Exam 3: Lecture 8 Flashcards
1
Q
Translation (General)
A
- 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
Q
Genetic Code/Codons
A
- 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
Q
FIrst and Last Exons
A
- 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
Q
Translatable Section
A
- 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
Q
Khorana, Nirenberg and Holley
A
- 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
Q
Transfer RNA (tRNA)
A
- 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
Q
Degeneracy in Genetic Code
A
- 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
Q
Wobble Effect
A
- 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
Q
Second Mechanism
A
- 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
Q
Missense Mutations
A
- 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
Q
Silent Mutation
A
- 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
Q
Nonsense Mutation
A
- 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
Q
Frameshift Mutations
A
- 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…
