Cell Division, Differentiation, and Stem Cells: Key Concepts for Biology

What is a chromosome?

A coiled structure of DNA and proteins found in the nucleus, containing many genes.

What is a gene?

A short section of DNA that codes for a specific protein, determining a characteristic.

How many chromosomes are in a human body cell?

46 chromosomes (23 pairs)

How many chromosomes are in a human gamete?

23 chromosomes

Why do chromosomes exist in pairs?

We inherit 23 from our mother and 23 from our father; pairs are homologous, same size, same genes but may have different versions.

What is the cell cycle?

The sequence of events a cell goes through to grow and divide, including interphase, mitosis, and cytokinesis.

What happens during interphase?

The cell grows, produces more organelles, and duplicates DNA, forming sister chromatids.

What happens during mitosis?

Sister chromatids are separated and pulled to opposite poles, dividing the nucleus into two identical nuclei.

What happens during cytokinesis?

The cytoplasm divides, forming two genetically identical daughter cells.

Define mitosis.

A type of cell division producing two genetically identical daughter cells from one parent cell.

List three purposes of mitosis.

Growth, repair of tissues, and asexual reproduction.

Why must DNA be duplicated before cell division?

So that each new cell receives a complete set of genetic information.

Give an example of mitosis for repair.

Replacing skin cells after a cut or mending a broken bone.

What does "genetically identical" mean in mitosis?

The daughter cells have the exact same DNA as the parent cell.

What is differentiation?

The process by which a cell becomes specialised for a specific function.

Why is differentiation important?

It allows cells to perform specialised roles, increasing organism efficiency.

Give examples of animal cell differentiation.

Nerve cells (carry signals), muscle cells (contract), red blood cells (carry oxygen).

Give examples of plant cell differentiation.

Xylem (transport water), root hair cells (absorb water/minerals).

Compare animal and plant differentiation timing.

Animals: mostly early in life, low flexibility; Plants: occurs throughout life, high flexibility, totipotent cells.

Why is plant cloning easier than animal cloning?

Many mature plant cells remain totipotent and can dedifferentiate; animal cells are harder to reprogramme.

What is tissue culture (micropropagation)?

A method to produce thousands of plant clones from a single explant.

List the steps of plant tissue culture.

Selection → Sterilisation → Plating → Callus formation → Transfer & differentiation → Hardening off.

Why must plant tissue culture be sterile?

Microbes grow faster on nutrient agar and can kill plant cells; sterility prevents contamination.

What is an explant?

A small piece of plant tissue used to start tissue culture, often from a meristem.

Define a stem cell.

An undifferentiated cell that can divide to produce more stem cells or differentiate into specialised cells.

Compare embryonic and adult stem cells.

Embryonic: pluripotent, can become any cell; Adult: multipotent, limited range of cell types.

Where are embryonic stem cells found?

In the inner cell mass of an early embryo (4-5 days old).

Where are adult stem cells found?

In bone marrow, brain, skin, liver, and other specific tissues.

What are plant stem cells called?

Meristems, located at tips of roots, shoots, and cambium.

What does pluripotent mean?

Can differentiate into any cell type in the body.

What does multipotent mean?

Can differentiate into a limited range of related cell types.

Give an example of a medical application of stem cells.

Replacing insulin-producing cells in Type 1 diabetes.

Explain how stem cells could help Parkinson's disease.

Differentiate stem cells into dopamine-producing neurons and transplant into the brain.

Explain therapeutic cloning.

Creating stem cells genetically identical to a patient using a donated egg cell and a body cell nucleus.

What is the main ethical debate about embryonic stem cells?

They involve destroying embryos, raising moral and religious concerns.

Give two arguments for embryonic stem cell research.

Potential to cure diseases; uses surplus IVF embryos that would otherwise be discarded.

Give two arguments against embryonic stem cell research.

Destroys potential human life; embryo cannot consent; slippery slope concerns.

Why take explants from meristem tissue?

Meristems are undifferentiated and free from viruses, ensuring healthy growth and cloning.

What is the role of the centromere during mitosis?

The attachment site for spindle fibres, pulling sister chromatids apart.

Why is animal cell differentiation usually one-way?

To maintain organ function; dedifferentiation would disrupt specialised tissues.

Explain the callus stage in plant tissue culture.

A lump of undifferentiated cells formed when hormones stimulate rapid division.

What is the purpose of hardening off plantlets?

Gradually acclimatising them to normal conditions before planting in soil.

Why is mitosis considered essential for growth?

It produces more cells for an organism to increase in size.

Why is mitosis essential for repair?

It replaces damaged or dead cells with identical ones.

Give an example of asexual reproduction using mitosis.

Yeast budding or starfish regenerating limbs.

What is homologous pair of chromosomes?

Two chromosomes of the same size carrying the same genes, one from each parent.

What is a totipotent cell?

A cell capable of differentiating into any cell type, including whole organisms in plants.

What is a pluripotent cell?

A stem cell that can form almost any body cell but not a whole organism.

