Translation, transcription, and transduction are processes involved in gene expression and DNA transfer. Translation is the process by which messenger RNA (mRNA) is decoded by ribosomes to produce a polypeptide. Transcription is the process where DNA is copied into mRNA by RNA polymerase. Transduction is the transfer of DNA from one bacterium to another mediated by bacteriophages through generalized or specialized transduction. These processes play important roles in protein production and genetic exchange.

1. Translation , Transcription and Transduction
The formationof mRNA formDNA template undercertainproteinsaswell asenzymes iscalled
Translation.
In translation, messenger RNA (mRNA)—produced by transcription from DNA—is decoded by aribosome to
produce a specific amino acid chain, or polypeptide. The polypeptide later folds into anactive protein and
performs its functions in the cell. The ribosome facilitates decoding by inducing the binding
of complementary tRNA anticodon sequences to mRNA codons. The tRNAs carry specific amino acids that are
chained together into a polypeptide as the mRNA passes through and is "read" by the ribosome. The entire
process is a part of gene expression.
In brief, translation proceeds in four phases:
Initiation: The ribosome assembles around the target mRNA. The first tRNA is attachedat the start codon.
Elongation: ThetRNA transfers an amino acid to the tRNA corresponding to the next codon.
Translocation:Theribosome then moves (translocates) to the next mRNA codon to continue the process,
creating an amino acid chain.
Termination: When a stop codon is reached, the ribosome releases the polypeptide.
In bacteria, translation occurs in the cell's cytoplasm, where the large and small subunits of
theribosome bind to the mRNA. In eukaryotes, translation occurs in the cytosol or across the
membrane of the endoplasmic reticulum in a process called vectorial synthesis. In many
instances, the entire ribosome/mRNA complex binds to the outer membrane of the rough
endoplasmic reticulum (ER); the newly created polypeptide is stored inside the ER for
later vesicle transport and secretion outside of the cell.
Many typesof transcribedRNA,suchas transferRNA,ribosomal RNA,andsmall nuclearRNA,donot
undergotranslationintoproteins.
A number of antibiotics act by inhibiting translation. These
include anisomycin, cycloheximide,chloramphenicol, tetracycline, streptomycin, erythromycin,
and puromycin. Prokaryotic ribosomes have a different structure from that of eukaryotic
ribosomes, and thus antibiotics can specifically target bacterial infections without any harm to a
eukaryotic host's cells.
Basic mechanisms

2. The basic processof proteinproductionisadditionof one amino acid at a time tothe endof a
protein.Thisoperationisperformedbya ribosome.The choice of aminoacidtype to add is
determinedbyan mRNA molecule.Eachaminoacidaddedismatchedto a three nucleotide
subsequenceof the mRNA.Foreachsuch tripletpossible,the correspondingaminoacidisaccepted.The
successive aminoacidsaddedtothe chainare matchedtosuccessive nucleotidetripletsinthe mRNA.In
thisway the sequence of nucleotidesinthe templatemRNA chaindeterminesthe sequence of amino
acidsin the generatedaminoacidchain.[1] Additionof anaminoacidoccurs at the C-terminus of
the peptide andthustranslationissaidtobe amino-to-carboxyldirected.[2]
The mRNA carries genetic informationencodedasaribonucleotide sequence fromthe chromosomes
to the ribosomes.The ribonucleotidesare "read"bytranslational machineryinasequence
of nucleotide tripletscalledcodons.Eachof those tripletscodesforaspecific amino acid.
The ribosome moleculestranslatethiscode toa specificsequence of aminoacids.The ribosome isa
multisubunitstructure containing rRNA andproteins.Itisthe "factory"where aminoacids are
assembledintoproteins.tRNAsare small noncodingRNA chains(74-93nucleotides) thattransport
aminoacidsto the ribosome.tRNAshave asite foraminoacid attachment,anda site calledan
anticodon.The anticodonisan RNA tripletcomplementaryto the mRNA tripletthatcodesfortheir
cargo amino acid.
Aminoacyl tRNA synthetases (enzymes) catalyze the bondingbetween
specific tRNAs andthe amino acids that theiranticodonsequencescall for.The productof this
reactionisan aminoacyl-tRNA.Thisaminoacyl-tRNAiscarriedtothe ribosome by EF-Tu,where
mRNA codonsare matchedthroughcomplementary base pairing to specific tRNAanticodons.
Aminoacyl-tRNA synthetasesthatmispairtRNAswiththe wrongaminoacidscanproduce mischarged
aminoacyl-tRNAs,whichcanresultininappropriate aminoacidsatthe respective positioninprotein.
This"mistranslation"[3] of the geneticcode naturallyoccursatlow levelsinmostorganisms,butcertain
cellularenvironmentscause anincrease inpermissivemRNA decoding, sometimestothe benefitof the
cell.
The ribosome has three sites for tRNA to bind. They are the aminoacyl site (abbreviated A), the
peptidyl site (abbreviated P) and the exit site (abbreviated E). With respect to the mRNA, the
three sites are oriented 5’ to 3’ E-P-A, because ribosomes move toward the 3' end of mRNA.
The A site binds the incoming tRNA with the complementary codon on the mRNA. The P site
holds the tRNA with the growing polypeptide chain. The E site holds the tRNA without its amino
acid. When an aminoacyl-tRNA initially binds to its corresponding codon on the mRNA, it is in
the A site. Then, a peptide bond forms between the amino acid of the tRNA in the A site and
the amino acid of the charged tRNA in the P site. The growing polypeptide chain is transferred
to the tRNA in the A site. Translocation occurs, moving the tRNA in the P site, now without an
amino acid, to the E site; the tRNA that was in the A site, now charged with the polypeptide

