Essential Biology: Genetics, Cell Division, Ecology

Mendel's Laws of Inheritance

Gregor Mendel's foundational work established the basic principles of heredity:

• 1. Law of Uniformity (First Law)

  If you cross two pure strains that differ in a single character, the first filial generation (F₁) will be uniform, presenting only the dominant character.

• 2. Law of Segregation (Second Law)

  The recessive character, which is not expressed in the F₁ generation, reappears in the F₂ generation in a ratio of 3 dominant to 1 recessive (3:1).

• 3. Law of Independent Assortment (Third Law)

  In heterozygotes for two or more characters, each character is transmitted to the next generation independently of any other character. This results in all possible combinations for these traits.

Functions of DNA

Deoxyribonucleic Acid (DNA) is crucial for life, performing several vital roles:

• Carries hereditary or genetic information.
• Controls the appearance and expression of an organism's characteristics.
• Transmits genetic information from one cell to its descendants during cell division.

Reproductive Strategies in Organisms

Organisms employ diverse reproductive strategies to ensure the survival of their species:

• High Birth Rate and High Offspring Mortality: Common in species like fish, where many offspring are produced, but few survive to adulthood.
• Low Birth Rate and Low Offspring Mortality: Typical of mammals, characterized by fewer offspring but significant parental care and protection, leading to higher survival rates.

Population Growth Patterns

Population growth can be observed in two primary patterns, influenced by resource availability:

• Unlimited Resources: Leads to J-shaped growth, often resulting in a rapid population explosion due to ideal conditions.
• Limited Resources: Results in S-shaped growth, where the population eventually stabilizes around the environment's carrying capacity.

Stages of Mitosis

Mitosis is a process of cell division that results in two genetically identical daughter cells. It consists of several distinct phases:

1. Prophase

   Chromatin condenses into visible chromosomes. The nuclear membrane disappears, and chromosomes disperse within the cell.

2. Metaphase

   Chromosomes, now shorter and thicker, align along the cell's equatorial (central) plane.

3. Anaphase

   Sister chromatids of each chromosome separate at the centromere and move towards opposite poles of the cell.

4. Telophase

   At each pole, a new nuclear membrane forms around the separated chromatids (now considered individual chromosomes), and the division of the nucleus is completed.

Following nuclear division, cytokinesis occurs, separating the cytoplasm by constriction (in animal cells) or by the formation of a cell plate (phragmoplast in plant cells).

Stages of Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells. It involves two main divisions:

1. Meiosis I (First Meiotic Division)

   Homologous chromosomes pair up and align along the central plane. One chromosome from each homologous pair moves to one pole, and the other moves to the opposite pole. Note: Sister chromatids remain attached during this stage. Each resulting cell receives a haploid set of chromosomes, each still consisting of two chromatids.

2. Meiosis II (Second Meiotic Division)

   Chromosomes align at the central plane. The centromeres divide, and sister chromatids separate, moving to opposite poles. At the end of Meiosis II, four daughter cells are produced, each containing half the number of chromosomes as the original parent cell.