TRANSLATION AHL Topic 7.4 IB Biology Miss Werba

AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS 7.1 DNA STRUCTURE 7.2 DNA REPLICATION 7.3 TRANSCRIPTION 7.4 TRANSLATION 7.5 PROTEINS 7.6 ENZYMES J WERBA – IB BIOLOGY 2

THINGS TO COVER Structure of tRNA tRNA activating enzymes Structure of ribosomes Process of translation Initiation, elongation, translocation, termination Polysomes, start codons, stop codons Direction of translation Structure of a peptide bond Position of ribosomes v function J WERBA – IB BIOLOGY 3

tRNA MOLECULES Command term = EXPLAIN 7.4.1 tRNA MOLECULES Command term = EXPLAIN The tRNA structure matches its function Function? To bring amino acids from the cytoplasm to the growing polypeptide and to attach them in the correct location. J WERBA – IB BIOLOGY 4

tRNA MOLECULES Command term = EXPLAIN 7.4.1 tRNA MOLECULES Command term = EXPLAIN It has a clover-leaf structure made from a single chain of RNA There are 3 loops and 2 ends The 3’end has a CCA base sequence (known as the ACC terminal) and acts as the attachment site for the amino acid The anticodon lies at the opposite end (on the centre loop). It is a sequence of 3 bases and will attach to the mRNA codons. J WERBA – IB BIOLOGY 5

tRNA MOLECULES Command term = EXPLAIN 7.4.1 tRNA MOLECULES Command term = EXPLAIN tRNA is activated by a tRNA activating enzyme There are 20 tRNA activating enzymes – one for each of the 20 amino acids The specific structure of each tRNA molecule means that each one only binds to the specific amino acid Addition of an amino acid to a tRNA molecule requires ATP J WERBA – IB BIOLOGY 6

tRNA MOLECULES Command term = EXPLAIN 7.4.1 tRNA MOLECULES Command term = EXPLAIN Ref: IB Biology HL, OSC J WERBA – IB BIOLOGY 7

STRUCTURE OF RIBOSOMES Command term = OUTLINE 7.4.2 STRUCTURE OF RIBOSOMES Command term = OUTLINE Made of protein and ribosomal RNA (rRNA) Have 2 subunits: large (50S) and small (30S) Have 3 tRNA binding sites on the large subunit: A, P and E sites Has a mRNA binding site Multiple ribsomes can work on the same mRNA at the same time to complete translation more efficiently J WERBA – IB BIOLOGY 8

DISTRIBUTION OF RIBOSOMES Command term = STATE 7.4.7 DISTRIBUTION OF RIBOSOMES Command term = STATE Free ribosomes synthesize proteins for use primarily within the cell and bound ribosomes synthesize proteins primarily for secretion. The distribution of ribosomes depends upon the function of the protein they make. Free floating ribosomes synthesise proteins for use within the cell. Ribosomes bound to the Rough ER produce proteins to be secreted out of the cell or for use in lysosomes. J WERBA – IB BIOLOGY 9

DIRECTION OF TRANSLATION Command term = STATE 7.4.4 DIRECTION OF TRANSLATION Command term = STATE Translation is carried out in a 5’→ 3’ direction. Like DNA replication & transcription, translation occurs in a 5’  3’ direction (of the mRNA). Why? b/c the start codon is at the 5’ end of the mRNA J WERBA – IB BIOLOGY 10

PROCESS OF TRANSLATION Command term = STATE 7.4.3 PROCESS OF TRANSLATION Command term = STATE Translation consists of initiation, elongation, translocation and termination. There are 4 main stages in translation: Initiation Elongation Translocation Termination J WERBA – IB BIOLOGY 11

PROCESS OF TRANSLATION Command term = EXPLAIN 7.4.6 PROCESS OF TRANSLATION Command term = EXPLAIN 1) Initiation The tRNA-activating enzymes activate the tRNA molecules pre-initiation. The mRNA binds to the small subunit of the ribosome at the ‘Met’ (AUG) codon – ie. START codon The first tRNA molecule is brought in carrying the complementary anticodon and the ‘Met’ amino acid This requires the help of the ATPase enzyme. Then the large subunit of the ribosome arrives. J WERBA – IB BIOLOGY 12

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 1) Initiation J WERBA – IB BIOLOGY 13

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN The larger subunit of the ribosome has 3 tRNA binding sites – A, P and E. The Arriving tRNA binds to the A site. The P site holds the tRNA that carries the growing Polypeptide strand. The tRNAs Exit the ribosome molecule via the E site. J WERBA – IB BIOLOGY 14

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 2) Elongation Elongation is divided into 2 steps: Codon recognition Bond formation J WERBA – IB BIOLOGY 15

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 2) Elongation Codon recognition tRNA binds to a codon in the A site The incoming tRNA contains the anticodon, which complements the codon Hydrogen bonds form between the base pairs. The tRNA carries the next amino acid in the chain. J WERBA – IB BIOLOGY 16

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 2) Elongation Bond formation The next amino acid is added The large subunit acts as a ribozyme, using the enzymes within it to join the amino acids together. The amino acids are joined by a peptide bond in a condensation reaction J WERBA – IB BIOLOGY 17

PEPTIDE BOND Command term = DRAW and LABEL 7.4.5 PEPTIDE BOND Command term = DRAW and LABEL J WERBA – IB BIOLOGY 18

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 3) Translocation tRNAs and the mRNA strand “move down one site”. The ribosome releases the used tRNA from the E site. Then the elongation process repeats.... the next tRNA and amino acid are brought in. J WERBA – IB BIOLOGY 19

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 4) Termination When a STOP codon on the mRNA is reached, translation is terminated. A release factor protein recognises and binds to the STOP codon. J WERBA – IB BIOLOGY 20

PROCESS OF TRANSLATION Command term = EXPLAIN 7.3.3 PROCESS OF TRANSLATION Command term = EXPLAIN 4) Termination The release factor allows the polypeptide to be released. Once this occurs, the ribosomal subunits and other factors break apart. The ribosome is free to pick up a new strand of mRNA. J WERBA – IB BIOLOGY 21

Sample questions Q1. What is the function of the tRNA activating enzyme? It links tRNA to ribosomes. It links tRNA to mRNA. It links tRNA to a specific amino acid. It links an amino acid on one tRNA to an amino acid on another tRNA. J WERBA – IB BIOLOGY 24

Sample questions Q2. The antisense strand on the DNA molecule coding for three codons of a gene is: TATCGCACG What are the anticodons of the three tRNA molecules that correspond to this sequence? UAU, CGC and ACG ATA, GCG and TGC AUA, GCG and UGC TAT, CGC and ACG J WERBA – IB BIOLOGY 25

Sample questions Q3. Translation occurs in living cells. Explain how translation is carried out, from the initiation stage onwards. [9] J WERBA – IB BIOLOGY 26

Sample questions A1. and A2. C J WERBA – IB BIOLOGY 27

Sample questions A3. translation involves initiation, elongation/translocation and termination; mRNA binds to the small sub-unit of the ribosome; ribosome slides along mRNA to the start codon; anticodon of tRNA pairs with codon on mRNA: complementary base pairing (between codon and anticodon); (anticodon of) tRNA with methionine pairs with start codon / AUG is the start codon; second tRNA pairs with next codon; peptide bond forms between amino acids; ribosome moves along the mRNA by one codon; movement in 5′ to 3′ direction; tRNA that has lost its amino acid detaches; another tRNA pairs with the next codon/moves into A site; tRNA activating enzymes; link amino acids to specific tRNA; stop codon (eventually) reached; 9 max J WERBA – IB BIOLOGY 28