3. chain, is moved to the P site. The tRNA in the E site leaves and another aminoacyl-tRNA enters
the A site to repeat the process.
Afterthe newaminoacidis addedtothe chain,andafter the mRNA isreleasedoutof the nucleusand
intothe ribosome'score,the energyprovidedbythe hydrolysisof aGTP boundto
the translocase EF-G (in prokaryotes) and eEF-2 (in eukaryotes) movesthe ribosome
downone codontowards the 3' end.The energyrequiredfortranslationof proteinsissignificant.For
a proteincontaining n aminoacids,the numberof high-energyphosphatebondsrequiredtotranslate it
is4n-1[citation needed]
. The rate of translationvaries;itissignificantlyhigherinprokaryoticcells(upto
17-21 aminoacidresiduespersecond) thanineukaryoticcells(upto6-9 aminoacid residuesper
second).
In activation,the correctaminoacidis covalently bonded to the correct transfer RNA
(tRNA).The aminoacidisjoinedbyitscarboxyl grouptothe 3' OH of the tRNA by an ester bond.
Whenthe tRNA has an aminoacid linkedtoit,itis termed"charged".Initiationinvolvesthe small
subunitof the ribosome bindingtothe 5' endof mRNA withthe helpof initiation factors (IF).
Terminationof the polypeptide happenswhenthe A site of the ribosome facesastopcodon(UAA,UAG,
or UGA). NotRNA can recognize orbindtothis codon.Instead,the stopcodoninducesthe bindingof
a release factor proteinthatpromptsthe disassemblyof the entire ribosome/mRNA complex.
Genetic code
AAs = FFLLSSSSYY**CC*WLLLLPPPPHHQQRRRRIIIMTTTTNNKKSSRRVVVVAAAADDEEGGGG
Starts = ---M---------------M---------------M----------------------------
Base1 = TTTTTTTTTTTTTTTTCCCCCCCCCCCCCCCCAAAAAAAAAAAAAAAAGGGGGGGGGGGGGGGG
Base2 = TTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGG
Base3 = TCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAG
Transcription
Transcription isthe firststepof gene expression,inwhicha particularsegmentof DNA iscopied
into RNA (mRNA) bythe enzyme RNA polymerase.
Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementarylanguage. The
two can be converted back and forth from DNA to RNA by the action of the correct enzymes. During
transcription, a DNA sequence is read by an RNA polymerase, which produces a
complementary, antiparallel RNA strand called a primary transcript.

