Bio

1. DNA Structure and Components

DNA is a double-helix molecule composed of repeating units called nucleotides. Each nucleotide consists of:

Deoxyribose: A five-carbon sugar that forms the backbone.
Phosphate Group: Along with deoxyribose, it forms the "uprights" of the DNA ladder via strong covalent bonds.
Nitrogenous Base: The "rungs" of the ladder, consisting of Adenine ( $A$ ), Thymine ( $T$ ), Guanine ( $G$ ), and Cytosine ( $C$ ).
Weak Hydrogen Bonds: These hold the nitrogenous bases together in the center, allowing the strands to be "unzipped" for replication.

Base Pairing Rules

According to the complementary base pairing rules:

Adenine ( $A$ ) always pairs with Thymine ( $T$ ).
Guanine ( $G$ ) always pairs with Cytosine ( $C$ ).

2. Chargaff’s Rule and Data Analysis

Chargaff’s rule states that the amount of Guanine should be equal to Cytosine, and the amount of Adenine should be equal to Thymine ( $A=T$ and $G=C$ ). Based on this, we can complete the table:

Organism	Adenine ( $A$ )	Guanine ( $G$ )	Thymine ( $T$ )	Cytosine ( $C$ )
Horse	$28.5\%$	$21.5\%$	$28.5\%$	$21.5\%$
Cow	$31.5\%$	$18.5\%$	$31.5\%$	$18.5\%$
Amoeba	$34.4\%$	$15.6\%$	$34.4\%$	$15.6\%$

Note: For the Amoeba, if $G = 15.6\%$ , then $C$ must also be $15.6\%$ . The remaining $68.8\%$ is split equally between $A$ and $T$ .

3. DNA Location and the Cell Cycle

In eukaryotic cells, DNA is located within the nucleus.

Phases of the Cell Cycle

Interphase: The longest stage where the cell grows and prepares.
- $G_1$ Phase: Cell growth and normal metabolic roles.
- $S$ (Synthesis) Phase: DNA replication occurs.
- $G_2$ Phase: Further growth and preparation for mitosis.
Mitosis: The division of the nucleus.
- Prophase: Chromosomes condense, and the nuclear envelope breaks down.
- Metaphase: Chromosomes line up in the middle of the cell.
- Anaphase: Sister chromatids are pulled apart to opposite poles.
- Telophase: Two new nuclear envelopes form.
Cytokinesis: The physical division of the cytoplasm, resulting in two distinct daughter cells.

Outcome: At the end of the cycle, each daughter cell enters its own $G_1$ phase and is genetically identical to the parent cell.

4. DNA Replication

DNA replication ensures that every new cell receives a complete set of genetic instructions.

Steps of Replication:
1. Unwinding: The enzyme Helicase breaks the hydrogen bonds, "unzipping" the double helix.
2. Base Pairing: DNA Polymerase adds complementary nucleotides to the exposed strands.
3. Joining: The sugar-phosphate backbone is sealed, resulting in two identical DNA molecules.
When and Where: Occurs during the S phase of Interphase in the nucleus.
Importance: It is vital because cells must double their DNA before dividing so that daughter cells have the full instruction manual for life.

5. Mitosis and Tissue Repair

Mitosis: The process of nuclear division. Cells perform mitosis for growth, tissue repair, and asexual reproduction.
Cytokinesis: The final step of cell division where the cell membrane pinches in to separate the two nuclei into two individual cells.
Repairing a Cut: When you cut your skin, the cells at the edge of the wound receive signals to enter the cell cycle. Through mitosis, they create new, identical skin cells to bridge the gap and replace the damaged tissue.

6. Mutations and Development

DNA Mutations: A mutation is a change in the DNA sequence. This can happen due to errors during replication or environmental factors (mutagens) like UV radiation or chemicals.
Effects of Mutations: Mutations can be neutral (no effect), harmful (leading to diseases like cancer), or occasionally beneficial (providing genetic variation for evolution).
Multicellular Development: Every organism starts as a single zygote. Mitosis allows that one cell to become trillions. The cell cycle ensures growth, while subsequent cell differentiation allows cells to specialize into tissues and organs that work together for the organism's survival.