Mitosis is the process of nuclear division whereby one nucleus divides to form two genetically identical daughter nuclei. It is a critical component of the cell cycle, ensuring genetic stability across generations of cells.
Phases of the Cell Cycle
Interphase: An active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication (S phase), and an increase in the number of mitochondria and/or chloroplasts.
G₁: Cell growth and normal metabolic roles.
S: DNA replication occurs.
G₂: Preparation for mitosis; replication of organelles.
M-Phase: Mitosis and cytokinesis.
Cells under light microscopyDNA replication during S-phase
⚡ HL Extension: Cyclins & Cell Cycle Control
Cyclins are a family of proteins that control the progression of cells through the cell cycle. Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches its threshold concentration.
Cyclins bind to enzymes called cyclin-dependent kinases (CDKs). These kinases then become active and attach phosphate groups PO₄³⁻ to other proteins in the cell, triggering them to carry out tasks specific to one of the phases of the cell cycle.
🔬 Application: Tumor Formation
Tumors are the result of uncontrolled cell division, which can occur in any organ or tissue. Mutagens, oncogenes, and metastasis are involved in the development of primary and secondary tumors.
2. Key Terminology (Interactive)
Hover over the cards to reveal the definitions of crucial terms for this topic.
Chromatin
Uncoiled, loosely packed DNA wrapped around histones, present during interphase. Allows machinery access for transcription.
Sister Chromatids
Two identical DNA molecules formed by DNA replication, held together at the centromere until anaphase.
Cytokinesis
The division of the cytoplasm to form two daughter cells, occurring after mitosis. (Cleavage furrow in animals; cell plate in plants).
3. Meiosis: Reduction Division (Core & HL)
Meiosis is a reduction division of a diploid nucleus to form four haploid nuclei. It involves two divisions: Meiosis I (separates homologous chromosomes) and Meiosis II (separates sister chromatids).
Creating Genetic Variation
Meiosis is fundamental for sexual reproduction and is the primary driver of genetic variation in offspring.
Crossing Over: During Prophase I, non-sister chromatids of a homologous pair exchange genetic material.
Random Orientation: During Metaphase I, the orientation of bivalents (homologous pairs) at the equator is random.
🧮 Skill: Calculating Genetic Variation
The number of possible chromosome combinations due to random orientation is given by the formula $2^n$, where $n$ is the haploid number of chromosomes.
For humans ($n = 23$), the number of possible gametes is $2^{23} \approx 8.38 \times 10^6$. When fertilisation occurs, the possible zygote combinations are $(2^{23})^2 \approx 70$ trillion!
⚡ HL Extension: Chiasmata Formation
Crossing over occurs at points called chiasmata. At these points, homologous non-sister chromatids break and recombine. This results in the formation of recombinant chromatids with novel allele combinations.
Mendel's Law of Independent Assortment states that alleles of two (or more) different genes get sorted into gametes independently of one another. This applies to unlinked genes and is physically explained by the random orientation of bivalents in Metaphase I.
4. Non-Disjunction & Karyotyping
Non-disjunction is the failure of chromosomes to separate properly during anaphase I or anaphase II of meiosis. This results in gametes with an abnormal number of chromosomes (aneuploidy).
Amniocentesis & CVS are used for prenatal karyotyping
Down Syndrome (Trisomy 21)
The most common viable aneuploidy in humans is Down Syndrome, caused by three copies of chromosome 21. The risk of non-disjunction increases exponentially with maternal age.
🔬 Application: Karyogram Analysis
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. Karyograms can be used to deduce sex (XX or XY) and diagnose conditions caused by chromosomal abnormalities.
Check Your Understanding
Test your knowledge of D2.1 Cell and Nuclear Division. Click the questions to reveal the answers.
What is the difference between anaphase of mitosis and anaphase I of meiosis?
In anaphase of mitosis, sister chromatids are pulled apart to opposite poles. In anaphase I of meiosis, homologous chromosomes are pulled apart, but the sister chromatids remain attached at their centromeres.
Explain how cyclins control the cell cycle. (HL focus)
Cyclins bind to and activate Cyclin-Dependent Kinases (CDKs). Once activated, CDKs phosphorylate target proteins, altering their activity to drive the cell into the next phase of the cell cycle. Different cyclins peak at different stages (e.g., Cyclin D triggers G1 to S phase transition).
Calculate the number of possible chromosome combinations in a gamete of an organism where 2n = 14. Show your working.
The diploid number ($2n$) is $14$, so the haploid number ($n$) is $7$.
Using the formula $2^n$:
Combinations $= 2^7 = 128$.
How does crossing over lead to genetic variation?
Crossing over (in Prophase I) involves the exchange of segments of DNA between non-sister chromatids of homologous chromosomes. This creates recombinant chromosomes containing new combinations of alleles that were not present in either parent.
