Transcription RNA processing, and translation part B

bio exam 2

how does a cell build the protein it needs

transcribes DNA into RNA, translates mRNA into protein

What is translation?

When the sequence of mRNA bases are converted to an amino acid sequence

What is used in the translation process?

ribosomes, mRNAs, and tRNAs

what is transfer RNA (tRNA)?

the adapter molecule used in translation

how is a polyribosome formed?

when multiple ribosomes attach to an mRNA

where does transcription take place in eukaryotes?

in the nucleus (mRNAs are synthesized and processed there)

where does translation take place in eukaryotes?

in the cytoplasm (mature mRNAs are transported to the cytoplasm for translation by ribosomes and polyribosomes form)

what is an adapter molecule?

a molecule that mediates the transfer between two different systems (mRNA → amino acid for tRNA)

What do tRNAs contain?

the amino acid side of the translation equation

How long are tRNAs?

relatively short: 75-95 nucleotides long

what secondary structure CAN tRNAs form?

a stem-and-loop structure

What sequence is at the 3’ end of the amino acid binding site?

CCA

where is the anticodon in tRNAs?

in the loop at the opposite end

what can the anticodon in tRNA base pair with?

the mRNA codon

What is required to attach tRNA to an amino acid?

ATP

what catalyses the addition of amino acids to tRNAs?

Aminoacyl-tRNA synthetases

For each amino acid, how many tRNAs are there?

one or more

How many tRNAs are there?

61 different codons but only about 40 tRNAs in most cell

what is wobble pairing?

A hypothesis that suggests that the anticodon’s third position can form a nonstandard base pair (one tRNA is able to read more than one codon)

what subunits make up ribosomes?

the small subunit and the large subunit

what does the small subunit do in ribosomes?

holds the mRNA in place

what does the large subunit do in ribosomes?

where the peptide bonds form

what makes up ribosomes?

proteins and ribosomal RNA (rRNA) - about equal parts by mass

How many tRNAs line up within the ribosome during translation?

three

what does the A (acceptor or aminoacyl) site do in the ribosome?

where the tRNA carries an amino acid

what does the P (peptidyl) site do in the ribosome?

holds the growing peptide chain

what does the E (exit) site do in the ribosome?

where the tRNAs without amino acids exit the ribosome

What is the order of the three sites in the ribosome?

A → P → E

How many steps happen when the ribosome synthesizes proteins?

three

What is the first step that ribosomes take when synthesizing proteins?

an aminoacyl tRNA enters the A site’ remains if there is a codon-anticodon match

What is the second step that ribosomes take when synthesizing proteins?

a peptide bond forms between the amino acid on the A-site tRNA and the polypeptide on the P-site tRNA

What is the third step that ribosomes take when synthesizing proteins?

the ribosome moves down the mRNA by one codon and all three tRNAs move down one position → the tRNA in the E site exits and the A site is available for another tRNA to bind

why can translation begin before transcription is complete in prokaryotes?

transcription and translation both occur in the cytoplasm

How many amino acids does a protein grow by with each repeat of the three steps of ribosomal protein synthesis

one

What end is the amino acid always added to of the polypeptide

the carboxyl end (C-terminus)

what are the three phases of translation

initiation, elongation, termination

Where does the initiation phase begin

near the AUG start codon

what is the Shine-Dalgarno Sequence

the ribosome binding site in bacteria

what is the first tRNA called

the initiator tRNA

how is the initiator tRNA different from other tRNA?

it carries a modified methionine (f-Met) in bacteria

What is the first step in translation initiation in bacteria?

the mRNA binds to a small ribosomal subunit

What is the second step in translation initiation in bacteria?

the initiator tRNA bearing f-Met binds to the start codon

What is the third step in translation initiation in bacteria?

the large ribosomal subunit binds so that the initiator RNA is in the P site

Where does the small ribosomal subunit bind to in eukaryotes to start translation?

the 5’ cap, then small ribosomal subunit + initiator tRNA slides along mRNA to find AUG

what happens at the start of elongation?

the initiator tRNA is in the P site, and the E and A sites are empty

where does an aminoacyl tRNA bind during the start of elongation?

to the codon in the A site

what does ribosomal RNA catalyze?

peptide bond formation

what is the active site of the ribosome entirely made of?

ribosomal RNA

is the ribosome a ribozyme?

yes

when does translocation occur?

when the ribosome slides one codon toward the 3’ end of the mRNA (elongation factors help move the ribosome)

What three things does translocation accomplish?

1. the uncharged tRNA from the P site moves into the E site and is ejected from the ribosome

2. the tRNA attached to the growing protein moves into the P site

3. opens the A site to expose a new codon, which is available to accept a new aminoacyl tRNA

What three steps of translocation repeat at each codon along the mRNA

1. arrival of the aminoacyl tRNA

2. peptide bond formation

3. translocation

when does the termination of translation occur?

when the A site encounters a stop codon

What enters the A site during termination?

a protein called a release factor

what does a release factor do?

hydrolyzes the bond linking the P-site tRNA to the polypeptide chain, resembles tRNAs in size and shape,

what happens at the end of termination?

The newly synthesized polypeptide, tRNAs, and ribosomal subunits separate from the mRNA

What are the differences in translation between prokaryotes and eukaryotes?

Prokaryotes: polycistronic messages possible, initiator tRNA = f-Met, Ribosome Binding Site (Shine-Dalgarno sequence)

Eukaryotes: Only monocistronic messages, initiator tRNA (special) contains methionine, ribosome scans 5’ UTR to find AUG codon

what is post-translational modification

an extensive series of processing steps proteins go through before they are functional

where does post-translational modification occur?

in various locations in a cell

does the ribosome or tRNA move during translation?

ribosome crawls along tRNA during translocation