Cell Biology and Genetics: Core Definitions and Principles

Essential Concepts in Biology and Heredity

Fundamental Definitions in Genetics

Gene

The basic unit of heredity passed from parent to child. Genes are made up of sequences of DNA and are arranged, one after another, at specific locations on chromosomes in the nucleus of cells.

Codominance

Codominance, as it relates to genetics, refers to a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual.

Cellular Components and Principles

Ribosomes

Small particles with two subunits, without membranes to divide them. They are responsible for building proteins using the information coded in a linear molecule of nucleic acid (RNA).

Mendel's Second Law of Segregation

Mendel's second law is the Law of Segregation. The law states that every pair of alleles or genes in parents divides, and a single gene is transferred from each parent to their offspring.

The Endosymbiotic Theory

In the beginning, bacteria performed anaerobic metabolism, which was not very efficient in terms of energy acquisition. When cyanobacteria developed photosynthesis, everything changed. Photosynthesis is a process that produces a lot of energy and releases a large amount of oxygen into the atmosphere. The oxygen released gave rise to a proliferation of bacteria which perform cellular respiration. The Endosymbiotic Theory proposes that a big cell started engulfing small cells until this relationship converted into a stable endosymbiotic relationship.

Karyotypes

A karyotype is the set of chromosomes that belong to a cell, individual, or species. There are two main types of chromosomes identified in a karyotype:

• Autosomes: Non-sex chromosomes.
• Heterochromosomes: Sex chromosomes (e.g., X and Y).

Tenets of the Modern Cell Theory

1. Cells are the structural units of living things.
2. Cells are the functional units of living things.
3. Cells are the unit of origin of living things (all cells come from pre-existing cells).
4. Cells are the hereditary unit.

Comparing Meiosis and Mitosis

Mitosis

• Takes place in somatic cells.
• Short process.
• The nucleus divides only once.
• Crossover does not occur.
• Sister chromatids separate in the anaphase.
• Produces two daughter cells with the same number of chromosomes (diploid) that are genetically identical to the parent cell.

Meiosis

• Takes place in germ cells (sex cells).
• Long process.
• The nucleus divides twice (Meiosis I and Meiosis II).
• Crossover occurs in homologous chromosomes (Prophase I).
• Homologous chromosomes separate in Anaphase I, and sister chromatids separate in Anaphase II.
• Produces four daughter cells (haploid) with half the parent's number of chromosomes and a different genetic combination.

Further Topics in Cellular Biology

• The Cell Cycle: Explanation and Diagram
• Cell Nucleus: Components and Structure