IB DP Biology Cell and nuclear division Study Notes

IB DP Biology Cell and nuclear division Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

How can large numbers of genetically identical cells be produced?
How do eukaryotes produce genetically varied cells that can develop into gametes?

Standard level and higher level: 3 hours
Additional higher level: 1 hour

IBDP Biology 2025 -Study Notes -All Topics

D2.1.1—Generation of new cells in living organisms by cell division

Cell division is a fundamental process that allows organisms to grow, repair tissues, and reproduce. It involves the division of a single cell into two daughter cells. This process ensures the continuity of life from one generation to the next.

The cell cycle is a series of events that take place in a cell leading to its division and duplication (replication). It is divided into two major phases: interphase and mitotic (M) phase. During interphase, the cell grows, replicates its DNA, and prepares for cell division. The mitotic phase involves the actual division of the cell into two daughter cells. This phase is further divided into four stages: prophase, metaphase, anaphase, and telophase.

D2.1.2—Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells

Cytokinesis is the process by which the cytoplasm of a cell is divided between two daughter cells during cell division. It occurs alongside nuclear division (mitosis or meiosis) to ensure that each daughter cell receives a complete set of chromosomes and cytoplasm.

Cytokinesis in Animal Cells:

In animal cells, cytokinesis involves the formation of a cleavage furrow. A ring of contractile proteins, composed of actin and myosin, constricts around the equator of the cell, pinching the cell membrane inward. This process continues until the cell is completely divided into two daughter cells.

Cytokinesis in Plant Cells:

Plant cells have a rigid cell wall, which prevents the formation of a cleavage furrow. Instead, cytokinesis involves the formation of a cell plate. Microtubules assemble into a scaffold that guides the formation of vesicles containing cell wall materials. These vesicles fuse together to form a cell plate, which eventually expands and merges with the existing cell wall, dividing the cell into two daughter cells.

Both processes ensure the equal distribution of cytoplasm and organelles between the daughter cells, allowing for the continued growth and development of the organism.

D2.1.3—Equal and unequal cytokinesis

Cytokinesis can be equal or unequal. In equal cytokinesis, the cytoplasm is divided equally between daughter cells, as seen in growing root tips. In unequal cytokinesis, one daughter cell receives most of the cytoplasm, as in budding yeast and oogenesis. This unequal division ensures that the larger cell has sufficient resources for development.

D2.1.4—Roles of mitosis and meiosis in eukaryotes

Mitosis and meiosis are two types of nuclear division that occur in eukaryotic cells. They differ in their purpose, the number of cell divisions, and the genetic makeup of the daughter cells.

Mitosis

Purpose: Growth, repair, and asexual reproduction.
Cell divisions: One
Daughter cells: Two
Chromosome number: Diploid (2n)
Genetic makeup: Genetically identical to the parent cell.

Mitosis is essential for maintaining genetic continuity and ensuring that all cells in an organism have the same genetic information. It allows for growth, tissue repair, and asexual reproduction.

Meiosis

Purpose: Production of gametes for sexual reproduction.
Cell divisions: Two
Daughter cells: Four
Chromosome number: Haploid (n)
Genetic makeup: Genetically different from the parent cell.

Meiosis is essential for sexual reproduction, as it generates genetic diversity among offspring. It involves two rounds of cell division, resulting in four haploid daughter cells. The genetic variation in offspring is generated through crossing over and independent assortment of chromosomes during meiosis.

D2.1.5—DNA replication as a prerequisite for both mitosis and meiosis

DNA Replication:

Purpose: To ensure that each daughter cell receives a complete set of genetic material.
Process: Before cell division, the DNA molecule is replicated, creating two identical copies called sister chromatids.
Chromosomes: The sister chromatids are held together at a region called the centromere.

Mitosis and Meiosis:

Mitosis:
- Results in two daughter cells that are genetically identical to the parent cell.
- Used for growth, repair, and asexual reproduction.
Meiosis:
- Results in four daughter cells, each with half the number of chromosomes as the parent cell.
- Used for sexual reproduction.
- Generates genetic diversity through crossing over and independent assortment.

Key Points:

DNA replication is essential for both mitosis and meiosis.
Sister chromatids are held together by cohesin proteins until they separate during anaphase.
The number of chromosomes in a species is characteristic and varies across different organisms.

