Genetics Essentials: Key Concepts — Prokaryotes vs Eukaryotes, DNA Packaging, Reproduction, and Chromosome Structure

2.1 Prokaryotic and Eukaryotic Cells Differ in Structure

• Life is classified into prokaryotes (bacteria, archaea) and eukaryotes; eukaryotes have a nucleus; prokaryotes do not.
• Genome organization:
  • Prokaryotes: typically a single circular chromosome; DNA generally not associated with histones; archaea have histone-like proteins; bacteria lack histones.
  • Eukaryotes: multiple linear chromosomes; DNA packaged with histones into chromatin; nucleus separates DNA from cytoplasm.
• Evolutionary note: bacteria, archaea, and eukaryotes are three major groups; archaea can be more similar to eukaryotes in some genetic processes.
• Extra DNA molecules exist:
  • Plasmids in many bacteria; some bacteria and archaea have multiple chromosomes.
  • Circular DNA in mitochondria and chloroplasts in some eukaryotes.

2.2 Cell Reproduction Requires the Copying of the Genetic Material, Separation of the Copies, and Cell Division

• Three essential events for reproduction: copy genetic information, separate copies, divide cytoplasm.
• Prokaryotic reproduction by Binary Fission:
  • Circular chromosome replicates; origin of replication; origins move toward opposite ends.
  • SMC complexes prevent tangling; new cell wall forms between chromosomes; two identical daughter cells.
  • Rate example: some bacteria divide every $20\ \text{min}$ under optimal conditions; potential to reach billions in $10\ \text{hours}$.
• Eukaryotic reproduction:
  • Multiple DNA molecules; nuclear membrane and nuclear matrix organize the nucleus.
  • Requires mechanisms to ensure exactly one copy of each DNA molecule ends up in each daughter cell.
• Eukaryotic chromosome concepts:
  • Most somatic cells are $2n$ (diploid) with two chromosome sets; humans have $46$ chromosomes and $23$ homologous pairs; sex chromosomes are XX or XY.

2.3 Eukaryotic DNA is Closely Associated with Histone Proteins (a) to Form Chromosomes

• In eukaryotes, DNA is packaged with histone proteins to form chromatin/ chromosomes.
• Histones regulate DNA accessibility to enzymes and reading machinery.
• Archaea have some histone-like proteins; bacteria lack histones.
• Chromosomes reside in the nucleus.

2.4 A Virus is a Simple Replicative Structure Consisting of Protein and Nucleic Acid

• Viruses are not cells; structure = outer protein coat encasing nucleic acid (DNA or RNA).
• Reproduction requires a host cell; evolutionary relationships to cellular life are uncertain.
• Viruses (e.g., bacteriophages) are used to study gene expression, recombination, and molecular evolution.

2.5 Diploid Eukaryotic Cells Have Two Sets of Chromosomes

• Diploid (2n): two homologous sets; each chromosome has a homolog at the same locus; alleles can differ.
• Haploid (n): single set; reproductive cells carry one set.
• Ploidy variation exists; humans: $2n=46$; $23$ homologous pairs; sex chromosomes are an exception (XX or XY).
• Some organisms are polyploid (more than two sets).

2.6 Each Eukaryotic Chromosome Has a Centromere and Telomeres

• A functional chromosome has: centromere, two telomeres, and origins of replication.
• Centromere: constricted region where kinetochores form; spindle microtubules attach during division.
• Kinetochore: multiprotein complex at the centromere.
• Telomeres: protective ends of linear chromosomes; prevent end degradation and regulate stability; linked to aging and cancer.
• Origins of replication: sites where DNA synthesis begins; not easily observed; discussed in more detail later.
• Replication produces sister chromatids held together at the centromere.

2.7 Eukaryotic Chromosomes Are Divided into Four Major Types Based on Centromere Position

• Centromere location defines chromosome type:
  • Metacentric
  • Submetacentric
  • Acrocentric
  • Telocentric
• Telomeres protect chromosome ends and ensure stability; origins of replication initiate DNA synthesis.
• Before cell division, each chromosome replicates to form two sister chromatids held at the centromere.