5.10 Chapter Summary
BioCast
1. Structure of a Chromosome
a. DNA (Deoxyribonucleic Acid)
- Function: Carries genetic information.
- Structure: Double helix composed of nucleotide sequences.
- Role in Chromosomes: DNA is tightly packed to form chromosomes during cell division.
b. Histone Proteins
- Function: Help in packaging DNA into a compact, structured form.
- Structure: Proteins around which DNA winds, forming nucleosomes.
- Role in Chromosomes: Facilitate DNA compaction and regulate gene expression.
c. Sister Chromatids
- Definition: Identical copies of a single chromosome connected by a centromere.
- Formation: Result from DNA replication during the S phase of interphase.
- Role: Ensure each daughter cell receives an identical set of chromosomes during mitosis.
d. Centromere
- Function: The region where sister chromatids are joined.
- Structure: A constricted area on the chromosome.
- Role in Mitosis: Serves as the attachment point for spindle fibers, guiding chromatids to opposite poles.
e. Telomeres
- Function: Protect chromosome ends from deterioration.
- Structure: Repetitive nucleotide sequences at each end of a chromosome.
- Role: Prevent loss of genetic information during DNA replication.
2. Importance of Mitosis in Producing Genetically Identical Daughter Cells
a. Growth of Multicellular Organisms
- Explanation: Mitosis allows organisms to increase in size by adding more cells.
- Outcome: Development from a single fertilized egg to a complex organism with trillions of cells.
b. Replacement of Damaged or Dead Cells
- Explanation: Mitosis replaces cells that are lost due to injury or natural cell death.
- Outcome: Maintains tissue integrity and function.
c. Repair of Tissues by Cell Replacement
- Explanation: Specialized tissues, such as skin and liver, use mitosis to regenerate after damage.
- Outcome: Ensures healing and restoration of normal tissue structure.
d. Asexual Reproduction
- Explanation: In unicellular and some multicellular organisms, mitosis enables reproduction without gametes.
- Outcome: Produces offspring genetically identical to the parent organism.
3. Mitotic Cell Cycle
The cell cycle comprises phases that prepare and execute cell division, ensuring accurate replication and distribution of genetic material.
a. Interphase
- Overall Function: Preparation for cell division; cell growth and DNA replication.
i. G₁ Phase (Gap 1)
- Activities: Cell growth, protein synthesis, and organelle production.
- Significance: Cells increase in size and prepare for DNA replication.
ii. S Phase (Synthesis)
- Activities: DNA replication occurs, doubling the genetic material.
- Significance: Ensures each daughter cell receives an identical set of chromosomes.
iii. G₂ Phase (Gap 2)
- Activities: Further cell growth and protein synthesis; preparation for mitosis.
- Significance: Final checks and readiness for chromosome segregation.
b. Mitosis
- Definition: The process of nuclear division where replicated chromosomes are separated into two new nuclei.
- Phases:
- Prophase: Chromosomes condense; spindle fibers form; nuclear envelope breaks down.
- Metaphase: Chromosomes align at the cell’s equatorial plate.
- Anaphase: Sister chromatids are pulled apart to opposite poles.
- Telophase: Nuclear membranes reform around each set of chromosomes; chromosomes de-condense.
c. Cytokinesis
- Definition: The division of the cytoplasm, resulting in two separate daughter cells.
Process:
- In Animal Cells: Formation of a cleavage furrow that pinches the cell into two.
- In Plant Cells: Formation of a cell plate that develops into a new cell wall.
4. Role of Telomeres in Preventing Gene Loss
Function of Telomeres
- Protection: Telomeres cap the ends of chromosomes, safeguarding them from deterioration and fusion with other chromosomes.
Prevention of Gene Loss
- Mechanism: During DNA replication, DNA polymerase cannot fully replicate the very ends of linear chromosomes.
- Role of Telomeres: Repetitive sequences serve as buffer zones, ensuring that essential genes are not lost with each cell division.
- Outcome: Maintains genomic integrity over successive generations of cells.
5. Role of Stem Cells in Cell Replacement and Tissue Repair by Mitosis
What are Stem Cells?
- Definition: Undifferentiated cells with the potential to develop into various specialized cell types.
- Types:
- Embryonic Stem Cells: Pluripotent; can become any cell type.
- Adult Stem Cells: Multipotent; typically differentiate into cell types of their tissue of origin.
Role in Cell Replacement
- Mechanism: Stem cells divide by mitosis to produce new cells needed for growth, maintenance, and repair.
- Differentiation: New cells can differentiate into specialized cells to replace damaged or lost cells.
