The Mitotic Cell Cycle - Study Notes
5.1 Replication and division of nuclei and cells
Overview
Division of existing cells gives rise to new cells in all organisms.
In unicellular organisms, cell division is the process by which the organism replicates (asexual reproduction).
In multicellular organisms, cell division enables normal turnover of cells and is essential for growth and development.
Nuclear division combined with cell division allows reproduction of cells and organisms.
Just before eukaryotic cell division, thread-like structures become visible in the nucleus: chromosomes.
5.1.1 Chromosome structure
Chromosomes are easily seen because they stain intensely; etymology: chromo = coloured, somes = bodies.
In eukaryotic cells, chromosomes are long, condensed DNA molecules associated with proteins.
Main proteins: histones (large, positively charged globular proteins) that organize and condense DNA to fit the nucleus.
Other proteins include enzymes used in copying and repairing DNA.
The condensed DNA-protein complex is called chromatin; chromatin makes up chromatids and chromosomes.
During interphase (S phase), DNA replicates to produce two identical strands of DNA called chromatids, joined at the centromere.
These two chromatids form a double structure of a chromosome and are known as sister chromatids.
Sister chromatids are identical (carry the same genes); during mitosis, one chromatid goes to each daughter cell, ensuring genetic identity.
Each chromatid is made of one long, condensed DNA molecule containing a sequence of genes.
Telomeres: protective structures at the ends of chromatids; ends of chromosomes are sealed.
The number of chromosomes is species-specific: e.g., humans have 46 chromosomes; fruit flies have 8.
Important concepts:
Chromatin = DNA + proteins (primarily histones) that package DNA.
Centromere: narrow region where sister chromatids are held together and where kinetochores attach during mitosis.
5.1.2 Mitosis
Mitosis produces two genetically identical daughter nuclei, identical to the parent nucleus.
Significance:
Growth of multicellular organisms: daughter cells are clones with the same chromosome number as the parent.
Growth can be whole-body or localized (e.g., plant meristems).
Replacement and repair: damaged tissues are replenished by genetically identical cells (e.g., skin and gut lining in humans).
Asexual reproduction: single parent produces offspring that are genetic clones (e.g., Amoeba);
in multicellular organisms, offspring may bud off from the parent (e.g., Hydra, yeast, runners from strawberries).
5.1.3 The cell cycle
Duration varies with environmental conditions and cell type; examples:
Onion root tip cells: division roughly every 20 ext{ h}.
Human intestinal epithelial cells: division roughly every 10 ext{ h}.
In animal cells, cytokinesis involves constriction of cytoplasm between two new nuclei (cleavage).
In plant cells, cytokinesis involves formation of a new cell wall between the two new nuclei.
Three main phases:
Interphase: G1, S, G2
Nuclear division
Cytokinesis
Interphase details:
G1: cells synthesize RNA, enzymes, and other proteins needed for growth; at the end, the cell commits to dividing or not.
S: DNA replication occurs; each chromosome consists of two identical chromatids.
G2: cell continues to grow; DNA checked and repaired; preparations for division.
M phase: Nuclear division; two nuclei form.
Growth stops temporarily during mitosis; cytokinesis completes the process, yielding two genetically identical cells.
5.1.4 Telomeres
Telomeres are formed at chromosome ends; composed of DNA with short repeated base sequences.
One strand is rich in guanine (G), the complementary strand rich in cytosine (C).
Function: ensure ends of DNA are included in replication; copying enzymes cannot reach the extreme ends, risking loss of information if not completed.
With each division, a small portion of information could be lost if ends were not preserved.
Telomerase enzyme adds bases at ends (telomeres) to act as a buffer, preventing loss of vital genes and enabling continued replication.
Some cells (often specialised cells) lack telomerase, so telomeres shorten over divisions, which is linked to aging.
5.1.5 Stem cells
A stem cell can divide by mitosis an unlimited number of times.
Each new cell has the potential to remain a stem cell or differentiate into a specialised cell (e.g., blood cell, muscle cell).
