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Prokaryote Reproduction

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How do bacteria reproduce?

Essentially, they grow and divide. Shown here is an example of Methicillin-resistant Staphylococcus aureus , or MRSA, bacteria. Notice how one bacterium is dividing into two.

Reproduction in Prokaryotes

Increases in the size of prokaryotes (cell growth) and their reproduction by cell division are tightly linked. Prokaryotes grow to a fixed size and then reproduce through binary fission.  

Binary Fission

Binary fission is a type of asexual reproduction. It occurs when a parent cell splits into two identical daughter cells. This can result in very rapid population growth. For example, under ideal conditions, bacterial populations can double every 20 minutes.

License: CC BY-NC 3.0

 1: The bacterium before binary fission has the DNA tightly coiled.

2: The DNA of the bacterium has replicated .

3: Each bacterial chromosome is attached to the inner plasma membrane (not shown here) and as the cell elongates the chromosomes separate.

4: The growth of a new cell wall and plasma membrane begins to separate of the bacterium. Cytoplasmic structures such as plasmids (when present) and ribosomes will be produced and separated at this time. 5: The new complete cell wall and plasma memberanes result in the complete split of the bacterium.

6: The new daughter cells have tightly coiled DNA, ribosomes, and plasmids.                                                                                           

  http://en.wikipedia.org/wiki/File:Binary_Fission.png   Creative Commons

 

Growth in Bacterial Populations

 Since bacteria can divide very rapidly, the presence of 1 bacterium can lead to the creation of a large population over a relatively short period of time.  We say that bacterial growth is exponential meaning that the bacteria will continue to divide and increase in numbers at a steady rate. This exponential growth will continue until the bacteria  reach the limit of their required resources (usually that is nutrientsand raw materials).  But does this growth begin as soon at that bacterium falls into your tuna salad?  Let's look at a typical bacterial growth curve for a closed population.  A closed population means that the bacteria can't leave and go somewhere else.  The bacteria in the container of tuna salad or the bacteria on a petri dish of nutient agar would both be examples of closed populations.

Bacterial Growth Curve

License: CC BY-NC 3.0

[Figure2]
                                                                                                                    http://en.wikipedia.org/wiki/File:Bacterial_growth_en.svg  Creative Commons

The phases of the growth curve:

Lag Phase  : the bacteria or even one bacterium are in a new environment.  During this phase they acclimate and become accustom to the new conditions of the environment, they increase in size and produce the substances needed to begin binary fission.

Exponential Phase : the bacteria begin to divide by binary fission at a steady rate This is eponential because 1 bacterium becomes two and then those two bacteria give rise to 4 and the 4 to 8, etc.   Theoretically the exponential phase would continue if all the necessary materials were present, but the number of nutrients and raw materials begins to decline and the amount of waste products begin to increase in a closed population.

Stationary Phase : at this point the rate of growth slows to the point that the number of new bacteria produced is equal to the number of bacteria that are dying.  The result is a steady population, however, nutrients and raw materials needed for continued growth are declining and waste products are increasing.

Death Phase : the lack of nutrients and raw materials along with the accumulation of waste products causes the number of bacterial deaths to exceed the number of new bacteria produced.  This phase will continue until the whole population of bacteria has died.

It should be noted that those bacteria that can produce endospores will begin producing those endospores toward the end of the stationary phase and during the death phase.  Remember that endospores allow some species of bacteria to form a dormant structure that encases the bacterial chromosome and can allow it to survive in a dormant state for a very long time.

 

 Bacterial Variation

Since bacteria only reproduce asexually. In asexual reproduction, all the offspring are exactly the same. This is the biggest drawback of this type of reproduction. Why? Lack of genetic variation increases the risk of extinction. Without variety, there may be no organisms that can survive a major change in the environment. Bacteria gain variations that allow them to adapt to changing conditions by several means:

Mutations, Transformation, Transduction, and Conjugation

Mutations

Mutations are changes in the DNA of the chromosome.  The change may cause genes to give instructions for new or slightly altered proteins.  Sometimes the new proteins are beneficial to the organism, sometimes the new proteins are detrimental.

Transformation

 Transformation occurs when a bacterium  takes in a short strand of DNA from the external environment and incorporates it into its own chromosome and then expresses the gene or genes that were present on that DNA fragment. (See Drawing below ) Only a very few bacteria are capable of transformation, but it does have the potential to give bacteria variation.  After the discovery of natural transformation in bacteria, a technique was perfected in the laboratory to intentionally introduce new DNA to bacteria as a form of genetic engineering.

 

License: CC BY-NC 3.0

 Transduction

Transduction occurs when DNA is transfered from one bacterium to another by a virus.( See drawing below ) The virus may pick up a small segment of DNA from a bacterium that it infects.  When copies of the virus are made by the infected cell, each new virus will contain viral DNA and may also contain a small segment of bacterial DNA.  The next bacterium infected by one of these viruses will not only get the viral DNA, but the small segment of the bacterial DNA carried by the virus.  This can give that bacterium the ability to produce new proteins and therefore increase variation in that bacterium. 

License: CC BY-NC 3.0

                                                                     http://en.wikipedia.org/wiki/Transduction_(genetics ) Creative Commons

 

Conjugation 

Bacterial conjugation occurs when bacterial cells directly exchange DNA (usually plasmids) with other cells. For example, as shown in Figure below , the donor cell makes a structure called an F pilus , or sex pilus. The F pilus attaches one cell to another cell. The membranes of the two cells merge and a copy of the genetic material, usually a plasmid ,  moves from the cell that develed the F pilus into the recipient cell. Genetic transfer makes bacteria very useful in biotechnology. It can be used to create bacterial cells that carry new genes.

It is important to note that this is NOT sexual reproduction.  Conjugation begins between two cells and ends with two cells.  There is no cellular reproduction taking place, but it does introduce variation to the recipient cell.

A flowchart showing bacterial conjugation.

A flowchart showing bacterial conjugation. The donor cell makes a structure called an F pilus, or sex pilus. The F pilus attaches one cell to another cell. The membranes of the two cells merge and genetic material, usually a plasmid, moves into the recipient cell.

 

 

Summary

  • Prokaryotic cells grow to a certain size. Then they divide by binary fission. This is a type of asexual reproduction.
  • Binary fission produces genetically identical offspring.
  • Bacteria in a closed population have a characteristic growth curve showing the rate of exponential growth and the point at which the number of bacteria begin to exceed the environments ability to sustain their rapid growth.
  • Genetic variation an occur in prokaryotes in several different ways..

Practice

Use this resource to answer the questions that follow.

  1. Describe the steps of binary fission.
  2. How quickly can some prokaryotes reproduce?
  3. What is the source of most genetic variation in prokaryotes?
  4. Describe the processes of transformation and conjugation.
  5. What is transduction?

Review

1. What is binary fission?

2. Why might genetic variation be important for the survival of prokaryote species?

3. Why might some of the methods of genetic transfer make genetic comparisons of prokaryotes difficult to interpret in studies of their evolution?

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0

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