AICE Biology Chapter 5

5.1 Growth and Reproduction

• Growth and reproduction are fundamental to all living organisms, as they are made up of cells that grow and divide.

• Cell division is crucial for passing genetic information to daughter cells, requiring precise control to avoid losing vital genetic information.

• The nucleus in eukaryotic cells plays a key role in controlling the cell’s activities and contains DNA, which acts as the instructions for life.

• In multicellular organisms, except for gametes, all cells are genetically identical, originating from a single zygote (fusion of gametes).

• Growth begins as the zygote divides through mitosis, creating genetically identical nuclei.

• The mitotic cell cycle, a repetitive cycle of nuclear division and cell division, creates the ~30 trillion cells in an average human body.

5.2 Chromosomes

• Chromosomes: Threadlike structures visible in the nucleus before cell division, named from “chromo” (colored) and “somes” (bodies).

• The number of chromosomes is species-specific:

• Humans: 46 chromosomes

• Fruit flies: 8 chromosomes

• Chromosomes consist of two chromatids joined at the centromere.

Structure of Chromosomes:

• Chromatids: Two identical parts of a chromosome, formed during interphase when DNA replicates.

• Each chromatid contains one DNA molecule, with identical genes in sister chromatids.

• Centromere: Holds the chromatids together, located in a specific position for each chromosome, and does not contain genes.

• DNA: A very long molecule storing genetic information:

• In humans, 46 chromosomes contain ~1.8 meters of DNA in a nucleus ~6 μm in diameter.

• DNA is compacted by wrapping around histone proteins, forming chromatin.

• Chromatin: Combination of DNA and proteins (e.g., histones, which are basic, allowing interaction with acidic DNA).

Key Features of Chromosomes:

1. Centromeres: Connect chromatids and play a key role during mitosis.

2. Telomeres: Found at the ends of chromosomes, essential for chromosome protection and successful nuclear division.

Key Terms:

• Chromatid: One of two identical parts of a chromosome, joined by a centromere, formed during interphase.

• Mitosis: Nuclear division process that produces identical nuclei for cell division.

• Chromatin: DNA-protein complex forming the structure of chromosomes.

5.3 The Cell Cycle

Overview:

• Mitosis: Nuclear division that produces two genetically identical daughter nuclei, each with the same number of chromosomes as the parent nucleus.

• Cell Cycle: A sequence of events between one cell division and the next.

• Three phases:

1. Interphase: Cell growth, DNA replication, and preparation for division.

2. Nuclear division (Mitosis/M phase): Division of the nucleus into two identical nuclei.

3. Cell division (Cytokinesis): Final division of the cell, producing two genetically identical cells.

Interphase:

1. G1 Phase (Gap 1):

• Cell grows to its normal size after division.

• Synthesizes RNA, enzymes, and proteins required for growth.

• At the end of G1, the cell either commits to division or exits the cycle.

2. S Phase (Synthesis):

• DNA replication occurs, creating chromosomes with two identical chromatids.

• A relatively short phase.

3. G2 Phase (Gap 2):

• Growth continues.

• DNA is checked for replication errors; most are repaired.

• Prepares for mitosis (e.g., increases production of tubulin to build microtubules for the mitotic spindle).

M Phase (Mitosis):

• Growth pauses temporarily.

• Mitosis stages divide the nucleus into two daughter nuclei, ensuring genetic identity.

• Followed by cytokinesis, where:

• Animal cells: Cytoplasm constricts between the new nuclei.

• Plant cells: A new cell wall forms between the new nuclei.

Cell Cycle Duration:

• Length depends on environmental conditions and cell type:

• Example: Root tip cells (onions) divide every ~20 hours.

• Example: Human intestinal epithelial cells divide every ~10 hours.

Key Terms:

• Mitosis: Division of a nucleus into two identical nuclei with the same chromosome number and type as the parent cell.

• Cell Cycle: Events from one cell division to the next, consisting of interphase, mitosis, and cytokinesis.

Figure Explanation:

• Figure 5.5 (Mitotic Cell Cycle):

• DNA replication happens during interphase:

• G1 (gap 1) → S (synthesis) → G2 (gap 2).

• Mitosis (M phase) follows interphase.

5.4 Mitosis

Overview:

Mitosis is the process of nuclear division that produces two genetically identical daughter nuclei. This process is continuous but is divided into four stages for easier study:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Stages of Mitosis:

1. Prophase

• Early Prophase:

• Chromatin coils and condenses into visible chromosomes.

• Each chromosome consists of two identical chromatids joined by a centromere.

• Nucleolus remains intact initially.

• Centrosomes replicate and start forming the spindle poles.