Potency: the ability of stem cells to differentiate into other cell types.
5.1.6 Cancer
Cancers illustrate why precise control of cell division is crucial; cancers arise from uncontrolled mitosis.
Cancer cells can divide far more times than normal due to expression of telomerase, which maintains telomeres.
Tumour: irregular mass of cancerous cells resulting from uncontrolled division.
Mutations: changes in genes that control cell division. A mutated gene that causes cancer is called an oncogene.
Most mutations are harmless or lead to cell death or immune destruction; cancer-causing mutations escape these controls and are inherited by descendants of the original cell.
A typical tumour contains around 10^9 cancerous cells by detection.
Carcinogens: agents that cause cancer (e.g., UV light, tobacco tar, X-rays); if an agent causes cancer, it is described as carcinogenic.
Tumour types:
Benign tumours: do not spread (non-cancerous).
Malignant tumours: spread (cancer) and invade/destroy tissues.
Malignant tumours and metastasis:
Metastasis is the spread of cancer cells to distant sites via blood or lymphatics, forming secondary tumours.
Metastasis makes cancers particularly dangerous and hard to detect and treat.
5.2 Chromosome behaviour in mitosis
5.2.1 The stages of mitosis
Mitosis yields two genetically identical daughter nuclei; stages: Prophase, Metaphase, Anaphase, Telophase.
To illustrate, diagrams often show an animal cell with a small number of chromosomes (e.g., 4) colored to show parental origin.
General sequence and key events:
Prophase: chromosomes condense and become visible; each chromosome consists of two identical sister chromatids joined at the centromere; centrosomes (replicated in G2) move to opposite poles; spindle fibres emerge; nuclear envelope breaks down.
Metaphase: centrosomes at opposite poles; spindle fibres attach to kinetochores at the centromeres; chromosomes align at the equator (metaphase plate), equidistant from the two poles.
Anaphase: sister chromatids separate at the centromere; centromere divides; spindle fibres shorten; sister chromatids (now chromosomes) are pulled to opposite poles.
Telophase: chromosomes arrive at the poles and decondense; nuclear envelopes reform around each set of chromosomes; spindle fibres break down.
5.2.2 Observing mitosis in root tips
Plant growth occurs in meristems; root tip meristem is a zone of active mitosis just behind the protective root cap.
Methods to study mitosis in root tips:
Use pre-prepared slides or prepare temporary slides with squash technique.
Example: Allium cepa (garlic/onion) root tips are commonly used; bulbs can be encouraged to grow roots by suspending over water for 1–2 weeks.
Procedure:
Cut ~1 cm from root tip and place in a suitable stain (e.g., warm, acidified acetic orcein) to stain chromosomes a deep purple.
Gently squash the stained root tip on a glass slide with a blunt instrument to spread cells.
Observe cells undergoing mitosis and draw them; annotate to indicate stages.
Micrographs can show onion root tip cells at different stages of mitosis.
5.2 Chromosome behaviour in mitosis (summary and practical notes)
Mitosis ensures genetic continuity between parent and daughter cells; the process is tightly coordinated with the cell cycle.
Key terminology:
Chromosome: Condensed DNA-protein complex carrying genetic information.
Chromatid: one of the two identical DNA copies formed during DNA replication; sister chromatids are the two copies of the same chromosome.
Centromere: region where sister chromatids are held together and where spindle fibres attach during mitosis.
Telomere: protective end sequence of a chromosome.
Telomerase: enzyme that extends telomeres to prevent end-loss and support continued division in certain cells.
Metaphase plate: imaginary plane where chromosomes align during metaphase.
Kinetochore: protein structure at the centromere where spindle microtubules attach.
Oncogene: mutated gene that can promote cancer when altered.
Practical implications:
Understanding mitosis helps explain tissue growth, regeneration, and the basis of many cancer therapies that target dividing cells.
Root-tip observation provides a hands-on approach to studying the stages of mitosis and the effects of staining on visualizing chromosomes.