Introduction to Mitotic Cell Division and Bacterial Binary Fission

Conceptual Framework:
- Cell division is categorized into two primary eukaryotic types: mitosis and meiosis.
- Though traditionally taught together due to shared terminology and procedural steps, mitosis and meiosis serve vastly different biological functions.
- Mitosis: Focused on creating identical copies of cells.
- Meiosis: Discussed in subsequent lectures as a distinct process for sex cell production.
- Binary Fission: The specific type of cell division utilized by bacteria (prokaryotes).

Reproduction in Unicellular Organisms:
- For single-celled organisms like the amoeba, mitotic division is the primary method of reproduction.
- It allows a cell to split into two, effectively resetting the biological clock if the parent cell is "wearing out."
Growth in Multicellular Organisms:
- Organisms do not grow by increasing the size of individual cells; they grow by adding more cells through mitosis.
- Humans consist of approximately trillions of cells ( $10^{12}$ ), all generated via mitotic divisions from a single starting cell.
- This process allows for the development of specialized organs and organ systems.
Tissue Renewal and Repair:
- Mitosis allows for the replacement of damaged or lost cells.
- A primary example is the human skin, which undergoes constant cell division to replace the outer layers lost to the environment.

Definition: Binary fission literally translates to "splitting in two" (binary = two; fission = split).
Simplicity and Speed:
- Bacteria lack the elaborate internal structures (like a nucleus) found in eukaryotes, meaning they do not use the complex spindle systems of mitosis.
- Bacteria typically possess only one circular chromosome.
The Process:
1. The single chromosome is replicated.
2. The cell splits into two "daughter cells."
3. Each daughter cell receives one copy of the DNA molecule.
Exponential Growth:
- Bacteria can divide very rapidly, sometimes every $20$ to $30$ minutes.
- This leads to exponential growth patterns: one cell becomes two, two become four, four become eight, etc.
- A single bacterium can produce hundreds of thousands to millions of descendants in a very short timeframe.

Genome: The collective information held within an organism's DNA, specifically its full collection of genes.
Chromosomal Variation by Species:
- Humans: $46$ total chromosomes.
- Cat: $38$ total chromosomes.
- Potato: $48$ total chromosomes.
- Pea Plant: Fewer than humans.
- Sunflower: More than humans.
- Puffer Fish: A count similar to humans.
Ploidy Levels:
- Diploid ( $2n$ ): The native state of typical body (somatic) cells, containing two copies of every chromosome (one maternal, one paternal).
  - In humans: $2n = 46$ .
- Haploid ( $n$ ): The state of sex cells (eggs and sperm), containing only one copy of each chromosome.
  - In humans: $n = 23$ .
- Triploid: Containing three copies of every chromosome (e.g., banana plants).
- High Variable Ploidy: Organisms like the paramecium may contain hundreds of copies of each chromosome.

Double Helix: The molecular level where DNA exists as a double-stranded, twisted polymer of nucleotides.
Histones: Proteins that acts as spools for DNA packaging.
- Yo-yo Metaphor: The histone is like the plastic part of a yo-yo, while the DNA is the string wrapped around it.
Nucleosome: The fundamental unit of DNA packaging, consisting of DNA wrapped approximately $2.5$ times around a histone protein.
Chromatin ("t-i-n"): The state of DNA when it is "stretched out" and not highly coiled.
- This is the functional state of DNA; it must be stretched out for enzymes to access genes for information.
Condensed Chromatin: As a cell prepares for division, it tightly coils the DNA to make it easier to move.
Chromosome: The highly condensed, packaged structure of DNA visible during cell division.

Somatic Cells: Typical body cells that are diploid ( $2n$ ).
Homologous Pairs (Homologs):
- A pair of chromosomes (e.g., "Pair Number 1") where one comes from each parent.
- They are the same length and contain the same matching set of genes along their length.
- While they have the same genes, they may have different versions (e.g., a dominant form from one parent and a recessive form from the other).
Autosomes: Chromosome pairs $1$ through $22$ .
Sex Chromosomes: The $23^{rd}$ pair.
- In humans, the options are typically $XX$ (genetically female) or $XY$ (genetically male).
- Females have $23$ homologous pairs (including $XX$ ).
- Males have $22$ homologous pairs; the $23^{rd}$ pair ( $XY$ ) is not homologous because $X$ and $Y$ carry different genes.
Karyotype: A diagnostic photograph of a cell's chromosomes.
- Chromosomes are spilled from a nucleus, stained to show banding patterns, and arranged from longest (Pair 1) to shortest (Pair 22).

Structure: A sequence of events in the "life" of a cell, consisting of two main phases: Interphase and the Mitotic (M) Phase.
Interphase: The longest portion of the cycle ( $90\%$ or more), where the cell is not actively dividing. It is subdivided into:
1. $G_1$ (Gap 1): The "typical" cell state where it performs its specialized function (e.g., a liver cell producing enzymes) and undergoes cell growth.
2. $S$ (Synthesis): The critical phase where DNA replication occurs. Each double helix makes an identical copy of itself.
3. $G_2$ (Gap 2): A second growth phase where the cell builds materials and duplicates organelles (mitochondria, chloroplasts) in anticipation of splitting.
Mitotic (M) Phase: A relatively short phase consisting of:
1. Mitosis: Nuclear division (making identical copies of the nucleus).
2. Cytokinesis: Division of the cytoplasm to form two distinct daughter cells.
Multinucleate Exceptions: Some cells, like human muscle cells, undergo mitosis without cytokinesis, resulting in a single cell with multiple nuclei.

DNA Replication: Happens during the $S$ phase of interphase, before mitosis begins.
Sister Chromatids: Two identical molecules of DNA produced by replication.
Centromere: The specific region where sister chromatids are held together.
Terminology Note: A "replicated chromosome" consists of two sister chromatids but is technically counted as ONE chromosome until the chromatids are pulled apart during division.
Mitosis Goal: To finish DNA replication at the centromere and pull the sister chromatids apart to give each new daughter cell an identical, complete set of DNA.