Cell Cycle and Mitosis: Study Notes for General Biology

Cell Division: Importance and Overview

Introduction to Cell Division

Cell division is a fundamental biological process essential for reproduction, growth, repair, and maintenance in all living organisms. It ensures that genetic information is accurately passed from one generation of cells to the next.

• Reproduction: Single-celled organisms reproduce by dividing, creating genetically identical offspring.

• Growth: Multicellular organisms grow from a single fertilized egg to a complex organism through repeated cell divisions.

• Repair and Maintenance: Cell division replaces worn-out or damaged cells, maintaining tissue health.

• Gamete Production: Specialized cell division (meiosis) produces gametes for sexual reproduction.

Genetic Information and Chromosomes

Genomes and Chromosomes

The genome of an organism is its complete set of DNA, containing all genetic instructions. In eukaryotes, DNA is organized into structures called chromosomes.

• Chromosome: A structure composed of one long DNA molecule and associated proteins (chromatin).

• Human Chromosome Numbers: Somatic (body) cells have 46 chromosomes; gametes (egg and sperm) have 23 chromosomes.

• Chromatin: The complex of DNA and proteins that forms chromosomes; condenses during cell division.

• Sister Chromatids: After DNA replication, each chromosome consists of two identical sister chromatids joined at a region called the centromere.

• Kinetochore: A protein structure at the centromere where spindle fibers attach during mitosis.

Example: Escherichia coli (a bacterium) has a single circular chromosome, while human cells have multiple linear chromosomes.

Cell Cycle: Phases and Regulation

Main Stages of the Cell Cycle

The cell cycle is the ordered sequence of events that leads to cell division and the production of two daughter cells. It consists of three main stages:

• Interphase: The cell grows, performs normal functions, and replicates its DNA in preparation for division.

• Mitosis: The process where sister chromatids are separated and distributed to two nuclei.

• Cytokinesis: The division of the cytoplasm, resulting in two distinct daughter cells.

Key Point: DNA replication occurs during Interphase, specifically in the S phase (Synthesis phase).

• G1 phase: Cell growth and normal activity.

• S phase: DNA replication.

• G2 phase: Preparation for mitosis.

Quiz Example: During which stage of the cell cycle is DNA replicated? Answer: Interphase (S phase)

Mechanisms of Cell Division

Binary Fission (Prokaryotes)

Prokaryotic cells, such as bacteria, divide by binary fission, a simpler process than mitosis.

• Binary Fission: The cell replicates its DNA, elongates, and splits into two identical daughter cells.

Example: Escherichia coli reproduces by binary fission.

Mitosis (Eukaryotes)

Mitosis is the process by which eukaryotic cells divide their nucleus and distribute identical genetic material to daughter cells.

• Purpose: Growth, repair, and asexual reproduction in multicellular organisms.

• Result: Two genetically identical daughter cells.

Stages of Mitosis

Overview of Mitosis Stages

Mitosis consists of several distinct phases, each with specific events:

• Prophase: Chromatin condenses into visible chromosomes; centrosomes move to opposite poles; mitotic spindle begins to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores; chromosomes begin moving toward the cell center.

• Metaphase: Chromosomes align along the metaphase plate; spindle fibers connect to kinetochores.

• Anaphase: Cohesins are cleaved; sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense; spindle breaks down.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Summary Table: Stages of Mitosis

Stage	Main Events
Prophase	Chromosomes condense, centrosomes move to poles, spindle forms
Prometaphase	Nuclear envelope breaks down, spindle fibers attach to kinetochores
Metaphase	Chromosomes align at metaphase plate
Anaphase	Sister chromatids separate and move to opposite poles
Telophase	Nuclear envelopes reform, chromosomes decondense
Cytokinesis	Cytoplasm divides, forming two daughter cells

Key Terms and Structures

• Centrosome: Organelle that organizes spindle fibers during mitosis.

• Spindle Fibers: Microtubules that separate chromosomes.

• Metaphase Plate: Imaginary plane where chromosomes align during metaphase.

• Cohesins: Proteins holding sister chromatids together.

• Separase: Enzyme that cleaves cohesins during anaphase.

Additional Info

• Length of DNA in Human Cell: If stretched end to end, the DNA in a human cell is about 2 meters long.

• Reference Text: Campbell Biology, sections 12.1, 12.2 (Mitosis), and 13.3, 13.4 (Meiosis).

Review Questions

• During which stage of the cell cycle is DNA replicated? Interphase (S phase)

• During which stage of mitosis do chromosomes align at the metaphase plate? Metaphase

• What is the function of the kinetochore? Attachment site for spindle fibers, enabling chromosome movement

Summary

Cell division is a tightly regulated process that ensures genetic continuity and organismal health. Understanding the cell cycle and mitosis is essential for studying growth, development, and disease in biology.