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Description and types of bacterial gene

Bacterial genes and eukaryotic genes have significant structural differences.

The majority of the bacterial genome is coding; about 90% of the DNA found in E. coli codes for protein. This figure can be as low as 3% in some eukaryotes.

Bacterial genes nearly always lack introns (non-coding DNA within a gene).

This means that genes are typically much closer together on the bacterial chromosome and clusters of genes (operons) can be transcribed together.

This is a faster and more straightforward way of producing proteins than the eukaryotic method. Due to the small quantity of non-coding DNA, the bacterial gene can be considered to be 'tidier' than the prokaryotic genome.

The human genome on the left contains large sections of non-coding DNA. Almost all of the prokaryotic genome on the right will code for protein.
The human genome on the left contains large sections of non-coding DNA. Almost all of the prokaryotic genome on the right will code for protein.

Bacterial genes and eukaryotic genes both code for proteins, but the processes involved are different.

In eukaryotes, translation occurs in the nucleus and only translation occurs in the cytoplasm.

Bacteria do not have nuclei. Both transcription and translation occur in the cytoplasm of the bacterium.

This means transcription and translation can occur simultaneously as there is no compartmentalisation of the two processes.

Bacterial gene organisation and gene expression are much simpler than eukaryotic gene organisation and gene expression; this reflects the relatively simple structure of bacteria.

Bacterial genes can be divided into two categories:

  • Structural genes code for RNA and protein products with structural functions.
  • Regulatory genes code for RNA and proteins that regulate the expression of structural genes.

Bacterial DNA is not confined to a nucleus. However, it is often confined to a particular region of the cell, called the nuclear body or nucleoid (this region is not bound by a membrane).

Bacteria carry the majority of their DNA in a single circular chromosome. As bacteria have only one copy of this chromosome, they are haploid (not diploid like most animal cells).

Some bacteria carry part of their genomes in plasmids.

Plasmids are sections of DNA which are not part of the chromosome and are able to replicate and be transmitted independently. Plasmids are important in maintaining genetic variation amongst bacterial cells.

Genome of a bacterium with plasmids
Genome of a bacterium with plasmids

Bacteria have both structural and regulatory genes.

Structural genes are genes that code for RNA and proteins that have structural functions, and enzymes. They are not involved in gene expression.

Structural genes are hugely diverse and are responsible for building the cell and controlling the reactions that occur within it.

The majority of structural genes code for proteins.

Some genes code for ribosomal RNA. rRNA is not a protein but is essential for translation as it forms part of the ribosome.

Regulatory genes control the genes that are involved with protein and trait expression.

Transcription factors are common regulatory genes. These influence the rate of transcription of other genes.

The bacterial chromosome has distinct features and is very different from the eukaryotic chromosome.

The bacterial chromosome is circular (rather than linear), has a single origin of replication and lacks structural proteins called histones (proteins that fold eukaryotic DNA).

The bacterial chromosome is sometimes said to not be a "true chromosome" because it lacks so many of the features that are characteristic of a eukaryotic chromosome.

The human genome on the left contains several linear chromosomes. The prokaryotic genome is organized as one large loop.
The human genome on the left contains several linear chromosomes. The prokaryotic genome is organized as one large loop.

Plasmids are small circular sections of DNA. These contain approximately 1000 to 1 000 000 nucleotide bases (1-100kbp).

A single plasmid usually carries between 5 and 100 genes.

Plasmids tend to code for strongly advantageous traits. For example, traits that enable the bacterium to survive in the presence of antibiotics, utilise new carbon sources or cause disease.

Plasmids can replicate and be transmitted independently.

This means that genes can be passed horizontally (between cells in a generation) as well as vertically (from parent to daughter cell).

Vertical transfer through binary fission is the primary method of plasmid inheritance.

Horizontal gene transfer allows advantageous traits to pass rapidly through populations. It also increases the genetic variation in a population, increasing adaptability.

Bacterial genome
Bacterial genome