6) Cell Division

Question 1

What are the two main phases of the eukaryotic cell cycle?

Answer 1

Interphase (growth and preparation) and the Mitotic phase (cell division).

Question 2

What happens during the three stages of interphase?

Answer 2

G₁: First growth phase – cell increases in size, organelles replicate.
S: Synthesis phase – DNA is replicated in the nucleus.
G₂: Second growth phase – cell continues growing, energy stores increase, DNA is checked for errors.

Question 3

What is G₀ phase?

Answer 3

A phase where the cell leaves the cell cycle either temporarily or permanently. Reasons include differentiation, DNA damage, or senescence.

Question 4

What are checkpoints in the cell cycle?

Answer 4

Control mechanisms that monitor and verify whether processes at each phase are completed accurately before the cell progresses to the next phase. Examples: G₁, G₂, and spindle assembly checkpoints.

Question 5

What is cytokinesis?

Answer 5

The division of the cytoplasm to form two separate cells after mitosis.

Question 6

Why is mitosis important?

Answer 6

For growth, tissue repair, replacement, and asexual reproduction in eukaryotic organisms.

Question 7

What are chromosomes made of before cell division?

Answer 7

DNA wrapped around histone proteins to form chromatin.

Question 8

What is a replicated chromosome made of?

Answer 8

Two identical sister chromatids joined at the centromere.

Question 9

What are the four stages of mitosis?

Answer 9

Prophase, Metaphase, Anaphase, Telophase (PMAT).

Question 10

What happens during prophase?

Answer 10

Chromosomes condense, nuclear envelope breaks down, nucleolus disappears, spindle fibres form.

Question 11

What happens during metaphase?

Answer 11

Chromosomes line up at the metaphase plate (equator of the cell) and are attached to spindle fibres.

Question 12

What happens during anaphase?

Answer 12

Centromeres divide, sister chromatids are pulled to opposite poles by shortening spindle fibres.

Question 13

What happens during telophase?

Answer 13

Chromosomes decondense, nuclear envelopes reform, nucleoli reappear, cytokinesis begins.

Question 14

How does cytokinesis differ in plant and animal cells?

Answer 14

Animal cells: Cleavage furrow forms and pinches the cell in two.
Plant cells: Cell plate forms from Golgi vesicles, developing into a new cell wall.

Question 15

What is meiosis?

Answer 15

A type of cell division that produces haploid gametes with half the chromosome number of the parent cell.

Question 16

What is the difference between diploid and haploid?

Answer 16

Diploid (2n): Two sets of chromosomes (one from each parent).
Haploid (n): One set of chromosomes.

Question 17

What are homologous chromosomes?

Answer 17

Chromosome pairs (one from each parent) that are the same size, shape, and carry the same genes at the same loci.

Question 18

What are alleles?

Answer 18

Different versions of the same gene.

Question 19

What happens during meiosis I?

Answer 19

Homologous chromosomes pair up, cross over, and are separated into two haploid cells.

Question 20

What happens during meiosis II?

Answer 20

Sister chromatids are separated (similar to mitosis), resulting in four haploid gametes.

Question 21

What is crossing over?

Answer 21

The exchange of genetic material between non-sister chromatids during prophase I of meiosis, leading to genetic variation.

Question 22

What is independent assortment?

Answer 22

The random orientation of homologous pairs in metaphase in meiosis I, leading to genetic variation.

Question 23

What is a chiasma (plural: chiasmata)?

Answer 23

The point where chromatids break and rejoin during crossing over.

Question 24

What is cell differentiation?

Answer 24

The process by which a cell becomes specialised to carry out a specific function.

Question 25

Give an example of a specialised animal cell and its adaptation.

Answer 25

Erythrocyte (red blood cell): Biconcave shape for large SA:V, no nucleus for more haemoglobin space, flexible to squeeze through capillaries.

Question 26

Give an example of a specialised plant cell and its adaptation.

Answer 26

Palisade mesophyll cell: Many chloroplasts for photosynthesis, rectangular shape for tight packing, thin walls for efficient gas exchange.

Question 27

What is a tissue?

Answer 27

A collection of differentiated cells working together to perform a specific function.

Question 28

What are stem cells?

Answer 28

Undifferentiated cells that can divide repeatedly and differentiate into various cell types.

Question 29

What is potency?

Answer 29

The ability of a stem cell to differentiate into different cell types.

Question 30

What are the three types of stem cell potency?

Answer 30

Totipotent: Can form any cell type, including extra-embryonic tissues (e.g., zygote). Pluripotent: Can form all tissue types but not a whole organism (e.g., embryonic stem cells). Multipotent: Can form a range of cells within a specific tissue (e.g., adult stem cells in bone marrow).