• Late Prophase:

• Nuclear envelope disintegrates into vesicles (invisible under a light microscope).

• Nucleolus disappears.

• Spindle microtubules attach to centromeres via kinetochores.

2. Metaphase

• Chromosomes align along the equator of the spindle (the metaphase plate).

• Spindle fibers attach to each chromatid via their kinetochores.

3. Anaphase

• Centromeres split, separating sister chromatids.

• Chromatids (now individual chromosomes) are pulled to opposite poles by shortening of spindle microtubules.

4. Telophase

• Chromatids reach the poles and start to uncoil.

• Nuclear envelope reforms around each group of chromosomes.

• Nucleolus reappears in each new nucleus.

• Spindle fibers break down.

Cytokinesis:

• Division of the cytoplasm to form two new daughter cells.

• In animal cells: The cytoplasm constricts from the edges of the cell.

• In plant cells: A new cell wall forms between daughter nuclei.

Centrosomes, Centrioles, and Centromeres:

• Centrosomes: Microtubule-organizing centers (MTOCs) located at spindle poles in animal cells.

• Centrioles: Structures within centrosomes, but not essential for spindle formation.

• Centromeres: The region holding chromatids together and where spindle microtubules attach via kinetochores.

Functions of Mitosis:

1. Growth in Multicellular Organisms

• Enables a single zygote to develop into a fully formed organism with ~30 trillion cells.

• Growth can occur throughout the body (e.g., animals) or in specific areas (e.g., meristems in plants).

2. Cell Replacement and Tissue Repair

• Replaces dead or damaged cells.

• Rapid in tissues such as skin or the gut lining.

• Enables regeneration (e.g., starfish regrowing arms).

3. Asexual Reproduction

• Produces offspring genetically identical to the parent organism.

• Examples:

• Unicellular organisms (e.g., Amoeba) divide for reproduction.

• Multicellular organisms form new individuals via budding, such as in vegetative propagation.

• Biotechnology applications like cloning.

4. Immune Response

• Mitosis is essential for the cloning of B- and T-lymphocytes during an immune response.

Key Words:

• Mitosis: Nuclear division producing two genetically identical nuclei.

• Asexual Reproduction: Formation of new individuals from a single parent organism.

• Kinetochore: Protein structure on chromatids where spindle fibers attach during mitosis.

5.5 The Role of Telomeres

Overview of Telomeres:

• Telomeres are repetitive DNA sequences found at the ends of chromosomes.

• Their role is often compared to the plastic tips on shoelaces, as they protect the ends of chromosomes during DNA replication.

Telomere Function:

1. Replication Challenge:

• DNA copying enzymes cannot fully replicate the very ends of a DNA molecule.

• Without telomeres, critical genetic information at the ends of DNA would be lost with each cell division.

2. Protection of Vital DNA:

• Telomeres contain multiple repeat sequences of non-coding DNA.

• These sequences ensure the meaningful DNA is fully copied by allowing the enzyme to complete replication.

• After replication, any un-copied DNA comes from the telomeres, not vital genes.

3. Telomerase Activity:

• Telomerase is an enzyme that replenishes telomere sequences.

• It adds extra bases to the telomeres during each cell cycle, preventing telomere shortening.

• This ensures the non-telomeric DNA remains intact and cell replication continues.

Role in Ageing:

1. Shortening Telomeres:

• Not all cells replenish their telomeres after replication, particularly fully differentiated (specialised) cells.

• With each division, telomeres shorten until vital DNA is no longer protected, leading to cell death.

2. Mechanism of Ageing:

• The progressive loss of telomere length may contribute to human ageing.

• Cells lose their ability to divide, causing tissue and organ function to decline over time.

3. Potential for Anti-Ageing:

• By preventing telomere shortening, scientists could potentially slow or halt the ageing process.

• This is an area of ongoing research.

Exceptions:

• Stem Cells:

• These cells maintain telomere length through constant telomerase activity, allowing them to divide indefinitely.

• Cancer Cells:

• Similar to stem cells, many cancer cells reactivate telomerase, allowing uncontrolled cell division.

Key Points:

• Telomeres protect chromosomes during replication by preventing vital genes from being lost.

• Telomerase replenishes telomeres, but not all cells can perform this.

• Shortening telomeres contribute to ageing and eventual cell death.

• Telomeres play a key role in the study of ageing, cell division, and diseases like cancer.

5.6 The Role of Stem Cells

What are Stem Cells?

• Stem cells are unspecialized cells capable of dividing indefinitely by mitosis.

