Unit 1: Structure of Chromosomes, Cell Cycle and Cell Division

Chapter Focus: Unit 1: Basic Biology – Structure of Chromosomes, Cell Cycle, and Cell Division.
Syllabus Topics:
- Cell Cycle and Cell Division.
- Structure of chromosomes.
Detailed Scope of Study:
- Cell Cycle: Includes Interphase ( $G_1$ , $S$ , and $G_2$ phases) and the Mitotic phase ( $M$ phase).
- Cell Division:
  - Mitosis and its various stages.
  - Meiosis: Basic understanding as a reduction division (specific stages are not required for study).
  - Homologous chromosomes and crossing over: A brief idea of how these lead to variations.
  - Significance and major differences between mitotic and meiotic divisions.
- Chromosome Structure: Basic structure with an elementary understanding of terminology including chromatin, chromatid, gene structure of DNA, and the centromere.

Characteristics of Life: Cell division is recognized as one of the most fundamental characteristics of living beings.
Perpetuation of Life: This method enables life to continue generation after generation across all spectrums of biological complexity.
Diversity of Organisms: Cell division occurs in:
- Simple organisms like the amoeba.
- Highly complex organisms such as humans ( $Homo\,sapiens$ ).
- Giant-sized animals like elephants.
- Tall plants like coconut trees.
Chromosomal Distribution: The most conspicuous events during division relate to chromosomes. During mitosis, duplicated chromosomes are distributed evenly into daughter cells.
Functional Consistency: Because all body cells result from a long chain of repeated mitotic divisions, they contain the same type and number of chromosomes, ensuring normal cellular functioning and the life of the organism.

Observation via Microscope:
- Unstained Cells: When a normal, living cell is observed under a light microscope without staining, the nucleus may appear empty or devoid of internal structures.
- Stained Cells: When specific dyes are applied, several structures become visible within the nucleus.
Chromosomal Composition: The nucleus contains the majority of the cell's DNA, organized into discrete units called chromosomes.
Molecular Association: Each chromosome consists of one long DNA molecule associated with many proteins.
Chromatin Definition: The complex formed by DNA and proteins is called chromatin.
Appearance in Non-Dividing Cells: Under an electron microscope, chromatin appears as a very long, extremely thin, and darkly stained fiber.
Condensation Process: As a cell prepares to divide, chromatin fibers coil and condense. They become thick enough to be distinguished as separate chromosomes (referencing Mitotic Prophase).
Etymology: The name "chromosome" is derived from Greek words: chroma meaning "coloured" and soma meaning "body," due to their ability to pick up certain dyes.
Formal Definition: Chromosomes are highly coiled, ribbon-like structures formed by the condensation of chromatin fibers during cell division.

Individual Contribution: Chromosomes in animals were first studied in $1882$ by the German Scientist, Walther Flemming.
Experimental Subject: He observed these structures in the rapidly dividing cells of salamander larvae (an amphibian).
Visual Observations: Using an older type of microscope, Flemming observed minute threads that appeared to be dividing lengthwise.
Terminology Origin: Flemming termed this division "mitosis," which literally translates to "thread."
Technological Advancement: Subsequent studies using more powerful microscopes and specialized techniques have allowed for the detailed study of chromosomes and their constituents.

Formative Substances: The chromatin material constituting the fiber is composed of two primary substances:
- DNA (Deoxyribonucleic acid): Comprises approximately $40\%$ .
- Histones: A specific type of protein that comprises approximately $60\%$ .

Historical Timeline of Discovery:
- $1953$ (Rosalind Franklin): First studied the shape of the DNA molecule.
- $1953$ (Watson and Crick): Proposed the definitive double-stranded helical structure.
- $1962$ (Nobel Prize): Awarded to Watson and Crick for their work on the double helix.
Physical Classification: A single DNA molecule is extremely large and is classified as a macromolecule.
Structural Layout: It consists of two complementary strands wound around each other in a double helix.
The Nucleotide Unit: Each single strand of DNA is composed of repeating units called nucleotides. A nucleotide consists of three components:
- Phosphate.
- Sugar (Pentose): Arranged lengthwise.
- Nitrogenous Base: Attached to the sugar inwards.
Nitrogenous Bases: There are four types of nitrogenous bases in DNA:
- Adenine ( $A$ ).
- Guanine ( $G$ ).
- Cytosine ( $C$ ).
- Thymine ( $T$ ).
Base Pairing Rules (Ladder Analogy):
- The two strands are joined by hydrogen bonds between nitrogenous bases, forming the "rungs" of the DNA ladder.
- Adenine pairs with Thymine ( $A = T$ ) using two hydrogen bonds.
- Guanine pairs with Cytosine ( $G \equiv C$ ) using three hydrogen bonds.

Role of Histones: These proteins facilitate the coiling and packaging of DNA into structural units.
Nucleosome Formation: The DNA strand winds around a core consisting of eight histone proteins, known as the histone octamer.
The Nucleosome Unit: The complex of the histone octamer and the DNA wound around it is called a nucleosome.
Structural Metaphor: A nucleosome can be visualized as a football with a long rope (DNA) wound around it in one or two loops.
Quantitative Data: A single human chromosome can contain approximately $1,000,000$ (one million) nucleosomes.