Chapter 16 Flashcards
(32 cards)
1
Q
Each amino acids consists of
A
- a central carbon atom bonded to an amino group (NH3+)
- a hydrogen atom
- a free carboxyl group
- an R (radical) group that differs for each amino acid
2
Q
peptide bonds
A
join amino acids to form polypeptide chains
3
Q
codon
A
- the basic unit of the genetic code
4
Q
the genetic code
A
the set of 3 bases that encode an amino acid
5
Q
one amino acid is encoded by
A
three consecutive nucleotides
6
Q
stop codon
A
- UAA, UGA, UAG
- also called nonsense codon or termination codon
- no tRNA molecules have anticodons that pair with termination codons
7
Q
degenerate
A
amino acids may be specified by more than one codon
8
Q
synonymous codons
A
codons that specify the same amino acid
9
Q
isoaccepting tRNAs
A
different tRNAs that accept the same amino acid but have different anticodons
10
Q
wobble
A
- different codons can sometimes pair with the same anticodon through flexibility in base pairing at the 3rd position of the codon.
- allows some tRNAs to pair with more than one codon on an mRNA, thus from 30-50 tRNAs can pair with 61 sense codons
11
Q
genetic code generally nonoverlapping
A
a nucleotide is only part of one codon
12
Q
reading frame
A
refers to how the nucleotides in a sequence are grouped into codons containing three nucleotides. Each sequence has three possible sets of codons, or reading frames
13
Q
initiation codon
A
- the first codon of the mRNA to specify an amino acid.
- usually AUG
- in bacterial cells only, AUG encodes N-formylmethionine
- when the start codon is at an internal position in a gene, it encodes unformylated methionine
14
Q
steps of protein synthesis
A
- tRNA charging
- initiation
- elongation
- termination
15
Q
tRNA charging
A
- the binding of the tRNA to the appropriate amino acid
16
Q
aminoacyl-tRNA synthetases
A
- provide the specificity between an amino acid and it tRNA
- adds a specific charge to the tRNA to attach amino acid
17
Q
tRNA charging steps
A
- the amino acid reacts with ATP producing aminocyl-AMP and PPi
- the amino acid is transferred to the tRNA and AMP is released
18
Q
translation initiation in bacteria
A
- Shine-Delgarno sequence base pairs with the 16S rRNA, allowing the small subunit to attach to the mRNA and positioning the ribosome directly over the AUG initiation codon
- initiator tRNA, fMet-tRNA attaches to initiation codon
- requires initiation factor 2 which forms a complex with GTP to produce the 30S initiation complex
- IF3 dissociates from the small subunit, allowing the large subunit to join
- GTP hydrolyzed to GDP and initiation factors dissociate
- when large subunit joined, we have the 70S initiation complex
19
Q
translation initiation in eukaryotes
A
- cap-binding proteins and poly(A) proteins bind to Cap and poly(A) tail of mRNA
- the small subunit of the ribosome, initiation factors, and the initiator tRNA with amino acids form a complex that recognizes and bind to the 5’ cap
- complex scans until it locates the first AUG codon
- identification of the start codon aided by the presence of a Kozak sequence that surrounds the start codon
20
Q
elongation in bacteria in eukarya (mostly)
A
- requires 70S (80S in eukarya) complex, charged tRNAs, several elongation factors, and GTP
- initiator tRNA immediately occupies P site, but all other tRNAs first enter A site
- charge tRNA binds to A site
- binding takes place when elongation factor TU joins with GTP and then with a charged tRNA
- enters A site where the anticodon on the tRNA pairs with the codon on the mRNA
- after the charged tRNA is in the A site, GTP is cleaved, and the EF-Tu-GDP complex is released
- Elongation factor TS regenerates EF-Tu-GDP to EF-Tu-GTP
- formation of peptide bond between amino acids in P site (property of large subunit) releases amino acid in the P site from its tRNA. enters E site then moves into cytoplasm
- movement of ribosome down the mRNA 5’ to 3’ (translocation) positions the ribosome over the next codon and requires elongation factor G and hydrolysis of GTP to GDP (costs energy to move)
21
Q
termination in bacteria and eukarya (mostly)
