Eukaryotic Genomes in Medical Biology

Genome Organization

• A genome is the complete genetic information of an organism, comprised of DNA.
• Eukaryotic genomes consist of one or more extremely long linear molecules of DNA assembled into chromosomes.
• The human genome consists of 46 linear DNA molecules with a total length of 2 meters and approximately 3.2 billion base pairs.
• Humans are diploid, meaning they have two sets of each chromosome.

Eukaryotic vs. Prokaryotic Genomes

• Eukaryotic cells have a nucleus with linear DNA molecules, while prokaryotic cells have a single circular DNA molecule with no nucleus.
• Prokaryotes generally have a single chromosome, whereas eukaryotes have multiple linear DNA molecules.

Human Genome

• The human genome contains approximately 20,500 genes, as determined by the Human Genome Project (2007).
• If the DNA molecules from all 46 chromosomes in a human cell were laid end to end, they would reach approximately 2 meters in length.

Prokaryotic Genome

• Prokaryotic genome is a single circular DNA molecule.
• Prokaryotes generally have a single chromosome with no mRNA post-transcriptional modification.

Eukaryotic Genome Paradoxes

• There are three genomic paradoxes:
  1. Complexity does not correlate with chromosome number.
  2. Complexity does not correlate with gene number.
  3. Complexity does not correlate with genome size.

Gene Structure

• A gene is a segment of DNA that encodes a functional cellular product, such as a polypeptide or RNA.
• Eukaryotic genes have a split structure, with coding sequences (exons) separated by noncoding sequences (introns).
• Control regions, such as promoters, enhancers, and silencers, are also present in eukaryotic genes.

Gene Expression

• The central dogma states that genes encode proteins indirectly through mRNA.
• Processing of eukaryotic mRNAs involves transcription of the entire gene, removal of introns by splicing, and addition of a cap and poly(A) tail.

Introns

• Introns are noncoding sequences present in most eukaryotic genes, but not in all eukaryotic genes (e.g., histone genes).
• Prokaryotic genes usually do not contain introns.
• Introns were discovered in 1977 by Sharp and Roberts, who received the Nobel Prize in 1993.

Functions of Introns

• Introns play important roles during evolution, such as the formation of new genes.
• Some introns encode functional RNA and proteins.
• Introns play a crucial role in controlling gene expression.