Chapter 6

Expression of Biological Information (Part IV)

By :

Biology UnitKedah Matriculation College

Concept review…

DNA & genetic

information DNA

replication

Protein synthesis

Gene regulation & expression

DNA structure

DNA as a carrier

Gene concept Definition

Models of DNA replication

DNA replication process

Enzyme involved

Transcription

Translation

Expression of Biological Information

concept

Components

Mechanism

Learning outcome

Explain translation in protein synthesis

Describe the stages involved:

i. initiation ii. elongation (codon recognition, peptide bond formation and translocation) iii. termination

Translation

Translation

Translation - a process

- whereby the genetic information in mRNA strand

- is translated into sequence of amino acids to form polypeptide/protein

Occurs in cytoplasm

Definition

Translation

RNA direct the synthesis of polypeptide.

cell interprets a genetic message and builds a protein accordingly.

The message is a series of codons along an mRNA molecule and interpreter is

transfer RNA

Structures that involve in translation

Ribosome tRNA

mRNA

triplet bases on tRNA: anticodon

Structures that involve in

translation:1. tRNA tRNA transfer amino acid

from cytoplasm to ribosome

tRNA can be used repeatedly.

Each type of tRNA molecule associates a particular mRNA codon with a particular amino acid.

Shape: 2-dimensional –cloverleaf shape.

tRNA includes the anticodon, the specialized base triplet that binds to a specific mRNA codon.

e.g.: mRNA codon UUU has AAA as its anticodon.

From the other end of the L-shaped tRNA

molecule protrudes its

3’end; the attachment site for

amino acid.

tRNA with amino acid; aminoacyl

tRNA.

2. Aminoacyl-tRNA synthetase

Catalyze the matching up of amino acid with the correct tRNA and using energy of ATP

(activation of amino acids)

Catalyzes the covalent attachment of amino acid to the 3’- OH of tRNA.

Resulting aminoacyl tRNA

Activation of aminoacyl tRNA

3. Ribosomes

Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein

synthesis.

A large and small subunit join to form a functional ribosome only when they attach to

mRNA molecule.

Each ribosomal subunit is consist proteins and ribosomal RNA (rRNA).

Ribosome has TWO binding site for mRNA:

P site (peptidyl-tRNA site)

Holds the tRNA carrying the growing polypeptide chain.

A site (aminoacyl-tRNA site)

Holds the tRNA carrying the next amino acid to be added to the chain.

Translation process

Overall Process…

3 steps of translation process,

synthesis of a polypeptide chain :

1. Initiation

2. Elongation

3. Termination

Bring together mRNA and tRNA bearing the first amino acid of polypeptide and two subunits of ribosome.

Small ribosomal subunit binds to mRNA that being

read from 5’ to 3’.

1. Initiation

Initiation

Then initiator tRNA base-pairs with the start codon

Arrival of a large subunit completes the

initiation complex

Initiation

initiation factor: required to bring all the translation components together

Initiator tRNA site in the P site of ribosome and vacant A site is ready for next tRNA.

initiation complex of translation

Ribosome (large)

tRNA

mRNA

Ribosome (small)

2. Elongation

Amino acid are added one by one to the initial amino acid. Each addition, occurs in 3 steps cycle.

i. Codon recognitionii. Formation od peptide bondiii. Translocation

i. Codon recognition

mRNA codon in A site forms hydrogen bonds with the anticodon of an incoming molecule of tRNA carrying its appropriate amino acid.

Elongation factors lead the tRNA into A site. hydrogen

bonds

ii. Peptide bond formation

Polypeptide detaches from tRNA (in P site) to which it was bond and is link to amino acid carried by the tRNA (in A site)

large ribosomal subunit catalyzes the formation of peptide bond between polypeptide extending from P site and newly arrived amino acid in A site.

2. Peptide bond Formation

3. Translocation

iii. Translocation

tRNA in P site dissociates from ribosome.

tRNA in A site now attached to the growing polypeptide, is translocated to the P site

as the tRNA changes sites, its anticodon remains hydrogen bonded to the mRNA codon, allowing the mRNA and tRNA molecules to move as a unit.

This movement, in turn, brings the next codon to be translated into A site.

Elongation is repeated as each amino acid is added to the chain until the polypeptide is completed.

iii. Translocation

Peptide bond formation

3. Termination Elongation continues until A site reaches

termination codon.

do not code for amino acids and stop translation.

- No aminoacyl tRNA recognize

TerminationProtein called ‘release factor’ binds to the termination codon in A site.

- freeing the polypeptide from the ribosome.- mRNA detaches

- ribosome separates

UAC

Met

Small Unit

Large Unit

Small Unit

P A

Initiation Complex

Initiation

5’ UUG UGG UAC AUG GAA GAG CAU … UGA 3’

Initiation codonAUG

UUU

ACU

5’ UUG UGG UAC AUG GAA GAG CAA ….UGA 3’

UAC

Met

CUU

CUC

Elongation

5’ UUG UGG UAC AUG GAA GAG CAU UGA 3’

GUG

ACU

Termination STOP codonUAA UAG UGA

copyright cmassengale 41

Messenger RNA (mRNA)

methionine glycine serine isoleucine glycine alanine stopcodon

protein

A U G G G C U C C A U C G G C G C A U A AmRNA

startcodon

Primary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

AmRNA U C C A U C A U G G G C U C C A U C G G

methionine

Codon 1

Polyribosomes

Polyribosomes/polysome

A group of several ribosomes attached to, and translating the same messenger RNA simultaneously

Significance of polyribosome

Increase the rate of same protein synthesis/protein produced rapidly

more of same protein can be made simultaneously

Animation

Can you tell the story?

DNA

pre-mRNA

ribosome

tRNA

aminoacids

polypeptide

mature mRNA

5' GTP cap

poly-A taillarge ribosomal subunit

small ribosomal subunit

aminoacyl tRNAsynthetase

E P A

5'

3'

RNA polymerase

exon introntRNA