Cell Division

Part One: Mitosis

In this lesson…

• Explain the difference between double and single stranded chromosome, chromatin and chromatid

• List the steps of mitosis and describe what happens in each

• Identify what stage of mitosis a cell is in by looking at the chromosomes

• List three reasons cells divide by mitosis

• Describe cytokinesis

In the nucleus

• In non-dividing cells, the genetic material is stored as thin DNA super coils called CHROMATIN

• When a cell divides, the chromatin will shorten and thicken into CHROMOSOMES

• One strand of a double stranded chromosome is called a CHROMATID

Mitosis

• MITOSIS: a process by which the nucleus of a cell divides while maintaining the chromosome number One cell two cells New cells have identical genetic material (DNA) of the parent cell

• Four stages of division (Prophase, Metaphase, Anaphase and Telophase - PMAT) plus a period of growth and metabolism called Interphase

The Cell Cycle

Interphase

• Period between divisions• Longest part of the cell cycle• Cell is growing and metabolizing• Nuclear membrane present• Genetic information in the form of chromatin and cannot be seen with a microscope

• Before division each strand of DNA will replicate (copy) itself to become double stranded

• Near the end of interphase the DNA begins to condense (shorten)

Interphase

Draw a double stranded chromosome. Label chromosome, chromatid and centromere

ChromosomeChromatid

Centromere

Chromatid

Phase One: Prophase

• Chromatin contracts and becomes visible (spaghetti). It is now called CHROMOSOMES

• Each is a double chromosome with a pair of SISTER chromatids which are joined to each other by a centromere

• Chromosomes begin to move towards the equator (center) of the cell

• Nuclear membrane disintegrates (breaks down)

• CENTRIOLES will form SPINDLE FIBERS that will attach to each centromere and move around the chromosomes

Prophase

Phase Two: Metaphase

• The centromeres of each chromosome line up along equator ( looks like praying hands)

• Centromeres divide so the doubled chromosomes become two identical single stranded sister chromatids

• Centrioles are now at the poles of the cell and are attached to each centromere by spindle fibers

Metaphase

Phase Three: Anaphase

• The spindle fibers begin to shorten and the chromosomes begin moving to opposite ends or poles of the cell (fingers)

• Each side gets one chromatid from each double stranded chromosome

Anaphase chromosomes

Phase Four: Telophase

• Begins when single stranded chromosomes reach the poles

• Chromosomes uncoil and turn into chromatin

• Nuclear membrane reappears• Reverse of prophase• Division of the cytoplasm or CYTOKINESIS is completed by pinching off in animals or by building a cell wall in plants

Telophase

What’s the point of Mitosis?

• Mitosis creates identical copies of cells for: 1. growth

2. Repair/regeneration of damaged tissue

3. Asexual reproduction (animals) or vegetative reproduction (plants)

Asexual Reproduction

• Reproduction that involves only one parent; parent and offspring have identical genetics

• No special reproductive cells or organs used to create offspring

• Occurs through mitosis and cytokinesis• Both single and multi-celled organisms,

plants and simple animals can reproduce asexually

• In multi-cellular organisms, the offspring develop from undifferentiated, unspecialized cells from the parent

• Usually a rapid form of reproduction

Binary Fission

• Simplest form of asexual reproduction

• Parent divides into two approximately equal sized daughter cells

• Bacteria: circular chromosome attaches to plasma membrane then replicates, cell wall separates each copy

• Protozoa: eg. Amoeba become circular and use mitosis

Budding

• New individuals develop from small outgrowths of the parent (buds)

• May develop colonies (sponges) or break off to be individuals (hydra, yeast)

• Some organisms can both bud and reproduce sexually

Spores

• Specialized single cells that are released from the parent organism, germinate and grow by mitosis

• New cells differentiate to form a new organism

• Can reproduce quickly and in large quantities

• Often have thick protective coats• Eg. Fungi, algae, protozoa

Regeneration

• The ability to regrow lost body parts

• Some animals can regrow entire new organisms from parts

• Ability to regenerate decreases as organisms increase complexity

• Even simple organisms that can regenerate entire organisms generally prefer to utilize a

different method to reproduce

Vegetative reproduction

• MERISTEM: area on plant with unspecialized cells (cells that can become any kind of cell) that frequently divide using mitosis

• Meristematic cells can be found in the vegetative structures of a plant (roots, stems, leaves)

• Given proper treatment, meristem cells can reproduce mitoticlly then differentiate into new independent plants

• Structures include bulbs, corms, tubers, runners, rhizomes

• Can also be artificially propagated using cuttings, layerings or grafting

Bulb

• short underground stem with thickened storage leaves

• small new bulbs sprout from the old ones

• Eg. onions, tulips

Corm

• short underground stems with no fleshy leaves

• Eg. gladiolas, crocuses

Tuber

• enlarged part of an underground stem that contains stored food

• potatoes (eyes are tiny buds)

Runner

• AKA stolon• is a stem that runs sideways and contains

buds• Eg. strawberry

Rhizome

• a stem that grows sideways under the ground

• ferns, irises

Cutting

• a stem, root or leaf cutting used to make a new plant

Layering

• part of a stem is bent and covered in soil

• once it roots the original can be cut off

• Eg. raspberries, roses

Grafting

• stem or bud removed from one plant and permanently joined to another plant

• Eg. grapes and many seedless fruits