Mitosis and Cell Division

• Mitosis results in two daughter cells, each with the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
• It is the process of nuclear division in eukaryotic cells.

Phases of Mitosis

• Mitosis is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.

Prophase

• Chromosomes condense and become visible.
• The nuclear envelope breaks down.
• The mitotic spindle forms from the centrosomes and microtubules.

Prometaphase

• The nuclear envelope completely breaks down.
• Microtubules from the mitotic spindle attach to the kinetochores on the chromosomes.

Metaphase

• Chromosomes align at the metaphase plate, the middle of the cell.
• Each chromosome is attached to a spindle fiber originating from opposite poles.

Anaphase

• Sister chromatids separate and are also pulled to opposite poles of the cell.
• The cell elongates during this phase.

Telophase

• Chromosomes arrive at the poles and begin to decondense.
• After the chromosomes arrive, the nuclear envelope reforms around each set of chromosomes.
• The mitotic spindle breaks down.
• Telophase typically overlaps with cytokinesis.

Cytokinesis

• Cytokinesis is the division of the cytoplasm to form two separate daughter cells.
• In animal cells, a cleavage furrow forms and pinches the cell in two.
• In plant cells, a cell plate forms and eventually becomes the new cell wall.

Significance of Mitosis

• Mitosis is essential for growth, repair, and asexual reproduction.
• Growth occurs because mitosis allows multicellular organisms to grow by increasing the number of cells.
• Damaged or dead cells are replaced through mitosis.
• In some organisms, mitosis is the basis of asexual reproduction, producing offspring that are genetically identical to the parent

Regulation of Mitosis

• The cell cycle, including mitosis, is tightly regulated by internal and external signals.
• Checkpoints ensure that the process is accurate.
• The G1 checkpoint checks for cell size, nutrients, growth factors, and DNA damage.
• The G2 checkpoint checks for DNA replication completeness and DNA damage.
• The M checkpoint, also known as the spindle checkpoint, checks for chromosome attachment to the spindle.

Errors in Mitosis

• Errors in mitosis can lead to cells with an abnormal number of chromosomes known as aneuploidy.
• Aneuploidy can result in genetic disorders or cancer.

Comparison with Meiosis

• Mitosis produces two diploid daughter cells, while meiosis produces four haploid daughter cells.
• Mitosis is for growth, repair, and asexual reproduction, while meiosis is for sexual reproduction.
• Mitosis involves one cell division, while meiosis involves two cell divisions.

Mitotic Spindle

• The mitotic spindle is a structure composed of microtubules, centrosomes, and associated proteins that is responsible for separating chromosomes during mitosis.
• Microtubules are polymers of tubulin that extend from the centrosomes to the chromosomes.
• Centrosomes are the microtubule-organizing centers in animal cells.
• Kinetochores are protein structures on chromosomes where microtubules attach.

Chromosome Structure

• A chromosome consists of DNA tightly coiled around histones.
• Sister chromatids are identical copies of a chromosome produced during DNA replication.
• The centromere is the region where sister chromatids are most closely attached.
• The kinetochore is the protein structure on the centromere where microtubules attach.

Cell Cycle

• The cell cycle is an ordered sequence of events in the life of a cell, including growth, DNA replication, and cell division.
• The cell cycle has two major phases: interphase and the mitotic (M) phase.
• Interphase includes the G1, S, and G2 phases.
• During the G1 phase the cell grows and performs normal functions.
• During the S Phase, DNA is replicated.
• During the G2 Phase, the cell prepares for mitosis.
• The M Phase includes mitosis and cytokinesis.

Plant Cell Mitosis

• Mitosis in plant cells is similar to that in animal cells, but there are some key differences.
• Plant cells lack centrosomes, but they still form a mitotic spindle.
• Cytokinesis in plant cells involves the formation of a cell plate, which becomes the new cell wall.

Control Mechanisms

• Cyclins and cyclin-dependent kinases, Cdks, are key regulatory molecules in the cell cycle.
• Cdks are enzymes that phosphorylate other proteins to regulate their activity.
• Cyclins bind to and activate Cdks, and their levels fluctuate during the cell cycle.

Mutations and Cancer

• Mutations in genes that regulate the cell cycle can lead to uncontrolled cell division and cancer.
• Proto-oncogenes are genes that promote cell growth and division.
• Tumor suppressor genes are genes that inhibit cell growth and division.

Research Applications

• Mitosis is a fundamental process in biology and is studied in various contexts.
• Understanding how mitosis is disrupted in cancer cells can lead to new therapies in Cancer Research.
• Mitosis is essential for embryonic development in developmental biology.
• Mitosis is used in cell culture and genetic engineering for biotechnology.

Variations in Mitosis

• While the general process of mitosis is conserved across eukaryotes, some variations do exist.
• Some organisms have variations in the duration or timing of specific phases of mitosis.
• Other organisms have variations in the structure of the mitotic spindle.

Spindle Assembly Checkpoint

• The spindle assembly checkpoint (SAC) is a critical control mechanism that ensures accurate chromosome segregation during mitosis.
• The SAC monitors the attachment of spindle microtubules to kinetochores.
• If chromosomes are not properly attached, the SAC prevents the cell from progressing to anaphase.

Kinetochore Function

• Kinetochores are complex protein structures that play a crucial role in chromosome segregation.
• They attach chromosomes to spindle microtubules.
• Kinetochores monitor microtubule attachment and tension.
• Proteins in the kinetochore signal the cell cycle to delay anaphase if attachments are incorrect.

Microtubule Dynamics

• Microtubules are dynamic structures that constantly polymerize and depolymerize.
• Dynamic instability is important for spindle assembly and chromosome movement.
• Microtubule-associated proteins, or MAPs, regulate microtubule dynamics.

Centrosome Maturation

• Centrosomes mature during prophase, increasing their microtubule-nucleating capacity.
• Maturation is regulated by various kinases and other proteins.
• Centrosome abnormalities can lead to mitotic errors.

Chromosome Condensation

• Chromosome condensation is essential for proper chromosome segregation.
• Condensins are protein complexes that play a key role in chromosome condensation.
• Defects in chromosome condensation can lead to mitotic errors.

Cohesion and Sister Chromatid Separation

• Cohesion is a protein complex that holds sister chromatids together until anaphase.
• During anaphase, cohesin is cleaved by separase, allowing sister chromatids to separate.
• Regulation of cohesin and separase is critical for accurate chromosome segregation.

Cell Cycle Arrest

• If problems are detected during mitosis, the cell cycle can be arrested.
• Allows time for repairs to be made before cell division continues.
• Cell cycle arrest is regulated by checkpoint pathways.