protein synthesis - copy

7/29/2019 Protein Synthesis - Copy

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PROTEIN SYNTHESIS

DECEMBER 13, 2010

CAPE BIOLOGY UNIT 1MRS. HAUGHTON


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GENE

Mendel 1866 described a gene as aunit of inheritance

Morgan defined it as the shortestsegment of a chromosome which

could be separated from adjacentsegments by crossing over.


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GENE

A gene can also be described as the shortest

segment of a chromosome responsible for the

production of a specific product (protein).

Genes are codes or blueprints for proteins.

A piece of DNA that codes for a polypeptide

chain.


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GENETIC CODE

Watson and Crick proposed that geneticinformation might be stored in the form of asequence of bases in the DNA molecule.

It was shown that DNA was a code for theproduction of proteins.

It then became clear that the sequence of basesin the DNA must be a code for the sequence ofamino acids in a polypeptide chain!


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GENETIC CODE

This relationship between bases and amino

acids is known as the genetic code.

Did the code really exist?

How was it to be broken?

How exactly was the code translated toprimary protein structure?


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TRIPLET CODON

Four bases hence fournucleotides arranged ona polynucleotide strandmaking up a DNAstrand.

This alphabet

arrangement isresponsible for carryingthe genetic code.


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TRIPLET CODON

There are 20 common amino acids usedto make proteins and the bases in DNAmust code for them.

If only one base determined the positionof an amino acid in the polypeptide

chain, then only _________amino acidswould be in the chain.


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41 = 4


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If two bases coded for an aminoacid, then only _________ aminoacids would make up thepolypeptide chain.


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42 = 16


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TRIPLET CODON

Lets list the 16 possible combinations of bases

if only pairs of bases (ATCG) were used.

AT AC AG AA TT TC TG TA

CC CA CG CT GG GA GT GC


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TRIPLET CODON

Obviously a code

composed of three

bases could incorporate

all 20 amino acids intothe structure of protein

molecules.

Such a code would

produce ________

combinations of bases.

43 = 64Lets look at

them


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PROOF OF TRIPLET CODON

Crick in 1961 produced DNA mutations calledframe-shifts by adding extra or deletingbases/nucleotides from the genetic code.


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Adding or deleting one base (+ or -) led to adifferent polypeptide chain entirely.


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Adding or deleting two bases (++ or --) led to a

different polypeptide chain entirely.


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PROOF OF TRIPLET CODON

But adding or deleting three bases (+++ or ---)

did not cause a different chain to be made,

only the deletion of a single amino acid from

the chain and this did not usually affect theprotein being made.


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FEATURES OF THE GENETIC CODE


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1

The code is a triplet of bases.

Theoretically, three bases represents an aminoacid.


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2

The triplet code is degenerate.

Some amino acids are coded for by several

codons.

For many amino acids, only the first 2 bases

appear to be significant so the number ofamino acids is less than the number ofavailable codons.


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3

The code is punctuated.

Three of the codons (e.g. UAA) act as full stops

determining where the coded message to betranscribed must end. These are stopcodons or stop signals.

Other codons are start codons or signals (e.g.AUG which codes for the a.a. methionine)


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4

The code is universal as all living organisms

contain the same 20 common amino acids and

the same five bases (ATCGU).


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5

The code is not overlapping.

E.g. AUUAUCGUUAGCCA is read AUU AUC CGU UAG CCA and not

AUU UUA UAU.. Or

AUU UAU AUC. etc.


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HOMEWORK

In 250 words or less, explain just how

scientists eventually determined which three

bases represented which amino acid or family

of amino acids (breaking the code).


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PROCESS OF PROTEIN SYNTHESIS

DNA makes RNA and RNA makes

PROTEIN which is responsible forhow we look and function


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Protein synthesis is a two-stage process.

1. Transcription the making of mRNA from

DNA. A length of DNA (a gene) is copied intoa mRNA molecule.

2. Translation translating the base sequencein mRNA into an amino acid sequence in aprotein.


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TRANSCRIPTION

The mechanism by which the base sequenceof a section of DNA representing a gene isconverted into a complementary base

sequence of mRNA.

The DNA double helix unwinds by breaking

the relatively weak H-bonds between thebases of the 2 strands exposing the singlestrand of the DNA.


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Only one of the strands can be selected as a

template for the formation of a

complementary single strand of mRNA.

This molecule is formed by the linking of free

nucleotides under the influence of RNA

polymerase according to the rules of base

pairing between DNA and RNA.


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When the mRNA molecules have been

synthesized, they leave the nucleus via the

nuclear pores and carry the genetic code to the

ribosomes.

When sufficient numbers of mRNA molecules

have been formed from the gene, the RNApolymerase molecule leaves the DNA and the two

strands zip up again reforming the double helix.


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TRANSLATION

This is the mechanism by which the sequence ofbases in the mRNA molecule is converted into asequence of amino acids in a polypeptide chain.

It occurs on ribosomes.

Several ribosomes may become attached to amolecule of mRNA like beads on a string(polysome/polyribosome).


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Each ribosome consists of a large and smallsubunit.

The first two mRNA codons (a total of 6 bases)enters the ribosome.

The first codons bind to the aminoacyl-tRNA

molecule having the complementary anti-codonand which is carrying the first amino acid which isusually Met (AUG).


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The second codon then also subtracts theaminoacyl-tRNA molecule showing thecomplementary anticodon.

The function of the ribosomes is to hold inposition the mRNA, tRNA and the associateenzymes controlling the process until apeptide bond forms between the adjacentamino acids.


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Once the new amino acid has been added tothe growing polypeptide chain the ribosomemoves one codon along the mRNA.

The tRNA molecule which was previouslyattached to the polypeptide chain now leavesthe ribosome and passes back to thecytoplasm to be reconverted into a newaminoacyl-tRNA molecule.


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This sequence of ribosome reading and

translating the mRNA code continues until it

comes to a codon signaling STOP.

These terminating codons are UAA, UAG and

UGA.


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At this point the polypeptide chain, now with itsprimary structure as determined by the DNA,leaves the ribosome and translation is complete.

As the polypeptide chains leave the ribosomethey may immediately assume either secondary,tertiary or quaternary structures.

If the ribosome is attached to ER, the proteinenters it and is transported.


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SUMMARY OF TRANSLATION

1. Binding of mRNA to ribosome.

2. Amino acid activation and attachment to

tRNA

3. Polypeptide chain initiation

4. Chain elongation

5. Chain termination6. Fate of mRNA


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HOMEWORK

In 100 words or less, explain what non-coding

DNA is.

In 100 words or less, explain what introns and

exons.

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