Study Notes on The Mitotic Cell Cycle and Related Topics

In the nucleus of each cell, DNA is packaged into thread-like structures called chromosomes.
- Each chromosome is made up of DNA coiled many times around proteins known as histones, which support its structure.

Chromosomes are not visible in the cell’s nucleus when the cell is not dividing.
- During cell division, DNA becomes more tightly packed, making chromosomes visible under a microscope.

Each chromosome features a centromere, which is a constriction point dividing the chromosome into two sections or “arms.”
- The short arm is labeled the p arm and the long arm is the q arm.
- The centromere's location contributes to the chromosome's characteristic shape and aids in gene location description.

A chromatid is one of the two identical strands of DNA that comprise a chromosome.
- Two chromatids are joined by a centromere.
- Before replication, chromosomes feature one DNA molecule; after the S phase of interphase, they consist of two DNA molecules.
- The identical copies of chromatids are known as homozygous, but they may contain slight mutations (termed heterozygous).

Telomeres are protective caps at the ends of chromosomes consisting of repetitive nucleotide sequences.
- They protect chromosomes from deterioration or fusion with neighboring chromosomes, essential for the integrity and stability of eukaryotic genomes.
- DNA replication cannot extend to the end of the chromosome, leading to gradual telomere shortening during cell division, which shields genes from truncation.

Mitosis is a nuclear division process that produces genetically identical cells, where the chromosome number remains constant.

Production of genetically identical cells - It maintains chromosome number and genetic stability in daughter cells.
Growth - A single cell divides to produce cells for the adult organism.
Repair of tissue and cell replacement - New cells replace damaged or old cells.
Asexual reproduction - It enables single parents to produce genetically identical offspring.

Mitosis specifically refers to the division of the nucleus; cytokinesis is the subsequent division of the cell.

The cell cycle is a continuous cycle of growth and mitotic division that consists of two major phases: Interphase and Mitotic Phase.

Interphase occurs between mitotic events and comprises three stages:
- G1 Phase (Gap 1): Cells monitor their environment, synthesizing RNA and proteins to promote growth.
- S Phase (Synthesis): DNA replication occurs; each chromosome, initially having 1 DNA molecule, now contains 2 identical DNA molecules (2 chromatids connected at the centromere).
- G2 Phase (Gap 2): Cells grow and prepare for mitosis; organelles such as mitochondria and chloroplasts are replicated.

The mother cell divides into two genetically identical daughter cells.
- Mitosis (Nuclear Division):
- The two chromatids split and move to opposite ends of the cell.
- A new nuclear envelope surrounds each group, producing 2 identical nuclei from one parent nucleus.
- Cytokinesis (Cell Division): The cell divides into 2 daughter cells which are genetically identical to each other and their parent cell.

Prophase:
- Nuclear membrane breaks down, forming small vesicles; the nucleolus disappears.
- Centrosome duplicates and moves to opposite ends; mitotic spindle formation begins.
- Chromosomes coil and are visible under a light microscope; each consists of 2 chromatids.
Metaphase:
- Chromosomes align at the metaphase plate; centromeres migrate to the cell's midpoint.
- Kinetochore formation occurs at each side of the centromere for spindle attachment.
Anaphase:
- Centromeres divide, the sister chromatids are pulled apart and move to opposite ends of the cell.
- These separate chromatids become daughter chromosomes.
Telophase:
- The nuclear membrane reforms around chromosomes at the poles;
- Chromosomes uncoil and diffuse, and spindle fibers disappear.
- Cytokinesis may follow, dividing the cell by membrane infolding in animal cells or forming a new cell wall in plant cells.

Cell division is regulated by genes that dictate when a cell should divide; improper control can lead to issues such as:
- Under-dividing: ceasing to grow or heal
- Over-dividing: tumor formation

Stem Cells: Undifferentiated cells capable of differentiating into specialized cells while maintaining the ability to produce more stem cells.
- Found during embryonic development and adult organisms, serving as a repair mechanism in tissues.
- Stem cells divide by mitosis to generate either two stem cells (expanding the stem cell pool) or one daughter that differentiates while the other retains stem cell properties.

Haploid cells contain one complete set of chromosomes, primarily associated with gametes or sex cells.
- They are produced from diploid cells via meiosis, resulting in daughter cells with half the original chromosome number.
- In humans, fusion of sperm and egg (each haploid with 23 chromosomes) forms a diploid zygote with 46 chromosomes.

Diploid cells contain two complete sets of chromosomes (one from each parent), totaling 46 chromosomes.
- These cells reproduce via mitosis, generating identical copies.
- Meiosis reduces diploid cells to haploid cells and occurs in the testes and ovaries before fertilization.