Cell Cycle, DNA Structure, and Chromosomes: Study Notes for Human Biology

Cell Cycle

Overview of Cell Growth and Division

The cell cycle is a fundamental process in eukaryotic cells, enabling growth, development, and reproduction. It consists of a series of events that result in the duplication and division of cells, ensuring continuity of life. According to cell theory, new cells are created from existing cells, which is central to biology.

• Cell Growth: Cells reproduce continuously, starting from a single fertilized egg. By birth, humans have over 10 trillion cells.

• Cell Theory: New cells arise from pre-existing cells.

Phases of the Cell Cycle

The cell cycle is divided into two main phases: Interphase and the Mitotic Phase.

• Interphase: The period of cell growth and DNA duplication, subdivided into G1, S, and G2 phases.

• Mitotic Phase: The nucleus and cytoplasm divide, resulting in two new cells.

Interphase Subdivisions

Interphase is the longest phase and is subdivided as follows:

• G1 Phase (First Gap): Occurs after cell division and before DNA replication. The cell is at its smallest size and undergoes most of its growth (6–9 hours).

• S Phase (Synthesis): DNA replication occurs, chromosomes are duplicated, and growth continues slowly (7–8 hours).

• G2 Phase (Second Gap): The cell continues to grow and prepares for division (6–9 hours).

G0 Phase

Many cells enter a non-dividing, non-growing stage called G0.

• G0 Phase: A healthy condition for many cell types. Some cells, such as neurons and osteocytes, remain in G0 permanently after adolescence.

Mitotic Phase

The mitotic phase is much shorter (30–45 minutes) and consists of:

• Mitosis: Duplicated DNA is divided into two sets, and the nucleus divides.

• Cytokinesis: The cytoplasm divides, forming two new cells.

DNA Structure and Chromosomes

DNA and Its Organization

Before cell division, an exact copy of parental DNA must be produced during the S-phase.

• DNA (Deoxyribonucleic Acid): The genetic material found in the nucleus of every cell.

• Base Pairs: DNA consists of 3 billion pairs of nucleotides (A-T, C-G) arranged in a double helix.

Chromosomes

DNA is organized into chromosomes, which vary in number across species.

• Humans: 46 chromosomes (23 pairs)

• Blue Whale: 44 chromosomes (22 pairs)

• Fruit Fly: 8 chromosomes (4 pairs)

Chromatin and Histones

During interphase, chromosomes are not visible and exist as chromatin.

• Chromatin: Long, thin strands of DNA combined with proteins called histones.

• Histones: Proteins that act as spools for DNA, keeping it compacted.

• Chromatin is duplicated during S-phase, and condenses in late G2, becoming visible in the mitotic phase.

Chromosomes in Mitotic Phase

Prior to nuclear division, chromosomes condense and become visible as identical sister chromatids.

• Sister Chromatids: Two identical copies of a chromosome held together by a centromere.

Genes

Genes are short segments of DNA that code for proteins.

• Gene: The smallest functional unit of DNA.

• Human Genome: Approximately 20,000–21,000 genes located on 46 chromosomes.

• Each gene occupies a specific location on a chromosome.

Summary Table: Cell Cycle Phases

The following table summarizes the main events and durations of each cell cycle phase:

Phase	Main Events	Duration
G1	Cell growth	6–9 hours
S	DNA replication	7–8 hours
G2	Preparation for division	6–9 hours
Mitotic Phase	Nucleus and cytoplasm divide	30–45 minutes
G0	Non-dividing stage	Variable

Key Equations

DNA Base Pairing:

• $A = T$

• $C = G$

Chromosome Number (Humans):

• $\text{Total chromosomes} = 2 \times \text{number of pairs} = 2 \times 23 = 46$

Example

During the S-phase of the cell cycle, a human cell with 46 chromosomes duplicates its DNA, resulting in 92 chromatids before mitosis.

Additional info: Academic context was added to clarify the phases, chromosome structure, and gene function.