Question 31

Where are stem cells found in plants?

Answer 31

In meristems (e.g., apical meristems at root and shoot tips, vascular cambium).

Question 32

What are some potential uses of stem cells in medicine?

Answer 32

Treating heart disease, type 1 diabetes, Parkinson’s, Alzheimer’s, spinal injuries, burns, and drug testing.

Question 33

What are the ethical issues surrounding embryonic stem cells?

Answer 33

Destruction of embryos is considered by some to be unethical; debates over when life begins and the rights of embryos.

Question 34

What are induced pluripotent stem cells (iPSCs)?

Answer 34

Adult stem cells that have been genetically reprogrammed to behave like embryonic stem cells (pluripotent).

Question 35

How are erythrocytes and neutrophils produced?

Answer 35

From multipotent haematopoietic stem cells in the bone marrow.

Question 36

Why do erythrocytes have a short lifespan?

Answer 36

They lack a nucleus and organelles, so they cannot repair themselves.

Question 37

What is the role of meristematic tissue in plants?

Answer 37

It contains stem cells that allow continuous growth and differentiation into various tissues (e.g., xylem, phloem).

Question 38

What is the significance of meiosis in life cycles?

Answer 38

It produces genetically varied haploid gametes for sexual reproduction, ensuring genetic diversity in offspring.

Question 39

How do kinases regulate the cell cycle?

Answer 39

Kinases are enzymes that catalyze phosphorylation—the addition of a phosphate group to proteins—changing their structure and activity. In the cell cycle, cyclin-dependent kinases (CDKs) are activated by binding to cyclins. CDKs then phosphorylate key regulatory proteins (e.g., proteins involved in DNA replication and mitosis), driving the cell through the phases of the cycle.

Question 40

What are cyclins? Describe their role in cell-cycle regulation

Answer 40

Cyclins are proteins whose concentration fluctuates during the cell cycle. They bind to and activate CDKs to form cyclin-CDK complexes. These complexes act as molecular switches: G₁ cyclins → activate CDKs to promote progression from G₁ to S phase. S cyclins → drive DNA replication. M cyclins → trigger entry into mitosis. Cyclin levels rise and fall at specific points in the cycle, ensuring precise timing of events.

Question 41

How is the activity of cyclin-CDK complexes regulated?

Answer 41

The activity of cyclin-CDK complexes is tightly controlled by: Cyclin availability – Cyclins are synthesized and degraded at specific times. Inhibitor proteins – e.g., CKIs (CDK inhibitors) like p21 and p27 block CDK activity. Phosphorylation/dephosphorylation – Regulatory kinases and phosphatases modify CDKs. Ubiquitin-mediated degradation – The anaphase-promoting complex (APC/C) targets cyclins for breakdown after mitosis, turning off CDK activity.

Question 42

What is the role of the anaphase-promoting complex (APC/C)?

Answer 42

The APC/C is a large protein complex that marks specific proteins (like mitotic cyclins) for degradation via ubiquitination. During late mitosis, it triggers the destruction of M cyclins, which inactivates CDKs and allows the cell to exit mitosis and proceed to cytokinesis. This ensures proper chromosome separation and completion of cell division.

Question 43

How does the cell-cycle machinery ensure orderly progression through the cycle?

Answer 43

The cell cycle is regulated by a network of interacting proteins: Cyclins accumulate and bind to CDKs. CDK-cyclin complexes phosphorylate target proteins to initiate phase transitions. Checkpoints monitor DNA integrity, chromosome attachment, and replication completion. Regulatory proteins (e.g., inhibitors, APC/C) ensure each step is completed before proceeding. This creates a sequential, self-checking system to prevent errors.

Question 44

What happens if cell-cycle regulation fails?

Answer 44

Failure of cell-cycle regulation can lead to uncontrolled cell division, resulting in tumour formation. For example: Overexpression of cyclins → continuous CDK activation → constant cell division. Mutation or loss of checkpoint proteins (e.g., p53, which regulates p21) → cells bypass damage checks. Defective APC/C → failure to degrade cyclins → prolonged mitosis. These disruptions contribute to cancer development.

Question 45

What is cancer, and how is it related to cell-cycle regulation?

Answer 45

Cancer is a disease caused by uncontrolled cell division due to mutations in genes regulating the cell cycle. These mutations may affect: Proto-oncogenes (e.g., cyclin genes) → become oncogenes, promoting excessive growth. Tumor suppressor genes (e.g., p53, RB) → lose function, removing brakes on the cell cycle. Result: Cells divide uncontrollably, forming tumours that may be benign (localized) or malignant (invasive and metastatic).

Question 46

Why are cyclins potential targets for cancer treatment?

