Cell Growth & Division

Cell Growth Limits and Surface Area to Volume Ratios

• Cell growth "Traffic" issues are directly related to the ratio of surface area ($SA$) to volume ($V$).

• The Surface Area of a cell refers to the membrane.

• The Volume of a cell refers to the activities and internal components inside.

• Numerical examples of SA:V ratios (comparing three different cell sizes):     - Cell 1: $SA = 96$, $V = 64$, Ratio = $1.5$.     - Cell 2: $SA = 24$, $V = 8$, Ratio = $3$.     - Cell 3: $SA = 6$, $V = 1$, Ratio = $6$.

• Comparison: A large SA:V ratio (e.g., $6$) is superior to a small SA:V ratio (e.g., $1.5$) for efficient cellular operation.

Questions and Reviews

• Review Questions:     1. What are the benefits of sexual reproduction? What are the cons?     2. What are the benefits of asexual reproduction? What are the cons?     3. For surface area-to-volume ratios, is a large ratio better or a small ratio better?

• Warm-UP Questions:     1. Which stage of the cell cycle (interphase or M Phase) does the cell spend the MOST time in? (Answer: Interphase).     2. When does DNA replication occur? (Answer: S phase of Interphase).     3. What stage happens AFTER M phase? (Answer: G1 phase or the beginning of Interphase).

Genetic Packaging and Chromosome Structure

• DNA Organization across domains:     - In Prokaryotes: DNA is packaged into a single, circular chromosome.     - In Eukaryotes: DNA is packaged into multiple chromosomes; specifically, humans maintain $46$ chromosomes.

• Building a Chromosome:     - DNA exists as a double helix.     - DNA combines with histone proteins to form a structure called a nucleosome.     - Nucleosomes coil together to form coils.     - Coils condense further to form supercoils.     - Supercoils form the duplicated chromosome.

• Structural Components:     - Sister Chromatids: Identical halves of a duplicated chromosome.     - Centromere: The localized region where sister chromatids are joined together.

Prokaryotic Cell Cycle: Binary Fission

• Prokaryotes undergo Binary Fission, which is a method of asexual reproduction involving simple division.

• Process Steps:     1. DNA duplicates.     2. The cell membrane pinches.     3. The cell divides.

• Critical Concept: DNA needs to replicate BEFORE the cell divides so that each resulting cell has a complete set of genetic instructions.

Eukaryotic Cell Cycle Components

• Cells spend the majority of their life cycle in Interphase, which is subdivided into three distinct phases:     1. G1 Phase: The growth phase.     2. S Phase: DNA synthesis phase where DNA is replicated and chromosomes are doubled.     3. G2 Phase: The phase where molecules and organelles replicate and the cell prepares for division.

• M Phase: Follows interphase and consists of cell division.

Stages of M Phase: Mitosis and Cytokinesis

• M Phase consists of the division of the nucleus (Mitosis) and the division of the cytoplasm (Cytokinesis).

• Mitosis consists of four phases (PMAT):     1. Prophase: The first and longest phase. The nuclear membrane breaks down, chromosomes condense to become visible, and the spindle begins to form from the centrioles.     2. Metaphase: The second phase. Chromosomes line up at the center (equator) of the cell, and spindle fibers connect the chromosome to the centriole at the centromere.     3. Anaphase: The third phase. Sister chromatids separate into individual chromosomes and move toward opposite poles of the cell along the spindle.     4. Telophase: The fourth and final phase. The nuclear envelope reforms around the DNA, the spindle breaks apart, and the cell membrane begins to pinch.

• Cytokinesis: The physical division of the cytoplasm occurs at approximately the same time as telophase.     - In Animals: The membrane pinches in the center to form a cleavage furrow.     - In Plants: A cell plate forms midway between the two nuclei.

Numerical Chromosome Tracking (Amoeba Sisters)

• Chromosomes and Chromatids in Humans throughout division stages:     - Before Interphase: $46$ Chromosomes and $46$ Chromatids.     - After Interphase (Post-Replication): $46$ Chromosomes and $92$ Chromatids.     - After Mitosis: $46$ Chromosomes and $46$ Chromatids per daughter cell.

• Visual Notation:     - I = Chromosome with $1$ chromatid.     - X = Chromosome with $2$ chromatids.

• Result: After cytokinesis, two daughter cells result, each with a $2n$ (diploid) chromosome count.

Mitosis Identification Exercise (Carolina®)

• Students must observe plant cells to identify the following stages:     - Interphase (Cells labeled A, B, C).     - Prophase.     - Metaphase (Chromosomes aligned with centromeres on the equator).     - Anaphase (Chromatids move to opposite poles).     - Telophase (Two nuclei reform at poles).

• Identification clues include the orientation of genetic material, presence of spindle fibers, and the physical shape of the dividing cell.