FORM FOUR BIOLOGY EXAM – TOPIC: GROWTH
Time: 3 Hours
Instructions
- Answer all questions in Sections A and B.
- Answer only two (2) questions from Section C.
- Write your answers clearly.
SECTION A (16 Marks)
1. Multiple Choice Questions (10 Marks)
Choose the correct answer and write its letter:
2. Matching Items (6 Marks)
Match the growth measurement tools or terms with their correct description:
I. Auxanometer — ___
II. Germination — ___
III. Dry mass — ___
IV. Growth curve — ___
V. Differentiation — ___
VI. Mitosis — ___
List B
a) Process of cells becoming specialized
b) Used to measure increase in size of a plant
c) Increase in cell number
d) Shows the pattern of growth over time
e) Initial phase in development from seed
f) Accurate measure of organic material
SECTION B (54 Marks)
Answer all questions.
3. (a) Describe two main differences between growth in plants and growth in animals. (4 marks)
(b) Explain three environmental factors that affect growth in organisms. (5 marks)
4. (a) A student planted bean seeds in two pots, one with water and one without. After five days, only the watered seeds germinated. Explain the results. (5 marks)
(b) Why is oxygen important in the process of germination? (4 marks)
5. (a) Outline the three main phases of the sigmoid growth curve. (6 marks)
(b) Why is dry mass preferred over fresh mass in measuring growth? (3 marks)
6. (a) Explain the importance of mitosis in the growth of multicellular organisms. (5 marks)
(b) Describe how hormones control growth in plants. (4 marks)
7. (a) Explain how insect growth through molting is different from continuous growth. (4 marks)
(b) Describe how light and temperature influence plant growth. (5 marks)
8. (a) Explain the difference between growth and development using two examples. (4 marks)
(b) A seed germinated successfully, but the seedling later died. List and explain possible causes. (5 marks)
SECTION C (30 Marks)
Answer two (2) questions only.
9. Discuss the process of germination in dicotyledonous plants, highlighting internal and external factors necessary for the process. (15 marks)
10. With the aid of a labeled diagram, describe the sigmoid growth curve and explain what occurs in each of its stages. (15 marks)
11. Explain five differences between mitosis and meiosis and describe how mitosis contributes to growth and repair in organisms. (15 marks)
FORM FOUR BIOLOGY EXAM – TOPIC: GROWTH (With Answers)
Time: 3 Hours
Instructions
- Answer all questions in Sections A and B.
- Answer only two (2) questions from Section C.
- Write your answers clearly.
SECTION A (16 Marks)
1. Multiple Choice Questions (10 Marks)
Choose the correct answer and write its letter:
2. Matching Items (6 Marks)
Match the growth measurement tools or terms with their correct description:
I. Auxanometer — b
II. Germination — e
III. Dry mass — f
IV. Growth curve — d
V. Differentiation — a
VI. Mitosis — c
Answers:
I. Auxanometer — b) Used to measure increase in size of a plant
II. Germination — e) Initial phase in development from seed
III. Dry mass — f) Accurate measure of organic material
IV. Growth curve — d) Shows the pattern of growth over time
V. Differentiation — a) Process of cells becoming specialized
VI. Mitosis — c) Increase in cell number
I. Auxanometer — b) Used to measure increase in size of a plant
II. Germination — e) Initial phase in development from seed
III. Dry mass — f) Accurate measure of organic material
IV. Growth curve — d) Shows the pattern of growth over time
V. Differentiation — a) Process of cells becoming specialized
VI. Mitosis — c) Increase in cell number
SECTION B (54 Marks)
Answer all questions.
3. (a) Describe two main differences between growth in plants and growth in animals. (4 marks)
Answer:
1. Location of growth: In plants, growth occurs mainly at specific regions called meristems (apical and lateral), while in animals growth occurs throughout the body.
2. Type of growth: Plants show indeterminate growth (can grow throughout their life), while most animals show determinate growth (stops after reaching maturity).
