http://workbench.concord.org/web_content/unitV/sickle_cell_worksheet_s.html (a model-based activity to make a sickle-cell mutation)
http://workbench.concord.org/web_content/unitV/muta_wksht_s.html (a model-based activity to make substitution & deletion mutations)
Protein synthesis is the manufacture/synthesis of proteins.
Occurs in the nucleus and cytoplasm .
The genetic codes for specific proteins are located on the DNA, e.g. codes for keratin, collagen, Hb, polymer ase , amyl ase , pepsin etc.
The triplet genetic codes on DNA are called codons . This is a sequence of three DNA bases.
The site for protein synthesis occurs on the ribosomes (rRNA and protein) in the cytoplasm.
Protein synthesis: divided into 2 processes:
(A) Transcription : copying the codons for a protein from a segment of the DNA strand to make mRNA (messenger RNA); requires 2 enzymes ( helic ase , RNA polymer ase ).
Includes steps of initiation , elongation and termination .
(B) Translation : translate the sequence of codons for the protein on the mRNA to build a protein at the ribosome; tRNA assists in this process.
Includes steps of initiation , elongation and termination .
B7– Roles of the Molecules Involved in Protein Synthesis
Contains the information/ genetic code / triplet codes / codons for the synthesis of proteins, e.g. TCG, ATG, ATT.
Provides a template for mRNA to be produced.
Let’s draw this together!!
Carries the coded message from the nucleus to the cytoplasm, e.g. codons UAA, CGC, AGU
Sets the order of amino acids for protein synthesis by the sequence of codons . ( Codon = 3 bases/ nucleotides in a DNA or RNA sequence which specify a single amino acid.)
Provides a site for mRNA and tRNA to join together by complementary base pairing.
Site for protein synthesis (i.e. translation) where it translates mRNA and allows amino acids to bond (peptide bond).
rRNA & protein make up the small and large subunits of ribosomes . The small subunit contains the binding site for mRNA and the large subunit contains 2 binding sites for tRNA ~ amino acid and a dehydration synthesis site!
Carries the specific amino acid to the ribosome where its anticodon complementary base pairs with the mRNA codon, e.g. mRNA codon- UUA ; anticodon- AAU .
(e) Amino acids
These are the monomers of proteins and are picked up by tRNA in the cytoplasm, i.e. tRNA~ amino acids.
mRNA Codon Table (see below) tells what 3 bases on mRNA code for each amino acid (64 combinations of 3 bases)
Methionine (AUG) on mRNA is called the ‘START codon’ because it triggers the linking of amino acids
UAA, UGA, & UAG on mRNA signal ribosomes to stop linking amino acids together
(involves steps of initiation, elongation & termination )
Requires 2 enzymes: helic ase and RNA polymer ase .
Here we go…..
(a) DNA containing genetic code for protein is in triplet codes.
(b) Initiation : RNA polymerase initiates transcription by locating and binding to the beginning of a gene/code (i.e. the promoter region ) .
(c) Elongation : - Helic ase unwinds, unzips DNA, breaks H-bonds and provides a template for mRNA formation.
RNA polymerase travels down the template and complementary base pairs RNA nucleotides with DNA codons forming a mRNA,
e.g. DNA – CAT; mRNA – GUA.
RNA polymer ase joins adjacent
nucleotides to mRNA, forming the S-P-S backbone.
(d) Termination : -RNA polymerase reaches termination sequence at end of gene/code and STOPS mRNA synthesis. RNA polymerase releases the mRNA and detaches from the DNA.
mRNA exits nucleus through its pores and enters the cytoplasm; DNA rejoins.
mRNA carries a sequence of codons (linear order of three RNA bases complementary to DNA triplet code) to the ribosome…
One is a “STOP” codon, e.g. UAA, UAG and UGA and a “START” codon, e.g. AUG (Methionine).
2. Translation (involves steps of initiation , elongation and termination )
Location: cytoplasm at the ribosome
(a) tRNAs in the cytoplasm attach the correct amino acid to one end (aided by enzymes).
(b) Initiation : tRNA~methionine binds to small subunit and this binds to the mRNA. tRNA anticodon (UAC) complementary pairs with mRNA “ START” codon (AUG). Large subunit binds to the small subunit assembling a ribosome.
(c) Elongation : -Another tRNA (i.e. tRNA~amino acid) with its anticodon complementary base pairs with mRNA codon on the ribosome. (It has 2 binding sites for incoming tRNA~aa)
Ribosomes move along one codon to
receive the next incoming tRNA~aa.
Amino acid undergoes dehydration synthesis and forms a peptide bond.
“ Empty” or outgoing tRNA will bond with another amino acid in the cytoplasm, i.e. tRNA~aa.
(d) Termination : Synthesis of polypeptide until a “STOP” codon on mRNA (UAA, UAG, UGA).
Termination of protein synthesis and the polypeptide chain and mRNA are released from the ribosome.
Ribosome subunits separate.
Ribosomes can synthesize 5-15 peptide bonds/sec!
Most proteins are 100-200 aa long & takes less than a minute to be synthesized!
http://nobelprize.org/educational_games/medicine/dna/b/translation/translation.html (click on animation icon)
Use the following table to answer questions 1 – 7.
