Protein Synthesis.ppt

8/13/2019 Protein Synthesis.ppt

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Nazabayev University UPCSE

Protein synthesis

11/19/2013 3:47 AM Protein Synthesis Pork Chop Willie


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How does DNA code for

proteins? The sequence of bases in the DNA tells

the cell which amino acids to link

together to form the protein Each geneis a sequence of bases on a

DNA molecule coding for a sequence of

amino acids in a polypeptide chain

Protein Synthesis Pork Chop Willie 211/19/2013 3:47 AM


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One base does not code for one amino

acid.

20 amino acids commonly found inproteins

The code carried by DNA is a three base

code (Triplet code)

Each adjacent group of three bases

codes for an amino acid

Triplet code


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Triplet code

Several triplets code for the same amino

acid.

Others are startand stopsignals. The fundamental coding system is found

in all organisms



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From DNA to Proteins

DNA in our chromosomes carries the

genetic information from one generation

to the next It determines the structure and function

of cells by telling them which proteins to

make



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From DNA to proteins

But DNA is in the nucleus and proteins

are made in the cytoplasm.

DNA cannot pass through the nuclearmembranes.



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How do the instructions get from

the nucleus to the cytoplasm?A copy of DNA is made.

This copy is not made from DNA but

made from RNA The RNA can leave the nucleus and

carry the information to the cytoplasm

where it is used to make proteins



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Types of RNA

mRNA messenger

tRNA transferrRNA ribosomal



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Protein Synthesis

Transcriptiontakes

place in the nucleus

DNA code is copied tomRNA

Translationtakes place

in the cytoplasm

Correct sequence of

amino acids are used

to build a new protein



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Transcription

Takes place in the nucleus

DNA double helix unwinds

Hydrogen bonds between bases break

Sequence of bases on one of the

strands (template strand) is used to

produce a mRNAmolecule



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Transciption

mRNAis built from free RNA nucleotides

which line up along the DNA template

strand.RNA nucleotides are activated by the

attachment of phosphate groups

There are four different activated RNA

nucleotides ATP,GTP,CTP,UTP



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Transcription

Because of complementary base pairing

the order of bases on the DNA exactly

determines the order of bases on themRNA

The triplet code on the DNA gives rise to

a complementary codonon the mRNA

C binds with G

A bind with U (not T)



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Codons, anticodons and complementary

base pairing



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Transciption

Template strand (antisense strand)

because once transcribed makes anmRNA molecule with the same base

sequence as the DNA coding strand

Coding strand (sense)



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Transcription

Helicase breaks hydrogen bonds in DNA

RNA polymerase attaches DNA at the

promoter site The complete mRNA leaves the nucleus

through a pore in the nuclear envelope



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Protein synthesis: Transcription



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Translation

After transcription the mRNA passes into

the cytoplasm and attaches itself to a

ribosome In eukaryotic cells ribosomes are usually

attached to endoplasmic reticulum

In prokaryotestakes place as soon as

mRNA is made

Translation starts once the mRNA

attaches to the ribosome



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Translation

Ribosomes are made up of two subunits

mRNA attaches to the smaller of the subunits so that

two mRNA codons face the binding site of the larger

subunit

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Translation

At one side of a tRNA molecule is a triplet base

sequence called an anticodon The three bases of the anticodonare complementary

to the mRNA codonof the amino acid



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Translation

mRNAcodons for lysineare AAA and

AAG

Within the cytoplasm free amino acidsbecome attached to the correct tRNA

molecule

Each amino acid has its own specific

RNA that carries it to the ribosome



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Translation

First codon exposed on the ribosome is

always the start code AUG

AUG codes for the amino acid methionine. The tRNAwith the complementary

anticodon is UAChydrogen bonds to the

codon.

The next codon attracts the tRNAwith the

complementary anticodon



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Translation

The ribosome holds the mRNA, tRNA, amino acid

and associated enzymes in place while a peptidebondforms between the two amino acids



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Translation

Once the peptide is formed the ribosome

moves along the mRNAto reveal anew

codon at the binding site The first tRNA returns to the cytoplasm

The process is repeated until the

ribosome reaches a stop signal: UAA,

UAC, or UGA



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Ribosome moves along the mRNA, allowing

another complementary base to bind



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Nature of the genetic code

The code is nonoverlapping, no base of

one triplet contributes to the next triplet.

64 possible combinations of 4 bases ifgrouped in a triplet

Several triplets can code for the same

amino acid

The code is therefore described as

degenerative



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Different proteins can be produced when different

coding sections of mRNA are spliced together



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mRNA splicing

Between transcription and translation mRNA

is often edited.

Some sections are removed and some are

spliced together.

Non-coding intronsare removed. The remaining sequences which will be

expressed are exons.

It means several proteins can be formed from

one single length mRNA spliced in different

ways.

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Protein Synthesis.ppt