Cell Division and homeostasis

Course Information

• Who teaches it: Dr. Isabelle Miletich

• How to reach her: isabelle.miletich@kcl.ac.uk

• Where she works: King's College London, Centre for Craniofacial and Regenerative Biology

What You'll Learn

This course is about understanding how cells divide and why it's super important for keeping our body's tissues healthy. Here are the main things you'll understand:

1. How DNA is Organized into Chromosomes:

   • We'll look closely at special proteins called histones that act like spools for our long DNA strands.

   • Learn about the basic package of DNA wrapped around histones, called a nucleosome.

   • Understand the special jobs of centromeres (the constricted part of a chromosome where sister chromatids join) and telomeres (the protective caps at the ends of chromosomes).

   • See how the structure of this DNA-protein mix (called chromatin) affects which genes are turned on or off (dictates gene expression).

2. What a Karyotype Is:

   • We'll define what a normal set of chromosomes (a karyotype) looks like.

   • Learn the difference between haploid ($n$) (half a set, like in sperm or egg cells) and diploid

   • ($2n$) (a full set, like in most normal body cells, which are somatic cells).

   • Learn how to identify and understand diseased conditions:

     • Aneuploidy: When a cell has an irregular number of chromosomes (either gained or lost individual chromosomes), not an exact multiple of the haploid set (e.g., $2n+1$ or $2n-1$).

     • Heteroploidy: This is a more general term for any deviation from the normal number of chromosomes (the euploid number).

3. Mitosis (Cell Splitting):

   • We'll meticulously cover each distinct stage where a cell divides into two identical copies:

     • Prophase

     • Prometaphase

     • Metaphase

     • Anaphase

     • Telophase

   • Understand how mitosis fits into the bigger cell cycle, focusing on crucial control points (regulatory checkpoints) and the cell's internal machinery (molecular mechanisms).

4. How Cell Division Keeps Tissues Healthy:

   • Understand how controlled cell growth (proliferation) and cell death maintain the overall health and proper working of tissues (tissue integrity and function).

5. Why This Matters for Dentistry:

   • Learn about the direct impact of these processes on oral health, dental diseases, and the treatments used in dentistry (therapeutic interventions).

Key Concepts of Cell Division

Cell division involves several fundamental biological processes essential for life:

• Proliferation: This is the rapid increase in cell numbers, typically through mitotic cell division. It's crucial for normal growth, development, and repairing damaged tissues.

• Differentiation: This is the process where an unspecialized cell changes to become a more specialized cell type. These specialized cells have distinct looks (morphological structures) and specific jobs (physiological functions).

• Cell Death: This mainly happens through apoptosis (programmed cell death), a systematic way of removing old, damaged, or unnecessary cells to keep cell populations in balance.

• Cellular Homeostasis: This is the body's remarkable ability to maintain a stable and tightly regulated internal environment at the tissue and organ level.

Relevance to Dentistry

The principles of cell division and tissue homeostasis are profoundly relevant to dentistry:

1. Oral Tissue Development and Maintenance:

   • This includes the intricate processes of tooth development (odontogenesis).

   • The continuous renewal and repair of the lining of your mouth (oral mucosa) and gums (gingival tissues).

   • The constant rebuilding (remodeling) of jaw bone in response to biting forces or injuries.

2. Wound Healing and Regeneration:

   • These principles are critical for effective wound healing and the regrowth of tissues following dental procedures (e.g., tooth extractions, periodontal surgery, implant placement).

3. Cancer and Pathology:

   • Problems in the tightly regulated cell cycle are a key feature (hallmark) of oral cancers.

   • Understanding these disruptions allows for the development of specific drug strategies (targeted pharmacological strategies) aimed at particular phases of the cell cycle for cancer treatment (therapy).

The Nucleus

• Definition: The definitive hallmark (a defining characteristic) of eukaryotic cells, which encapsulates (stores) the organism's genetic material (DNA).

• Size: It's the largest and most prominent part of the cell, typically measuring between $5-8 \,\mu m$ (micrometers) in diameter.

• Membrane: It's crucially separated from the rest of the cell's contents (cytoplasm) by a double membrane system called the nuclear envelope, which has tiny holes called nuclear pores.

• Staining and Visualization:

  • Hoechst Stain: This special dye sticks to DNA, causing nuclei to glow a distinct blue color under ultraviolet light.

  • This allows scientists to observe cellular features like divided nuclei and tightly compacted chromosomes.

Genetic Material and Chromatin

• DNA Content: Each human nucleus contains approximately 2 meters of DNA. Individual human chromosomes contain DNA molecules ranging from $1.7 \, cm$ to $8.5 \, cm$ in length.

• Complexing with Proteins: DNA is intricately wrapped around various proteins, primarily histones, to form chromatin.

