The Genome

Core Concepts

Prokaryotic versus eukaryotic genomes

Prokaryotes (bacteria and archaea) typically carry their genome as one circular chromosome in a cytoplasmic region called the nucleoid (not enclosed in a membrane). Many also carry smaller DNA circles called plasmids, often bearing extras like antibiotic resistance genes. Eukaryotes, by contrast, have multiple linear chromosomes in a membrane-bound nucleus. The linear form creates a special problem: the strand ends, called telomeres, need protection, and chromosome tips shorten with each round of replication — a topic that returns in discussions of aging. The nuclear envelope and protein-packed chromosomes also explain why eukaryotic division is so much more elaborate than prokaryotic division.

DNA packaging: from double helix to chromosome

Eukaryotic DNA is never naked in the nucleus; it is wrapped around proteins. The packaging ladder works like this:

1. Double helix. The DNA molecule itself, about 2 nm across.
2. Nucleosome. DNA wraps about 1.7 turns around a core of eight histone proteins (two each of H2A, H2B, H3, H4), forming a bead about 10 nm across; short linker segments connect the beads — the classic "beads on a string" appearance.
3. 30-nm fiber. The beads coil into a thicker fiber with the help of histone H1.
4. Loops and higher-order folding. The fiber forms loops on a protein scaffold, folding further into the compact chromosome.
5. Metaphase chromosome. At its most condensed — during cell division — each chromosome is a dense X-shaped structure (the two identical sister chromatids still joined after replication).

The same DNA can exist in different packaging states: loosely packed, gene-rich regions called euchromatin are accessible for transcription, while tightly packed, gene-poor heterochromatin is largely inactive. Packaging is not just storage — it is a control dial for gene expression.

Genes within the genome

A gene is a segment of DNA carrying information for a functional product, usually a protein (or an RNA molecule). A single chromosome is one long DNA molecule containing many genes — in humans, chromosome 1 alone holds thousands (commonly taught estimates; exact counts depend on the reference annotation). Only a small fraction of the human genome encodes proteins; the rest includes regulatory sequences, noncoding RNA genes, and DNA whose functions are still being mapped. This is why genome size does not equal complexity or gene count: organisms with much larger genomes than humans exist, often because of duplicated or noncoding DNA.

Chromosomes, karyotypes, and ploidy

Each species has a characteristic chromosome number; humans have 46 chromosomes: 23 pairs (a commonly taught reference value). The two members of a pair are homologous chromosomes — one from each parent — carrying matching genes at matching positions (though possibly different versions, called alleles). A cell with paired homologs is diploid (2n); a cell with one of each pair, like a gamete, is haploid (n). A karyotype displays a cell's chromosomes arranged in pairs by size and shape, and it is how chromosomal conditions are diagnosed — for example, an extra copy of chromosome 21 in Down syndrome (trisomy 21). Errors in chromosome number arise from mistakes in the cell division covered later in this chapter — exactly why this topic precedes them.

Telomeres and chromosome stability

Linear chromosomes have ends, and ends are a problem: the replication machinery cannot copy the very tip of a DNA strand, so chromosomes would shorten with every division. Telomeres — repetitive DNA sequences at the tips, bound by protective proteins — cushion this loss, and when they get critically short, cells stop dividing. An enzyme called telomerase can extend telomeres; it is active in stem cells and germ cells and is also found in many cancer cells, which use it to keep dividing. Telomere biology links this topic to aging, cancer, and the cell cycle.

Common Confusions