• After division, each new cell can either:

1. Remain a stem cell.

2. Differentiate into a specialized cell (e.g., blood cells, muscle cells).

Stem Cell Potency

The potency of a stem cell determines the variety of cells it can produce:

1. Totipotent Stem Cells:

• Definition: Can differentiate into any type of cell in the organism, including extra-embryonic tissues like the placenta.

• Examples:

• The zygote (fertilized egg).

• All cells up to the 16-cell stage in humans.

2. Pluripotent Stem Cells:

• Definition: Can form all types of cells in the embryo and adult, except for placental tissues.

• Examples: Embryonic stem cells.

3. Multipotent Stem Cells:

• Definition: Can produce a limited range of cell types, often related to their specific tissue.

• Examples: Adult stem cells like those in bone marrow, which can produce various types of blood cells (e.g., red blood cells, neutrophils).

Specialization and Division

• As tissues and organs develop, stem cells specialize into specific roles and lose their potency.

• Most adult cells cannot divide. However, small populations of multipotent stem cells remain to support:

• Growth.

• Repair and replacement of cells.

Example: Blood Cell Replacement

• Bone marrow stem cells produce blood cells.

• Importance:

• 250 billion red blood cells and 20 billion white blood cells are replaced daily.

Sources of Adult Stem Cells

Adult stem cells are found in various tissues and organs, such as:

• Bone marrow.

• Skin.

• Gut.

• Heart.

• Brain.

Medical Applications of Stem Cells

Research into stem cells has advanced medical treatments and possibilities, including:

1. Stem Cell Therapy:

• The introduction of new adult stem cells into damaged tissues.

• Example:

• Bone marrow transplantation (already widely used).

• Treats blood disorders, bone marrow diseases, and blood cancers like leukaemia.

2. Potential Treatments:

• Diabetes.

• Muscle and nerve damage.

• Brain disorders (e.g., Parkinson’s and Huntington’s diseases).

3. Laboratory Applications:

• Experiments with growing tissues or even entire organs from isolated stem cells.

Summary

• Stem cells are crucial for development, repair, and replacement of cells in the body.

• Stem cell potency ranges from totipotent to multipotent, with varying potential for differentiation.

• Current and future research holds promise for transformative medical treatments.

5.7 Cancers

What are Cancers?

• Cancers result from uncontrolled mitosis, where cells divide repeatedly and form a tumour, an irregular mass of cells.

• These diseases arise from a breakdown in the control mechanisms regulating cell division.

• Cancers are no longer considered a single disease, with over 200 distinct types identified.

Global Impact of Cancer

• In high-income countries, cancers are responsible for 1 in 4 deaths.

• Worldwide, they account for 1 in 6 deaths, killing approximately 9.6 million people in 2018.

• Cancer is the second-leading cause of death globally, after cardiovascular diseases.

• Lung cancer is the leading cause of cancer-related deaths worldwide.

Causes and Characteristics of Cancer Cells

1. Mutations and Oncogenes

• A mutation is a change in the genes that control cell division.

• Mutated genes that cause cancer are called oncogenes (from the Greek “onkos,” meaning bulk or mass).

• Mutations causing cancer can be:

• Inherited or acquired over a lifetime.

• Most mutations do not lead to cancer, as the body:

• Removes mutated cells through immune responses.

• Ensures cell death (apoptosis) when necessary.

2. Cancerous Cells

• Cancerous cells evade normal destruction and survive, passing on their mutations to descendants.

• A typical tumour contains around a billion cancer cells by the time it is detected.

• Cancer cells often exhibit abnormal shapes and lose their specialized characteristics.

3. Carcinogens

• A carcinogen is an agent (e.g., asbestos, radiation) that causes cancer.

• Carcinogens are described as carcinogenic.

Types of Tumours

• Benign Tumours

• Do not spread from their origin.

• Example: Warts.

• Usually harmless, but can sometimes cause minor complications.

• Malignant Tumours (Cancerous)

• Spread to other tissues, invading and destroying normal cells.

• Can block organs (e.g., intestines, blood vessels, lungs) or disrupt their function.

• Cells may detach and spread via the bloodstream or lymphatic system, leading to:

• Metastasis: Formation of secondary tumours in other body parts.

• Metastasis is the most dangerous characteristic of cancer as it is difficult to find and treat secondary tumours.

Key Vocabulary

• Cancer: A disease caused by uncontrolled cell division, resulting in tumours, with possible spread (metastasis).

• Tumour: An abnormal mass of cells caused by uncontrolled division.

• Benign Tumour: A non-cancerous tumour that does not spread.

• Malignant Tumour: A cancerous tumour that invades nearby tissues and spreads through metastasis.

• Mutation: A random change in the DNA’s base sequence or chromosome structure/number.

