Mitosis and Asexual Reproduction

The Cell Cycle and Mitosis

Reproduction and Sustainability

Reproduction ensures that life exists beyond its present generation.

Reproduction transfers genetic information from parents to offspring.

Sustainability: The Ability of the environment and living things it supports to endure into the future.

Sustainability of living things depends on reproduction.

*recall the Cell Theory: all cells come from pre-existing cells

Continuity: How each species of organism continues to exist over time

Cell Replacement and Development

Human life begins as a fertilized egg, which divides to make a multicelled organism

Many cells divide your whole life(skin cells, liver cells, blood cells), but others stop dividing when you become an adult(muscle and nerve cells)

Different types of cells have different division rates

The Cell Cycle

All eukaryotic cells reproduce through the cell cycle(replace older and/or damaged cells for multicellular organisms, produce new offspring in single-celled organisms)

Cell cycle: stages in a cell’s life

A) Interphase - cells carry out the functions necessary for survival and cells that divide prepare for division

B) Mitosis - duplicated nuclear material divides into two equal parts

C) Cytokinesis - separates the two nuclei and cell contents into two daughter cells

A) Interphase

Phases of Interphase:

1) Growth and preparation: Cell increases in size

Makes proteins and molecules necessary for the cell to function

2) Replication: DNA makes a “replica”(copy) of itself

i) DNA unwinds/”unzips”

ii) Each side becomes a template for a new side to form

iii) New bases pair with bases on original DNA (A joins with T, C joins with G)

The result is two identical DNA molecules

3) Continued growth and preparation

B) Mitosis

Shortest stage of the cell cycle:

• The contents of a cell’s nucleus divide

• Results in two daughter nuclei, each with the same number and types of chromosomes as the original cell

• Daughter cells are identical to the parent cell (unless mutations occur)

• Sister chromatids of chromosomes that replicate during interphase are joined by a centromere

• It occurs in 4 stages: Prophase, Metaphase, Anaphase and Telophase(PMAT)

Stages of Mitosis:

1. Prophase - DNA condenses into chromosomes(x-shaped) - each chromosome contains two copies of the same DNA

   1. nuclear membrane disappears

   2. spindle fibres (tiny tube-like structures) form and attach to chromosomes at their centromeres

2. Metaphase - spindle fibres pull chromosomes into a line across the middle of the cell

3. Anaphase - centromeres are pulled apart and sister chromatids move to opposite ends of the cell

4. Telophase - Spindle fibres disappear and a nuclear membrane forms around each set of chromosomes

C) Cytokinesis

The final stage of the cell cycle:

Separates the two nuclei into two identical daughter cells

In animal cells, the cell membrane pinches together to divide the cytoplasm and organelles

In plant cells, a cell plate grows toward the cell membrane forming a new cell membrane for each daughter cell

Checkpoints in the Cell Cycle

Some stages/checkpoints monitor the cell cycle

Proteins at checkpoints monitor cell activities and send the information to the nucleus

The nucleus tells the cell whether or not to divide

The cell will not divide if:

• There are not enough nutrients to support cell growth

• DNA has not been replicated

• DNA is damaged

The Cell Cycle and Cancer

If a mutation occurs in a gene that produces instructions for a checkpoint protein, then the cell may divide uncontrollably

Cancer: a disease that results from very uncontrolled cell division

Cancer cells:

1. Do not respond to messages from neighbouring cells telling them to stop dividing, so the cancer cells grow in multiple layers, forming a tumour

2. Have large abnormal nuclei because chromosomes do not divide correctly

3. Are not specialized and do not function as part of the body

4. Can release chemicals to attract small nearby blood vessels that branch into tumours and deliver nutrients to them, allowing tumour cells to increase the speed of division

Cancer can spread to other areas of the body if some tumour cells break away and are carried by blood vessels to a new location

Cancer researchers are trying to:

• Find out how cancer disrupts the cell cycle

• Identify treatments that prevent tumour cells

Asexual Reproduction

There are two types of reproduction:

