Overview of the Eukaryotic Cell Cycle: Structure, Phases, and Chromosome Organization

Overview of The Cell Cycle

Introduction

The eukaryotic cell cycle is a highly regulated, repeating process of cell growth, DNA replication, and division. It ensures accurate duplication and distribution of genetic material in all eukaryotic cells and is conserved across species.

• Major Phases: Interphase (G1, S, G2) and M phase (mitosis and cytokinesis)

• Cycle Duration: About 24 hours in mammals

• Regulation: Controlled by cell cycle checkpoints and regulatory proteins

Interphase

G1 (Gap 1/Growth 1) Phase

G1 is a period of cell growth and active gene expression, occurring at a particularly high level. The cell produces RNA and proteins needed for DNA replication.

• Duration: ~10 hours in a typical 24-hour cycle

• Key Activities: Cell growth, biosynthesis, preparation for DNA replication

G0 (Gap 0/Growth 0) Phase

G0 is a resting or nondividing state within the cell cycle. Cells may remain here temporarily or indefinitely (quiescent).

• Entry/Exit: Cells can re-enter the cycle when stimulated by injury, growth signals, or cell turnover

S Phase

During S phase, DNA replication occurs, doubling the genetic material in the nucleus. Each chromosome now consists of two identical sister chromatids joined at the centromere, forming a dyad.

• Duration: ~9 hours

• Key Activities: DNA synthesis, formation of sister chromatids

• Chromosome Number: Remains the same (2n for diploid organisms), but DNA content doubles

G2 (Gap 2/Growth 2) Phase

G2 is the phase where organelles are duplicated and spindle microtubules begin to form. The cell contains twice the DNA (4c) but maintains the same chromosome number (2n).

• Duration: ~4 hours

• Preparation: Final checks before mitosis

Interphase Recap

• Characterized by the absence of visible chromosomes

• DNA exists as uncoiled chromatin, not condensed chromosomes

• The cell could be in G0, G1, S, or G2 phase

Chromosome Structure and Terminology

Dyads and Sister Chromatids

• Dyad: A duplicated chromosome made of two identical sister chromatids joined at the centromere

• Sister Chromatids: Identical copies of a single chromosome formed after DNA replication

• Homologous Chromosomes: Matching pairs of chromosomes (one from each parent) carrying the same genes but possibly different alleles

• Non-sister Chromatids: Chromatids from different homologous chromosomes (not identical)

Chromosome Number and DNA Content

• n: Number of chromosomes (e.g., human skin cell has 46 chromosomes = 2n)

• c: Amount of DNA (e.g., 2c before replication, 4c after S phase in diploid cells)

Interpreting a Human Karyotype

Definition and Features

• Karyotype: A light micrograph image of stained, condensed chromosomes from a dividing cell

• Chromosomes are arranged in homologous pairs by size and centromere position

• Used to study chromosome number and detect large structural changes

• Karyotypes are always shown after S phase, when chromosomes are replicated and condensed

M Phase: Mitosis and Cytokinesis

Overview

M phase is the stage of the cell cycle when the cell divides its duplicated genetic material.

• Karyokinesis: Separates replicated chromosomes and partitions them into two identical daughter nuclei

• Cytokinesis: Divides the cytoplasmic contents, producing two genetically identical daughter cells

• Subphases: Prophase, Prometaphase, Metaphase, Anaphase, Telophase

Key Questions and Concepts

• Which phase includes active gene expression and cell growth before DNA replication? Answer: G1 phase

• Which statements correctly describe S phase? Answer: Each chromosome forms two identical sister chromatids; DNA replication occurs

• A diploid cell in G2 has 2n = 4 and 4c DNA content. What does this mean? Answer: The cell has the same chromosome number but twice the amount of DNA

• During interphase, chromosomes are visible under a light microscope as distinct X-shaped structures. Answer: False

• Which two processes occur during M phase to complete cell division? Answer: Karyokinesis and cytokinesis

Table: Chromosome Terminology Comparison

Term	Definition	Example
Dyad	Duplicated chromosome made of two identical sister chromatids joined at the centromere	Post-S phase chromosome
Sister Chromatids	Identical copies of a single chromosome formed after DNA replication	Both arms of a dyad
Homologous Chromosomes	Matching pair of chromosomes from each parent, same genes, possibly different alleles	Chromosome 1 from mother and father
Non-sister Chromatids	Chromatids from different homologous chromosomes	One chromatid from each homolog

Key Equations

• Chromosome number (diploid): $2n$

• DNA content before replication: $2c$

• DNA content after S phase: $4c$

Summary

• The cell cycle consists of interphase (G1, S, G2) and M phase (mitosis and cytokinesis)

• DNA replication during S phase is essential for accurate chromosome segregation

• Chromosome structure and terminology are key for understanding genetic inheritance and cell division

• Karyotypes are used to study chromosome number and structure, always post-S phase

Additional info: The notes expand on brief points with academic context, definitions, and examples for clarity and completeness.