Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (M phase), which includes mitosis and cytokinesis.

• G1 phase (Gap phase 1): Normal cell function and growth; longest phase.

• S phase (Synthesis): Replication of DNA.

• G2 phase (Gap phase 2): Replication of organelles; microtubules organize.

• Mitosis: Division of the nucleus, subdivided into 5 phases.

• Cytokinesis: Separation of two new cells.

Example: In fruit fly embryos, the cell cycle can be as short as 8 minutes, while a typical mammalian cell takes about 24 hours.

Duration and Regulation

The time required to complete one cell cycle varies greatly among cell types and organisms. Most growth and cellular function occur during the G1 phase. DNA organization changes throughout the cycle, balancing between accessibility for transcription and compactness for cell division.

• Less compact DNA is easier to read and transcribe into proteins.

• More compact DNA takes up less space, important during cell division.

DNA Structure

Macromolecule Composition

DNA (deoxyribonucleic acid) is a nucleic acid macromolecule composed of nucleotide monomers. Each nucleotide consists of:

• Five-carbon sugar: Deoxyribose

• Phosphate group: Attached to the 5' carbon

• Nitrogen base: Adenine (A), Thymine (T), Cytosine (C), or Guanine (G)

Example: The structure of a nucleotide includes a deoxyribose sugar, a phosphate group, and a nitrogenous base.

Polymerization of DNA

Nucleotides are joined together by dehydration synthesis, forming phosphodiester bonds. The DNA strand grows in the 5' to 3' direction.

• Phosphodiester bond: Links the 3' OH of one nucleotide to the 5' phosphate of the next.

• Energy source: The phosphate group provides energy for bond formation; the process is endothermic and anabolic.

Equation:

Double Helix Model

Watson & Crick (1953) proposed the double helix structure of DNA using inductive reasoning and evidence from other scientists such as Rosalind Franklin and Erwin Chargaff.

• Double helix: Two antiparallel strands held together by hydrogen bonds between complementary bases.

• Phosphodiester backbone: Sugar-phosphate units joined by phosphodiester bonds.

• Antiparallel orientation: Strands run in opposite directions (5' to 3' and 3' to 5').

• Base pairing: Sequence on one strand determines the sequence on the other (A pairs with T, C pairs with G).

Additional info: The double helix allows for efficient packing and accurate replication of genetic information.

Key Vocabulary

Summary Table: Cell Cycle Phases

References and Historical Context

• Watson & Crick (1953): Proposed the double helix model of DNA structure.

• Rosalind Franklin: Provided X-ray diffraction images crucial for understanding DNA's structure.

• Erwin Chargaff: Discovered base pairing rules (A=T, C=G).