Note: most of the amino acids are coded with 2, 3 or even 4 different codons. Of these 64 codons , 61 are used to program the 20 amino acids. Three of the 64 codons are used as signals to start or stop the program.
http:// learn.genetics.utah.edu / Visit the Genetics Reference Series and click on link to ‘The Basics and Beyond’. Follow the directions for synthesis of a protein.
Ok Let’s Try it Out!
1.) What are the RNA triplet codes for Lysine? Leucine?
using the mRNA table, the codons for Lysine are AAA or AAG.
using the mRNA table, the codons for Leucine are CUU, CUC, CUA, CUG, UUA and UUG.
2.) What is the DNA triplet code for Tryptophan?
mRNA = UGG (from table)
DNA = ACC.
3.) What are the mRNA triplet codons for “STOP”?
using the mRNA table, the codons are UAA, UAG and UGA.
4.) The anticodon on a tRNA is GCU. Determine the name of the amino acid and its triplet codon.
tRNA anticodon GCU would complementary base pair with mRNA CGA at the ribosome.
using the table, the amino acid code for CGA is Arginine.
5.) DNA contains the following genetic code: TACAAGATT . Determine the amino acid sequence.
DNA = TAC|AAG|ATT
mRNA = AUG|UUC|UAA
= methionine - phenylalanine - stop
B8 – Environmental Mutagens Causing Mutations
UV radiation, X-rays, gamma rays.
Industrial chemicals, e.g. PCB’s, pollutants, pesticides, and food additives, i.e. carcinogens
Heavy metals such as Lead (Pb), Mercury (Hg).
Viruses can cause mutations by adding or deleting a nucleotide or by adding a new section of DNA from another organism; (viruses change a proto-oncogene (normal gene) into an oncogene (cancer-causing gene), e.g. HIV.
B8 – DNA Mutations Affecting Protein Synthesis and Possible Genetic Disorders
Altering the code for the synthesis of proteins, may result in different codons combining different amino acids with different protein shapes and therefore function is impaired
(e.g. Sickle-cell anemia causing sickle-shaped RBC’s, cystic fibrosis, muscular dystrophy and other diseases/disorders).
Also, if the protein is an enzyme, E + S (enzyme + substrate) reaction may not occur or if the protein is a membrane protein, the cell membrane will not function properly.
Three types of gene/DNA Mutations:
One or more nucleotides are deleted.
This alters the code and therefore alters the polypeptide and its function for the cell.
E.g. Deletion of N-base, Cytosine from codon:
Normal DNA: TAC|GGG|ATG|TCA|
Mutation: TA C G|GGA|TGT|CA
One or more nucleotides are added.
This pushes all bases back one code and therefore alters the polypeptide and its function for the cell.
E.g. Addition of an Adenine base to codon:
Normal DNA: TAC|GGG|ATG|TCA|
Mutation: A TA|CGG|GAT|GTC|A
Involves a change in a single nucleotide and a change in a specific codon.
When substituting a base the results are variable.
E.g.1: C substituting for U:
Normal RNA: UAU = Tyrosine.
Substitute: UA C = Tyrosine.
No noticeable effect on these codons as they are both for tyrosine.
E.g.2: G substitute for C:
Normal RNA: UAC = Tyrosine.
Substitute: UA G = Stop.
Drastic effect as UAG is a Stop codon & the resulting protein may be too short and/or unable to function for the cell.
E.g.3: Sickle-cell anemia
Single base substitution mutation .
Mutation and triplet code for one amino acid is altered. Glutamate ( CTT or CTC ) is changed to Valine (CAT, CAG, CAA, or CAC).
Normal DNA: ......GGA| CTT
Amino acid ….. - Proline - Glutamate .
Substituting A for T.
Normal DNA: .......GGA| CTT
Mutated DNA: ......GGA|C A T
mRNA: .......CCU|G U A
Amino acid …… - Proline - Valine.
This alters ONE codon and changes Glutamate to Valine. Their chemical properties differ which causes red blood cells to become abnormally sickle-shaped, causing sickle-cell anemia.
B8 – Mutation Problems Using the “Table of mRNA codons”
Use the mRNA table in B7 to answer the following examples of mutation problems.
1.) The base sequence of a section of DNA is
A A G C C T G C A.
Which of the following represents mRNA produced from this DNA after a mutation has occurred?
a) TTC GGA CGT b) TTC GUU CGT
c) UUC GGA CGU d) UUC GGA CUU
2.) The DNA strand C G A T G C G A C A T T undergoes a mutation in which the section coding for the amino acid threonine is lost. Which of the following would be the correct codons after this mutation?
using the table, the codons for threonine are __________, __________, ___________, ___________.
DNA sequence is CGA /______/______/______
mRNA sequence is ______/______/______/______ .
Now DELETE threonine from the mRNA. Therefore, the correct sequence of codons is
a. ACG/ CUG/ UAA b. GCU/ ACG/ CUG
c. GCU/ CUG/ UAA d. GCU/ ACG/ UAA
3.) The following is a DNA base sequence:
GCA CCT ATA GGA ACC
Explain any three things that would occur during the translation of this gene if ATA underwent a mutation and was converted to A T T .