Role in Tissue Repair
Process:
- Activation: Stem cells are activated in response to tissue damage.
- Proliferation: Stem cells undergo mitosis to produce progenitor cells.
- Differentiation: Progenitor cells differentiate into specific cell types needed for repair.
- Outcome: Restoration of normal tissue structure and function.
6. Uncontrolled Cell Division and Tumour Formation
What is Uncontrolled Cell Division?
- Definition: When cells divide without the normal regulatory mechanisms, leading to excessive cell proliferation.
Causes of Uncontrolled Division
- Genetic Mutations: Alterations in genes that regulate the cell cycle (e.g., oncogenes, tumor suppressor genes).
- Environmental Factors: Exposure to carcinogens (e.g., radiation, chemicals).
Formation of Tumours
- Process:
- Initiation: Genetic mutations disrupt normal cell cycle control.
- Promotion: Mutated cells proliferate excessively.
- Progression: Accumulation of further mutations leads to malignant characteristics.
Types of Tumours
- Benign Tumours: Non-cancerous; grow slowly and do not invade surrounding tissues.
- Malignant Tumours (Cancers): Cancerous; grow rapidly, invade nearby tissues, and can metastasize to distant sites.
Implications
- Health Impact: Malignant tumours can disrupt organ function and are often life-threatening.
- Treatment Challenges: Requires medical interventions like surgery, chemotherapy, or radiation therapy.
7. Chromosome Behaviour in Plant and Animal Cells During Mitosis
Overview of Mitosis
Mitosis is a fundamental process for growth, development, and tissue repair in eukaryotic organisms. It ensures that each daughter cell receives an identical set of chromosomes. Mitosis is divided into distinct stages: Prophase, Metaphase, Anaphase, and Telophase.
A. General Cell Structures Involved:
- Chromosomes: Structures composed of DNA and proteins, visible during mitosis.
- Nuclear Envelope: Membrane surrounding the nucleus, disassembles and reassembles during mitosis.
- Spindle Fibers: Microtubule structures that segregate chromosomes.
- Cell Surface Membrane: Maintains cell integrity; in animal cells, it pinches during cytokinesis; in plant cells, a cell plate forms.
B. Stages of Mitosis:
- Prophase:
- Chromosomes Condense: Chromatin coils into visible chromosomes, each consisting of two sister chromatids joined at the centromere.
- Spindle Formation: Mitotic spindle begins to form from centrosomes (in animal cells) or spindle pole bodies (in plant cells).
- Nuclear Envelope Breakdown: The nuclear membrane disintegrates, allowing spindle fibers to interact with chromosomes.
- Differences:
- Animal Cells: Centrosomes move to opposite poles.
- Plant Cells: Lack centrosomes; spindle forms from microtubule organizing centers.
- Metaphase:
- Chromosome Alignment: Chromosomes align at the metaphase plate (equatorial plane) due to spindle fiber attachment at kinetochores.
- Checkpoint: Ensures all chromosomes are properly attached before proceeding.
- Anaphase:
- Sister Chromatids Separation: Cohesin proteins are cleaved, allowing chromatids to move to opposite poles via shortening of spindle fibers.
- Pole Movement: Poles move further apart as chromatids are pulled towards them.
- Telophase:
- Nuclear Envelope Reformation: New nuclear membranes form around each set of chromosomes at the poles.
- Chromosome Decondensation: Chromosomes uncoil back into chromatin.
- Spindle Disassembly: Mitotic spindle breaks down.
C. Cytokinesis:
- Animal Cells: Cleavage furrow forms, pinching the cell into two.
- Plant Cells: Cell plate develops, leading to the formation of a new cell wall separating the daughter cells.
8. Comparative Behaviour in Plant vs. Animal Cells
Spindle Formation:
- Animal Cells: Utilize centrosomes with centrioles to organize spindle fibers.
- Plant Cells: Spindle fibers form from microtubule organizing centers without centrioles.
Cytokinesis Mechanism:
- Animal Cells: Involves the formation of a cleavage furrow.
- Plant Cells: Involves the formation of a cell plate that develops into a new cell wall.
Nuclear Envelope:
- Both cell types disassemble during prophase and reassemble during telophase.
9. Identifying Stages of Mitosis Through Visuals
A. Prophase:
- Visual Cues:
- Condensed, visible chromosomes.
- Disintegrating nuclear envelope.
- Beginning of spindle fiber formation.
B. Metaphase:
- Visual Cues:
- Chromosomes aligned along the metaphase plate.
- Spindle fibers extending from opposite poles attaching to kinetochores.
