The Cell Cycle and DNA Structure: Study Notes for Human Biology

The 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. In humans, cell division begins with fertilization and continues throughout development and adulthood.

• Cell Theory: All cells arise from pre-existing cells through division.

• Continuous Replication: Over 10 trillion cells are formed by birth; replication continues through childhood and adolescence.

• Cell Cycle Duration: Actively dividing cells complete the cycle in approximately 18–24 hours.

Phases of the Cell Cycle

The cell cycle is divided into two main phases: Interphase and the Mitotic Phase. Interphase is further subdivided into G1, S, and G2 phases, while the Mitotic Phase includes mitosis and cytokinesis.

• Interphase: Period of cell growth and DNA duplication.

• Mitotic Phase: Division of the nucleus (mitosis) and cytoplasm (cytokinesis).

Interphase Subdivisions

• G1 Phase (First Gap): Cell grows and prepares for DNA replication. Most growth occurs here (6–9 hours).

• S Phase (Synthesis): DNA is replicated, chromosomes are duplicated. Growth is slow (7–8 hours).

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

G0 Phase

Many cells enter a non-dividing, non-growing stage called G0. This is a healthy state for certain cell types, such as neurons and osteocytes, which remain in G0 permanently after adolescence.

• Reversible vs. Irreversible G0: Some cells can re-enter the cycle, others cannot.

• Example: Neurons and osteocytes remain in G0 forever.

Mitotic Phase

The mitotic phase is much shorter, lasting about 30–45 minutes. It consists of two stages:

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

• Cytokinesis: Cytoplasm divides, forming two new cells.

DNA Structure and Chromosomes

DNA and Its Organization

Before cell division, an exact copy of the parental DNA must be produced during the S-phase. DNA, or deoxyribonucleic acid, is the genetic material found in the nucleus of every cell, consisting of 3 billion pairs of nucleotides (A-T, C-G) arranged in a double helix.

• Base Pairs: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

• Chromosomes: DNA is packed into 46 chromosomes in humans (23 pairs).

Chromosomes 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 long, thin strands called chromatin. Chromatin contains proteins called histones, which act as spools for DNA, keeping it compacted. Chromatin is duplicated during S-phase, and begins to condense in late G2, becoming visible as chromosomes during the mitotic phase.

Chromosomes in Mitotic Phase

Prior to nuclear division, chromosomes condense and can be seen under a microscope. Each chromosome consists of two identical sister chromatids held together by a centromere.

Genes

A gene is a short segment of DNA that contains the code for making one or more proteins. Genes are the smallest functional units of DNA, and humans have approximately 20,000–21,000 genes distributed across the 46 chromosomes. Each gene occupies a specific location on a chromosome.

Summary Table: Cell Cycle Phases

Phase	Main Event	Duration
G1	Cell growth, preparation for DNA replication	6–9 hours
S	DNA replication, chromosome duplication	7–8 hours
G2	Cell growth, preparation for division	6–9 hours
Mitotic Phase	Nucleus and cytoplasm divide	30–45 minutes
G0	Non-dividing, non-growing stage	Variable

Key Equations

DNA Base Pairing

The number of base pairs in human DNA:

$3 \times 10^9$ base pairs

Chromosome Number

Humans:

$2n = 46$ chromosomes

References

• Johnson, M.D. (2017). Human biology: Concepts and current issues (8th ed). Pearson Education Inc.