Understanding DNA replication and cell division is crucial for understanding the transmission of genetic information and the diversity of life.

D2.1.6—Condensation and movement of chromosomes as shared features of mitosis and meiosis

During both mitosis and meiosis, chromosomes must be condensed and moved to opposite poles of the cell to ensure that each daughter cell receives a complete set of genetic material.

Chromosome Condensation:

DNA Packaging: The long, thin DNA molecules are packaged into compact structures called chromosomes. This involves wrapping the DNA around histone proteins to form nucleosomes, which are further compacted into chromatin fibers.
Shorter Structures: Chromosome condensation allows the long DNA molecules to fit within the nucleus and facilitates their movement during cell division.

Chromosome Movement:

Microtubules: Microtubules, composed of tubulin proteins, form a spindle apparatus that extends from the poles of the cell.
Attachment: Microtubules attach to the chromosomes at the kinetochore, a protein structure on the centromere of each chromosome.
Separation: Microtubules pull the chromosomes towards opposite poles of the cell, ensuring that each daughter cell receives a complete set of chromosomes.

This coordinated process of chromosome condensation and movement is essential for both mitosis and meiosis, ensuring accurate distribution of genetic material to daughter cells.

D2.1.7—Phases of mitosis

Mitosis is a complex process that involves four distinct phases:

1. Prophase:

Chromatin condenses into visible chromosomes.
Nuclear envelope breaks down.
Spindle fibers begin to form.

2. Metaphase:

Chromosomes align at the equator of the cell.
Spindle fibers attach to the centromeres of chromosomes.

3. Anaphase:

Sister chromatids separate and are pulled towards opposite poles of the cell by spindle fibers.

4. Telophase:

Chromosomes decondense and nuclear envelopes reform around each set of chromosomes.
Cytokinesis occurs, dividing the cytoplasm and forming two daughter cells.

Each phase of mitosis is essential for ensuring that each daughter cell receives a complete and identical copy of the parent cell’s genetic material. This process is crucial for growth, repair, and asexual reproduction in eukaryotic organisms.

D2.1.8—Identification of phases of mitosis

To identify the phases of mitosis in a stained slide, you need to observe the following characteristics of the cells:

Prophase:

Chromosomes start to condense and become visible.
Nuclear envelope begins to break down.
Spindle fibers start to form.

Metaphase:

Chromosomes align at the equator of the cell.
Spindle fibers attach to the centromeres of chromosomes.

Anaphase:

Sister chromatids separate and move towards opposite poles of the cell.

Telophase:

Chromosomes reach the poles and¹ begin to decondense.
Nuclear envelopes reform around each set of chromosomes.
Cytokinesis occurs, dividing the cytoplasm and forming two daughter cells.

By carefully examining the stained slide under a microscope, you can identify cells in different stages of mitosis based on these characteristics.

Tips for Identifying Phases:

Chromosome Condensation: Look for the progressive condensation of chromosomes from prophase to metaphase.
Spindle Fiber Formation: Observe the formation and attachment of spindle fibers to chromosomes.
Chromosome Movement: Pay attention to the movement of chromosomes during anaphase.
Nuclear Envelope Breakdown: Look for the disappearance of the nuclear envelope during prophase and its reformation during telophase.
Cytokinesis: Identify the formation of the cleavage furrow in animal cells or the cell plate in plant cells.

D2.1.9—Meiosis as a reduction division

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing haploid gametes (sperm and egg cells). This process is essential for sexual reproduction, as it ensures that the offspring inherit the correct number of chromosomes.

Key stages of meiosis:

Interphase: The cell replicates its DNA, resulting in two identical copies of each chromosome (sister chromatids).
Meiosis I:
- Prophase I: Chromosomes condense, and homologous chromosomes pair up, forming tetrads. Crossing over occurs, exchanging genetic material between homologous chromosomes.
- Metaphase I: Homologous pairs of chromosomes line up at the equator of the cell.
- Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell.
- Telophase I:² Two daughter cells are formed, each with a haploid set of chromosomes.
Meiosis II:
- Prophase II: Chromosomes condense.
- Metaphase II: Chromosomes align at the equator of the cell.
- Anaphase II: Sister chromatids separate and move to opposite poles.³
- Telophase II: Four haploid daughter cells are formed.

