Genetics Chpt 6

process performed by the ribosome, of reading mRNA, and synthesizing a protein

amino acids are attached to tRNA by enzymes

happens when the ribosome hits a stop codon on the mRNA (UAA, UAG, UGA)

the RNA that has an amino acid attached to it and binds to the codon on the mRNA

Shine-Dalgarno sequence, tRNA f Met

Kozak sequence

GTP hydrolysis, mRNA, release factor, UAA codon, tRNA

IF3 binds to 30S subunit which prevents its association with the 50S subunit, The 16S rRNA base-pairs with the Shine-Dalgarno sequence tRNA fMet, with IF2 and GTP binds to the start codon, IF1 binds to A site of 30S subunit the 50S subunit joins the complex Initiation factors are released from the complex a 70S initiation complex is formed

A,P, E

is attached to the tRNA occupying the A site

It is catalyzed by peptidyl transferase.

a formyl group is attached to the initiating methionine

complementary base pairing between mRNA and DNA

The small ribosomal subunit binds to an mRNA sequence near the 5' end of the transcript

AUG

UGA, UAG, UAA

3

3

1

1

3'-UTR, 5'-cap, start codon, stop codon, 5'UTR, AAUAAA, poly-A tail

Eukaryotic large subunits

A tRNA that carries the next amino acid to be added to the polypeptide chain

A tRNA that carries a specific amino acid

A short conserved nucleotide sequence upstream of the AUG start codon that serves to align the mRNA on the bacterial small ribosomal subunit

To the partial P-site on the small ribosomal subunit

Union of the large subunit with the small subunit to form the intact ribosome

Binding of initiator tRNA to the AUG start codon

mRNA that encodes multiple different polypeptide products

20

The genetic code consists of nonoverlapping codons.

contain multiple ribosomal RNAs (rRNAs) and proteins translate mRNA in the 5-3 direction composed of two subunits, the large ribosomal subunit and the small ribosomal subunit

Svedberg units (S), a property based on size, shape, and hydration state

sequence dictates the resulting amino acid sequence

start codon- N-terminus stop codon- C-terminus

5'UTR and 3'UTR segments of mRNA outside of translated regions

mostly bacteria, small subunit- 30S (21 proteins and one 16S rRNA molecule) large subunit - 50S (31 proteins, small 5S rRNA, large 23S rRNA) fully assembled ribosome is 70S

Aminoacyl site : binds new tRNA (containing an amino acid) to the growing polypeptide chain

Peptidyl site holds tRNA to polypeptide chain

Exit site provides an exit for tRNA after amino acid has been added to the chain

mammalian ribosomes small subunit - 40S (3 proteins, 1 18S rRNA) large subunit- 60S (45-50 proteins, 3 rRNA molecules(5S, 5.8S, 28S)) fully assembled 80S ribosome

begins when small ribosomal subunit binds near 5' end of the mRNA and finds start codon

carries first amino acid, binds to start codon (AUG) with the help of Initiation factor proteins (IF) large subunit joins small subunit to form intact ribosome and translation begins powered by GTP

tRNAs used during translation, attached to amino acids

six components: mRNA, small subunit, large subunit, initiator tRNA, 3IF, GTP 30S and IF3 subunit bind near 5' end of mRNA and search for Shine-Dalgarno sequence. 1) small subunit binds near 5' end of mRNA and identifies start codon 2) initiator tRNA carrying the amino acid binds to start codon 3) large subunit joins small subunit completing ribosome

prevents 30S small subunit from binding with 50S large subunit

forms when the 16S rRNA molecule from the small subunit base pairs with a sequence of six nucleotides upstream of the start codon (5'UTR)

purine rich (A/G) about six nucleotides upstream of start codon on mRNA

binds to start codon carrying a modified amino acid (fMet) and is charged IF2 and a GTP molecule bind to tRNA fMet and IF1 joins the complex completing the 30S initiation complex

50S subunit joins 30S by the hydrolysis of GTP to GDP IF1, IF2, IF3 dissociate and create 70S complex

40S subunit uses eukaryotic initiation factor eIF proteins eIF1A, eIF3 and charged tRNA fMet bind the small subunit to form pre initiation complex recruited by 5' cap region of mRNA

pre initiation complex joins group of 4 eIF4 proteins that assemble 5' cap of mRNA initiation complex scans 5' UTR for start codon ATP hydrolysis required for scanning

start codon is found in Kozak sequence, which leads to 60S subunit to complex using GTP hydrolysis for energy and eIF proteins dissociate.

recruits EF (elongation factor) use energy from GTP hydrolysis recruits charged tRNAs to A site forms peptide bonds between sequential amino acids Translocates the ribosome in 3' direction along mRNA

charged tRNAs with an EF inspect the open A-site and tRNA with correct anticodon enters A site enzyme peptidyl transferase catalyzes peptide bonds between amino acids at P and A sites elongated polypeptide is transferred to tRNA at A site while tRNA in P site exits through E site Efs then translocate the ribosome moving it three nucleotides toward 3' end which moves tRNA at A site to P site and opens new A site

UAA, UAG, UGA

(RF) used by both eukaryotes and bacteria to bind a stop codon in the A site polypeptide bound to the tRNA at the P-site is released when the GTP complexed to the RF is hydrolyzed bacteria- release factor RF1 recognizes UAG and UAA and RF2 recognized UAA and UGA Eukaryotic termination- accomplished by single release factor eRF recognizes all 3 stop codons

containing groups of ribosomes all actively translating the same mRNA

allows ribosomes to begin translating mRNAs not yet completed

mRNAs are produced in the nucleus

multiple polypeptide-producing segments each contains a Shine-Dalgarno site

catalyze the addition of correct amino acid to tRNAs large molecule contacts several points on the tRNA

refers to the specific codon sequence as determined by the start codon

amino acids Phe (phenylalanine)

The genetic code derives its specificity from the interaction between tRNA and mRNA The fidelity of the aminoacyl-tRNA synthetases in recognizing each of their tRNAs and correctly charging them, is extremely important

modifies polypeptides into functional proteins by removal or chemical alteration of amino acids

uses signal sequences (also called leader sequences) to direct proteins to their cellular destinations

Unfolded Protein Response

Translation begins toward the 5 end of the mRNA, which corresponds to the amino end of the nascent polypeptide.

Upstream of the AUG start codon on the mRNA The Shine-Dalgarno sequence is located a few nucleotides upstream of the AUG start codon on the mRNA. It base pairs with a complementary sequence near the 5 end of the16S rRNA.

Initiator tRNAmet Initiator tRNAmet is localized to the P site of the ribosome. Initiation is the only time a tRNA bearing a single amino acid is found in the P site.

Large ribosomal subunit In both prokaryotes and eukaryotes, the large ribosomal subunit is the last component incorporated in the process of translational initiation.

U - C Wobble at the third base of the codon allows imperfect base pairing, but only for purine - pyrimidine combinations. U and C are both pyrimidines, which is not permitted by wobble.