Microbial genetics

1. Microbial genetics
Bio 433
By
Dr. Mona Othman Albureikan

2. Translation (The Genetic Code)
• The genetic code used to produce
proteins is carried by DNA or mRNA.
• A code is needed to convert the
language of mRNA into the language
of proteins ( Amino acids).
• There are 21 naturally occurring
amino acids found in the structure of
proteins, and mRNA has only 4 bases
(AUCG).

4. • Scientist found that a group of 3
nucleotides codes for 1 amino acid.
• Each set of 3 nucleotides that code
for an amino acid is called a
codon.
• It was discovered that information
about amino acid is carried by
codons.
Translation (The Genetic Code)

6. • 64 codon combinations are possible based
on the original 4 DNA nucleotide triplet
pairs (43).
• 61 of these code for amino acids and 3
(UAA, UAG and UGA) have not associated
amino acids, thus serve as stop codons,
since they cause ribosomes to stop the
process of translation.
Translation (The Genetic Code)

7. • The mRNA code is "redundant,"
since with the exception of the
amino acids methionine and
tryptophan, there are multiple
codons that can code for the
same amino acid.
Translation (The Genetic Code)

8. The protein synthesis (Translation, mRNA)
• Translation is the process cells use to synthesize polypeptides, using the
codon sequence on the mRNA as a template.
• In the bacterial cell, several related polypeptide sequences are carried on
the same mRNA template.

9. • Each sequence on the mRNA strand has its own ribosome binding site, start
and stop codons.
• Since transcription occurs in the cytoplasm, translation can begin even
before transcription is complete.
The protein synthesis (Translation, mRNA)

10. The protein synthesis (Translation, mRNA)
• Eukaryotic mRNA differs from that of
prokaryotes in that it;
• (1) undergoes post-transcriptional modification.
• (2) carries instructions for the synthesis of only
one polypeptide.
• (3) is not translated until after it has been
transcribed completely, modified, and has left
the nucleus.

11. • Transfer RNA (tRNA) is a sequence of
~75 ribonucleotides that fold to form a
three-dimensional structure.
• Each tRNA has an acceptor stem that
forms a temporary bond with one amino
acid and an anticodon that can bind to
its complementary codon on mRNA.
The protein synthesis (Translation, tRNA)

12. • Is the base sequence of tRNA
which pairs with codon of mRNA
during translation is called
anticodon.
The protein synthesis (Translation, tRNA)
Anticodon;

13. • Though there are 62 codons on mRNA,
there are only about 40 tRNA molecules.
• Each tRNA anticodon can bind to a
mRNA codon provided that at least the
first two mRNA nucleotides are
complementary to its first two
nucleotides.
The protein synthesis (Translation, tRNA)

14. A tRNA molecule. In this series of diagrams, the same tRNA molecule—in this case a tRNA specific for the amino acid phenylalanine
(Phe)—is depicted in various ways. (A) The cloverleaf structure, a convention used to show the complementary base-pairing (red lines)
that creates the double-helical regions of the molecule. The anticodon is the sequence of three nucleotides that base-pairs with a codon
in mRNA. The amino acid matching the codon/anticodon pair is attached at the 3′ end of the tRNA. tRNAs contain some unusual bases,
which are produced by chemical modification after the tRNA has been synthesized. For example, the bases denoted Y (for
pseudouridine) and D (for dihydrouridine) are derived from uracil. (B and C) Views of the actual L-shaped molecule, based on x-ray
diffraction analysis. Although a particular tRNA, that for the amino acid phenylalanine, is depicted, all other tRNAs have very similar
structures. (D) The linear nucleotide sequence of the molecule, color-coded to match A, B, and C.