3. Cell division: Plant cells can differentiate and then divide again, while animal cells typically become specialized and then lose ability to divide (with some exceptions).
1. Location of growth: In plants, growth occurs mainly at specific regions called meristems (apical and lateral), while in animals growth occurs throughout the body.
2. Type of growth: Plants show indeterminate growth (can grow throughout their life), while most animals show determinate growth (stops after reaching maturity).
3. Cell division: Plant cells can differentiate and then divide again, while animal cells typically become specialized and then lose ability to divide (with some exceptions).
(b) Explain three environmental factors that affect growth in organisms. (5 marks)
Answer:
1. Temperature: Affects enzyme activity in metabolic processes. Each species has an optimal temperature range for growth.
2. Light: For plants, light is essential for photosynthesis which provides energy for growth. Light quality and duration affect growth patterns.
3. Water: Essential for all metabolic activities, maintains cell turgidity in plants, and is a medium for biochemical reactions.
4. Nutrients: Availability of essential minerals (N, P, K for plants) and proper nutrition (proteins, vitamins for animals) affects growth rate.
5. Oxygen: Required for cellular respiration which provides energy for growth processes.
1. Temperature: Affects enzyme activity in metabolic processes. Each species has an optimal temperature range for growth.
2. Light: For plants, light is essential for photosynthesis which provides energy for growth. Light quality and duration affect growth patterns.
3. Water: Essential for all metabolic activities, maintains cell turgidity in plants, and is a medium for biochemical reactions.
4. Nutrients: Availability of essential minerals (N, P, K for plants) and proper nutrition (proteins, vitamins for animals) affects growth rate.
5. Oxygen: Required for cellular respiration which provides energy for growth processes.
4. (a) A student planted bean seeds in two pots, one with water and one without. After five days, only the watered seeds germinated. Explain the results. (5 marks)
Answer:
The results demonstrate that water is essential for seed germination. Water plays several critical roles:
1. Activates enzymes: Water activates hydrolytic enzymes that break down stored food in the seed.
2. Softens seed coat: Water softens the testa, allowing the radicle to emerge.
3. Medium for metabolic reactions: All biochemical reactions require water as a medium.
4. Transport medium: Water helps transport dissolved food materials to growing embryo.
5. Cell elongation: Water is needed for cell expansion during growth.
The seeds without water remained dormant because these essential processes couldn't occur without water.
The results demonstrate that water is essential for seed germination. Water plays several critical roles:
1. Activates enzymes: Water activates hydrolytic enzymes that break down stored food in the seed.
2. Softens seed coat: Water softens the testa, allowing the radicle to emerge.
3. Medium for metabolic reactions: All biochemical reactions require water as a medium.
4. Transport medium: Water helps transport dissolved food materials to growing embryo.
5. Cell elongation: Water is needed for cell expansion during growth.
The seeds without water remained dormant because these essential processes couldn't occur without water.
(b) Why is oxygen important in the process of germination? (4 marks)
Answer:
Oxygen is crucial for germination because:
1. Aerobic respiration: The growing embryo needs energy from respiration which requires oxygen.
2. ATP production: Oxygen is the final electron acceptor in the electron transport chain, enabling efficient ATP production.
3. Metabolic activity: Oxygen supports the high metabolic activity during germination and growth.
4. Breakdown of stored food: Oxygen is needed for the oxidation of stored lipids and carbohydrates to release energy.
Oxygen is crucial for germination because:
1. Aerobic respiration: The growing embryo needs energy from respiration which requires oxygen.
2. ATP production: Oxygen is the final electron acceptor in the electron transport chain, enabling efficient ATP production.
