Chromosome and Extrachromosomal Structure

Overview

Earlier lessons in this module followed the central dogma at the level of individual molecules — how a strand is replicated, transcribed, and translated. This lesson zooms out to ask where that DNA actually lives. A molecular assay does not interrogate a free-floating gene; it interrogates a gene packaged into a chromosome inside a nucleus, or carried on a small circular genome inside a mitochondrion, or borne on a plasmid inside a bacterium. The physical form and, crucially, the copy number of the target determine how much template a test starts with — which in turn sets how sensitive the assay can be. This lesson surveys the human nuclear genome, the mitochondrial genome, and the extrachromosomal and microbial elements a molecular laboratory encounters.

The human nuclear genome

A human somatic cell is diploid: it carries two copies of each chromosome, one inherited from each parent. The nuclear genome is organized into 46 chromosomes — 22 pairs of autosomes plus one pair of sex chromosomes (XX in a typical female, XY in a typical male) ¹. The haploid complement (one of each chromosome, as found in a gamete) totals roughly 3.2 billion base pairs; the diploid genome of a somatic cell therefore contains about twice that amount of DNA ¹.

If stretched end to end, the DNA in a single cell would span about two meters, yet it is confined within a nucleus only micrometers across. This is achieved by hierarchical packaging: the double helix wraps around histone proteins to form nucleosomes, which coil and fold into progressively higher-order structures and, during cell division, condense into the compact chromosomes visible under a microscope ¹. For the molecular laboratory, the key consequence is that a typical gene is present at only two copies per cell — the baseline against which other targets are compared.

Chromosome anatomy

Each condensed chromosome has a recognizable shape defined by a few landmarks:

        p arm (short)
   ===============
            ||
         [======]   <- centromere (constriction)
            ||
   =========================
        q arm (long)

  telomere ......... telomere
  (protective cap)   (protective cap)

• Centromere — the constricted region that holds the two sister chromatids together and serves as the attachment point for the spindle during division. Its position divides the chromosome into two arms.
• p arm and q arm — the short arm (p, from French petit) and the long arm (q) extend from the centromere ¹.
• Telomeres — repetitive sequences that cap each chromosome end, protecting it from degradation and from being mistaken for a broken end ¹.

Karyotype and band nomenclature

A karyotype is the ordered display of an individual’s full chromosome set, arranged by size and centromere position. When chromosomes are stained, they show a reproducible pattern of light and dark bands, and this banding gives each region an address. In standard cytogenetic nomenclature, a location is written as the chromosome number, the arm (p or q), and then the band and sub-band — for example, 7q31 denotes chromosome 7, long arm, region 3, band 1 ¹. This shorthand lets a result name precisely where on a chromosome a feature lies. The mechanics of cytogenetic and molecular-cytogenetic testing are developed in later coursework; here the goal is only to read the address.

The mitochondrial genome

Not all human DNA resides in the nucleus. Each mitochondrion carries its own small, circular genome (mtDNA), a remnant of the organelle’s bacterial ancestry ¹. Several features distinguish it from nuclear DNA and matter directly for testing:

• Maternal inheritance — mtDNA is transmitted through the egg, so it is passed essentially only from mother to offspring ¹.
• Self-encoded translation machinery — the mitochondrial genome encodes a set of its own tRNAs and rRNAs along with a handful of proteins of the respiratory chain; most mitochondrial proteins, however, are encoded in the nucleus and imported ¹.
• High copy number — a cell contains many mitochondria, and each mitochondrion holds multiple copies of the genome, so mtDNA is present at hundreds to thousands of copies per cell rather than the two copies of a nuclear gene ².

That abundance is the reason mitochondrial targets are attractive when sample material is scarce or degraded: with far more starting template per cell, an mtDNA target can be detected from inputs that would yield too little signal from a single-copy nuclear gene ³.

Extrachromosomal and microbial elements

Molecular laboratories routinely work with genetic material that is not part of any chromosome at all. These elements are central both to the biology under test and to the laboratory’s own tools.

Bacterial plasmids are small, circular, double-stranded DNA molecules that replicate independently of the bacterial chromosome. They often carry genes that confer a selective advantage — notably antibiotic-resistance genes — and they can spread between bacteria ¹. Because plasmids are easy to manipulate and replicate, they are the workhorse cloning vectors of the molecular laboratory: a sequence of interest is inserted into a plasmid, the plasmid is introduced into bacteria, and the bacteria are grown to produce many copies ³. Like mtDNA, plasmids can be present in many copies per cell, depending on the plasmid.

Bacteriophage (“phage”) are viruses that infect bacteria. Their genomes, too, have been adapted as cloning vectors and as tools in molecular biology ¹.

More broadly, viral genomes vary in chemistry and form. A virus may carry its genetic information as DNA or RNA, and that genome may be single-stranded or double-stranded ¹. This diversity is the reason a single nucleic-acid extraction or detection chemistry does not fit every pathogen — the assay must match the genome it targets. The specifics of detecting viral and other infectious agents are taken up as a later topic in infectious-disease testing.

Why structure and copy number matter in the laboratory

The recurring theme across these genomes is target copy number, because the amount of template a specimen contains sets a floor on assay sensitivity:

  Target                         Copies per cell         Relative abundance
  ---------------------------    --------------------     ------------------
  Single-copy nuclear gene       2 (diploid)              lowest
  Mitochondrial genome           hundreds to thousands    high
  Plasmid (per bacterium)        one to many              variable, often high

A test aimed at a single-copy nuclear gene begins with very little template per cell and therefore demands efficient extraction and amplification to reach a detectable signal. A multi-copy mitochondrial target or a high-copy plasmid supplies far more starting material, so a positive result can be obtained from smaller or more degraded samples ³. Knowing whether a target is nuclear, mitochondrial, or extrachromosomal — and how many copies of it a cell carries — is thus a first step in reasoning about what an assay can and cannot detect. The amplification and detection methods that exploit these differences are developed in later modules.