15. • This can occur because of "wobble," a phenomenon associated with
the ability of the third nucleotide on an
anticodon to hydrogen bond to another
nucleotide different from its complement.
• A wobble base pair is a pairing between
two nucleotides in RNA molecules that does not follow Watson-Crick
base pair rules.
The protein synthesis (Translation, tRNA)

16. • It is a single tRNA can recognize more than one codon.
• Unconventional base pair in the third base of the
codon/ anticodon.
• This redundancy enables the cell to produce
the proper polypeptide even if minor mutations
(errors) occur in the DNA during replication or the mRNA during
transcription.
The protein synthesis (Translation, tRNA)

17. Differences between codon & anticodon
• Codon could be present in both DNA & RNA,
but anticodon is always present in RNA & never
in DNA.
• Codons are written in 5 to 3 direction whereas
anticodons are usually written in 3 to 5 direction.
• Anticodon of some tRNA molecules have to pair
with more than one codon.

18. • Codons are sequentially arranged in nucleic acid
strand while anticodons are discretely present in
cells with amino acids attached or not.
• Codon defines which anticodon should come next
with an amino acid to create the protein strand.
• Anticodon helps in bringing a particular amino acid
at its proper position during translation.
Differences between codon & anticodon

19. “Codon-anticodon recognition”
• The codon of the mRNA
is recognized by the
anticodon of tRNA.
• They pair with each other
in antiparallel direction
(5’- 3’ of mRNA with 3’-
5’ of tRNA).

20. • Prokaryote ribosomes are 70S in size,
composed of a 30S small subunit and
a 50S large subunit.
• Eukaryote ribosomes are 80S, having
a 40S small subunit and a 60S large
subunit. The ribosomes in
mitochondria and chloroplasts are,
however, 70S like those of bacteria.
The protein synthesis (Translation, rRNA)

21. • Prokaryote ribosomes are synthesized in the cytoplasm, while eukaryote
ribosome subunits are synthesized in the nucleolus and assemble
themselves in the cytoplasm or along the rough endoplasmic reticulum.
The protein synthesis (Translation, rRNA)

22. • The differences in the size and rRNA between the two types serves as a
target for selectively toxic antimicrobials such as erythromycin, that binds
to the 23S rRNA portion of the bacterial ribosome (the eukaryote rRNA
portion is 28S).
The protein synthesis (Translation, rRNA)

23. The protein synthesis (structure of ribosomes)
Ribosomes:
1- P site (peptidyl-tRNA site);
holds tRNA carrying growing
polypeptide chain.
2- A site (aminoacyl-tRNA site)
holds tRNA carrying next amino
acid to be added to chain.
3- E site (exit site) empty tRNA
leaves ribosome from exit site.

24. The protein synthesis (Translation steps)

25. The protein synthesis (Translation steps)

26. The protein synthesis (Translation steps)

27. • Prokaryote translation
always begins with the
amino acid
N-formylmethionine (f-met),
while eukaryote translation
always begins with
methionine (met).
The protein synthesis (Translation steps)

28. • In both eukaryotes and prokaryotes,
elongation of the polypeptide chain is
powered by guanosine triphosphate (GTP)
and terminates at a stop codon with the
help of release factor proteins that breaks
the bonds between the tRNA and final
amino acid in the sequence, allowing the
ribosome subunits to detach.
The protein synthesis (Translation steps)

29. Comparison of genetic processes

30. References
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Champness ISBN 10: 1555816274 ISBN 13: 9781555816278.
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 Genetics of Bacteria, Sheela Srivastava,(2013) ISBN: 978-81-322-1089-4
 Microbial Genetics. (1994). Jones and Bartlett Series in Biology. Jones and Bartlett Publishers, Inc.; 2nd
edition, ISBN-10: 0867202483, ISBN-13: 978-0867202489, 484 pages.
 Microbial genetics. (2008). Jones and Bartlett series in biology
Series of books in biology. David Freifelder, publisher, Jones and Bartlett, 1987. 601 pages.
 Molecular Biology: Genes to Proteins Hardcover . (2007). Burton E. Tropp, Publisher: Jones & Bartlett Publishers; 3
edition, ISBN-10: 0763709166, ISBN-13: 978-0763709167, 1000 pages .