Significance of Meiosis:

Genetic Diversity: Meiosis generates genetic diversity through crossing over and independent assortment of chromosomes, leading to unique combinations of genes in offspring.
Sexual Reproduction: Meiosis produces haploid gametes, which fuse during fertilization to form a diploid zygote. This process allows for the combination of genetic material from two parents, increasing genetic variation.

D2.1.10—Down syndrome and non-disjunction

Non-disjunction is the failure of chromosomes to separate properly during cell division, leading to cells with abnormal numbers of chromosomes. This can occur during either meiosis I or meiosis II.

Consequences of Non-disjunction:

Aneuploidy: The resulting cells may have an extra chromosome (trisomy) or a missing chromosome (monosomy).
Down Syndrome: Trisomy 21, caused by an extra copy of chromosome 21, is a common example of aneuploidy.
Other Aneuploidies: Other aneuploidies can affect sex chromosomes, leading to conditions like Klinefelter syndrome (XXY) and Turner syndrome (X).

Non-disjunction can occur during both meiosis I and meiosis II. If it occurs during meiosis I, both homologous chromosomes may move to the same pole, resulting in one daughter cell with an extra chromosome and the other with a missing chromosome. If it occurs during meiosis II, sister chromatids may fail to separate, leading to similar outcomes.

It’s important to note that non-disjunction is a random event and can occur in any chromosome. However, the effects of aneuploidy are often more severe for larger chromosomes.

D2.1.11—Meiosis as a source of variation

Random Orientation of Bivalents:

During metaphase I of meiosis, homologous pairs of chromosomes align randomly at the equator of the cell.
This random orientation leads to different combinations of chromosomes in the resulting daughter cells.
The number of possible combinations is 2^n, where n is the haploid number of chromosomes.

Crossing Over:

Crossing over involves the exchange of genetic material between homologous chromosomes.
It occurs during prophase I of meiosis when homologous chromosomes pair up and form tetrads.
The exchange of genetic material creates new combinations of alleles on chromosomes, further increasing genetic diversity.

Together, random orientation of bivalents and crossing over generate a vast number of possible genetic combinations in offspring, promoting genetic diversity and enabling adaptation to changing environments.

D2.1.12—Cell proliferation for growth, cell replacement and tissue repair

Cell proliferation is a rapid increase in the number of cells through cell division. This process is essential for multicellular organisms’ growth, development, and tissue repair.

Key points:

Growth: Cell proliferation is crucial for the growth of organisms. In animals, it occurs during embryonic development and juvenile phases. In plants, it takes place in specialized regions called meristems.
Cell Replacement: Cell proliferation replaces old, damaged, or dying cells, ensuring the continuous renewal of tissues.
Tissue Repair: After injury, cell proliferation plays a vital role in repairing damaged tissues.
Mitosis: The process of mitosis ensures the accurate distribution of genetic material to daughter cells, maintaining genetic continuity.

Specific Examples:

Animal Growth: During embryonic development, rapid cell division leads to the formation of tissues and organs. In juvenile animals, cell proliferation in growth plates contributes to bone growth.
Plant Growth: Apical meristems are responsible for the growth of plant roots and shoots. Cell division in these meristems generates new cells that differentiate into various tissues.

Cell proliferation is a fundamental process that underpins the development and maintenance of multicellular organisms.

D2.1.13—Phases of the cell cycle

The cell cycle is a series of events that a cell undergoes to grow, replicate its DNA, and divide. It consists of two major phases: interphase and mitosis.

Interphase:

G1 phase (Gap 1): The cell grows and carries out its normal functions.
S phase (Synthesis): DNA replication occurs, resulting in the¹ duplication of chromosomes.
G2 phase (Gap 2): The cell prepares for mitosis, synthesizing proteins and organelles.

Mitosis:

Prophase: Chromosomes condense, the nuclear envelope breaks down, and spindle fibers form.
Metaphase: Chromosomes line up at the equator of the cell, attached to spindle fibers.
Anaphase: Sister chromatids separate and move to opposite poles of the cell.
Telophase: Nuclear envelopes form around the separated chromosomes, and cytokinesis begins.