3. Metabolic activity: Oxygen supports the high metabolic activity during germination and growth.
4. Breakdown of stored food: Oxygen is needed for the oxidation of stored lipids and carbohydrates to release energy.
5. (a) Outline the three main phases of the sigmoid growth curve. (6 marks)
Answer:
The sigmoid (S-shaped) growth curve has three main phases:
1. Lag phase (Slow growth phase):
- Organism is adapting to the environment
- Cell division begins but population increases slowly
- Cells are maturing and preparing for rapid growth
2. Log phase (Exponential growth phase):
- Rapid cell division and growth
- Optimal conditions allow maximum growth rate
- Resources are plentiful
- Population doubles at regular intervals
3. Stationary phase (Plateau phase):
- Growth rate slows and eventually stops
- Birth rate equals death rate
- Limited by resources or environmental factors
- Organisms reach carrying capacity of environment
The sigmoid (S-shaped) growth curve has three main phases:
1. Lag phase (Slow growth phase):
- Organism is adapting to the environment
- Cell division begins but population increases slowly
- Cells are maturing and preparing for rapid growth
2. Log phase (Exponential growth phase):
- Rapid cell division and growth
- Optimal conditions allow maximum growth rate
- Resources are plentiful
- Population doubles at regular intervals
3. Stationary phase (Plateau phase):
- Growth rate slows and eventually stops
- Birth rate equals death rate
- Limited by resources or environmental factors
- Organisms reach carrying capacity of environment
(b) Why is dry mass preferred over fresh mass in measuring growth? (3 marks)
Answer:
1. Water content varies: Fresh mass includes variable water content which can fluctuate without actual growth occurring.
2. More accurate measure: Dry mass represents the actual organic matter (proteins, carbohydrates, lipids) accumulated through growth.
3. Consistency: Dry mass eliminates differences caused by hydration levels, allowing more consistent comparisons between measurements.
4. Structural growth: Better reflects true growth as it measures the permanent biomass rather than temporary water content.
1. Water content varies: Fresh mass includes variable water content which can fluctuate without actual growth occurring.
2. More accurate measure: Dry mass represents the actual organic matter (proteins, carbohydrates, lipids) accumulated through growth.
3. Consistency: Dry mass eliminates differences caused by hydration levels, allowing more consistent comparisons between measurements.
4. Structural growth: Better reflects true growth as it measures the permanent biomass rather than temporary water content.
6. (a) Explain the importance of mitosis in the growth of multicellular organisms. (5 marks)
Answer:
Mitosis is crucial for growth in multicellular organisms because:
1. Cell proliferation: It allows for the increase in number of cells, enabling organisms to grow in size.
2. Genetic consistency: Produces genetically identical daughter cells, maintaining the organism's genetic identity.
3. Tissue repair: Replaces damaged or dead cells, maintaining tissue integrity during growth.
4. Development: Enables differentiation as cells can specialize after dividing.
5. Uniform growth: Ensures balanced growth as all new cells contain the same genetic material.
6. Cell replacement: In constantly renewing tissues (like skin), mitosis maintains tissue while organism grows.
Mitosis is crucial for growth in multicellular organisms because:
1. Cell proliferation: It allows for the increase in number of cells, enabling organisms to grow in size.
2. Genetic consistency: Produces genetically identical daughter cells, maintaining the organism's genetic identity.
3. Tissue repair: Replaces damaged or dead cells, maintaining tissue integrity during growth.
4. Development: Enables differentiation as cells can specialize after dividing.
5. Uniform growth: Ensures balanced growth as all new cells contain the same genetic material.
6. Cell replacement: In constantly renewing tissues (like skin), mitosis maintains tissue while organism grows.
(b) Describe how hormones control growth in plants. (4 marks)
Answer:
Plant hormones (phytohormones) control growth through:
1. Auxins: Promote cell elongation in shoots, apical dominance, and root initiation.
2. Gibberellins: Stimulate stem elongation, seed germination, and fruit growth.
3. Cytokinins: Promote cell division, delay senescence, and work with auxins to control differentiation.
4. Abscisic acid: Inhibits growth during stress conditions (dormancy).
5. Ethylene: Controls fruit ripening and leaf abscission, affecting overall growth patterns.
These hormones work in combination, with their ratios determining growth responses to environmental stimuli.
