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Summary Genetics Chapter 7
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A summary of Genetics Chapter 7 of the book 'Genetics Analysis& Principles'.
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7 Genetic transfer and mapping inbacteria
7.1 Overview of genetic transfer in bacteria (table 7.1)
Genetic transfer= transfer pf genetic material from 1 bacterial cell to another-> enhances the genetic diversity of
bacterial species-> can occur in 3 ways:
Conjugation= direct physical interaction between 2 bacterial cells (donor’s genetic material recipient cell).
Transduction= virus (bacteriophage) transfers bacterial DNA from 1 bacterium to another.
Transformation= bacterial cell takes up genetic material from the environment.
7.2 Bacterial conjugation
Bacteria can transfer genetic material during conjugation (fig 7.1)
Lederberg+ Tatum: discovered genetic transfer in bacteria; E. coli strains+ different nutritional growth requirements.
Minimal medium= type of growth medium for microorganisms that contains a mixture of nutrients that are
required for growth; nothing additional has been added.
Auxotroph= strain that can't synthesize a particular nutrient; needs to be added to its growth medium.
Prototroph= strain that doesn’t need a particular nutrient included in its growth medium.
EXPERIMENT LEDERBERG+ TATUM
Met− bio− thr+ leu+ thi+ strain: auxotroph for met+ bio (needs to be added to its growth medium to grow).
Met+ bio+ thr− leu− thi− strain: auxotroph for thr, leu+ thi (3 defective genes)-> prototroph for met+ bio.
Differences in nutritional requirements; variations in the genetic material of the 2 strains.
No colony growth; medium doesn’t contain the nutrients needed.
2 strains mixed: colony growth-> genotype colonies: met + bio+ thr+ leu+ thi+.
Not due to mutations; genetic material transferred between the 2 strains.
Conjugation required direct physical contact (fig 7.2)
U-tube+ filter with pores small enough to let genetic material through (but permits passage bacterial cells).
The 2 different strains↑ on each side+ pressure/ suction for movement of liquid through the filter.
Bacteria couldn’t move through the filter; genetic material could.
Grown on minimal medium lacking met, bio, thr, leu+ thi (but contained all other nutrients for growth).
No bacterial colony growth bacterial strains must make direct physical contact for transfer to occur.
In this case, conjugation= DNA transfer between bacterial cells that require direct cell-to-cell contact.
Not all bacteria species can conjugate.
An F+ strain transfers an F factor to an F− strain during conjugation (fig 7.3+
7.4)
Only certain E. coli strains can act as donor cells; contain F factor (fertility factor; plasmid)= small circular segment of
genetic material found in certain bacteria strains; carries genes that allow the bacteria to conjugate.
Proteins encoded by F factor genes: needed to transfer DNA strand from the donor cell to a recipient cell.
F+ (bacteria strains with F factor)+ F- (bacteria strains without F factor).
Molecular events that occur during conjugation in E. coli:
1. Contact between donor+ recipient cells : mediated by sex pili (attachment site F- bacteria).
Contact made: pili shorten; drawing donor+ recipient cells closer together.
Conjugation bridge formed between the 2 cells.
2. Transfer of F factor DNA: genes within F factor encode relaxosome.
Relaxosome cut at the origin of transfer; begins to separate the DNA strands (making the T DNA).
3. Export nucleoprotein from donor to recipient cell : T DNA/ relaxase complex (nucleoprotein) recognized by
coupling factor+ transferred to exporter.
4. Exporter pumps nucleoprotein through conjugation bridge into recipient cell.
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