Lecture_17_-_Mitosis_and_Meiosis_2024

Introduction to Genetics

• Understanding Mitosis and Meiosis

• Key Questions to Address:

  1. Definitions of DNA, chromosomes, genes, alleles, heritable traits, genotypes, and phenotypes.

  2. Relationship between genetic information and heritable traits.

  3. Concept of homologous chromosomes and the definition of "diploid".

Genetic Information and Heritable Traits

DNA

• Defined as a molecule containing genetic instructions for protein production.

• Sequence of nitrogenous bases determines protein synthesis.

Chromosomes

• Chromosomes consist of DNA molecules.

• Each chromosome contains many genes.

Genes and Alleles

• Gene: A segment of DNA that encodes for a specific protein or trait.

• Allele: A variant form of a gene differing in base sequence.

• Variations in alleles create diversity in heritable traits within a population.

Genotype and Phenotype

• Genotype: The specific sequence of nitrogenous bases, which determines the corresponding allele.

• Phenotype: The observable physical trait or characteristic resulting from the genotype.

Chromosomal Basics

Chromosome Structure

• Eukaryotic species have varying chromosome numbers.

• Most organism's chromosomes exist in pairs, referred to as diploid (2n).

• Homologous chromosomes: Pairs of chromosomes that are similar in size, shape, and gene content, but may carry different alleles.

• Example: Drosophila (fruit fly) has 4 pairs of chromosomes.

The Process of Mitosis

Overview

• Mitosis occurs in all eukaryotic cell types, excluding germ cells.

• It results in the production of new somatic cells and is essential for growth and asexual reproduction.

Outcomes of Mitosis

• Produces two genetically identical daughter cells from one parent cell.

• Critical for embryonic development and cell replacement in somatic tissues.

Stages of Mitosis

1. Interphase - Preparation phase where chromosomes replicate.

2. Prophase - Chromatids condense; spindle apparatus begins to form.

3. Metaphase - Chromosomes align at the center of the cell.

4. Anaphase - Sister chromatids are pulled apart to opposite poles.

5. Telophase - Nuclear envelope reforms, and chromosomes de-condense.

6. Cytokinesis - Cytoplasm divides, completing the formation of two daughter cells.

The Process of Meiosis

Overview

• Meiosis occurs in germ cells to produce gametes (sperm and eggs).

• Involves two rounds of cell division, resulting in four daughter cells, each haploid (n).

Outcomes of Meiosis

• Produces male (sperm) and female (egg) gametes with half the chromosome number of the parent cell.

• Crucial for sexual reproduction and genetic diversity.

Mechanisms of Genetic Diversity

1. Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I.

2. Independent Assortment: Random distribution of maternal and paternal chromosomes to daughter cells during metaphase I.

Stages of Meiosis

1. Interphase - Chromosomes replicate in the parent cell.

2. Prophase I - Homologous chromosomes pair and exchange segments.

3. Metaphase I - Chromosomes align at the equator.

4. Anaphase I - Homologous chromosomes are pulled to opposite poles.

5. Telophase I and Cytokinesis - Nuclear membranes reform and the cell divides.

6. Meiosis II - Similar to mitosis but results in four haploid cells.

Comparison: Mitosis vs. Meiosis

Advantages of Mitosis

• Produces genetically identical offspring quickly.

• Essential for growth and tissue repair.

• More efficient under stable environmental conditions.

Advantages of Meiosis

• Generates genetic diversity through sexual reproduction.

• Enhances survival through varied offspring in changing environments.

Conclusion

• The processes of mitosis and meiosis are integral to the life cycle of organisms.

• Understanding these concepts is crucial for studying genetic inheritance and evolutionary biology.