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8/7/2019 transcript and translate

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DNA Transcription and

Translation


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Definition DNA Transcription :

Is a process that involves thetranscribing of genetic informationfrom DNA to RNA


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The transcribed DNA message isused to produce proteins

The information in the DNA is notdirectly converted into proteins, butmust be copied into RNA first


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HOW ????

3 m a in step s in th e D N A tra n scrip tion:process

.1 R N A Po lym e ra se B in d s to D N A

D N A is tran scrib e d b y a n e n zym e.ca lled R N A p o lym e ra se S p ecific

n u cleo tid e seq u en ce s te llR N A p o lym e ra se w h e re to b e g in an d w he re to

.e n d R N A p o lym e ra se a ttach es to th e D N A a t a sp ecific a re a ca lled th e

.p rom o te r reg io n


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2 . Elongation

Transcription factors (a type of protein)unwind the DNA strand and allow RNApolymerase to transcribe only a single strand of DNA into a single stranded RNA polymer calledmessenger RNA (mRNA). The strand that servesas the template is called the antisense strand.The strand that is not transcribed is called thesense strand. Like DNA, RNA is composed of nucleotide bases.RNA however, contains the nucleotides adenine,guanine, cytosine and uricil (U). When RNApolymerase transcribes the DNA, guanine pairswith cytosine and adenine pairs with uricil.


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3. TerminationRNA polymerase moves along the DNA

until it reaches a terminatorsequence. At that point, RNApolymerase releases the mRNApolymer and detaches from the DNA.


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Components of RNApolymerase

Prokaryotic a) Core enzyme = 3 different types of subunits (2;

1; 1) (1) - binds incoming nucleotides

(2) binds DNA (3) - helps with enzyme assembly; interacts withother transcriptional activator proteins.

b) Holoenzyme = core + factor (recognizes thepromoter)

c) factors Initially, people thought that there wasonly one factor that functioned to direct RNAP tothe promoters of genes. Later, different classes of factors were found. Each factor directs RNAP toa different type of promoter (differentiated by aspecific DNA sequence in the promoter).


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Eukaryotic RNA polymerases Much more complex

than prokaryotic RNAP (numerousadditional factors required, multiplepolymerases )

a) RNAP I synthesizes ribosomal RNA b) RNAP II synthesizes messenger RNA c) RNAP III synthesizes transfer RNA and 1

type of rRNA 2.Eukaryotic RNAPs have subunits that are

homologous to , , and of prokaryoticRNAP; however, eukaryotic RNAP alsocontain many additional subunits.


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Translation Construction of proteins by mRNA in

the cytoplasm.

In translation, mRNA along withtransfer RNA (tRNA), andribosomes, work together toproduce proteins.


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After leaving the nucleus, mRNA must

undergo several MODIFICATIONSbefore being translated. Sections of the mRNA that do not code

for amino acids, called INTRONS, areremoved.

A poly-A tail, consisting of severaladenine bases, is added to one end of

the mRNA, while a guanosinetriphosphate cap is added to the otherend.

These modifications remove unneededsections and protect the ends of the


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Transfer RNA (tRNA) Transfer RNA plays a huge role in

translation. Its job is to translate the message

within the nucleotide sequence of mRNA to a specific amino acidsequence.

Transfer RNA is shaped like a clover leaf with three loops. It contains an aminoacid attachment site on one end anda special section in the middle loopcalled the anticodon site.

The anticodon recognizes a specificarea on a mRNA called a codon.


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Ribosomes Ribosomes consist of two parts,

a large subunit a small subunit.

They contain a binding site for mRNAand two binding sites for tRNA located

in the large ribosomal subunit


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Whats happening now..

During translation, a small ribosomal subunit attaches to a mRNA

molecule. At the same time an initiator tRNAmolecule recognizes and binds to a specific

codon sequence on the same mRNAmolecule. A large ribosomal subunit then joins the newly

formed complex. The initiator tRNA resides inone binding site of the ribosome called the P

site, leaving the second binding site, theA

site, open. When a new tRNA moleculerecognizes the next codon sequence on themRNA, it attaches to the open A site. Apeptide bond forms connecting the aminoacid of the tRNA in the P site to the aminoacid of the tRNA in the A binding site.


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As the ribosome moves along the mRNAmolecule, the tRNA in the P site isreleased and the tRNA in the A site istranslocated to the P site. The A binding

site becomes vacant again until anothertRNA that recognizes the new mRNAcodon takes the open position. Thispattern continues as molecules of tRNAare released from the complex, new tRNA

molecules attach, and the amino acidchain grows. The ribosome will translatethe mRNA molecule until it reaches atermination codon on the mRNA. Whenthis happens, the growing protein called a

polypeptide chain is released from thetRNA molecule and the ribosome splits


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The newly formed polypeptide chainundergoes several modifications beforebecoming a fully functioning protein.

Some proteins will be used in themembrane of the cell, while others willremain in the cytoplasm or be transportedout of the cell.

Many copies of a protein can be made fromone mRNA molecule.

This is because several ribosomes cantranslate the same mRNA molecule at thesame time. These clusters of ribosomesthat translate a single mRNA sequence

are called polyribosomes or polysomes.


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