Microbiology
Notes…
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Microbial Genetics Lecture.2
Flow of Genetic Information
Genetic information can be transferred either:
▪ Within the cell itself (in this case the genetic information is used to produce proteins
needed for the cell function).This
is called “Gene expression”
▪
Or from parent cell to daughter cells (during cell division). This is called “Replication”
▪ Or from one cell to another cell of same generation, resulting in new combinations
of genes ”Recombination”
Gene Expression and Protein Synthesis
Gene (DNA) contains all the information needed by cell to make a particular protein.
But, how the gene changes to a protein?
Central Dogma of Molecular Genetics
Transcription and processing of the newly made mRNA occurs in the nucleus of the cell
(eukaryotic cell) or nuclear area (prokaryotic cell)
Once a mature mRNA transcript is made it is transported to the cytoplasm for translation
into protein
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Microbiology
Notes…
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Transcription
Double stranded DNA is transcribed into single stranded messenger RNA (mRNA)
Transcription: Initiation
In case of eukaryotic cell:
Ist step in transcription process is the binding of Transcription Factors to promoter region
of gene.
This is followed by binding of RNA polymerase to Transcription Factors.
In case of prokaryotic cell:
There is no transcription factors and RNA polymerase binds directly to promoter region of
gene.
Transcription: Elongation
RNA polymerase begins to move downstream in 5’ to 3’ direction , unwinding and making
RNA copy of DNA
Transcription: Termination
RNA polymerase reaches Terminator region
RNA polymerase detaches
mRNA Transcript is released
Transcription Factors
Microbiology
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Messenger RNA (mRNA)
Ways in which RNA & DNA differ:
mRNA is single stranded
Sugar is ribose
(U) instead of (T)
A-U base pairing
Now the mRNA moves into cytoplasm to be
translated into a protein.
Genetic Code
How does the nucleotide sequence of mRNA specify the specific order of amino acids in
a protein? The answer lies in what is known as the genetic code.
DNA: triplet code e.g. TAC CAC CCC GCC ATC
mRNA: codon AUG GUG GGG CGG UAG
(complimentary to triplet code of DNA)
Amino acid sequence Met- Val- Gly- Arg- Stop
Codons: code for the production of a specific amino acid
20 amino acids
3 base code
Degenerative: more than 1 codon codes for an amino acid
Universal: in all living organisms
Microbiology
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Translation:
Initiation
. 1
st
step in translation is the attachment of the small ribosomal subunit (30S) to mRNA at
the start codon (AUG)
.This is followed by the recruitment of large subunit (50S)
. On the assembled ribosome (70s) , tRNA carries amino acid to the ribosome.
.Each tRNA matches and binds to a specific codon.
. If codon does not match, tRNA rejected.
On assembled ribosome, a tRNA carrying the first amino acid is paired with the start codon
on the mRNA. The place on ribosome where the 1st tRNA is located is called P site.
tRNA carrying the 2
nd
amino acid approaches.
Microbiology
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Translation : Elongation
A site next to P site, the second codon of the mRNA pairs with a tRNA carrying the second
amino acid (A site).
The 1
st
amino acid joins the
second amino acid by a peptide
bond
The 1
st
tRNA is released.
The ribosome moves along the mRNA until 2
nd
tRNA is in P site, and process continues.
As ribosome continues to move along mRNA, new amino acids are added to polypeptide.
Microbiology
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Translation : Termination
When the ribosome reaches a stop codon, the polypeptide is released.
Stop codons are UGA, UAA, UAG.
Finally, the last tRNA is released, and the ribosomes comes apart. The released
polypeptide forms a new protein.
Types of RNA
Three types:
▪ mRNA: messenger RNA ADK20042020s_
▪ Contains 3 bases ( codon)
▪ rRNA: ribosomal RNA
▪ Comprises the 70 S ribosome
▪ tRNA: transfer RNA
▪ Transfers amino acids to ribosomes for protein synthesis
▪ Contains the anticodon (3 base sequence that is complimentary to
codon on mRNA)