Plant hormones (phytohormones) control growth through:
1. Auxins: Promote cell elongation in shoots, apical dominance, and root initiation.
2. Gibberellins: Stimulate stem elongation, seed germination, and fruit growth.
3. Cytokinins: Promote cell division, delay senescence, and work with auxins to control differentiation.
4. Abscisic acid: Inhibits growth during stress conditions (dormancy).
5. Ethylene: Controls fruit ripening and leaf abscission, affecting overall growth patterns.
These hormones work in combination, with their ratios determining growth responses to environmental stimuli.
7. (a) Explain how insect growth through molting is different from continuous growth. (4 marks)
Answer:
Insect growth through molting differs from continuous growth in several ways:
1. Intermittent growth: Insects grow in distinct stages (instars) separated by molts, rather than continuously.
2. Exoskeleton limitation: The rigid exoskeleton must be shed (molted) for growth to occur, creating growth steps.
3. Metamorphosis: Many insects undergo dramatic body changes between life stages (larva to adult).
4. Growth pattern: Growth occurs rapidly after molting when the new exoskeleton is soft, then stops until next molt.
5. Determinate growth: Insects stop growing after reaching adult stage, unlike continuously growing organisms.
Insect growth through molting differs from continuous growth in several ways:
1. Intermittent growth: Insects grow in distinct stages (instars) separated by molts, rather than continuously.
2. Exoskeleton limitation: The rigid exoskeleton must be shed (molted) for growth to occur, creating growth steps.
3. Metamorphosis: Many insects undergo dramatic body changes between life stages (larva to adult).
4. Growth pattern: Growth occurs rapidly after molting when the new exoskeleton is soft, then stops until next molt.
5. Determinate growth: Insects stop growing after reaching adult stage, unlike continuously growing organisms.
(b) Describe how light and temperature influence plant growth. (5 marks)
Answer:
Light influences plant growth through:
1. Photosynthesis: Provides energy for growth via light-dependent reactions.
2. Phototropism: Direction of light affects growth patterns (stems grow toward light).
3. Photoperiodism: Day length affects flowering and other growth stages.
4. Chlorophyll synthesis: Light is needed for chlorophyll production.
Temperature influences plant growth through:
1. Enzyme activity: Affects rate of metabolic reactions.
2. Membrane fluidity: Affects nutrient uptake and transport.
3. Transpiration rate: Higher temperatures increase water loss.
4. Growth rate: Each species has optimal temperature range for growth.
5. Dormancy: Temperature changes can trigger growth or dormancy periods.
Light influences plant growth through:
1. Photosynthesis: Provides energy for growth via light-dependent reactions.
2. Phototropism: Direction of light affects growth patterns (stems grow toward light).
3. Photoperiodism: Day length affects flowering and other growth stages.
4. Chlorophyll synthesis: Light is needed for chlorophyll production.
Temperature influences plant growth through:
1. Enzyme activity: Affects rate of metabolic reactions.
2. Membrane fluidity: Affects nutrient uptake and transport.
3. Transpiration rate: Higher temperatures increase water loss.
4. Growth rate: Each species has optimal temperature range for growth.
5. Dormancy: Temperature changes can trigger growth or dormancy periods.
8. (a) Explain the difference between growth and development using two examples. (4 marks)
Answer:
Growth refers to increase in size/mass, while development refers to increasing complexity and specialization.
Example 1: Human
- Growth: Increase in height and weight from infant to adult
- Development: Acquisition of motor skills, puberty changes, brain maturation
Example 2: Plant
- Growth: Stem elongation, leaf expansion
- Development: Formation of flowers from vegetative buds, differentiation of xylem and phloem
Growth can be measured quantitatively (cm, kg), while development involves qualitative changes in structure and function.
Growth refers to increase in size/mass, while development refers to increasing complexity and specialization.