C. Anaphase:
- Visual Cues:
- Separation of sister chromatids moving toward opposite poles.
- Spindle fibers shortening.
D. Telophase:
- Visual Cues:
- Two distinct nuclei with reformed nuclear envelopes.
- Chromosomes decondensing into chromatin.
- Spindle fibers disassembling.
E. Cytokinesis (often considered alongside Telophase):
- Animal Cells: Appearance of a cleavage furrow.
- Plant Cells: Formation of a cell plate.
10. Tips for Interpreting Photomicrographs and Diagrams
- Identify Chromosome Condensation: Highly condensed chromosomes indicate early stages (Prophase, Metaphase).
- Look for Nuclear Envelope: Presence suggests Telophase; absence suggests Prophase or Metaphase.
- Check Chromosome Arrangement:
- Central alignment indicates Metaphase.
- Chromatids moving apart indicate Anaphase.
- Observe Spindle Fibers:
- Visible and attaching to chromosomes during Prophase and Metaphase.
- Shortening during Anaphase.
- Cytokinesis Features:
- Cleavage furrow in animal cells.
- Cell plate in plant cells.
Practice Exercise:
- Examine labeled diagrams and identify each stage based on the presence or absence of the nuclear envelope, chromosome alignment, and spindle fiber status.
- Use unlabeled images to practice identifying stages by matching visual features to descriptions above.
Practise Questions 1
a) Question: Describe the structure of chromosomes.
Answer:
- Chromosomes are made of chromatin, a combination of DNA and proteins (histones).
- DNA wraps tightly around histones, forming a compact structure that can fit within the cell nucleus.
- Structure Before Cell Division:
- Each chromosome is composed of two identical sister chromatids.
- Sister chromatids are held together by a centromere.
- Each chromatid contains one DNA molecule.
- Telomeres are repetitive sequences at the ends of chromosomes that protect the DNA during replication.
b) Question: Outline the cell cycle and explain the sequence of events that lead body cells to grow and divide.
Answer:
- The cell cycle is the sequence of events that a cell undergoes from one division to the next, enabling growth and division.
- Phases of the Cell Cycle:
- G₁ Phase (Gap 1): The cell grows in size and synthesizes proteins necessary for DNA replication.
- S Phase (Synthesis): DNA is replicated, producing two identical DNA copies in preparation for cell division.
- G₂ Phase (Gap 2): The cell continues to grow, checks DNA for errors, and begins preparations for mitosis.
- M Phase (Mitosis): The nucleus divides, followed by cytokinesis, where the cell divides into two genetically identical daughter cells.
c) Question: Describe the behaviour of chromosomes during mitosis, along with the roles of the nuclear envelope, cell surface membrane, and spindle.
Answer:
- Prophase:
- Chromosomes condense and become visible as sister chromatids joined by a centromere.
- The nuclear envelope disintegrates, allowing spindle fibers to access chromosomes.
- Spindle fibers form from centrosomes and move toward opposite poles.
- Metaphase:
- Chromosomes align along the cell’s equatorial plane.
- Spindle fibers attach to the centromere of each chromosome.
- Anaphase:
- Centromeres split, and sister chromatids are pulled to opposite poles by shortening spindle fibers.
- Telophase:
- Chromatids reach opposite poles and begin to de-condense into chromatin.
- The nuclear envelope reforms around each new nucleus.
- Cytokinesis (division of the cytoplasm) occurs, leading to separation into two cells with the formation of the cell surface membrane between them.
d) Question: How can the stages of mitosis be identified in photomicrographs, diagrams, and microscope slides?
Answer:
- Prophase: Chromosomes are visible as condensed structures within the dissolving nuclear envelope.
- Metaphase: Chromosomes align at the cell equator in a line, attached to spindle fibers.
- Anaphase: Chromosomes appear as chromatids separating and moving to opposite poles.
- Telophase: Chromatids reach the poles, nuclear envelopes reform around each set of chromosomes, and cytokinesis begins to divide the cell.
e) Question: Explain the importance of mitosis in living organisms.
Answer:
- Growth: Mitosis enables organisms to grow by increasing cell numbers.
- Repair and Replacement: Replaces damaged or dead cells, maintaining tissue integrity (e.g., skin and blood cells).
- Asexual Reproduction: Allows certain organisms to reproduce by producing genetically identical offspring.
- Cloning in Immune Response: Cloning of B- and T-lymphocytes during immune responses helps fight infections.
f) Question: Outline the role of telomeres in chromosome protection and cell division.
Answer:
- Telomeres are repetitive DNA sequences at the ends of chromosomes.