What is a multipotent cell?

A stem cell that can form several closely related cell types.

Explain why plant cells can dedifferentiate.

They retain all genetic information and can revert to a meristem-like state under the right conditions.

What is the main difference in flexibility between plant and animal cells?

Plant cells remain totipotent; animal cells mostly lose differentiation ability after early development.

What is the purpose of nutrient agar in tissue culture?

Provides sugars, minerals, and hormones for plant cell growth.

Why is aseptic technique crucial in tissue culture?

Prevents contamination by bacteria or fungi that could kill the explant.

What are sister chromatids?

Two identical strands of DNA formed during DNA replication, joined at the centromere.

What happens if mitotic spindle fails?

Chromatids cannot separate; mitosis is incomplete and daughter cells are not formed.

Explain interphase in detail.

Cell grows, organelles replicate, DNA duplicates forming sister chromatids; prepares cell for mitosis.

What is cytokinesis in plants?

Formation of a new cell plate that becomes the cell wall, dividing cytoplasm.

What is cytokinesis in animals?

The cell membrane pinches inwards, dividing the cytoplasm.

Give three examples of specialised animal cells and their functions.

Nerve cells (signals), muscle cells (contract), red blood cells (carry oxygen).

Give two examples of specialised plant cells.

Xylem (water transport), root hair cells (absorption of water/minerals).

What is therapeutic cloning used for?

To create patient-specific stem cells for tissue or organ replacement.

Explain how therapeutic cloning reduces transplant rejection.

Stem cells are genetically identical to the patient, so immune system does not attack.

Why are embryonic stem cells more versatile than adult stem cells?

They are pluripotent and can form any cell type; adult stem cells are multipotent.

What are surplus IVF embryos?

Embryos created during IVF that are not implanted and often destroyed.

What is the major risk of adult stem cell research focus only?

Limited versatility; some diseases may not be treatable.

Why is ethical evaluation important in GCSE Biology?

To show critical thinking and ability to balance arguments for high-grade answers.

Give an example of an exam phrase for evaluation.

"However..., On the other hand..., Conversely..., In contrast..., Nevertheless..., Therefore, I conclude that..."

Why is DNA replication crucial before mitosis?

Ensures each daughter cell receives a full, identical set of genes.

What is the main purpose of differentiation?

To allow cells to specialise for efficient functioning of the organism.

Explain why plant tissue culture is useful in agriculture.

Rapid production of clones, disease-free plants, preserving rare species, overcoming seed dormancy.

Why do meristem cells remain totipotent?

They are undifferentiated and retain all genetic information needed to form any plant cell.

Define growth in multicellular organisms.

Permanent increase in size and mass, achieved by producing more cells via mitosis.

Explain the difference between animal and plant cell differentiation flexibility.

Animals: low flexibility after early life; Plants: high flexibility, differentiation continues throughout life.

What is the ethical concern about embryo destruction?

Many believe embryos are potential human life with moral rights.

Give one argument that supports stem cell research using surplus IVF embryos.

It uses embryos that would otherwise be discarded, potentially saving lives.

Why is using sterile instruments important in tissue culture?

To prevent contamination by microorganisms that could kill plant cells.

What is the role of hormones in plant tissue culture?

Stimulate cell division (callus formation) and later differentiation into plantlets.

What is the difference between interphase and mitosis?

Interphase: growth and DNA replication; Mitosis: division of nucleus.

What is the difference between plant and animal cytokinesis?

Plant: cell plate forms; Animal: membrane pinches inwards.

Explain why differentiated cells cannot usually dedifferentiate in animals.

Once specialised, animal cells lose the ability to divide into other cell types to maintain tissue stability.

What is a callus?

A mass of undifferentiated plant cells produced during tissue culture.

What are two medical conditions that stem cells could potentially help?

Diabetes (Type 1), Parkinson's Disease.

Explain the step-by-step process of plant tissue culture.

1. Take explant 2. Sterilise 3. Place on nutrient agar with hormones 4. Callus forms 5. Transfer to new agar with different hormones 6. Plantlets hardened off.

Explain why plant stem cells are totipotent throughout life.

Plants retain undifferentiated meristems at root/shoot tips, allowing continuous growth and regeneration.

What is a Grade 9 insight about chromosome inheritance?

Homologous chromosomes may carry different alleles from each parent, explaining variation in traits like eye color.

Explain one advantage of therapeutic cloning over organ transplant.

No immune rejection because new tissue is genetically identical to patient.

What is a common mistake students make about stem cell cures?

Claiming diseases are already cured; most research is experimental or in clinical trials.

Why is differentiation compared to division of labour?

Specialised cells perform specific tasks efficiently, like workers with distinct jobs.

Give an example of a "for" argument in ethical evaluation of stem cells.

Stem cells could cure debilitating diseases, relieving immense human suffering.

Give an example of an "against" argument in ethical evaluation.

Embryos have moral status as potential humans; destroying them is ethically wrong.