Ch_05_Lecture_on_Inheritance

Chapter 5: Chromosomes and Inheritance

5.1 Cells: The Building Blocks of Life

Cell Theory:

• All living organisms, whether plants, animals, fungi, or microorganisms, consist of one or more cells, which serve as the fundamental units of life.

• All cells arise from preexisting cells through cell division, emphasizing the continuity of life.

• Organisms may be unicellular, consisting of a single cell (e.g., bacteria, yeast), or multicellular, consisting of trillions of cells organized into complex structures.

5.2 Cell Division: Creating New Cells

Importance of Cell Division:

• Cell division is crucial for organismal growth, increasing the number of cells and facilitating the development of specialized tissues.

• It plays a vital role in healing, allowing damaged or dead cells to be replaced efficiently and maintaining homeostasis.

• Reproduction can occur in two primary ways:

  • Asexual Reproduction: Involves a single parent organism producing genetically identical offspring, known as clones, ensuring the rapid proliferation of species.

  • Sexual Reproduction: Involves two parent organisms contributing genetic material, leading to genetically diverse offspring through the combination of their distinct genomes.

Types of Nucleic Acids:

• DNA (Deoxyribonucleic Acid): The hereditary material that carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms.

• RNA (Ribonucleic Acid): Plays crucial roles in translating DNA information into proteins and is involved in various cellular processes.

5.3 Chromosomal Structure in Humans

Human Chromosome Count:

• Humans typically have 46 chromosomes, organized into 23 pairs, inherited equally from each parent. This number can vary in other species, influencing their genetic behavior.

Role of Mitosis:

• Mitosis is a carefully regulated process of cell division enabling somatic cells to replicate, thus generating two genetically identical daughter cells and ensuring the maintenance of genetic stability through generations.

5.4 The Cell Cycle

Phases:

• Interphase (90% of the cell's lifetime): The phase characterized by normal cellular functions, growth, and preparation for division, made up of:

  • G1 Phase: Cell growth and normal metabolic roles.

  • S Phase: DNA replication occurs, resulting in two copies of each chromosome.

  • G2 Phase: Further growth and preparation for mitosis.

• Mitotic Phase: Consists of active cell division (Mitosis and Cytokinesis).

Mitosis Steps:

1. Prophase: Chromosomes condense and become visible under a microscope, and the nuclear envelope begins to break down.

2. Metaphase: Chromosomes align at the cell's equatorial plane, ensuring accurate segregation.

3. Anaphase: Sister chromatids are pulled apart towards opposite poles by the spindle fibers.

4. Telophase: Chromatids reach the poles, and the nuclear envelope reforms around each set of chromosomes.

5. Cytokinesis: Division of the cytoplasm follows, resulting in two distinct cells. The process differs in plant and animal cells:

• Animal Cells: The cytoplasm pinches at the cleavage furrow, leading to cell separation.

• Plant Cells: A cell plate forms, eventually leading to the division into two cells.

5.5 Implications of Cancer

Cancer Development:

• Cancer arises when normal mitotic processes become uncontrolled, often due to genetic mutations that affect cell cycle regulation. Uncontrolled division results in the formation of tumors that may invade surrounding tissues.

• Cancer cells can also spread throughout the body through a process called metastasis, leading to invasive cancer.

Treatment Options:

• Combating cancer may involve various treatment methodologies, such as:

  • Surgery: Physically removing tumors.

  • Radiation Therapy: Using high-energy particles to destroy cancerous cells.

  • Chemotherapy: Employing chemical substances to inhibit cell division and selectively target rapidly dividing cells.

5.6 Meiosis: The Basis for Sexual Reproduction

Meiosis Overview:

• Meiosis is a specialized form of cell division used to produce gametes—sperm and egg cells—reducing the chromosome number by half, creating haploid cells (23 chromosomes).

Stages of Meiosis:

1. Meiosis I: Homologous chromosomes pair up and segregate into two new cells, reducing the chromosome number.

2. Meiosis II: Sister chromatids separate, resulting in four genetically unique haploid cells.

Variation Among Offspring:

• Genetic variation is achieved through processes such as independent assortment during metaphase I, random fertilization of gametes, and crossing over where homologous chromosomes exchange genetic material during prophase I.

5.7 Human Sex Determination

Chromosome Configuration:

• The sex of individuals is determined by their chromosomal configuration: females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

Karyotype Analysis:

• This technique allows visualization of an individual’s complete set of chromosomes, facilitating the detection of chromosomal abnormalities associated with genetic disorders.

5.8 Genetic Abnormalities

Nondisjunction Events:

• Nondisjunction occurs when chromosomes fail to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers, leading to conditions such as Down syndrome (Trisomy 21) or Turner syndrome (missing an X chromosome).

Common Characteristics:

• Down Syndrome: Individuals may exhibit a distinct set of physical traits and developmental challenges, including delays in cognitive and physical development.

• Turner Syndrome: Characterized by various developmental issues, including short stature and infertility, due to the absence of one X chromosome in females.

5.9 Summary of Sexual Reproduction

• Mitosis is essential for the growth and development of multicellular organisms, while meiosis is specifically responsible for the production of gametes for sexual reproduction.

5.10 Mechanisms of Variation

• Independent Assortment: Chromosomes assort independently during gamete formation, allowing for new combinations of genes.

• Random Fertilization: Any sperm can fertilize any egg, contributing to genetic diversity within a population.

• Crossing Over: During meiosis I, the exchange of genetic material between homologous chromosomes leads to increased genetic variability among offspring.

5.11 Conclusion

• Cancer can arise from mutations affecting normal cell division processes and may threaten health with potential severe consequences.

• Preventive strategies and treatments involve lifestyle choices, regular screenings, and a variety of medical interventions to combat the onset and progression of cancer.