Example 1: Human
- Growth: Increase in height and weight from infant to adult
- Development: Acquisition of motor skills, puberty changes, brain maturation
Example 2: Plant
- Growth: Stem elongation, leaf expansion
- Development: Formation of flowers from vegetative buds, differentiation of xylem and phloem
Growth can be measured quantitatively (cm, kg), while development involves qualitative changes in structure and function.
(b) A seed germinated successfully, but the seedling later died. List and explain possible causes. (5 marks)
Answer:
Possible causes for seedling death after germination:
1. Water stress: Either drought (lack of water) or waterlogging (oxygen deprivation).
2. Nutrient deficiency: Lack of essential minerals in soil for continued growth.
3. Temperature extremes: Too hot or too cold for the seedling's requirements.
4. Disease/pathogens: Fungal infections like damping-off disease.
5. Light insufficiency: Inadequate light for photosynthesis after seed reserves are depleted.
6. Soil conditions: Poor soil structure, salinity, or pH imbalance.
7. Herbivory: Being eaten by insects or other animals.
8. Genetic factors: Inherited defects affecting post-germination growth.
Possible causes for seedling death after germination:
1. Water stress: Either drought (lack of water) or waterlogging (oxygen deprivation).
2. Nutrient deficiency: Lack of essential minerals in soil for continued growth.
3. Temperature extremes: Too hot or too cold for the seedling's requirements.
4. Disease/pathogens: Fungal infections like damping-off disease.
5. Light insufficiency: Inadequate light for photosynthesis after seed reserves are depleted.
6. Soil conditions: Poor soil structure, salinity, or pH imbalance.
7. Herbivory: Being eaten by insects or other animals.
8. Genetic factors: Inherited defects affecting post-germination growth.
SECTION C (30 Marks)
Answer two (2) questions only.
9. Discuss the process of germination in dicotyledonous plants, highlighting internal and external factors necessary for the process. (15 marks)
Answer:
Process of Germination in Dicots:
1. Imbibition: Seed absorbs water, swelling and rupturing the testa.
2. Enzyme activation: Water activates hydrolytic enzymes (amylases, proteases, lipases).
3. Mobilization of reserves: Stored food (in cotyledons) is broken down:
- Carbohydrates → simple sugars
- Proteins → amino acids
- Lipids → fatty acids + glycerol
4. Radicle emergence: First visible sign, grows downward to anchor plant.
5. Hypocotyl growth: The stem portion elongates, forming a hook that pulls cotyledons upward.
6. Epicotyl expansion: Develops into first true leaves above cotyledons.
7. Photosynthesis begins: Once leaves emerge, plant becomes autotrophic.
Internal Factors:
1. Viable embryo: Living, undamaged embryonic tissue.
2. Enzyme systems: Complete set of enzymes for food mobilization.
3. Hormonal balance: Proper levels of gibberellins, auxins, and abscisic acid.
4. Food reserves: Adequate stored nutrients in cotyledons or endosperm.
External Factors:
1. Water: For imbibition, enzyme activation, and metabolic processes.
2. Oxygen: For aerobic respiration during high metabolic activity.
3. Temperature: Appropriate range for enzyme activity (species-specific).
4. Light: Some seeds require light (positive photoblastic), others are inhibited by it.
5. Soil conditions: Proper texture, pH, and absence of toxins.
The successful germination and establishment of a dicot seedling depends on the coordinated interaction of these internal and external factors.
