TRANSLATION: Translation: is the process by which a ribosome interprets a genetic message on mrna to place amino acids in a specific sequence in order to synthesize polypeptide. 3 Stages to translation, can you guess what they are? 1. Inititiation 2. Elongation 3. Termination
TRANSLATION Interpreters transfer RNA transfer amino acids from cytoplasm s amino acid pool to a ribosome. Molecules of trna are not identical a unique trna molecule associates with a particular mrna codon with a particular amino acid.
TRANSLATION: One end of trna is amino acid, other is an anticodon complimentary sequence. If mrna is UUU than anticodon on trna is AAA, and amino acid is phenylalanine.
TRANSLATION: Some codons do not actually code for an amino acid BUT are in a sense "punctuation marks". Eg.:UAA, UAG, UGA = "STOP" That is when any of these three codons appear in the message the "reading" process is stopped. There is also a "START" codon AUG also codes for Met (methionine) begins all poly s Met can be within a.a. sequence.
trna transcribed from DNA template. Used repeatedly 80 nucleotides long Folding hydrogen bonding 3D structure
REDUNDANCY OF THE GENETIC CODE: Each codon specifies one of 20 different amino acids. There are 64 codons (representing all the ways you can combine 4 letters into groups of 3). Some amino acids have more than one codon, which specifies them. Eg.: Serine = AGU, AGC There are 45 trnas because some anticodons can recognize more than one codon.
Wobble hypothesis Relaxed base pairing rule for third codon and corresponding base. trna with inosine (modified adenine) I can pair with U C or A trna with anticodon CCI can bind to codons GGU, GGC and GGA all code for glycine. Explains third base redundancy.
Aminoacyl-tRNA synthetase Enzyme responsible for recognition of correct match between trna and codon.
Ribosomes P-site holds trna carrying the growing peptide chain A-site aminoacyl-trna site Prok. and Euk. differ drugs (tetracycline)
INITIATION: Brings together mrna, trna with a.a. and two subunits of ribosome. Ribosome attaches at sequence upstream (5 ) The ribosome moves onto the mrna at the initiation codon (AUG) Initiator trna carrying methionine attaches to initiator codon. Used 1 GTP to form initiation complex
ELONGATION: Codon recognition Incoming aminoacyl trna binds to the codon in A site. Peptide bond formation ribosome catalyzes peptide bond between new amino acid and carboxyl end of polypeptide
Translocation: trna in A site is moved to P site, taking mrna along. trna in P site moves out and is released from ribosome mrna shifts its position on the ribosome by one codon Repeat with new aminoacyl trna
TRANSLATION
ELONGATION: As the ribosome base pairs the appropriate codons (mrna) with the trna anticodon a peptide bond is formed between the two adjacent amino acids. At this point the trna portion of the first amino acid falls off and returns to the trna pool to become charged with another Amino acid and the process continues... A transfer RNA molecule complete with its specific amino acid is referred to as a charged trna.
TERMINATION: When the ribosome reaches the UAA terminator codon the translation process stops. Other ribosomes may also translate the mrna into other copies of the protein OR the mrna can be degraded and the nucleotides returned to the nucleotide pool. Polyribosomes
RELEASE FACTOR: protein the releases the two RNA subunits
Polyribosomes
Mutation Caused by failure of DNA repair A permanent change in an organism s DNA that results in A different codon = Different amino acid sequence Change in protein function
Single-base substitutions A single base, becomes replaced by another. Also called point mutations. If one purine (A or G) is replace by a purine it is called a transition. If a purine is replaced by a pyrimidine (or vice-versa) a transversion.
Mis-sense mutations The new nucleotide alters the codon Producing an altered amino acid product. Ex: sickle cell anemia ACT CCT GAG GAG Codon# 4 5 6 7 ACT CCT GTG GAG The replacement of A by T at 17 th nucleotide for beta chain of hemoglobin.
Normal RBC round donut thru capillary S.S.A. RBC pointed ends sickle shape problems in capillaries joint, chest pain Anemic b/c RBC break
Nonsense mutations New nucleotide specifies STOP codon mrna transcript will end prematurely Shortened protein - depending earlier the mutation more likely to affect function. Silent mutations Ex third base in codon TCT changed to any other: TCA, TCG, TCC all code for no change in product, not detected
Splice-site mutations Removal of intron sequences requires signals at splice sites if mutation alters these signals Intron not removed, remains in final mrna alters sequence of protein product.
Insertions and Deletions Extra base pairs may be added or removed from DNA range 1 1000. If they involve one, or two base pairs or their multiples can be devastating. Frameshift can change translation:.gagccgcaacttc. By shifting the reading frame one nucleotide to the right, changes amino acid sequence.
Frame-shift mutations Original a.a. sequence: Glu-Pro-Gln-Leu Shifted sequence: Ser-Arg-Asn-Phe Frame-shifts often create STOP codons. Insertions/deletions of three nucleotides are less serious since reading frame is preserved.
Insertions of tri-nucleotide repeats Huntington s and Fragile X are caused by multiple copies of the same triplet. Fragile X string of repeats of CGG as few as 5 or as many as 50 normal phenotype. 80 90 repeats appear normal but gene unstable offspring repeats increase. Abnormal phenotypes as many as 4000 repeats. Males more likely affected mental retardation.
Duplications Can occur during meiosis (crossing over) sister chromatids not aligned can cause one chromatid to have a duplicated gene, and other to have two genes with deletions. Translocations Transfer of a piece of one chromosome to a nonhomologous chromosome reciprocal: Destroy gene function, or enhance expression.
Inversions Chromosome is broken and re-attached at an orientation opposite the original.
Frequency of mutations Humans inherit 3 x 10 9 base pairs from each parent. Substitutions occur most often when DNA is being copied during S phase of cell cycle. Estimated mutation rate 1 in every 50 million nucleotides, for each new cell 120 new mutations.