- Functions:
- Protect chromosomes from losing essential genetic information during DNA replication, as the replication enzyme cannot copy DNA to the very end.
- Prevent chromosomes from sticking to each other.
- As cells divide, telomeres shorten, which eventually leads to cell death. This process is part of aging.
g) Question: Outline the role of stem cells in growth, development, and tissue repair.
Answer:
- Stem Cells: Undifferentiated cells with the ability to divide indefinitely and differentiate into specialized cell types.
- Functions:
- Development: Crucial for growth from a single zygote to a fully developed organism.
- Tissue Repair and Regeneration: Replace cells lost due to injury or cell death, especially in tissues with high turnover (e.g., blood, skin).
- Types of Potency:
- Totipotent: Can form all cell types, including extraembryonic (placental) cells.
- Pluripotent: Can form all body cell types, found in embryos.
- Multipotent: Limited to certain cell types, found in adults (e.g., blood stem cells in bone marrow).
h) Question: Explain how uncontrolled cell division can lead to the development of cancer.
Answer:
- Carcinogens: Substances like tobacco smoke, radiation, and certain chemicals increase mutation rates and cancer risk.
- Cancer: A disease caused by uncontrolled mitosis, leading to the formation of tumors.
Mechanism:
- Mutations in genes controlling cell division can lead to unregulated cell growth.
- Oncogenes: Mutated genes that promote cell division even when it is not needed.
- Tumour Formation: Mutated cells divide repeatedly, forming a mass of abnormal cells (tumor).
Types of Tumors:
- Benign: Non-cancerous, do not spread.
- Malignant: Cancerous, invade nearby tissues and spread to other body parts (metastasis).
Practise Questions 2
Question 1
What is chromatin, and how does it relate to chromosomes? (4 marks)
Mark Scheme:
- Chromatin is composed of DNA tightly wrapped around histone proteins. (1 mark)
- It forms the structure of chromosomes during nuclear division. (1 mark)
- During mitosis, chromatin condenses into visible chromosomes. (1 mark)
- Each chromosome consists of two identical chromatids held together by a centromere. (1 mark)
Question 2
Describe the structure of a chromosome during mitosis. (5 marks)
Mark Scheme:
- A chromosome is made of two identical sister chromatids joined at a centromere. (1 mark)
- Each chromatid contains one DNA molecule. (1 mark)
- Chromosomes become visible during mitosis as chromatin condenses. (1 mark)
- The centromere allows spindle fibers to attach during mitosis. (1 mark)
- This structure ensures proper separation of chromatids during anaphase. (1 mark)
Question 3
Explain the role of telomeres in protecting chromosomes during cell division. (5 marks)
Mark Scheme:
- Telomeres are repetitive DNA sequences at the ends of chromosomes. (1 mark)
- They protect genes from being lost during DNA replication. (1 mark)
- DNA polymerase cannot replicate the very ends of DNA strands, leading to gradual shortening. (1 mark)
- Telomeres act as caps, preventing chromosome degradation over successive cell divisions. (1 mark)
- Without telomeres, critical genetic information would be lost, affecting cell function and viability. (1 mark)
Question 4
Outline the stages of mitosis and describe what happens during each stage. (8 marks)
Mark Scheme:
- Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle fibers form. (1 mark)
- Metaphase: Chromosomes align at the cell equator; spindle fibers attach to centromeres. (1 mark)
- Anaphase: Centromeres split, and sister chromatids (now chromosomes) are pulled to opposite poles. (1 mark)
- Telophase: Chromosomes reach opposite poles and uncoil; nuclear envelope reforms around each set of chromosomes. (1 mark)
- Mitosis ensures equal distribution of genetic material to daughter cells. (1 mark)
- Cytokinesis: The cytoplasm divides, resulting in two genetically identical daughter cells. (1 mark)
- In animal cells, cytokinesis occurs via cleavage furrow, while in plant cells, a cell plate forms. (1 mark)
- The sequence maintains genetic continuity and supports growth, repair, or asexual reproduction. (1 mark)
Question 5
What are stem cells, and why are they important in multicellular organisms? (5 marks)
Mark Scheme:
- Stem cells are undifferentiated cells capable of dividing indefinitely and differentiating into specialized cells. (1 mark)
- They are essential for growth from the zygote to an adult organism. (1 mark)
- In adults, stem cells support cell replacement and tissue repair. (1 mark)
- Unlike specialized cells, stem cells retain the ability to divide and produce various cell types. (1 mark)
- Example: Bone marrow stem cells produce red and white blood cells to replace those lost through turnover. (1 mark)
Question 6
How can chromosome behavior during mitosis be observed in a laboratory? (4 marks)
Mark Scheme:
