Mitosis and Meiosis

Introduction to Cell Division

Cell division is essential for growth, repair, and reproduction in multicellular organisms. Two main types of cell division are mitosis and meiosis, each serving distinct biological purposes.

• Mitosis: Responsible for growth and repair/replacement of tissues by producing genetically identical cells.

• Meiosis: Produces gametes (sperm and egg), ensuring genetic diversity and maintaining the species' chromosome number across generations.

Cell Growth and Division

• Hyperplasia: Accelerated growth via increased number of normal cells (possess contact inhibition).

• Hypertrophy: Increase in the size of cells (possess contact inhibition).

• Cancer: Uncontrolled cell division of abnormal cells (do not possess contact inhibition).

Genetic Terminology

Basic Genetic Terms

• Gene: A segment of DNA that codes for a specific protein.

• Allele: Alternative forms of a gene found at the same locus on homologous chromosomes.

• Locus: The specific location of a gene on a chromosome.

• Chromosome: A threadlike structure of nucleic acids and proteins carrying genetic information.

Chromosome Structure and Karyotype

• Karyotype: The complete set of chromosomes in a cell, arranged in pairs and ordered by size.

• Homologous Chromosomes: Chromosome pairs with identical structures and gene loci, but not necessarily identical alleles; one member of each pair is inherited from each parent.

Allelic Relationships

• Heterozygous: Having two different alleles at the same gene locus on homologous chromosomes.

• Homozygous: Having identical alleles at the same gene locus on homologous chromosomes.

• Genotype: The combination of alleles possessed by an individual at a specific locus; determines the phenotype for a given trait.

• Phenotype: The observable trait resulting from a particular genotype, such as eye color or blood type.

Dominance Relationships

• Recessive Allele: An allele that is not phenotypically expressed in the presence of a dominant allele; represented with a lowercase letter (e.g., a).

• Dominant Allele: An allele that is phenotypically expressed even in the presence of any other allele; represented with a capital letter (e.g., A).

Patterns of Inheritance

• Codominance: Both alleles are fully expressed in the phenotype when present together (e.g., ABO blood types).

• Incomplete Dominance: The phenotype is intermediate between those produced by either allele alone (e.g., hair texture: curly, wavy, straight).

Sex Chromosomes and Autosomes

• Sex Chromosomes: Chromosomes (X and Y) that determine a person's genetic sex.

• Autosomes: All chromosomes except the sex chromosomes; occur in homologous pairs.

• Carrier: An individual who carries a recessive allele but does not phenotypically express it.

Summary Table: Key Genetic Terms

Applications and Examples

• ABO Blood Types: Demonstrates codominance (both A and B alleles are expressed in AB individuals).

• Hair Texture: Shows incomplete dominance (heterozygotes have wavy hair, intermediate between curly and straight).

• Sex-linked Traits: Traits determined by genes on the X or Y chromosome, such as color blindness and hemophilia.

Additional info:

• Contact inhibition is a regulatory mechanism that functions to keep cells growing into a layer one cell thick (monolayer). Loss of contact inhibition is a hallmark of cancer cells.

• Meiosis involves two sequential divisions (meiosis I and II) and results in four genetically unique gametes, each with half the chromosome number of the original cell.