring tissue

2. Malignant Tumors is a mass of cancer cells that may invade surrounding tissues or spread to distant areas of the body. Cancer cells replace normal functioning cells in distant sites:

e.g. replacing blood forming cells in the bone marrow, replacing bones leading to increased calcium levels in the blood, or in the heart muscles so that the heart fails.

1. Image is a normal CT. Images 2, 3 & 4Are PET scans, Light green/blue areasshow cancer cells