E

DNA Replication, Binary Fission, and Mitosis

  • This chapter discusses the processes of cell division, the replication of DNA, and the implications of these processes in living organisms.

Section 8.1: Importance of Cell Division

  • Overview of Cell Division:

    • All living organisms must undergo cell division to reproduce.

    • Unicellular organisms:

    • Divide as a method of reproduction.

    • Multicellular organisms:

    • Divide to produce replacement cells, aiding in the maintenance of tissues that may include dead or damaged cells.

The Sexual Life Cycle of Eukaryotic Cells

  • Eukaryotic Cell Division via Mitosis:

    • After fertilization, the fertilized egg (zygote) undergoes mitosis to generate additional cells and tissues.

    • Each of these cells contains 46 chromosomes.

    • As development proceeds, the fetus grows, transitioning into a mature adult comprised of numerous cells.

  • Eukaryotic Sex Cells Divide by Meiosis:

    • Meiosis: A specific division process for germ cells (sperm and egg).

    • Meiosis reduces the genetic material from 2 copies to 1 copy, resulting in 23 chromosomes.

    • At fertilization, the zygote receives DNA from both gametes:

      • 23 chromosomes from the egg + 23 chromosomes from the sperm = total 46 chromosomes.

Roles of Mitosis

  • Functions of Mitotic Divisions:

    • Allows organisms to grow, repair tissues, and regenerate lost body parts.

    • Certain organisms reproduce asexually through mitosis.

Apoptosis: Programmed Cell Death

  • Apoptosis serves a key function in the lifecycle of cells:

    • It is a programmed cell death process that can sculpt distinct structures during the developmental phase.

    • Examples include the formation of fingers, toes, ears, and nostrils.

Section 8.2: DNA Replication and Cell Division

  • Link Between DNA Replication and Cell Division:

    • It is essential for cells to replicate their DNA before division.

    • The entire genome must be replicated ensuring that each progeny cell receives a complete copy of the DNA.

Structure of DNA

  • Composition of DNA:

    • DNA consists of two nucleotide chains known as strands, which are complementary.

    • Pairing of nitrogenous bases occurs as follows:

      • Adenine (A) pairs with Thymine (T)

      • Guanine (G) pairs with Cytosine (C)

    • Each strand acts as a template, enabling both to be utilized in constructing a new DNA molecule.

Semiconservative Nature of DNA Replication

  • Semiconservative Mechanism:

    • The term “semiconservative” indicates that during replication, the original DNA strand is retained while new DNA is synthesized concurrently.

    • After replication, each DNA molecule is composed of one parental strand and one newly synthesized daughter strand.

Role of Enzymes in DNA Replication

  • Key Enzymes Involved in DNA Replication:

    • Helicases: Unwind the DNA helix structure.

    • DNA Polymerases: Synthesize new DNA strands by adding nucleotides complementary to the template strands.

    • Ligases: Join short strands together to form longer DNA strands.

    • DNA replication requires significant energy, specifically in the form of ATP.

Origin and Direction of DNA Replication

  • Initiation of DNA Replication:

    • Begins simultaneously at numerous places on the chromosomes, termed origins of replication.

    • Replication extends bi-directionally from each origin site.

Section 8.3: Binary Fission in Prokaryotes

  • Binary Fission Description:

    • Binary fission is a type of asexual reproduction observed in bacteria and archaea.

    • This process allows for the replication of DNA and its distribution into two daughter cells.

Genetic Exchange in Prokaryotes

  • Methods for Genetic Variation:

    • Prokaryotic cells can acquire new DNA by several methods:

    • A donor cell may directly transfer genetic material to a recipient cell.

    • Healthy cells can take up DNA released from dead cells and incorporate it into their own genomes.

Section 8.4: Eukaryotic Chromosome Structure and Organization

  • DNA Condensation During Cell Division:

    • Following replication, DNA is compactly coiled prior to cell division, allowing for easier segregation of genetic material.

    • Each chromosome comprises two identical sister chromatids representing each DNA copy.

    • Eukaryotic chromosomes exhibit a high level of condensation during division.

Visibility of Eukaryotic Chromosomes

  • Chromatin and Chromosomes:

    • Before mitosis, DNA is present in a less condensed form known as chromatin.

    • Part of the composition of chromosomes:

    • Sister Chromatids: Identical copies of a chromosome.