Answer 46

Because cyclins are essential for activating CDKs and driving cell division, overexpression of cyclins (due to gene mutations) can cause uncontrolled proliferation. Targeting cyclins or CDKs with chemical inhibitors (e.g., CDK inhibitors) can block abnormal cell division in cancer cells. By reducing CDK activity, these drugs may halt or slow tumor growth.

Question 47

What is squamous epithelium, and where is it found?

Answer 47

Squamous epithelium is made of flat, thin, scale-like cells that form a single layer. It is also called pavement epithelium due to its appearance. It is only one cell thick and allows rapid diffusion across a surface. It lines the lungs, where it enables efficient gas exchange between air and blood in the alveoli.

Question 48

Describe the structure and function of ciliated epithelium.

Answer 48

Ciliated epithelium consists of columnar cells with hair-like cilia on their surface. These cilia move rhythmically to sweep mucus and trapped particles (like dust or bacteria) away from sensitive areas. Found in the trachea (windpipe), it helps remove pathogens and debris from the respiratory tract. Goblet cells within this tissue secrete mucus to trap unwanted particles.

Question 49

What is cartilage, and what are its key features?

Answer 49

Cartilage is a type of connective tissue found in joints, the nose, ears, and at the ends of bones. It contains chondrocytes (cartilage cells) embedded in an extracellular matrix made of collagen fibers and elastic proteins. The matrix is firm yet flexible, allowing support without rigidity. Unlike bone, cartilage does not contain blood vessels or nerves.

Question 50

What is skeletal muscle, and how is it structured?

Answer 50

Skeletal muscle is a voluntary muscle attached to bones by tendons. It is responsible for movement. Under the microscope, it appears as long, cylindrical, multinucleated cells with striations (stripes) due to repeating units of actin and myosin filaments. These are called myofibrils. Skeletal muscle contracts in response to nerve signals.

Question 51

Describe the structure of a palisade mesophyll cell.

Answer 51

Palisade mesophyll cells are cylindrical, tightly packed cells located just beneath the upper epidermis of a leaf. They contain many chloroplasts (rectangular in shape), which absorb light for photosynthesis. Their thin cell walls and large vacuole help maintain turgor pressure, keeping the leaf upright. The chloroplasts can move within the cytoplasm to optimize light absorption.

Question 52

Why are palisade cells important for photosynthesis?

Answer 52

Palisade cells are specialized for maximum light absorption. Their: High number of chloroplasts Close packing Position near the upper surface of the leaf All increase efficiency in capturing sunlight. This makes them the primary site of photosynthesis in most leaves.

Question 53

What is a root hair cell, and how does it aid water uptake?

Answer 53

Root hair cells are extensions of epidermal cells at the tip of roots. They have long, thin projections called root hairs that greatly increase the surface area of the root. This enhances the absorption of water and minerals from the soil via osmosis. The large central vacuole lowers water potential, driving water into the cell.

Question 54

How do guard cells control gas exchange in leaves?

Answer 54

Guard cells are pairs of kidney-shaped cells surrounding stomata (tiny pores). They regulate the opening and closing of stomata to allow gas exchange (CO₂ in, O₂ out) and control water loss (transpiration). When guard cells take up water (by osmosis), they swell and become turgid, pulling apart and opening the stoma. When they lose water, they become flaccid and close the pore.

Question 55

What structural features allow guard cells to open and close stomata?

Answer 55

Guard cells have: Thicker inner wall than outer wall Chloroplasts to perform photosynthesis and produce glucose (which lowers water potential) Cellulose cell walls that resist stretching unevenly When guard cells gain water, the thicker inner wall resists expansion, causing the cell to bend outward and open the stoma. When water is lost, the cells shrink and close the pore.

Question 56

Explain cytokinesis in animal cells.

Answer 56

A cleavage furrow forms from a microfilament ring pulling the membrane inwards until it fuses in the middle.

Question 57

Explain cytokinesis in plant cells.

Answer 57

Cleavage furrows can't form because of the cell wall. Vesicles from the Golgi Apparatus assemble in the centre and fuse with each other, and the cell surface membrane. New cell wall forms along the new sections of membranes.

Question 58

What happens in Prophase I?

Answer 58

The nuclear envelope breaks down. The chromatin condenses into chromosomes. Homologous chromosomes containing the two chromatids come together, joining at their centromeres This is when “crossing over” occurs, which creates genetic variation.

Question 59

How do yeast asexually reproduce?

Answer 59

Yeast usually asexually reproduce by a method called budding. A small knob or bud forms on the parent cell, grows, and finally separates to become a new yeast cell. This new yeast cell is genetically identical to the parent cell.