Exam 3: Lecture 8 Flashcards Preview

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Translation (General)

-eukaryotes: process of translating mRNA transcript into protein occurs in cytoplasm

-involves ribosomal complex "reading" information that is encoded within mRNA strand and executing these instructions by generating proteins that consist of amino acids that are placed in appropriate order


Genetic Code/Codons

-ribosome will bind to specific location within 5' end of mRNA

-reads template in sets of 3 bases (codons)

-each codon codes for amino acid (chart)

-first codon read in each eukaryotic transcript is AUG and codes for Methionine amino acid (Met)

-last codon to be read is on of the three stop sequences (UAA, UAG, or UGA).

-when ribosome encounters one of these it will disassociate from mRNA transcript


FIrst and Last Exons

-of each mRNA transcript contains sequences that will not be translated by ribosome

-referred to as 5' and 3' untranslated regions (UTR)

-ribosome must not only be guided to first exon but also be prevented from translating 5' UTR portion of first exon


Translatable Section

-once ribosome has reached this of first exon it will need to start translating first AUG and then continue to read in sets of three until reaches stop codon

-(look for first UGA and then start reading it) (look at reading sequences on second slide)


Khorana, Nirenberg and Holley

-awarded Nobel Prize for

-(1) showing mRNA was template used during translation

-(2) demonstrating that a codon consisted of three nucleotides

-(3) determining genetic code

-synthesized RNA polymers consisted of polyA, polyC, polyG, and polyU and then mixed it with translation machinery -proteins they recovered from this in vitro reaction consisted of Lysine, Proline, GLycine, and Phenylalanine respectively

-went through and generated RNA polymers that contained repeats of all possible codons and determined entire genetic code


Transfer RNA (tRNA)

-transcribed by RNA Pol III and encode set of non-coding RNA species

-each is folded into cloverleaf-like structure

-two most critical regions: (1) three base anticodon which forms complementary base pairing within mRNA and (2) 3' acceptor arm which is bound to amino acid

-sequence of anticodon determines which amino acid will be bound at acceptor arm -codon and anticodon bound antiparallel

-each type encoded by multiple genes

-level of redundancy ensures translation won't stop in case that any one tRNA gene is inactivated by mutation -also ensures that enough tRNAs produced to handle high translation volumes


Degeneracy in Genetic Code

-4 different nucleotides used in mRNA transcripts and since codon consists of 3 nucleotides there are 64 different 3 base combos

-however, only 20 amino acids -nearly all amino acids are coded by more than one codon

-Serine (Ser) having the most at 6 codons

-redundancy means genetic code is degenerate

-two mechanisms by which these 64 codons can direct addition of 20 amino acids to proteins


Wobble Effect

-mechanism makes use of fact that single tRNA can interact with multiple codons

-occurs because 5' nucleotide of anticon can base pair with more than one type of base at 3' nucleotide of codon


Second Mechanism

-centers around the fact that a single amino acid can be attached to multiple tRNAs

-ex: four different tRNAs bound to Leucine (Leu)

-each will recognize one of four codons predicted to encode Leu


Missense Mutations

-most changes to coding portion of DNA sequence will have eventual effect on protein sequence

-most common class is missense mutations

-in these cases single base change within DNA will lead to change in identity of single amino acid with protein

-can lead to complete inactivation of protein or in reduction in activity level

-since not all amino acid residues are not required for proper protein function there can be some missense mutations that do not affect activity of protein

-ex: CAT repeat after transcription and translation results in His amino acid

-if mutation changes this sequence to CCT resulting amino acid will be Pro


Silent Mutation

-since genetic code is degenerate, several amino acids are encoded by multiple codons

-means that depending upon position of mutation within DNA sequence may not change amino acid

-ex: CCG triplet transcribed into GGC which specifies Gly amino acid.

-error/mutagen changes sequence to CCA will be transcribed int GGU which still translated into GLY

-change does not change protein sequence


Nonsense Mutation

-errors in replication or chemical mutagens can also change DNA sequence such that a stop codon is introduced prematurely within coding sequence

-ex: CAG triplet converted to TAG due to mutation

-causes stop codon to be generated instead of Gln coding codon -premature stop codon incorporated into mRNA which causes truncation of protein


Frameshift Mutations

-during replication slippage additional bases may be inserted into newly synthesized strand or removed from template strand.

-depending on number of bases added or subtracted (1 or 2) can lead to a change in reading frame (frameshift mutation)

-ex: adenine base added, after transcription and translation string of His residues changes so that Threonine and Serine amino acids added to growing protein

-due to reading frame shifting from repeating CAT to ACA followed by TCA, TCA...