Process of Germination in Dicots:
1. Imbibition: Seed absorbs water, swelling and rupturing the testa.
2. Enzyme activation: Water activates hydrolytic enzymes (amylases, proteases, lipases).
3. Mobilization of reserves: Stored food (in cotyledons) is broken down:
- Carbohydrates → simple sugars
- Proteins → amino acids
- Lipids → fatty acids + glycerol
4. Radicle emergence: First visible sign, grows downward to anchor plant.
5. Hypocotyl growth: The stem portion elongates, forming a hook that pulls cotyledons upward.
6. Epicotyl expansion: Develops into first true leaves above cotyledons.
7. Photosynthesis begins: Once leaves emerge, plant becomes autotrophic.
Internal Factors:
1. Viable embryo: Living, undamaged embryonic tissue.
2. Enzyme systems: Complete set of enzymes for food mobilization.
3. Hormonal balance: Proper levels of gibberellins, auxins, and abscisic acid.
4. Food reserves: Adequate stored nutrients in cotyledons or endosperm.
External Factors:
1. Water: For imbibition, enzyme activation, and metabolic processes.
2. Oxygen: For aerobic respiration during high metabolic activity.
3. Temperature: Appropriate range for enzyme activity (species-specific).
4. Light: Some seeds require light (positive photoblastic), others are inhibited by it.
5. Soil conditions: Proper texture, pH, and absence of toxins.
The successful germination and establishment of a dicot seedling depends on the coordinated interaction of these internal and external factors.
10. With the aid of a labeled diagram, describe the sigmoid growth curve and explain what occurs in each of its stages. (15 marks)
Answer:
Sigmoid Growth Curve Description:
[Diagram would show an S-shaped curve with x-axis as Time and y-axis as Size/Population, divided into four phases with labels] Stages of Sigmoid Growth Curve:
1. Lag Phase (A):
- Slow initial growth as organism adapts to environment
- Cells prepare for rapid division (enzymes, organelles synthesized)
- In populations, few individuals are reproducing
- Characteristic of organisms in new environment or seeds beginning germination
2. Log Phase (Exponential Phase) (B):
- Rapid, exponential growth under ideal conditions
- Maximum reproductive potential is realized
- Population doubles at regular intervals
- Straight line when logarithm of population is plotted against time
- In individual organisms, cells divide rapidly with abundant resources
3. Deceleration Phase (C):
- Growth rate begins to slow as limiting factors come into play
- Competition for resources increases
- Waste products may accumulate
- In populations, birth rate starts to decline while death rate increases
4. Stationary Phase (Plateau) (D):
- Population/Size stabilizes at carrying capacity
- Birth rate = death rate in populations
- In organisms, mature size is reached with balanced cell production/loss
- Limited by environmental resistance factors (food, space, predation)
5. Death Phase (Decline Phase) (E - not always present):
- Population/size may decline if environment deteriorates
- Death rate exceeds birth rate
- In organisms, senescence processes begin
Factors affecting each phase:
- Lag phase duration: Depends on adaptation time and initial conditions
- Slope of log phase: Determined by intrinsic growth rate of species
- Stationary level: Set by carrying capacity of environment
Applications:
- Understanding bacterial growth in medicine
- Predicting population changes in ecology
- Managing crop growth in agriculture
- Studying tumor growth in cancer research
Sigmoid Growth Curve Description:
[Diagram would show an S-shaped curve with x-axis as Time and y-axis as Size/Population, divided into four phases with labels] Stages of Sigmoid Growth Curve:
1. Lag Phase (A):
- Slow initial growth as organism adapts to environment
- Cells prepare for rapid division (enzymes, organelles synthesized)
- In populations, few individuals are reproducing
- Characteristic of organisms in new environment or seeds beginning germination
2. Log Phase (Exponential Phase) (B):
- Rapid, exponential growth under ideal conditions
- Maximum reproductive potential is realized
- Population doubles at regular intervals
- Straight line when logarithm of population is plotted against time
- In individual organisms, cells divide rapidly with abundant resources
3. Deceleration Phase (C):
- Growth rate begins to slow as limiting factors come into play
- Competition for resources increases
- Waste products may accumulate
- In populations, birth rate starts to decline while death rate increases
4. Stationary Phase (Plateau) (D):
