GRQ 9: Cell Division and Genetics

GRQs: Genetics Reading Objectives

  • Contrast asexual and sexual reproduction in outcome and types of organisms/cells that use each.

  • Recognize/describe the stages of mitosis, contrasting animal and plant cells.

  • Articulate how the cell cycle differs in normal, dividing cells compared to nerve cells or cancer cells.

  • Explain the significance of a mutated BRCA-1 gene in terms of risks and consequences.

Module 8.1: Cell Division and Reproduction

Asexual vs Sexual Reproduction

  • Asexual Reproduction

    • Produces genetically identical offspring, often referred to as clones.

    • Methods:

    • Budding

    • Fission

    • Regeneration

    • Example: An African violet reproducing from a cutting.

  • Sexual Reproduction

    • Results in genetically unique offspring due to the fusion of gametes (egg and sperm).

    • Process involves fertilization, creating a diploid zygote with a unique combination of genes.

    • Example: Sea anemones can reproduce both sexually and asexually, switching modes based on environmental conditions.

Cell Division and Embryonic Development

  • Mitosis and Meiosis play critical roles in cell division.

  • Cell division is essential for:

    • Growth

    • Specialization of cells

    • Maintenance of the organism.

Module 8.2: Binary Fission in Prokaryotes

Characteristics of Prokaryotic Division

  • Prokaryotic division is called BINARY FISSION.

  • The single circular chromosome contains most of the cell's genetic information and must be divided and distributed to daughter cells.

  • True or False: Prokaryotes have one circular chromosome.

    • Answer: True.

Module 8.3: Eukaryotic Chromosomes

Eukaryotic Chromosome Characteristics

  • True or False Statements:

    • True: Eukaryotes have more genes than prokaryotes.

    • False: Intelligence does not correlate with the number of chromosomes.

    • False: Each human chromosome does not have only one gene; it contains hundreds or even thousands of genes.

    • False: Humans do not have over 100,000 genes; we have just under 21,000 genes.

DNA Compaction

  • DNA compaction/coiling is analogous to moving to a new apartment:

    • Packing and Organizing: Histones help coil DNA, forming structures like nucleosomes, similar to packed boxes.

    • Multilevel System: DNA undergoes several levels of coiling and folding, creating 30-nm fibers and looped domains to fit within the nucleus.

    • Accessibility: Tightly packed DNA is less accessible for gene expression, like items at the back of a closet being harder to reach.

Module 8.4: Cell Cycle Phases

Interphase - Growth and Preparation

  • Interphase Behavior:

    • Composed of three phases:

    1. G1 Phase (First Gap): The cell grows and performs normal functions; a period of intense biochemical activity.

    2. S Phase (Synthesis): DNA replication occurs, resulting in two sister chromatids per chromosome.

    3. G2 Phase (Second Gap): The cell continues to grow and prepares for mitosis by synthesizing proteins and organelles.

Mitotic Phase

  • The other part of the cell cycle is called MITOTIC PHASE:

    • Mitosis: The nucleus divides, distributing duplicated chromosomes into daughter nuclei.

    • Cytokinesis: The cytoplasm divides, yielding two genetically identical daughter cells.

Module 8.5: Dynamic Changes During Cell Division

Spindle Formation

  • For a spindle to form, the nuclear envelope must dissolve during prophase and prometaphase.

Stages of Mitosis

  1. Prophase: Chromosomes condense and become visible; the nuclear envelope begins to dissolve; mitotic spindle starts to form.

  2. Prometaphase: Nuclear envelope dissolves completely; spindle fibers attach to kinetochores of fully condensed chromosomes.

  3. Metaphase: Chromosomes align at the cell's equatorial plane (metaphase plate).

  4. Anaphase: Sister chromatids are pulled apart towards opposite poles of the cell.

  5. Telophase: Chromosomes reach poles and begin to de-condense; nuclear envelope re-forms around each chromosome set.

Module 8.7: Factors Affecting Cell Division

Cell Cultures and Division Factors

  • Cell Culture Maintenance: Many cells do not cooperate in division outside a body.

  • Three factors that influence normal cell division:

    1. Anchorage Dependence: Animal cells need a solid surface to attach to for division.

    2. Density-Dependent Inhibition: Cells stop dividing when crowded; they receive signals to cease division when they touch each other.

    3. Growth Factors: Proteins that stimulate cell division; essential nutrients or specific growth factors need to be present for cells to divide.

Module 8.8: Cell Cycle Control System

Cell Cycle Checkpoints

  • Checkpoints in the cell cycle function like traffic lights:

    • Red Light: Stop signals that regulate the cell cycle.

    • Green Light: Go-ahead signals that encourage progression through the cell cycle.

Growth Factors and the G1 Checkpoint

  • Growth factors bind to receptors, initiating signal transduction pathways that trigger the cell cycle control system:

    • Signal Transduction Pathway: Involves interactions that convey a “go signal”.

    • Cell Cycle Control System: Overrides inhibitory signals to progress beyond the G1 checkpoint. If activated, cells will proceed to the S phase and divide.

Non-Dividing Cells

  • Certain cells never receive the go-ahead signal, resulting in permanent non-dividing states:

    • Mature Nerve Cells: Highly specialized; do not divide, leading to irreversible damage post-injury.

    • Muscle Cells: Mature muscle cells, including cardiac cells, also do not divide.

    • Liver Cells: Generally non-dividing but can re-enter the cell cycle if liver damage occurs.

Module 8.9: Cancer Cells and Tumors

Characteristics of Malignant Tumors

  • Malignant tumors comprise abnormally reproducing cells that invade neighboring tissues.

  • Immortal Cells: In cell biology, this term refers to cancer cells that can divide indefinitely.

Tumors vs. Cancer

  • The terms are not interchangeable:

    • Tumor: A mass of abnormally growing cells that do not spread.

    • Cancer: Malignant tumors that can spread to other body parts.

Chemotherapy Mechanisms

  • Taxol: Freezes the mitotic spindle after formation, preventing cancer cells from progressing past metaphase.

  • Vinblastine: Prevents the mitotic spindle from forming, stopping cell division before it starts.

  • Targeted Cells: These drugs mainly affect rapidly dividing cancer cells, but also impact other growing cells, leading to side effects like nausea and hair loss.

Module 8.10: Individual Responses to Cancer Treatment

Study Design

  • This study is characterized as a prospective observational study, where researchers followed a cohort over time.

Scientific Inquiry

  • The study aimed to understand how growth factors regulate the cell cycle control system and their influence on cell division.

Findings and Societal Impact

  • Growth factors are crucial for promoting cell division and understanding these interactions can aid in developing better cancer prevention strategies.

BRCA Gene Mutation and Cancer Risk

Risks Associated with BRCA-1 and BRCA-2 Mutations

  • Individuals with a BRCA mutation have an 87% risk of developing breast cancer by age 70, compared to a general population risk of 13.1%.

  • In addition to breast cancer, mutated BRCA genes significantly increase the risk of ovarian cancer in women.

Gene Testing Candidates

  • Good candidates for gene testing include:

    • Individuals with a personal history of breast, triple-negative breast, or ovarian cancer.

    • Men with a personal history of breast cancer.

    • Individuals of Ashkenazi Jewish descent with a family history of breast, ovarian, prostate, or pancreatic cancer.

    • Close relatives diagnosed with breast cancer before age 50 or with ovarian cancer at any age.

    • Males in the family with breast cancer or cases of two separate breast cancer diagnoses in the same person or family branch.