PROTEIN SYNTHESIS

DECEMBER 13, 2010CAPE BIOLOGY UNIT 1

MRS. HAUGHTON

GENE

• Mendel 1866 described a gene as a unit of inheritance

• Morgan defined it as the shortest segment of a chromosome which could be separated from adjacent segments by crossing over.

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.

GENETIC CODE

• Watson and Crick proposed that genetic information might be stored in the form of a sequence of bases in the DNA molecule.

• It was shown that DNA was a code for the production of proteins.

• It then became clear that the sequence of bases in the DNA must be a code for the sequence of amino acids in a polypeptide chain!

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 to

primary protein structure?

TRIPLET CODON

• Four bases hence four nucleotides arranged on a polynucleotide strand making up a DNA strand.

• This “alphabet” arrangement is responsible for carrying the genetic code.

TRIPLET CODON

• There are 20 common amino acids used to make proteins and the bases in DNA must code for them.

• If only one base determined the position of an amino acid in the polypeptide chain, then only _________amino acids would be in the chain.

•41 = 4

• If two bases coded for an amino acid, then only _________ amino acids would make up the polypeptide chain.

42 = 16

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

TRIPLET CODON

• Obviously a code composed of three bases could incorporate all 20 amino acids into the structure of protein molecules.

• Such a code would produce ________ combinations of bases.

43 = 64Let’s look at

them

PROOF OF TRIPLET CODON

• Crick in 1961 produced DNA mutations called frame-shifts by adding extra or deleting bases/nucleotides from the genetic code.

• Adding or deleting one base (+ or -) led to a different polypeptide chain entirely.

• Adding or deleting two bases (++ or --) led to a different polypeptide chain entirely.

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 the protein being made.

FEATURES OF THE GENETIC CODE

1

• The code is a triplet of bases.

• Theoretically, three bases represents an amino acid.

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 of amino acids is less than the number of available codons.

3

• The code is punctuated.

• Three of the codons (e.g. UAA) act as full stops determining where the coded message to be transcribed must end. These are “stop codons” or “stop signals”.

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

4

• The code is universal as all living organisms contain the same 20 common amino acids and the same five bases (ATCGU).

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.

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).

PROCESS OF PROTEIN SYNTHESIS

DNA makes RNA and RNA makes PROTEIN which is responsible for

how we look and function

• Protein synthesis is a two-stage process.

1. Transcription – the making of mRNA from DNA. A length of DNA (a gene) is copied into a mRNA molecule.

2. Translation – translating the base sequence in mRNA into an amino acid sequence in a protein.

TRANSCRIPTION

• The mechanism by which the base sequence of a section of DNA representing a gene is converted into a complementary base sequence of mRNA.

• The DNA double helix unwinds by breaking the relatively weak H-bonds between the bases of the 2 strands exposing the single strand of the DNA.

• 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.

• 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 RNA polymerase molecule leaves the DNA and the two strands zip up again reforming the double helix.

TRANSLATION

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

• It occurs on ribosomes.

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

• Each ribosome consists of a large and small subunit.

• 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-codon and which is carrying the first amino acid which is usually Met (AUG).

• The second codon then also subtracts the aminoacyl-tRNA molecule showing the complementary anticodon.

• The function of the ribosomes is to hold in position the mRNA, tRNA and the associate enzymes controlling the process until a peptide bond forms between the adjacent amino acids.

• Once the new amino acid has been added to the growing polypeptide chain the ribosome moves one codon along the mRNA.

• The tRNA molecule which was previously attached to the polypeptide chain now leaves the ribosome and passes back to the cytoplasm to be reconverted into a new aminoacyl-tRNA molecule.

• 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.

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

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

• If the ribosome is attached to ER, the protein enters it and is transported.

SUMMARY OF TRANSLATION

1. Binding of mRNA to ribosome.2. Amino acid activation and attachment to

tRNA3. Polypeptide chain initiation4. Chain elongation5. Chain termination6. Fate of mRNA

HOMEWORK

• In 100 words or less, explain what non-coding DNA is.

• In 100 words or less, explain what introns and exons.