Exam 3: Lecture 8 Flashcards Preview

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Flashcards in Exam 3: Lecture 8 Deck (13):
1

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

2

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

3

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

4

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)

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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...