• Oncogene: A mutated gene that promotes cancer development.

• Carcinogen: A substance or environmental factor that causes cancer.

Summary

• Cancer occurs due to uncontrolled cell division, forming tumours.

• It is caused by mutations in genes regulating the cell cycle.

• Not all tumours are cancerous—malignant tumours invade and spread, causing metastasis.

• Research and early detection focus on managing this highly lethal condition, which remains a major global health challenge.

SUMMARY: Chromosomes are made of chromatin. Chromatin consists mainly of DNA wrapped around basic protein molecules called histones. During nuclear division chromosomes become visible and are seen to consist of two chromatids held together by a centromere. Each chromatid contains one DNA molecule. Growth of a multicellular organism is a result of cells dividing to produce genetically identical daughter cells. During cell division, the nucleus divides first, followed by division of the whole cell. Division of a nucleus to produce two genetically identical nuclei is achieved by the process of mitosis. Mitosis is divided into four phases: prophase, metaphase, anaphase and telophase. Mitosis is used in growth, repair, asexual reproduction and cloning of cells during an immune response. The period from one cell division to the next is called the cell cycle. It has four phases: G1 is the first growth phase after cell division; S phase is when the DNA replicates; G2 is a second growth phase; M phase is when nuclear division takes place (followed by cell division). The ends of chromosomes are capped with special regions of DNA known as telomeres. Telomeres are needed to prevent the loss of genes from the ends of chromosomes during replication of DNA. Many specialised cells lose the ability to divide, but certain cells known as stem cells retain this ability. Stem cells are essential for growth from zygote to adult and for cell replacement and tissue repair in the adult. The behaviour of chromosomes during mitosis can be observed in stained preparations of root tips, either in section or in squashes of whole root tips. Cancers are tumours resulting from repeated and uncontrolled mitosis. They are thought to start as the result of mutation.

This comprehensive summary provides a detailed breakdown of key concepts related to growth, cell division, and cancer, focusing on topics such as:

Growth and Cell Division (5.1 - 5.6):

1. Growth and Reproduction (5.1):

• Growth and reproduction depend on controlled cell division to maintain genetic consistency.

• Mitosis produces genetically identical daughter cells for growth, repair, and replacement.

• Multicellular organisms originate from a single zygote through mitotic divisions.

2. Chromosomes (5.2):

• Chromosomes, made of chromatin (DNA + histones), appear during cell division.

• Each chromosome contains two identical chromatids joined by a centromere after DNA replication.

3. The Cell Cycle (5.3):

• Comprised of interphase (G1, S, G2 phases), mitosis (M phase), and cytokinesis.

• Interphase prepares cells for division, replicating DNA and ensuring error-free genetic material.

4. Mitosis (5.4):

• Divided into four stages:

1. Prophase: Chromosomes condense; spindle fibers attach to kinetochores.

2. Metaphase: Chromosomes align along the metaphase plate.

3. Anaphase: Chromatids separate and move to opposite poles.

4. Telophase: Nuclear envelopes reform; cells prepare for cytokinesis.

• Functions: Growth, repair, asexual reproduction, and immune cell production.

5. Telomeres (5.5):

• Repetitive DNA sequences protecting chromosome ends, preventing loss of vital genes during replication.

• Telomere shortening is linked to ageing; telomerase sustains telomeres in stem and cancer cells.

6. Stem Cells (5.6):

• Totipotent, pluripotent, and multipotent stem cells vary in differentiation potential.

• Applications include tissue repair, blood cell regeneration, and treatments for various disorders.

Cancers (5.7):

• Uncontrolled Mitosis: Mutations in oncogenes disrupt normal cell cycle regulation, forming tumours.

• Tumour Types:

• Benign Tumours: Non-cancerous and localized, like warts.

• Malignant Tumours: Cancerous, invade tissues, and spread via metastasis (through blood/lymph).

• Global Impact: Second leading cause of death worldwide (lung cancer most common).

• Causes:

• Carcinogens: Environmental factors (e.g., asbestos, UV radiation).

• Mutations: Spontaneous or inherited changes in DNA.

• Significance of Metastasis: Secondary growths make cancer challenging to treat.

Summary:

• Chromosomes are composed of DNA and histones, forming visible chromatids during division.

• Mitosis ensures consistent genetic information in growth, repair, and reproduction.

• The cell cycle governs division; telomeres protect chromosomes, ensuring genomic stability.

• Stem cells support regeneration, offering therapeutic potential in medicine.

• Cancer arises from mutations causing uncontrolled mitosis and tumour formation, highlighting the need for understanding cellular processes for treatment and prevention.