1. Asexual reproduction - only one parent is required to produce offspring(offspring are identical to the parent)

2. Sexual reproduction - two parents contribute genetic information to produce a zygote(produces offspring that are genetically unique)

Types of Asexual Reproduction

Asexual reproduction occurs in many living things through a variety of methods:

1. binary fission

2. budding

3. fragmentation

4. vegetative reproduction

5. spore formation

Binary fission

A single parent duplicates its genetic material and divides/splits into two equal parts(cell division)

Only single-celled organisms(e.g. bacteria), some protists(e.g. amoeba) and some algae use binary fission to reproduce

Allows for rapid population growth (in ideal conditions, it can reproduce every 20 mins)

Budding

The parent undergoes repeated mitosis and cell division, developing a “bud”(outgrowth) that pinches off to become an identical organism

Budding occurs in single-celled organisms(e.g. yeast) and multi-cellular organisms(e.g. hydra)

Fragmentation

A small part of an organism breaks off and grows into an organism identical to the parent

Some animals, such as sea stars and flatworms, can reproduce asexually from fragments

Most plants can reproduce asexually from fragments as long as the environment is suitable and enough nutrients are available

Vegetative propagation

Special cells in plants, usually in stems, roots, or leaves, divide repeatedly to form structures that develop into a new plant identical to the parent

• Tulips and daffodils produce bulbs that become new plants, identical to the parent plant

• Strawberries have stem runners that grow roots and become new plants, identical to the parents

• Artificial vegetative propagation uses techniques to produce plants with specific characteristics

  • e.g. grafting - a bud, stem, or root is cut from one plant and joined to another

    • used to produce trees with high-quality fruit or resistance to disease

Spore formation

An organism produces single-celled spores that grow into new individuals by mitosis

Many fungi (e.g. moulds, mushrooms), and some plants (e.g. ferns, mosses) reproduce by forming large numbers of spores

Spores are released into the air from a structure called a sporangium

Spores are lightweight and rely on water or wind to be carried away from the parent plant

If environmental conditions are good(warm and moist), a spore will grow into a new organism, identical to the parent

Advantages and Disadvantages of Asexual Reproduction

Human assisted Cloning

Complex multi-cellular organisms are unable to naturally clone themselves because they have many different specialized cells

Human assisted plant and animal cloning can be used to:

• Save genetic information of endangered species

• Mass- produce an organism with a desirable trait

Reproductive Cloning (adult DNA cloning)

a genetic duplicate of an existing or previously existing organism with desirable qualities is made

Problem: Only 10% of the clones survive and clones have higher rates of infections and cancer

Therapeutic Cloning:

Use embryonic and adult stems to correct health problems

Stem cells:  

Cells that have the potential to become many different types of cells 

• Embryonic stem cells can become any of the 200 types of body cells

• Adult stem cells can only become certain types of cells

• Stem cells can be used to replace damaged cells in patients with diabetes, spinal injuries, Parkinson’s disease

The Catholic Church

The catholic church is against embryonic stem-cell research because it destroys human embryos

Canadian Law: Bill C-6, February 11, 2004

This stated that creating embryos for in- vitro fertilization is permitted, creating human embryos only for research purposes is prohibeted

    Human cloning is prohibitted

🔬 CELL CYCLE & ASEXUAL REPRODUCTION — REVIEW ANSWERS

1. Why is cell division necessary in unicellular organisms?

Because it is their only method of reproduction. When a unicellular organism divides, it creates a new organism.

2. Why is cell division necessary in multicellular organisms?

To allow the organism to grow, repair damaged cells, and replace old or dead cells.

3. Explain why the skin cells of an adult must divide.

Skin cells are constantly exposed to friction and damage, so they must divide to replace dead or damaged cells and maintain a protective barrier.

