Genetic Transfer and Recombination

Genetic recombination, the rearrangement of genes from separate groups of genes, usually involves DNA from different organisms; it contributes to genetic diversity.

In crossing over, genes from two chromosomes are recombined into one chromosome containing some genes from each original chromosome.

Vertical gene transfer occurs during reproduction when genes are passed from an organism to its offspring.

Horizontal gene transfer in bacteria involves a portion of the cell’s DNA being transferred from donor to recipient.

Transformation

Conjugation

Transduction

When some of the donor’s DNA has been integrated into the recipient’s DNA, the resultant cell is called a recombinant.

Transformation in Bacteria

Let's take a look at Griffith's experiment:

Transformation was first demonstrated in Griffith’s experiment, using Streptococcus pneumoniae, and is a process that occurs naturally among a few genera of bacteria.
		(a) Inject living encapsulated bacteria into mice, mice die, encapsulated bacteria isolated from dead mice.   (b) Inject living nonencapsulated bacteria into mice, mice remain healthy, a few non-encapsulated bacteria can be isolated from the living mice – most phagocytized by leukocytes.   (c) Inject heat-killed encapsulated bacteria into mice, mice remain healthy, no bacteria isolated from the living mice.   (d) Inject living non-encapsulated and heat-killed encapsulated bacteria into mice, mice die, isolated encapsulated bacteria from dead mice.
Resurrection? No, transformation, a process in which genes are transferred from one bacterium to another as “naked” DNA in solution.
Some of the non-encapsulated bacteria took up pieces of DNA from dead encapsulated bacteria, some of the pieces contained the capsule gene and recombined with the recipient's chromosome, transforming the avirulent non-encapsulated bacteria into the encapsulated virulent strain.

Conjugation in Bacteria

This process requires contact between living cells. Gram-negative cells produce sex pili that bring the cells into contact; gram-positive cells produce surface molecules that interact to form a "mating bridge".

The following describes the process as it occurs in E. coli.

One type of genetic donor cell is an F⁺; recipient cells are F^-. F⁺ cells contain plasmids called F factors; these are transferred to the F^- cells during conjugation and convert the F^-cell into an F⁺ cell.

If the plasmid becomes incorporated into the chromosome, the cell is called an Hfr (high-frequency recombinant).

During conjugation, an Hfr can transfer chromosomal DNA to an F^-. Usually, the Hfr chromosome breaks before it is fully transferred.

Click here for McGraw-Hill conjugation animation

Transduction in Bacteria

In this process, DNA is passed from one bacterium to another in a bacteriophage and is then incorporated into the recipient’s DNA.

In generalized transduction, any bacterial genes can be transferred.

Click here for McGraw-Hill transduction animation

Plasmids and Transposons

Plasmids are self-replicating circular molecules of DNA carrying genes that are not usually essential for survival of the cell.

There are several types of plasmids:

a. Conjugative plasmids – genes for sex pili and conjugation

b. Dissimulation plasmids – genes for enzymes that catabolize unusual organic molecules (Pseudomonas species – toluene, camphor, petroleum products)

c. Plasmids carrying genes for toxins or bacteriocins

d. Plasmids carrying genes for resistance (R) factors

i. Consist of two sets of genes – RTF (resistance transfer factor) and specific resistance genes (r-determinant)

Click here for McGraw-Hill transposon animation

Genes and Evolution

Diversity is the precondition of evolution.

Genetic mutation and recombination provide a diversity of organisms, and the process of natural selection allows the growth of those best adapted for a given environment.