Transcription and

Translation AHL

IB Biology HL

Outcomes - Transcription

• 7.3.1 State that transcription is carried out in a 5’ to 3’

direction.

• 7.3.2 Distinguish between the sense and antisense strands

of DNA.

• 7.3.3 Explain the process of transcription in prokaryotes,

including the role of the promoter region, RNA

polymerase, nucleoside triphosphates and the terminator.

• 7.3.4 State that eukaryotic RNA needs the removal of

introns to form mature mRNA.

Outcomes - Translation

• 7.4.1 Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy.

• 7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites.

• 7.4.3 State that translation consists of initiation, elongation, translocation and termination.

• 7.4.4 State that translation occurs in a 5’ to 3’ direction.

• 7.4.5 Draw and label a diagram showing the structure of a peptide bond between two amino acids.

• 7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons and stop codons.

• 7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes.

Let’s see an overview

• http://www.youtube.com/watch?feature=player_e

mbedded&v=itsb2SqR-R0

Transcription – the details

• RNA Polymerase can only work in a 5’ to 3’

direction – in other words, it can only add to a 3’

end so… which strand of DNA is used as a

template?

• The template strand is called the antisense strand

Sense and Antisense Strands

Remember this… 5’ to 3’ direction (always!)

Let’s see…

• http://www.stolaf.edu/people/giannini/flashanima

t/molgenetics/transcription.swf

Promoter and Terminator

RNA Polymerase

• RNA Polymerase will begin at the promoter and it

will read the antisense strand and build a

complementary mRNA strand with free nucleoside

triphosphates until it reaches the terminator.

• Once the terminator is reached, the mRNA will be

released and the DNA will re-anneal unchanged

• The mRNA will then undergo post-transcriptional

modifications

Introns removed

Post-Transcriptional

Modification

http://bcs.whfreeman.com/thelifewire/content/ch

p14/1401s.swf

Translation – the details!

• Once Introns are removed, the mRNA will travel to

the cytoplasm to a ribosome

tRNA

• Each tRNA molecule has a 5’

and a 3’ end just like DNA and

mRNA strands.

• Hydrogen bonds cause the

folding of and resulting three

dimensional shape of the tRNA

molecule.

• The anticodon is located on one

of three loops in the tRNA clover

leaf shape. The anticodon pairs

with a codon located on the

mRNA strand.

tRNA Enzymes

• The 3’ end of the tRNA contains the base sequence

CCA and this is the amino acid attachment site.

• There is a group of 20 enzymes collectively known as

tRNA activating enzymes.

• These enzymes bind each of the 20 amino acids to the

amino acid attachment site of a tRNA molecule with

the help of energy supplied by ATP.

• Each enzyme is specific to a certain amino acid and a specific

tRNA molecule.

• Once the amino acid is attached it is called an activated amino

acid.

• The tRNA can now

deliver the amino

acid to a ribosome

where it can be used

in a polypeptide.

Translation Overview

• http://www-

class.unl.edu/bio

chem/gp2/m_bio

logy/animation/

m_bio_ga.html#

Ribosomes

• The structure of ribosomes

consists of a large subunit and a

small subunit.

• The subunits are composed of

rRNA and many distinct, small

proteins.

• Ribosomes are constructed in the

nucleolus and exit the nucleus

through pores in the nuclear

membrane.

Ribosome binding sites

• Located in the space between the two subunits there are

binding sites for mRNA and three binding sites for

tRNA.

• The rRNA is responsible for binding the mRNA and

tRNA.

Site Function

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

P site Holds the tRNA carrying the elongating polypeptide

E site The site from which tRNA that has lost its amino acid is discharged from the ribosome.

Three sites for tRNA Stages of Translation

• Similar to transcription, the process of translation

involves several stages including:

• Initiation

• Elongation

• Translocation

• Termination

Direction of Translation

• Translation also occurs in a 5’ to 3’ direction similar

to that of DNA replication and transcription.

Peptide bond between amino acids

KNOW THIS!!!

The Process of Translation

• We will discuss 4 steps in Translation:

• Initiation

• Elongation

• Translocation

• Termination

Initiation

• Initiation, the first step in translation, begins when the

activated amino acid methionine combines with a

mRNA molecule and the small sub unit of the ribosome.

• When the ribosome reads AUG (start codon) hydrogen

bonds begin forming between the initiation tRNA and the

start codon. Now the larger subunit of the ribosome

attaches along with proteins called initiation

factors. These are attached using GTP (guanosine

triphosphate ).

Elongation

• The tRNA molecules start bringing in amino acids

to the mRNA ribosomal complex in the order set out

by the mRNA.

• Proteins known as elongation factors bind the

tRNA’s to the A-site. The initiator tRNA moves to

the P-site and a peptide bond is formed between the

two amino acids.

Translocation

• This phase technically occurs during the elongation

phase.

• Translocation is the movement of the tRNA

molecules from one site to the next as the

polypeptides is being built.

Termination

• This phase begins when one of three stop codons appear

in the A-site.

• A protein known as a release factor enters the A-site

and it catalyzes the release of the now complete

polypeptide from the tRNA in the P-site.

• The released polypeptide signals the separation of the

ribosomal subunits and the mRNA strand.

• A polysome is a string of ribosomes all going through

the process of translation on one single mRNA strand.

Free and bound Ribosomes

• Free ribosomes are found in the cytoplasm and they

synthesize proteins that are used within the cell.

• Ribosomes that are bound would be attached to the

endoplasmic reticulum and the proteins synthesized

here would travel in vesicles to the Golgi apparatus

where they would be modified.