4. List the three stages of the cell cycle.

1. Interphase

2. Mitosis

3. Cytokinesis

5. Why is DNA replication so important?

Because each new cell must receive a complete and identical copy of the DNA to function properly.

6. How does the cell prepare for cell division?

During interphase, the cell:

• Grows

• Duplicates its DNA

• Makes extra organelles

• Checks for errors

7. What are the phases of mitosis?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

8. What do the nucleus and chromosomes look like during prophase?

• The nuclear membrane breaks down

• Chromatin condenses into visible chromosomes

• Each chromosome has two sister chromatids

9. How does cytokinesis differ in plant and animal cells?

• Animal cells: the cell membrane pinches in (cleavage furrow).

• Plant cells: a cell plate forms because the cell wall is too rigid to pinch.

10. What is the importance of checkpoints in the cell cycle?

Checkpoints ensure the cell:

• Is healthy

• Has correctly replicated DNA

• Only divides when safe

They prevent mutations and uncontrolled division.

11. What may happen when checkpoint proteins no longer function?

Cells may divide uncontrollably → cancer (tumour formation).

12. Outline the activities in the cell during interphase.

• G1: Cell grows, makes proteins

• S: DNA replicates

• G2: More growth, organelles duplicated, final checks

13. List the steps in DNA replication.

1. DNA unzips

2. New nucleotides pair with exposed bases

3. Two identical DNA molecules are produced

14. Is mitosis constantly occurring in your cells? Explain.

No. Only certain cells divide often (skin, stomach lining). Others rarely divide (nerve or muscle cells).

15. What is the function of the spindle fibres?

They attach to chromosomes and pull sister chromatids apart during mitosis.

16. Sequence of mitosis phases (beginning to end):

Prophase → Metaphase → Anaphase → Telophase

17. Why is it important that DNA be tightly coiled during mitosis?

Tightly coiled DNA:

• Prevents tangling

• Makes chromosomes easier to separate accurately

18. What might happen if DNA replication and mitosis were not highly controlled?

• Mutations

• Incorrect chromosome numbers

• Cancer

• Cell death

19. What are some environmental factors that can contribute to the development of cancer?

• UV radiation

• Smoking and chemicals

• Pollution

• Certain viruses

• High radiation exposure

• Some pesticides or toxins

20. What causes a cell to become cancerous?

Mutations in DNA that damage the genes controlling the cell cycle, causing uncontrolled division.

21. How is a cancer cell different from a normal cell?

• Divides uncontrollably

• Ignores checkpoints

• Does not die when it should

• Uses extra nutrients

22. How does a cancer cell look compared to a normal cell?

• Abnormally shaped

• Larger or irregular nucleus

• Disorganized

• Cells often overlap instead of forming neat layers

23. How do blood vessels help cancer cells multiply?

Tumours stimulate angiogenesis (new blood vessel growth), giving them:

• More oxygen

• More nutrients
  This lets them grow even faster.

24. Describe the result of mitosis.

Two genetically identical daughter cells from one parent cell.

25. List ways plants can reproduce asexually.

• Runners (strawberries)

• Tubers (potatoes)

• Bulbs (onions)

• Cuttings

• Grafting

• Rhizomes

• Plantlets

26. Why do organisms that reproduce asexually often produce large numbers of offspring?

Because they do not need a mate and can reproduce very quickly, using simple cell division.

27. What limits the number of times a bacterium can divide?

• Lack of nutrients

• Waste buildup

• Space limitations

• Temperature

• Competition

• Being killed by environmental conditions

28. Why are most multicellular organisms unable to reproduce by budding?

Their bodies are too complex and have many different specialized cells, making simple cloning impossible.

29. Sea stars were cut into pieces and thrown back into the ocean. Predict what happened.

Many pieces regenerated into new sea stars, increasing the population instead of decreasing it.

30. Give three reasons for human-assisted cloning.

• To save endangered species

• To produce genetically identical organisms for research

• To mass-produce plants or animals with desirable traits

• (Bonus) To create tissues or organs for medical use

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