Module 1: Cellular Reproduction and Chromosomal Basis of Inheritance

Course Overview and Learning Outcomes

This module introduces the foundational concepts of cell structure, cellular reproduction, and the chromosomal basis of inheritance, which are essential for understanding genetics and evolutionary biology. Students will learn about the differences between prokaryotic and eukaryotic cells, the flow of genetic information, and the mechanisms of cell division.

• Key Learning Outcomes:

  • Define and describe the molecular nature of genes.

  • Explain inheritance and evolutionary processes.

  • Describe gene regulation, replication, mutation, and expression.

  • Analyze genetic material and understand genetic diseases.

  • Understand principles of inheritance and genetic exchange.

Cell Structure

Prokaryotic vs. Eukaryotic Cells

Cells are the basic units of life and can be classified as prokaryotic or eukaryotic based on their structural features.

Both cell types contain ribosomes, DNA, cell membrane, cytosol, RNA, proteins, lipids, and polysaccharides.

Example: Escherichia coli is a prokaryote; human cells are eukaryotic.

The Four Important Classes of Biological Molecules

Macromolecules and Their Functions

Cells are composed of four major classes of biological molecules, each with distinct structures and functions:

• Carbohydrates: Provide energy and structural support (e.g., glucose, starch).

• Proteins: Perform a wide range of functions, including catalysis (enzymes), transport, and structural roles (e.g., alcohol dehydrogenase).

• Nucleic Acids: Store and transmit genetic information (DNA, RNA).

• Lipids: Not true polymers; provide membrane structure, energy storage, and signaling (e.g., phospholipids, hormones).

Flow of Genetic Information

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a cell: DNA is transcribed into RNA, which is then translated into protein.

• Transcription: The process by which a gene's DNA sequence is copied to messenger RNA (mRNA).

• Translation: The process by which ribosomes synthesize proteins using the sequence encoded in mRNA.

Example: In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm at ribosomes.

Chromosomes, Genes, and Genomes

Definitions and Organization

Genetic information is organized into discrete units:

• DNA: A double-stranded helix composed of nucleotide monomers (adenine, guanine, cytosine, thymine).

• Genome: The complete set of DNA in a cell.

• Chromosome: A DNA molecule with part or all of the genetic material of an organism. Prokaryotes usually have one circular chromosome; eukaryotes have multiple linear chromosomes.

• Gene: A segment of DNA that encodes a functional product, typically a protein.

Example: Human somatic cells have 46 chromosomes (23 pairs).

Cell Division and the Cell Cycle

Purpose and Overview

Cell division is essential for reproduction, growth, development, and tissue repair. The cell cycle describes the ordered sequence of events that a cell undergoes to divide and produce two daughter cells.

• In unicellular organisms: Cell division is a means of reproduction.

• In multicellular organisms: Enables growth, development, and tissue renewal.

Key Point: Daughter cells are genetically identical to the parent cell (except in meiosis).

The Eukaryotic Cell Cycle

The cell cycle consists of interphase (G1, S, G2 phases) and the mitotic phase (mitosis and cytokinesis).

• G1 phase: Cell growth and normal function.

• S phase: DNA replication.

• G2 phase: Further growth, preparation for division, and centrosome duplication.

• Mitotic phase: Division of the nucleus (mitosis) and cytoplasm (cytokinesis).

Mitosis

Mitosis is the process by which a eukaryotic cell divides its chromosomes into two identical sets, followed by division of the cytoplasm (cytokinesis).

• Phases of Mitosis (PPMAT):

  1. Prophase: Chromosomes condense, spindle forms.

  2. Prometaphase: Nuclear envelope breaks down, spindle attaches to chromosomes.

  3. Metaphase: Chromosomes align at the metaphase plate.

  4. Anaphase: Sister chromatids separate to opposite poles.

  5. Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, forming two daughter cells. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.

Prokaryotic Cell Division: Binary Fission

Prokaryotes reproduce by binary fission, a simpler process than mitosis.

• DNA replication begins at a single origin and proceeds bidirectionally.

• Each origin is pulled to opposite sides of the cell.

• The cell elongates and divides, forming two genetically identical daughter cells.

Ploidy and Chromosome Number

Definitions and Examples

Ploidy refers to the number of sets of chromosomes in a cell.

• Diploid (2n): Two sets of chromosomes (e.g., humans: 2n = 46).

• Haploid (n): One set of chromosomes (e.g., gametes: n = 23 in humans).

• Polyploid: More than two sets of chromosomes (common in plants).

Example: Peas have 14 chromosomes; some butterflies have ~450 chromosomes.

Practice Questions

• Where in the eukaryotic cell is DNA transcribed to mRNA?

  • Answer: In the nucleus

• Proteins destined for secretion from the cell are generally translated in:

  • Answer: Ribosomes found on the endoplasmic reticulum (ER)

• The nuclear membrane is impermeable.

  • Answer: False (it contains nuclear pores for transport)

Summary

• Prokaryotic and eukaryotic cells differ in structure and complexity.

• Genetic information flows from DNA to RNA to protein.

• The cell cycle ensures accurate duplication and division of genetic material.

• Mitosis produces genetically identical cells; binary fission is the prokaryotic equivalent.

• Ploidy describes the number of chromosome sets in a cell.