• This protein-DNA complex is the fundamental material that makes up chromosomes, allowing efficient packing (compaction) and regulated access to the genetic information.

Structure of Chromatin

1. Nucleosome Formation:

   • This is the fundamental repeating unit of chromatin, like a bead on a string.

   • Approximately 146 base pairs of DNA are wrapped nearly twice around a core made of an octamer (a group of eight) of histone proteins (H2A, H2B, H3, H4).

   • These nucleosomes are regularly spaced, separated by about 50 base pairs of linker DNA, appearing roughly every 200 base pairs.

2. 30 nm Fiber Structure:

   • The nucleosomes, with help from another linker histone H1 protein, coil up even further into a more condensed $30 \, nm$ (nanometer) fiber.

   • This fiber undergoes further looping and folding, anchoring to a non-histone chromosomal scaffold protein structure. This extensive packing (condensation) leads to the very compact and visible mitotic chromosomes during cell division.

Chromatin Types

• Heterochromatin:

  • This is DNA that is highly condensed and tightly packed.

  • It's mainly found in genetically inactive regions (e.g., centromeres, telomeres).

  • Generally, genes in heterochromatin are turned off (transcriptionally inactive).

• Euchromatin:

  • This is a more open and less condensed form of chromatin.

  • It's easier for messenger proteins (transcription factors) and the enzyme RNA polymerase to access these genes.

  • Genes in euchromatin are usually turned on (transcriptionally active), where the majority of gene expression occurs.

Gene Expression Regulation

• The dynamic interplay and reversible conversion between heterochromatin (closed) and euchromatin (open) plays a critical role in controlling which genes are active (gene expression regulation).

• The physical structure of chromatin directly controls how tightly packed it is locally (local chromatin compaction), thereby modulating (adjusting) how easily genes can be accessed for the process of copying genetic information into RNA (transcription).

Chromosomal Features

Essential Components of a Functional Chromosome

Every working eukaryotic chromosome needs two indispensable (absolutely essential) specialized regions:

• Centromere: This is the crucial constricted region that serves as the attachment point for sister chromatids (the two identical copies of a replicated chromosome) and the assembly site for the kinetochore.

• Telomere: These are specialized protective caps at the ends of chromosomes, consisting of tandem repeats of short, G-rich DNA sequences (e.g., GGGTTA in humans) and associated proteins.

Centromeres

1. Functions:

   • Holding sister chromatids together from S phase (when DNA is copied) until their precise separation during anaphase (a stage of cell division).

   • Serving as the assembly site for the kinetochore, a complex protein structure that mediates (helps with) attachment to mitotic spindle microtubules (the fibers that pull chromosomes apart).

2. Citations:

   • Kursel, L. E., & Malik, H. S. (2016). Current Biology.

Telomeres

• Role: They prevent chromosome fusion (chromosomes sticking together) and solve the 'end replication problem' inherent in linear DNA molecules.

• End Replication Problem: Due to the way DNA is copied, an RNA primer at the 5' end of the lagging strand template cannot be replaced, leading to progressive shortening of chromosome ends with each division.

• Telomerase Activity:

  • This is a unique reverse transcriptase enzyme that adds special hexanucleotide (six-nucleotide: GGGTTA) repeats to telomeres, extending them.

  • It is highly active and found primarily in germ cells (sperm and egg cells).

  • Generally inactive or expressed at very low levels in most somatic cells (body cells), leading to telomere shortening.

  • This shortening in somatic cells limits their ability to divide (replicative lifespan), triggering cellular senescence (cells stop dividing and age) or apoptosis (programmed cell death).

  • Telomerase is often abnormally re-expressed or reactivated in many cancer cells, which helps them divide indefinitely (unlimited proliferative potential).

Telomeres and Cell Division

• Role as Mitotic Clock: The progressive shortening of telomeres in somatic cells acts as a signal, indicating the number of times a cell has divided.

• Cessation of Division: When telomeres become critically short, they trigger a DNA damage response, signaling the cell to stop dividing and enter either senescence or apoptosis.

• Telomerase Levels:

  • High: In cells that need extensive division potential (e.g., germ cells, embryonic stem cells, rapidly dividing adult stem cells).

  • Low/Undetectable: In most differentiated somatic cells, limiting their ability to replicate and contributing to the aging process.

Chromosome Visibility

• During interphase (the period between cell divisions), chromosomes exist as decondensed chromatin and are not visible under a light microscope.

• During M phase (mitosis), they undergo dramatic condensation (become tightly packed) into highly compact, rod-like structures, becoming distinctly visible.

• This condensation is essential for accurate and efficient separation (segregation) to daughter cells.

Karyotype

1. Definition and Human Karyotype:

   • Definition: A characteristic snapshot of an organism's chromosome complement, defined by their number, size, and morphological shape, which is specific to each species.