- Population/Size stabilizes at carrying capacity
- Birth rate = death rate in populations
- In organisms, mature size is reached with balanced cell production/loss
- Limited by environmental resistance factors (food, space, predation)
5. Death Phase (Decline Phase) (E - not always present):
- Population/size may decline if environment deteriorates
- Death rate exceeds birth rate
- In organisms, senescence processes begin
Factors affecting each phase:
- Lag phase duration: Depends on adaptation time and initial conditions
- Slope of log phase: Determined by intrinsic growth rate of species
- Stationary level: Set by carrying capacity of environment
Applications:
- Understanding bacterial growth in medicine
- Predicting population changes in ecology
- Managing crop growth in agriculture
- Studying tumor growth in cancer research
11. Explain five differences between mitosis and meiosis and describe how mitosis contributes to growth and repair in organisms. (15 marks)
Answer:
Differences between Mitosis and Meiosis:
How Mitosis Contributes to Growth and Repair:
1. Organismal Growth:
- Allows multicellular organisms to increase in size by increasing cell number
- Enables balanced growth of all tissues and organs
- During development, mitosis creates the millions of cells needed from the zygote
2. Tissue Maintenance and Repair:
- Replaces cells that die naturally (e.g., skin, gut lining)
- Repairs damaged tissues after injury (e.g., wound healing)
- Maintains constant cell numbers in renewing tissues (e.g., blood cells)
3. Mechanisms:
- Cell cycle control: Precise regulation ensures proper timing and frequency of divisions
- Stem cell function: Stem cells divide by mitosis to produce both new stem cells and differentiated cells
- Positional information: Cells divide according to developmental patterns to maintain tissue organization
4. Importance in Different Organisms:
- Plants: Meristematic cells divide to allow continuous growth
- Animals: Enables determinate growth patterns and tissue specialization
- Simple organisms: Basis of asexual reproduction (e.g., budding in Hydra)
5. Cellular Level Benefits:
- Maintains genetic consistency in somatic cells
- Allows for controlled proliferation when needed
- Works with apoptosis (programmed cell death) to shape tissues and organs
Mitosis is therefore fundamental to both the development and maintenance of multicellular organisms, ensuring proper growth patterns and the ability to repair damage throughout life.
Differences between Mitosis and Meiosis:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete formation) |
| Number of divisions | One division | Two successive divisions (Meiosis I and II) |
| Daughter cells | 2 diploid cells (identical to parent) | 4 haploid cells (genetically different) |
| Chromosome number | Maintained (2n → 2n) | Reduced by half (2n → n) |
| Genetic variation | None (clones produced) | Extensive (crossing over, independent assortment) |
| Synapsis | No pairing of homologous chromosomes | Homologous chromosomes pair in prophase I |
| Occurrence | In somatic cells throughout life | Only in germ cells at specific times |
1. Organismal Growth:
- Allows multicellular organisms to increase in size by increasing cell number
- Enables balanced growth of all tissues and organs
- During development, mitosis creates the millions of cells needed from the zygote
2. Tissue Maintenance and Repair:
- Replaces cells that die naturally (e.g., skin, gut lining)
- Repairs damaged tissues after injury (e.g., wound healing)
- Maintains constant cell numbers in renewing tissues (e.g., blood cells)
3. Mechanisms:
- Cell cycle control: Precise regulation ensures proper timing and frequency of divisions
- Stem cell function: Stem cells divide by mitosis to produce both new stem cells and differentiated cells
- Positional information: Cells divide according to developmental patterns to maintain tissue organization
4. Importance in Different Organisms:
- Plants: Meristematic cells divide to allow continuous growth
- Animals: Enables determinate growth patterns and tissue specialization
- Simple organisms: Basis of asexual reproduction (e.g., budding in Hydra)
5. Cellular Level Benefits:
- Maintains genetic consistency in somatic cells
- Allows for controlled proliferation when needed
- Works with apoptosis (programmed cell death) to shape tissues and organs
Mitosis is therefore fundamental to both the development and maintenance of multicellular organisms, ensuring proper growth patterns and the ability to repair damage throughout life.
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