4. Transcription proceeds in the following general steps:
One or more sigma factor protein binds to the RNA polymerase holoenzyme, allowing it to bind to promoter
DNA.
RNA polymerase createsa transcription bubble, which separates the two strands of the DNA helix. This is
done by breaking the hydrogen bonds between complementary DNA nucleotides.
RNA polymerase adds matching RNA nucleotides to the complementary nucleotides of one DNA strand.
RNA sugar-phosphate backbone forms with assistance from RNApolymerase to form an RNAstrand.
Hydrogen bonds of the untwisted RNA-DNA helix break, freeing the newly synthesized RNA strand.
If the cell has a nucleus, the RNA may be further processed. This may includepolyadenylation, capping,
and splicing.
The RNA may remain in the nucleus or exit to the cytoplasm through the nuclear porecomplex.
The stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least
one gene. If the gene transcribed encodes a protein,messenger RNA (mRNA) will be transcribed; the mRNA
will in turn serve as a template for the protein's synthesis through translation. Alternatively, the transcribed
gene may encode for either non-coding RNA (such as microRNA), ribosomal RNA (rRNA), transfer RNA (tRNA),
or other enzymatic RNA molecules calledribozymes.[1] Overall, RNA helps synthesize, regulate, and process
proteins; it therefore plays a fundamental role in performing functions within a cell.
In virology, the term may also be used when referring to mRNA synthesis from an RNA molecule (i.e., RNA
replication). For instance, the genome of a negative-sense single-stranded RNA (ssRNA -) virus may be
template for a positive-sense single-stranded RNA (ssRNA +). This is because the positive-sense strand
contains the information needed to translate the viral proteins for viral replication afterwards. This process is
catalysed by a viral RNA replicase.
Reverse transcription
Some viruses (such as HIV, the cause of AIDS), have the ability to transcribe RNA into DNA. HIVhas an
RNA genome that is reverse transcribed into DNA. The resulting DNAcan be merged with the DNA genome of
the host cell. The main enzyme responsible for synthesis of DNA from an RNA template is called reverse
transcriptase.
In the case of HIV, reverse transcriptase is responsible for synthesizing a complementary DNA strand
(cDNA) to the viral RNA genome. The enzyme ribonuclease H then digests the RNA strand, and reverse
transcriptase synthesises a complementary strand of DNA to form a double helix DNA structure ("cDNA").
The cDNA is integrated into the host cell's genome by the enzyme integrase, which causes the host cell to
generateviral proteins that reassemble into new viral particles. In HIV, subsequent to this, the host cell
undergoes programmed cell death, or apoptosis of T cells.[21] However, in other retroviruses, the host
cell remains intact as the virus buds out of the cell.

6. Transduction:
Definition - Unlike transformation in which the naked DNA is transferred in transduction
DNA is carried by a bacteriophage. OR
In transduction, DNA is transferred from cell to cell through the agency of viruses
NOTE :- All phages can be transducer and not all bacteria are transducible
EXAMPLES OF BACTERIA
Transduction has been found to occur in a variety of prokaryotes, including certain species of
the Bacteria: Desulfovibrio, Escherichia, Pseudomonas, Rhodococcus, Rhodobacter, Salmonella,
Staphylococcus, and Xanthobacter, as well as Methanobacterium thermoautotrophicum.
Mechanismof Transduction
Bacteriphage
Firstly Descoveredin1915 by FredrickTwortand two yearslaterbyFelix d’Herelle.
Means bacteriaeater.
A virusthat infectscertaintype of bacteriaandreplicateswithinthem

7. Types ofbacterio-phage
Virulent:capable of causinginfectionandeventuallydestructionanddeathof the bacterial cell.These
followthe lyticcycle.e.g.T4host E.coli.
Temperate:doesnot cause destrupticinfectioninsteadphage DNA isincorporatedintobacteriumDNA
and isreplicatedwithitandaftersome cycle become virulentcause lysis.
e.g.lambdaphage.
Life cycle of bacteriophage
There are two typesof transduction:
Generalizedtransduction:ADNA fragment is transferredfrom one bacteriumto anotherby a lytic
bacteriophage that isnow carrying donor bacterial DNA due to an error in maturation duringthe lytic
life cycle
1. A lytic bacteriophage adsorbs to a susceptible bacterium.

8. 2. The bacteriophage genome enters the bacterium. The genome directs the bacterium's
metabolic machinery to manufacture bacteriophage components and enzymes
3. Occasionally,a bacteriophage heador capsid assemblesarounda fragmentof donor bacterium's
nucleoidinsteadof a phage genome by mistake.
4. The bacteriophagesare released.

9. 5. The bacteriophage carrying the donor bacterium's DNA adsorbs to a recipient bacterium
6. The bacteriophage inserts the donor bacterium's DNA it is carrying into the recipient
bacterium .
• It transfers geneticmaterial from one bacterial cell to another and alter the genetic
characteristics.
• For example:In specialisedtransductionthe gal gene,a cell lacking ability to metabolize
galactose couldaquire the ability.
• It shows the evolutionaryrelationshipbetweenthe prophage and host bacterial cell.
• Prophage can existin a cell for a long periodsuggestsa similarpossible mechanismfor the
viral originof cancer.
• It providesa way to study the gene linkage.