After mitosis, the cell cycle can either continue with another round of interphase and mitosis, or the cells can enter G0 phase, a non-dividing state.

D2.1.14—Cell growth during interphase

Interphase is a crucial phase in the cell cycle, during which the cell grows, replicates its DNA, and prepares for cell division. It is divided into three subphases.

G1 (Gap 1) Phase:

The cell grows in size and synthesizes proteins and organelles.
The cell also carries out its normal functions.

S (Synthesis) Phase:

The DNA is replicated, creating two identical copies of each chromosome.
This ensures that each daughter cell will receive a complete set of genetic information.¹

G2 (Gap 2) Phase:

The cell continues to grow and synthesize proteins needed for cell division.
The cell also checks for any errors in DNA replication and makes repairs if necessary.

Interphase is an active period in the cell cycle, with various metabolic processes taking place. It is essential for the proper functioning and development of cells.

D2.1.15—Control of the cell cycle using cyclins

The cell cycle is a tightly regulated process that ensures that cells divide in an orderly manner. Cyclins are a family of proteins that play a crucial role in controlling the progression of the cell cycle.

How Cyclins Work:

Cyclin-Dependent Kinases (CDKs): These are enzymes that are activated by cyclins.
Phosphorylation: CDKs phosphorylate other proteins, activating or inactivating them.
Cell Cycle Progression: The phosphorylation of specific proteins triggers the events of each phase of the cell cycle, such as DNA replication, chromosome condensation, and cell division.

The Role of Checkpoints:

Checkpoints are control points within the cell cycle that ensure proper cell division. They monitor the cell’s internal state and external conditions, such as DNA damage or nutrient availability. If conditions are unfavorable, the cell cycle can be paused until the issue is resolved.

The Importance of Cell Cycle Regulation:

Proper cell cycle regulation is essential for preventing uncontrolled cell growth, which can lead to cancer. Mutations in genes that control the cell cycle can cause cells to divide uncontrollably, forming tumors.

By understanding the mechanisms that regulate the cell cycle, scientists can develop strategies to prevent and treat cancer.

D2.1.16—Consequences of mutations in genes that control the cell cycle

Mutations in genes that control the cell cycle can lead to uncontrolled cell growth and the development of tumors. There are two main types of genes involved in this process:

1. Proto-oncogenes: These genes normally promote cell growth and division. When mutated, they can become oncogenes, which are hyperactive and drive uncontrolled cell proliferation.

2. Tumor-suppressor genes: These genes normally inhibit cell growth and division, acting as brakes on the cell cycle. Mutations in tumor-suppressor genes can lead to a loss of control over cell proliferation.

When both proto-oncogenes and tumor-suppressor genes are mutated, the cell cycle becomes dysregulated, resulting in the formation of tumors. This multi-step process is known as carcinogenesis.

Mutagens, such as certain chemicals and radiation, can increase the likelihood of mutations in these genes, contributing to cancer development. It’s important to be aware of these factors and take steps to minimize exposure to mutagens to reduce the risk of cancer.

D2.1.17—Differences between tumours in rates of cell division and growth and in the capacity for metastasis and invasion of neighbouring tissue

Benign Tumors:

Slow Growth: Benign tumors grow slowly and remain confined to their original location.
Cell Adhesion: Cells in benign tumors adhere to each other, preventing them from spreading to other tissues.
Limited Harm: Benign tumors generally do not cause significant harm unless they grow large enough to compress surrounding tissues or organs.

Malignant Tumors (Cancer):

Rapid Growth: Malignant tumors grow rapidly and invade surrounding tissues.
Loss of Cell Adhesion: Cancer cells can break away from the primary tumor and spread to other parts of the body through the bloodstream or lymphatic system.
Metastasis: The spread of cancer cells to distant sites, forming secondary tumors, is a hallmark of malignant tumors.

It is important to note that not all tumors are cancerous. Benign tumors, while they can cause problems, are not life-threatening. However, malignant tumors can be life-threatening if left untreated. Early detection and treatment of cancer are crucial for successful outcomes.

IB DP Biology Cell and nuclear division Study Notes | New Syllabus