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  • Like other organisms, bacteria1 use double-stranded DNA2 as their genetic3 material. However, bacteria organise their DNA differently to more complex organisms.

    Bacterial DNA – a circular chromosome plus plasmids

    The DNA of most bacteria is contained in a single circular molecule4, called the bacterial chromosome. The chromosome5, along with several proteins and RNA molecules6, forms an irregularly shaped structure called the nucleoid. This sits in the cytoplasm7 of the bacterial cell.

    In addition to the chromosome, bacteria often contain plasmids – small circular DNA molecules. Bacteria can pick up new plasmids from other bacterial cells (during conjugation) or from the environment. They can also readily lose them – for instance, when a bacterium8 divides in two, one of the daughter cells9 might miss out on getting a plasmid.

    Every plasmid10 has its own ‘origin of replication’ – a stretch of DNA that ensures it gets replicated (copied) by the host bacterium. For this reason, plasmids can copy themselves independently of the bacterial chromosome, so there can be many copies of a plasmid – even hundreds – within one bacterial cell11.

    Plasmids help bacteria to survive stress

    Plasmids contain just a few genes12, but they make a big difference to their host13 bacterium. The genes are usually not essential for the bacterium’s day-to-day survival – instead, they help the bacterium to overcome occasional stressful situations. For instance, many plasmids contain genes that, when expressed, make the host bacterium resistant to an antibiotic (so it won’t die when treated with that antibiotic14). Other plasmids contain genes that help the host to digest unusual substances or to kill other types of bacteria.

    Plasmids make themselves indispensable

    Keeping a plasmid is hard work for a bacterial cell, because replicating DNA (including plasmid DNA) uses up energy. However, by protecting its bacterial host from stress-related death, a plasmid maximises its chances of being kept around. Under stressful conditions15, bacteria with the plasmid will live longer – and have more opportunity to pass on the plasmid to daughter cells or to other bacteria. Bacteria without the plasmid are less likely to survive and reproduce.

    Some plasmids take extreme measures to ensure that they are retained within bacteria. For example, some carry a gene that makes a long-lived poison16 and a second gene17 that makes a short-lived antidote18. These plasmids are effectively holding their host bacterial cell hostage – if they are ever lost from the cell, they won’t be able to provide the antidote and the cell will die.

    Plasmids in biotech – delivering DNA

    Plasmids have been key to the development of molecular biotechnology. They act as delivery vehicles, or vectors19, to introduce foreign DNA into bacteria. Using plasmids for DNA delivery began in the 1970s when DNA from other organisms was first ‘cut and pasted’ into specific sites within the plasmid DNA. The modified plasmids were then reintroduced into bacteria.

    Why plasmids are excellent DNA delivery vectors

    Decades after their first use, plasmids are still crucial laboratory tools in biotechnology:

    • Scientists can force bacteria to keep them. Virtually all plasmids that are used to deliver DNA contain genes for antibiotic resistance. Once bacteria have been treated with a plasmid, scientists grow them in the presence of antibiotic. Only those cells that contain the plasmid will survive, grow and reproduce. The others will be killed by the antibiotic.
    • They are copied independently. Plasmids can be copied numerous times, regardless of whether the bacterial host is replicating its own DNA, and every time a plasmid vector is replicated, so is the introduced DNA that it contains.
    • They are circular. DNA that is circular is well suited to incorporate extra DNA sequences. That’s because it can be cut open without falling apart, then snap back together once new DNA has been incorporated.

    Read more about how to add foreign DNA to bacteria.

    1. bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
    2. DNA: Deoxyribonucleic acid (DNA) is a molecule that contains the instructions needed for an organism to develop and function. These instructions are stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T).
    3. genetic: Of, relating to, or determined by genes.
    4. molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
    5. chromosome: A structure within the cell nucleus made of a single coiled piece of DNA that contains the genetic blueprint of an organism.
    6. molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
    7. cytoplasm: All of the contents of a cell outside of the nucleus.
    8. bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
    9. daughter cell: One of two (or more) new cells that is formed when a cell divides.
    10. plasmid: A short circular DNA sequence found in bacterial cells and occasionally in other cells. Plasmids can replicate independently of the bacterial chromosome.
    11. cell: 1. Building block of the body. A human is made of millions of cells, which are adapted for different functions and can reproduce themselves exactly. 2. A simple electrolytic device that enables chemical energy to be transformed into electrical energy.
    12. genes: A segment of a DNA molecule that carries the information needed to make a specific protein. Genes determine the traits (phenotype) of the individual.
    13. host: An organism that contains a parasite or other dependent organism. Hosts usually provide the dependent organism with food and shelter.
    14. antibiotic: A substance produced by an organism that kills bacteria.
    15. condition: An existing state or situation; a mode or state of being.
    16. poison: A substance that, through its chemical action, usually kills, injures or impairs an organism.
    17. genes: A segment of a DNA molecule that carries the information needed to make a specific protein. Genes determine the traits (phenotype) of the individual.
    18. antidote: A remedy to counteract the effects of poison.
    19. vector: Biology: a carrier, i.e. a mosquito is a vector for malaria. Biocontrol: a vector carries the control agent to the target organisms, i.e. blowflies (vector)​ spread calicivirus amongst rabbits. Genetic engineering: a tool used to carry a gene of interest. Vectors are small pieces of DNA, often a plasmid, and are used to carry foreign DNA into a cell. Physics: a quantity that has both magnitude and direction.
    Published 25 March 2014 Referencing Hub articles
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        bacteria

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      2. (Singular: bacterium) Single-celled microorganisms that have no nucleus.

        molecule

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      4. Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.

        daughter cell

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      6. One of two (or more) new cells that is formed when a cell divides.

        genes

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      8. A segment of a DNA molecule that carries the information needed to make a specific protein. Genes determine the traits (phenotype) of the individual.

        condition

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      10. An existing state or situation; a mode or state of being.

        vector

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      12. Biology: a carrier, i.e. a mosquito is a vector for malaria. Biocontrol: a vector carries the control agent to the target organisms, i.e. blowflies (vector)​ spread calicivirus amongst rabbits. Genetic engineering: a tool used to carry a gene of interest. Vectors are small pieces of DNA, often a plasmid, and are used to carry foreign DNA into a cell. Physics: a quantity that has both magnitude and direction.

        DNA

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      14. Deoxyribonucleic acid (DNA) is a molecule that contains the instructions needed for an organism to develop and function. These instructions are stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T).

        chromosome

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      16. A structure within the cell nucleus made of a single coiled piece of DNA that contains the genetic blueprint of an organism.

        plasmid

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      18. A short circular DNA sequence found in bacterial cells and occasionally in other cells. Plasmids can replicate independently of the bacterial chromosome.

        host

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      20. An organism that contains a parasite or other dependent organism. Hosts usually provide the dependent organism with food and shelter.

        poison

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      22. A substance that, through its chemical action, usually kills, injures or impairs an organism.

        genetic

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      24. Of, relating to, or determined by genes.

        cytoplasm

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      26. All of the contents of a cell outside of the nucleus.

        cell

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      28. 1. Building block of the body. A human is made of millions of cells, which are adapted for different functions and can reproduce themselves exactly.

        2. A simple electrolytic device that enables chemical energy to be transformed into electrical energy.

        antibiotic

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      30. A substance produced by an organism that kills bacteria.

        antidote

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      32. A remedy to counteract the effects of poison.