1.3 Transmission of mutations by vertical gene transfer

Vertical gene transfer is the transfer of genetic information, including any genetic mutations, from a parent to its offspring. As you briefly saw in Week 1, bacteria reproduce by binary fission, where the cell divides into two identical daughter cells. As in humans, the genetic information in bacteria is encoded in DNA, which is packed into chromosomes. During binary fission, the chromosomal DNA is copied, so that each new daughter cell inherits an exact copy of the parent cell’s chromosomes (Figure 3).

Figure 3 The stages of binary fission.

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This figure is a schematic diagram showing the process of binary fission. The bacteria cell is represented as a blue oval surrounded by a blue plasma membrane and a brown cell wall. The chromosomal DNA is represented as a purple squiggle inside the bacterial cell. The process begins with duplication of the chromosomes, represented by an increase in the amount of purple. This DNA is then separated, represented by the presence of two purple squiggles. During this process the cell grows, represented by elongation of the oval. The cell divides giving two cells that are identical to the first image.

Figure 3 The stages of binary fission.

Activity 1 Exploring vertical transmission

Timing: Allow about 10 minutes

Begin by watching the following animation which illustrates the process of binary fission in E. coli. Then complete the activity below.

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Transcript: Video 3 Binary fission in E. coli.

TEXT ON SCREEN

Schematic representation of an E. coli bacterium. The region of the cytoplasm containing the single circular genomic DNA molecule is called the nucleoid. Between divisions, the bacterium grows by elongation from the middle of the cell, increasing cell mass. When the cell reaches a certain size, the circular DNA molecule is replicated (copied). The two DNA molecules each attach to a different part of the cell membrane, so that as the cell elongates, they segregate to opposite ends of the cell. The cell membrane constricts and new cell walls are formed at a point between the two DNA molecules. And the two identical cells finally separate.

End transcript: Video 3 Binary fission in E. coli.

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Video 3 Binary fission in E. coli.

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Apply what you have learned and the information about binary fission in the animation to complete the following sentences. Select the appropriate word from the list.

Select the answer for

Question 1a

here

(a) The DNA in both of the daughter cells is [identical/similar/different] to the DNA in the parent cell.

identical

similar

different

 

a. 

identical

b. 

similar

c. 

different

The correct answer is a.

Answer

During binary fission, the genetic material (DNA) is copied so that each new daughter cell inherits an exact copy of the parent cell's DNA.

Select the answer for

Question 1b

here

(b) If an E. coli bacterium contains a genetic mutation in a chromosomal pbp gene, both of its daughters will [always/sometimes/never] contain a mutation in the pbp gene.

sometimes

never

always

 

a. 

sometimes

b. 

never

c. 

always

The correct answer is c.

Answer

During binary fission, the genetic material (DNA) is copied, so that each new daughter cell inherits an exact copy of the parent cell’s DNA. When the parent DNA is copied during binary fission, any genetic mutations will also be copied, and consequently inherited, by both of the daughter cells.

Select the answer for

Question 1c

here

(c) If the parent bacterial cell contains a genetic mutation that results in resistance to β-lactam antibiotics, both of the daughter cells will [always/sometimes/never] be resistant to β-lactam antibiotics.

sometimes

always

never

 

a. 

sometimes

b. 

always

c. 

never

The correct answer is b.

Answer

During binary fission, the genetic material (DNA) is copied, so that each new daughter cell inherits an exact copy of the parent cell's DNA. When the parent DNA is copied during binary fission, any genetic mutations will also be copied, and consequently inherited, by both of the daughter cells. If these genetic mutations give rise to antibiotic resistance in the parent bacteria, they will also result in antibiotic resistance in both of the daughters.

Vertical gene transfer is only one of the ways in which bacteria can spread antibiotic resistance genes. In the next section you will look at another – horizontal transfer.