Horizontal gene transfer
Normally genes pass vertically: parent to offspring. Horizontal gene transfer (HGT) is the movement of genes between living cells that are not parent and child, even of different species. In bacteria it happens in three main ways:
- Transformation: a cell takes up free DNA from its surroundings.
- Conjugation: two cells join by a thin tube and copy a plasmid (a small ring of DNA) across.
- Transduction: a virus carries DNA from one bacterium to another.
HGT spreads useful genes quickly, such as antibiotic resistance. Many bacterial genomes contain a good share of genes that came from other species. HGT also exists in other life, for example some genes in plants and animals came from bacteria.
Viruses as gene vectors
A vector is something that carries a gene into a cell. Viruses are natural vectors: they inject their genes into a host and use the cell’s machinery to copy them. Some viruses, such as retroviruses, make a DNA copy of their RNA and insert it into the host chromosome. If that happens in a cell that makes eggs or sperm, the viral DNA is passed to children and becomes endogenous (part of the genome). About 8% of the human genome comes from such viruses. A few of these genes were put to use: one (called syncytin) helps build the placenta.
Viruses can also move host genes between cells (transduction). Scientists copy this idea in gene therapy and in biotechnology, using harmless viruses to deliver useful genes.
Endosymbiotic origin of organelles
The endosymbiotic theory says that mitochondria and chloroplasts were once free-living bacteria that were taken inside a larger cell, lived there helpfully (symbiosis) and over time became organelles. Mitochondria came from a bacterium that could use oxygen; chloroplasts from a photosynthetic cyanobacterium.
Evidence: both have a double membrane, their own circular DNA, their own small (70S) ribosomes, they divide on their own like bacteria, and their DNA looks like bacterial DNA. Over time many of their genes moved to the nucleus, so they cannot live alone now. This was a huge jump in complexity: the host gained a power plant or a solar panel.
Genome duplications
Gene duplication happens when a copy of a gene or a stretch of DNA is made by mistake, for example when chromosomes pair unevenly and swap pieces unequally. Now one copy can keep the old job while the other is free to mutate. The spare may gain a new function, share the old job, or break and become a pseudogene. The globin gene family (haemoglobin and its cousins) arose this way.
Sometimes the whole genome is doubled (polyploidy), when chromosomes fail to separate in cell division. This is common in plants: bread wheat is hexaploid (six sets of chromosomes, 42 in total). Early vertebrates also went through genome doublings. After doubling, many genes are lost again, but some stay and give new functions.
Key formulas and definitions
- Ploidy = total chromosomes ÷ chromosomes in one basic set
- Genome length after duplication = old length + length of duplicated piece
- Share of genome from a source = (DNA from source ÷ total DNA) × 100
- Vertical = parent → child; Horizontal = cell → unrelated cell
Worked examples
1. Bread wheat has 42 chromosomes and one basic set has 7. Find its ploidy.
Ploidy = 42 ÷ 7 = 6, so wheat is hexaploid.
2. The human genome is about 3000 million base pairs (Mb) and about 8% came from retroviruses. How much is that?
8% of 3000 = 0.08 × 3000 = 240 Mb.
3. A bacterial genome has 4000 genes. It receives a plasmid with 20 genes by conjugation. How many genes now, and what is the percentage increase?
New total = 4020. Increase = 20 ÷ 4000 × 100 = 0.5%.
4. List two pieces of evidence that mitochondria were once bacteria.
They have their own circular DNA and a double membrane; they divide by themselves and have bacteria-like ribosomes.
Common mistakes
- Confusing horizontal transfer with sexual reproduction. HGT moves genes between unrelated cells, not parents and children.
- Thinking viruses always harm. Some viral genes became useful parts of our genome.
- Thinking mitochondria are just parts made by the cell. They have their own DNA and came from a bacterium.
- Assuming a duplicate gene always creates something new. Many extra copies are lost or become pseudogenes.