- Chromosome behavior is studied using stained root tips from rapidly dividing tissues (e.g., onion or garlic root tips). (1 mark)
- The root tips are prepared using squashes or sections for microscopic observation. (1 mark)
- Different stages of mitosis (prophase, metaphase, anaphase, telophase) are identifiable based on chromosome visibility and position. (1 mark)
- Staining highlights chromatin and chromosomes, allowing clear visualization of mitotic processes. (1 mark)
Question 7
What causes cancer, and how does it result in tumour formation? (6 marks)
Mark Scheme:
- Cancer results from uncontrolled cell division due to mutations in genes regulating the cell cycle. (1 mark)
- Mutations in proto-oncogenes transform them into oncogenes, allowing unchecked division. (1 mark)
- Carcinogens, such as UV light or tobacco tar, increase mutation rates, raising cancer risk. (1 mark)
- Rapidly dividing cancerous cells form abnormal masses called tumours. (1 mark)
- Benign tumours remain localized, while malignant tumours invade other tissues and may metastasize. (1 mark)
- Malignant tumours disrupt normal organ function and create secondary tumours in distant body parts. (1 mark)
Question 8
Explain the significance of the S phase of the cell cycle. (5 marks)
Mark Scheme:
- The S phase is the period of DNA synthesis and replication within the cell cycle. (1 mark)
- During this phase, each chromosome produces an identical copy, forming sister chromatids. (1 mark)
- This ensures that, after mitosis, each daughter cell receives a complete set of genetic material. (1 mark)
- Accurate replication during the S phase is crucial to prevent mutations or genetic errors. (1 mark)
- The S phase prepares the cell for mitosis, ensuring genetic continuity between generations of cells. (1 mark)
Question 9
Compare mitosis and its role in growth, repair, and asexual reproduction. (6 marks)
Mark Scheme:
- Mitosis produces two genetically identical daughter cells from a single parent cell. (1 mark)
- In growth, mitosis increases the number of cells, allowing organisms to develop from a single zygote. (1 mark)
- In repair, mitosis replaces damaged or dead cells, maintaining tissue integrity. (1 mark)
- In asexual reproduction, mitosis generates clones, ensuring genetic uniformity in offspring. (1 mark)
- Example: Regeneration of skin cells involves mitosis to replace lost or damaged cells. (1 mark)
- Mitosis ensures genetic stability, supporting the continuity of cellular functions in multicellular organisms. (1 mark)
Question 10
What are carcinogens, and how do they contribute to cancer development? (4 marks)
Mark Scheme:
- Carcinogens are substances or environmental factors that increase the risk of cancer. (1 mark)
- They cause mutations in DNA, potentially leading to the activation of oncogenes. (1 mark)
- Examples include UV radiation, tobacco tar, X-rays, and asbestos. (1 mark)
- These mutations disrupt normal cell cycle regulation, allowing uncontrolled cell division and tumour formation. (1 mark)
Quizzes
Test 1
1. What is the term for the end of a chromosome?
2. What are the identical copies of DNA that are attached together called?
3. What are the proteins around which DNA is coiled?
4. What is the region where identical copies of DNA remain attached after DNA has duplicated?
5. What is the function of mitosis?
6. Which of the following best describes a gene?
7. What happens to the telomeres each time DNA is copied?
8. What is the role of histones in chromosomes?
9. Which of the following is NOT a function of mitosis?
10. What happens when the DNA in a chromosome is duplicated?
Correct Answers: 0%
Test 2
1. What are stem cells?
2. What term describes the ability of a stem cell to form all types of specialised cells?
3. What type of stem cells are found in the bone marrow?
4. What happens if the control of cell division goes wrong?
5. What happens during prophase of mitosis?
6. What happens during metaphase of mitosis?
7. What structure forms and attaches to chromosomes during mitosis?
8. What is the result of incorrect cell division?
9. What is the primary function of stem cells in adults?
10. During which stage of mitosis do the chromosomes condense?
Correct Answers: 0%
Test 3
1. During which phase of the mitotic cell cycle does DNA replication occur?
2. What is the purpose of mitosis in multicellular organisms?
3. During which phase do chromatids split apart and move to opposite ends of the cell?
4. What is the role of totipotent stem cells in an early embryo?
5. Why is it important for the cell cycle to be carefully controlled?
6. What happens during cytokinesis?
7. Which phase of interphase occurs immediately after cell division?
8. What limits the ability of adult stem cells to replace damaged tissues?
9. What is the outcome of mitosis and cytokinesis?
10. What happens if the control of cell division fails?
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