Cell Division Overview

Cell Division - Overview

• Cell division is an essential characteristic of life and is key to the continuity of life.
• Unicellular organisms use cell division for reproduction.
• Multicellular organisms rely on cell division for growth, development, and repair.
• Cell division forms part of the cell cycle, the process that encompasses the life of a cell from its formation to its division.
• Most cell division results in daughter cells containing identical genetic information as the parent cell.
• Meiosis is a specialised cell division process resulting in gametes (sperm and eggs), which contain half the number of chromosomes of the parent cell.

Organization of Genetic Material

• Each cell's collective DNA is known as its genome.
• Prokaryotes typically have a single DNA molecule, while eukaryotes have multiple.

Eukaryotic Chromosomes

• DNA is packaged into chromosomes inside eukaryotic cells.
• Chromosomes consist of chromatin: a combination of DNA and proteins.
• Chromatin condenses during cell division.
• Each species has a specific number of chromosomes within its cells.
• Human somatic cells (non-reproductive cells) have two sets of chromosomes (46 in total).
• Human gametes (reproductive cells) have one set of chromosomes (23 in total).

Chromosomes and Cell Division

• In preparation for cell division, DNA is replicated and chromosomes condense.
• Duplicated chromosomes are made of two sister chromatids which are joined copies of the original chromosome.
• Sister chromatids are linked by cohesin proteins.
• The centromere is the narrow constricted region where the two chromatids are most tightly bound.
• Sister chromatids separate during cell division, moving into two separate nuclei, and are then called chromosomes.

Types of Cell Division

• Eukaryotic cell division consists of mitosis and cytokinesis.
• Mitosis is the division of the nucleus.
• Cytokinesis is the division of the cytoplasm.
• Meiosis is a variation of cell division that produces gametes (sperm and eggs).
• Meiosis creates non-identical daughter cells with one set of chromosomes, half the number of the parent cell.

Cell Cycle Phases

• The cell cycle has two phases: Interphase and Mitotic (M) phase.
• Interphase constitutes 90% of the cell cycle.
• Interphase includes three sub-phases: G1 (first gap), S (DNA synthesis), and G2 (second gap).
• Cellular growth occurs during all three sub-phases of interphase, but DNA replication only takes place during S phase.

Mitosis: Five Phases

• Mitosis is a continuous process but can be broken down into 5 distinct stages:
  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase

Interphase G2

• The nuclear envelope is intact.
• The nucleolus is present, serving as the site for rRNA synthesis and ribosomal subunit assembly.
• Replicated chromosomes are present, but not yet condensed.
• Two centrosomes, each containing two centrioles, are present.

Prophase

• The nucleoli disappear.
• The nuclear envelope remains intact.
• Chromosomes begin to condense and sister chromatids become visible.
• Centrosomes move to opposite poles and begin forming the mitotic spindle.

Prometaphase

• The nuclear envelope breaks down.
• Highly condensed chromosomes interact with microtubules.
• Kinetochores, protein complexes that connect centromeres to microtubules, appear.
• Microtubules extend from the centrosomes and attach to chromosomes at kinetochores (kinetochore microtubules).
• Microtubules also extend from the centrosomes to interact with other microtubules from the opposite pole (non-kinetochore microtubules).

Metaphase

• The nuclear envelope is absent.
• Chromosomes align in the middle of the cell along the metaphase plate.
• Centrosomes remain at opposite poles.
• The spindle apparatus is formed by mitotic spindles, with kinetochore microtubules interacting with kinetochores on chromosomes and non-kinetochore microtubules from opposing poles interacting with each other.

Anaphase

• The nuclear envelope is absent.
• Sister chromatids separate and move to opposite poles as individual chromosomes.
• Centromeres lead the way during separation.
• Kinetochore microtubules shorten, while non-kinetochore microtubules lengthen.

Telophase & Cytokinesis

• Two daughter nuclei begin to form.
• The nuclear envelope reforms.
• The nucleolus reappears.
• Chromosomes decondense.
• One centrosome is present in each daughter cell.
• Cytokinesis: The formation of a cleavage furrow pinches the cell in half.

Mitotic Spindle

• The mitotic spindle is a structure for the organisation of microtubules, which control chromosome movement during mitosis.
• Spindle microtubules begin to assemble at the centrosome (microtubule organizing center) in animal cells.
• Two centrosomes, which replicate during interphase, move to opposite ends of the cell during prophase and prometaphase.