    • Centromere: The region that connects sister chromatids together (often referred to as the “glue”).

    • Genes: Segments of DNA that encode for traits.

Section 8.5: The Cell Cycle

  • Overview of Mitosis:

    • Eukaryotic cell division is comprised of a process known as mitosis.

    • Mitosis is a fundamental component of the cell cycle.

    • At the conclusion of mitosis, two daughter cells containing genetically identical DNA to the parent cell are formed.

Structure of the Cell Cycle

  • Components of the Cell Cycle:

    1. Interphase:

      • A preparatory phase where no division occurs, and the cell undergoes duplication.

    2. Mitosis:

      • The phase for genetic material division occurring in four stages (PMAT).

    3. Cytokinesis:

      • The process of cytoplasmic division, resulting in two distinct daughter cells.

Stages of Interphase

  • Interphase Components:

    • G1 Phase:

    • Represents normal growth and cellular functions; includes protein synthesis.

    • S Phase:

    • DNA replication occurs; the cell is now committed to division.

    • G2 Phase:

    • Involves the production of specialized proteins necessary for mitosis to occur.

Detailed Steps of Mitosis

  • Mitosis Phases:

    • Prophase:

    • Chromosomes become visible, uncoil, and are held together by centromeres.

    • Centrioles migrate toward opposite poles and spindle fibers form, attaching to the centromeres.

    • The nuclear membrane begins to break down.

    • Metaphase:

    • Centromeres divide, and chromatids are linked to spindle fibers.

    • Chromatids align along the equatorial plane of the cell.

    • Anaphase:

    • Chromatids begin migrating towards opposite poles.

    • Cytokinesis initiates during this phase.

    • Telophase:

    • Chromosomes recoil into their chromatin form, spindle fibers dissolve, and nuclear envelopes reform around the separated DNA.

    • Cytokinesis nears completion, resulting in two identical daughter cells.

Cytokinesis Mechanism

  • Cytokinesis in Different Cell Types:

    • Animal Cells:

    • A cleavage furrow forms as proteins contract to separate the daughter cells.

    • Plant Cells:

    • A cell plate is formed to create a new wall separating the plant daughter cells.

Section 8.6: Control of the Cell Cycle

  • Cell Cycle Control Mechanism:

    • The cell cycle is regulated by a series of chemical checkpoints that ensure:

    • All DNA has been accurately replicated.

    • DNA remains undamaged throughout division.

    • Chromosomes align and separate correctly during division.

Cancer and the Cell Cycle

  • Cancer Cell Characteristics:

    • In cancer, cells divide uncontrollably, bypassing regulatory checkpoints that maintain normal division.

    • This unregulated division can lead to tumor formation.

Tumor Classification

  • Tumor Types:

    • Benign Tumors:

    • Non-cancerous; remain localized and do not spread.

    • Malignant Tumors:

    • Cancerous; possess the capability to invade surrounding tissues and metastasize.

Cancer Treatment Options

  • Common Treatments for Cancer:

    • Surgical Tumor Removal: Physical extraction of the tumor.

    • Chemotherapy: Utilizes anticancer drugs to inhibit or slow down cell division processes.

    • Radiation Therapy: Targets and kills tumor cells directly.

Reducing Cancer Risk

  • Cancer Risk Factors and Reduction Strategies:

    • Cancer develops from mutations in genes that oversee cell division checkpoints.

    • Certain mutations can be inherited, while others can result from environmental factors such as:

    • Unhealthy Diet:

      • Reduce saturated fats; increase consumption of fruits and vegetables.

    • Obesity:

      • Maintain a healthy body weight and exercise regularly.

    • Tobacco Use:

      • Quit smoking or tobacco usage to minimize risk.

    • Environmental Toxins:

      • Limit exposure to hazardous chemicals, especially in the home or workplace.

    • Ultraviolet Radiation:

      • Avoid excessive exposure to UV radiation from the sun and tanning beds.

    • Cancer-causing Viruses:

      • Use protection to decrease the risk of STIs linked to cancer (like HPV).

    • Early Detection:

      • Engage in self-testing and regular medical examinations for early identification of possible malignancies.

    • Family History:

      • Genetic predispositions cannot be altered.

    • Aging:

      • Aging is an unavoidable risk factor.