A
- terminates when ribosome translocates to a termination codon
- no tRNAs with anticodons complementary, no tRNA enters A site
- release factors bind to the ribosome and cleave tRNA in the P site from the polypeptide chain and release it
22
Q
polyribosome (polysome)
A
- translates mRNA molecules simultaneously
- mRNA with several ribosomes attached
- successfully attaches to the 5’ end of the mRNA and moves toward the 3’ end and the polypeptide associated becomes progressively longer
23
Q
Beadle and Tatum experiment
A
- induced mutations in fungus
- placed individual spores into different culture tubes containing complete medium
- transferred spores from each culture to tubes containing minimal media to identify auxotrophic mutants
- determined effects of each mutation
- if the spores grew in the tube, they were able to identify the added substances as the biological molecular whose synthesis had been affected by the mutation
24
Q
one gene one enzyme hypothesis
A
- genes function by encoding enzymes, and each gene encodes a separate enzyme
- modified to become the one gene, one polypeptide hypothesis after it became known that some proteins are composed of more than one polypeptide chain and that different chains are encoded by separate genes
25
aminoacyl (A) site
where new tRNAs enter and match up with codons
26
peptidyl (P) site
where the growing amino acid chain is held
27
Exit (E) site
where uncharged tRNA goes before leaving the ribosome
28
translation initiation in bacteria
- initiation factors bind and prevent large subunit from binding
- Shine-Delgarno sequence base pairs with the 16S rRNA, allowing the small subunit to attach to the mRNA and positioning the ribosome directly over the AUG initiation codon
- initiator tRNA, fMet-tRNA attaches to initiation codon
- requires initiation factor 2 which forms a complex with GTP to produce the 30S initiation complex
- IF3 dissociates from the small subunit, allowing the large subunit to join
- GTP hydrolyzed to GDP and initiation factors dissociate
- when large subunit joined, we have the 70S initiation complex
29
Exit (E) site
where uncharged tRNA goes before leaving the ribosome
30
translation initiation in eukaryotes
- cap-binding proteins and poly(A) proteins bind to Cap and poly(A) tail of mRNA to enhance binding of small subunit to 5' end
- the small subunit of the ribosome, initiation factors, and the initiator tRNA with amino acids form a complex that recognizes and bind to the 5' cap
- complex scans until it locates the first AUG codon
- identification of the start codon aided by the presence of a Kozak sequence that surrounds the start codon
31
elongation in bacteria in eukarya (mostly)
- requires 70S (80S in eukarya) complex, charged tRNAs, several elongation factors, and GTP
- initiator tRNA immediately occupies P site, but all other tRNAs first enter A site
- charge tRNA binds to A site with help of elongation factor
- enters A site where the anticodon on the tRNA pairs with the codon on the mRNA
- after the charged tRNA is in the A site, GTP is cleaved, and the complex is released. Regenerated by elongation factor.
- formation of peptide bond between amino acids in P site (property of large subunit) releases amino acid in the P site from its tRNA. enters E site then moves into cytoplasm
- movement of ribosome down the mRNA 5' to 3' (translocation) positions the ribosome over the next codon and requires elongation factor G and hydrolysis of GTP to GDP (costs energy to move)
32
Beadle and Tatum experiment
- induced mutations in fungal spores by irradiation
- placed individual spores into different culture tubes containing complete medium
- transferred spores from each culture to tubes containing minimal media to identify auxotrophic mutants
- determined effects of each mutation
- moves mutants to a series of tubes with minimal medium plus one variety of essential molecules (such as amino acids).
- if the spores grew in the tube, they were able to identify the added substances as the biological molecule whose synthesis had been affected by the mutation