Cytokinesis in Animal Cells

• Involves the formation of a cleavage furrow.
• The cleavage furrow is created by a contractile ring of actin microfilaments.

Cytokinesis in Plant Cells

• Involves the formation of a cell plate.
• The cell plate forms from vesicles carrying cell wall materials.

Binary Fission

• Prokaryotic cells (bacteria and archaea) reproduce through binary fission.
• The chromosome replicates, starting at the origin of replication, and the two daughter chromosomes move apart.
• The plasma membrane pinches inward, dividing the cell into two.

Regulation of the Cell Cycle

• The frequency of cell division varies depending on the type of cell.
• Cellular differences are regulated by molecular signals.
• These signals are thought to be specific chemical signals present in the cytoplasm.

Evidence for Cytoplasmic Signals

• Fusion of an S phase cell with a G1 phase cell: The G1 cell enters S phase and begins DNA synthesis.
• Fusion of an M phase cell with a G1 phase cell: The G1 cell's chromosomes condense without DNA replication.
• Fusion of an M phase cell with a G1 phase cell: The G1 cell forms a spindle apparatus.

Cell Cycle Control System

• There is a distinct cell cycle control system that governs the order of events in the cell cycle.
• The cell cycle control system is regulated both internally and externally.
• Checkpoints mark specific points in the cell cycle where it is paused until a go-ahead signal is received.

Checkpoints

• Three key checkpoints in the cell cycle regulate progression: G1 checkpoint, G2 checkpoint, and M checkpoint.
• If the go-ahead signal is received at G1, the cell usually completes the S, G2 and M phases and divides.
• If the go-ahead signal is not received at G1, the cell will exit the cell cycle and enter G0, a non-dividing phase.

Checkpoints

• Go-ahead signals at checkpoints are regulated by the interaction of two types of regulatory proteins: cyclins and cyclin-dependent kinases (Cdks).

Cyclins & Cyclin-Dependent Kinases

• Cdks act as the go-ahead signal, but only when attached to a cyclin protein.
• MPF (Maturation-promoting factor) is an example of a cyclin-Cdk complex.
• MPF triggers the cell's transition from G2 to the M phase.
• The activity of cyclin-Cdk complexes fluctuates throughout the cell cycle.

MPF and G2 Checkpoints

• Cyclin accumulates during late S phase.
• Cyclin binds with a Cdk creating active MPF.
• MPF triggers the go-ahead signal at G2 to enter the M phase.
• Cyclin is degraded at the end of M phase to stop mitosis.
• Cdk is recycled and ready to bind to a new cyclin during the next S phase.

Internal Checkpoint Signals

• Checkpoint signals originate from both inside and outside the cell.
• A non-attached kinetochore to a microtubule is an example of an internal signal.
• This lack of attachment sends a signal to delay anaphase.

External Checkpoint Signals

• External signals include various factors that stimulate cell division:
  • Growth factors
  • Anchorage dependence
  • Density-dependent inhibition

Growth Factors

• Proteins that stimulate cell division.
• For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells.

Density-Dependent Inhibition and Anchorage Dependence of Cell Division

• Most animal cells exhibit anchorage dependence and need to be attached to a surface to divide.
• Crowded cells exhibit density-dependent inhibition, where they stop dividing when in close contact with other cells.
• Density-dependent inhibition is demonstrated in cultured human fibroblast cells.

Cancer

• Cancer cells do not exhibit density-dependent inhibition or anchorage dependence.
• Cancer cells do not respond normally to the control mechanisms in the cell cycle.
• For example, they do not cease dividing when growth factors are removed and ignore normal cell cycle checkpoints.
• Cancer cells form tumors: masses of abnormal cells within normal tissue.

Benign Tumours

• If abnormal cells remain in the original site, the lump is called a benign tumor.
• Benign tumors usually grow slowly.

Malignant Tumours and Metastasis

• Malignant tumors (cancer) invade surrounding tissues, a process called metastasis.
• Cancer cells first spread to adjacent regions.
• Cells may spread via the lymphatic system or bloodstream.

Cancer

• Metastasis spreads cancer cells to other parts of the body, leading to additional tumors.
• Localized tumors can be treated with high-energy radiation, which damages the DNA of cancer cells.
• Chemotherapies that target the cell are used to treat metastatic cancers.

Explore the fundamental concepts of cell division, including its importance in unicellular and multicellular organisms. This quiz covers the cell cycle, genetic material organization, and the role of meiosis in reproduction. Test your understanding of how DNA is organized within eukaryotic cells.