   • Human Karyotype: A total of 46 chromosomes (23 homologous pairs).

     • 22 pairs of autosomes (non-sex chromosomes).

     • 1 pair of sex chromosomes (XX for females or XY for males).

2. Staining Techniques:

   • Specialized techniques (e.g., G-banding) create distinct banding patterns along chromosomes.

   • This allows scientists (cytogeneticists) to accurately identify homologous pairs and detect structural abnormalities (like deletions, duplications, or translocations).

3. Chromosome Origins:

   • Each homologous pair consists of one chromosome from the maternal parent and one from the paternal parent.

   • Meiosis (a type of cell division) produces haploid gametes (sperm and egg cells).

   • Fertilization (when sperm and egg combine) restores the diploid status, ensuring genetic diversity.

Polyploidy and Aneuploidy

1. Polyploidy:

   • Cells possess more than the typical diploid ($2n$) chromosome number (e.g., $3n$ or $4n$).

   • This results from DNA replication without subsequent cell division, or from problems during meiosis.

   • It's lethal (deadly) in humans, but common in plants.

2. Aneuploidy:

   • Cells have an abnormal chromosome count (gained or lost individual chromosomes), not an exact multiple of the haploid set (e.g., $2n+1$ or $2n-1$).

   • It's associated with severe developmental conditions (e.g., Down's syndrome, caused by trisomy 21, meaning three copies of chromosome 21).

   • Dental Implications: Can show up as delayed tooth eruption and various dental anomalies affecting tooth size, shape, or number.

Cell Cycle and Mitosis

1. Duration and Interphase Phases:

   • Mitosis and Cytokinesis: Typically about 1 hour to complete in mammalian cells.

   • Interphase: The period between two successive mitotic divisions.

     • G1 Phase ('Gap 1'): Initial growth phase. The cell grows, makes proteins, and carries out its normal metabolic functions.

     • S Phase ('Synthesis'): This is the critical stage where DNA replication occurs, duplicating all chromosomal DNA.

     • G2 Phase ('Gap 2'): Follows S phase. The cell continues to grow, synthesizes proteins and organelles needed for division, and prepares for M phase.

2. Terminology Clarification:

   • G1 and G2 phases are accurately referred to as "Gap" phases, not "Growth" phases, which is a common mislabeling.

Cell Cycle Regulation

• The cell cycle is primarily regulated during the G1 phase, influenced by protein growth factors (e.g., PDGF, EGF, FGF).

• Lack of sufficient growth factors may cause cells to exit the cycle and enter a resting state (quiescent state or G0 phase).

• Restriction Point (R): This is a critical decision point in G1, marking the cell's commitment to complete the cell cycle.

M Phase: Mitosis and Cytokinesis

1. Mitosis (Karyokinesis): This is the separation of daughter chromatids and the division of the nucleus.

2. Cytokinesis: This is the division of the cytoplasm, helped by a contractile ring formed by actin filaments and dependent on microtubules.

Stages of Mitosis

1. Prophase: Chromosomes condense (become tightly packed), becoming progressively more visible.

2. Prometaphase: The nuclear envelope breaks down; chromosomes attach to the mitotic spindle.

3. Metaphase: Chromosomes align precisely along the spindle at the metaphase plate (the cell's equator).

4. Anaphase: Sister chromatids separate and move to opposite poles of the cell; cytokinesis often initiates.

5. Telophase: The spindle apparatus disappears; the nuclear envelope reforms around the separated chromosomes; chromosomes decondense (loosen up).

How Often Do Cells Divide?

Human cells divide at varying rates:

• Somatic Cells: Constantly replicate and divide, balancing with the steady loss of old cells to maintain tissue integrity.

• Somatic Stem Cells: These are special cells that can differentiate (change) into various cell types, contributing to tissue repair and renewal.

• Differentiated Cells: Many often exit the division cycle permanently or divide only infrequently.

Skin Stem Cell Dynamics

1. Location in Skin: Stem cells are found within the stratified epithelium (the outer layers of skin), making up 2-7% of cells in the basal layer (the deepest layer).

2. Skin Cell Cycle: A dynamic process:

   • New skin cells are created approximately every 14 days.

   • Mature cells push older cells to the surface, where they deteriorate and flake off.

3. Age-Related Changes in Turnover: The rate at which skin cells are replaced (turnover) varies significantly throughout life, with noticeably longer turnover times observed as individuals age.

Stem Cell Progeny

1. Homeostasis vs. Injury:

   • Stem cells divide (proliferate) to maintain tissue homeostasis under normal conditions.

   • They also respond to injury by moving to damaged areas and proliferating to help with healing.

2. Cell Types: Different types of progeny cells (daughter cells) result from stem cells differentiating in response to normal homeostasis versus injury conditions.