*2.7: Transcription & Translation

*Review DNA replication & Transcription

Transcription

The synthesis of mRNA

Transcription is always carried out in a 5' - 3' direction

So the 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule that

is already synthesized.

Have a look at these…

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_2_.html

Theory follows these slides….

DNA consists of two polynucleotide strands, only one of which is transcribed into RNA

The antisense strand is transcribed into RNA. This is the TEMPLATE STRAND.

The sense strand is not transcribed into RNA. It has the SAME BASE SEQUENCE as the mRNA.

Again, you do not need to draw this from memory but you do need to be able to explain

what is going on!Also on page 202 (old book) page 117 new book.

In Summary

Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence and involves a number of steps:

1. RNA polymerase binds to the promoter and causes the unwinding and separation of the DNA strands

2. Nucleoside triphosphates (NTPs) bind to their complementary bases on the antisense strand (uracil pairs with adenine, cytosine pairs with guanine)

3. RNA polymerase covalently binds the NTPs together in a reaction that involves the release of two phosphates to gain the required energy

4. RNA polymerase synthesises an RNA strand in a 5' - 3' direction until it reaches the terminator

5. At the terminator, RNA polymerase and the newly formed RNA strand both detach from the antisense template, and the DNA rewinds

6. Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts

7. Post-transcriptional modification is necessary in eukaryotes. (Introns must be removed to form mature mRNA.)

Translation

*The synthesis of polypeptides on

ribosomes.

Remember that…

A gene is a sequence of DNA which is transcribed into RNA and contain three main parts:

Promoter:  Responsible for the initiation of transcription (in prokaryotes, a number of genes may be regulated by a single promoter - this is an operon)

Coding Sequence:  The sequence of DNA that is actually transcribed (may contain introns in eukaryotes)

Terminator:  Sequence that serves to terminate transcription (mechanism of termination differs between prokaryotes and eukaryotes)

Ribosomes Viewed with an electron microscope

Large subunit + small subunit

Subunits composed of ribosomal RNA (rRNA) as well as many proteins

Ribosomes formed in nucleolus of eukaryotic cells. Exit nucleus through pores.

Translation occurs between the subunits.

http://highered.mheducation.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html

What do the sites do?

Site A – holds tRNA carrying next amino acid to be added to the chain

Site P – holds tRNA carrying growing polypeptide chain

Site E – tRNA lost amino acid, ready to be discharged.

There are four stages of translation

First some important back ground info.

Codon AUG on the 5’ end of all mRNA. Start codon. Hydrogen bonds form in four places of the tRNA strand – forms the ‘clover leaf structure’ (fig. 7.10 pg 205).

One of the loops has an exposed anticodon, this is unique to the tRNA and pairs with the mRNA.

There are 20 amino acids which can bond to the tRNA. Bonding occurs due to enzymes. How many enzymes are there?

1. InitiationThe process starting

An activated amino acid (methionine) attached to a tRNA with the anitcodon UAC. Combines with a small ribosomal subunit and an mRNA strand.

Subunits moves down mRNA strand until start codon is reached (AUG)

Hydrogen bonds form between the initiator tRNA and the start codon.

A large ribosomal subunit combines as well, producing a translation initiation complex.

Initiation factors are proteins which join the complex together.

Energy in this case does not come from ATP. It comes from Guanosine triphosphate (GTP) which is another energy rich molecule – very similar to ATP.

2. Elongation

the chain growing

tRNA’s bring amino acids to the mRNA-ribosomal complex. Order specified by the codons.

Proteins called elongation factors assists binding tRNA’s to the mRNA codons at site A.

Initiator tRNA then moves to site P.

Ribosomes catalyse the formation of peptide bonds between amino acids.

3. TranslocationtRNA relocating within the

ribosome

Takes place during elongation

tRNA moving from site A P E.

Occurs in a 5’ to 3’ direction so ribosomal complex is moving along the mRNA strand towards the 3’ end.

4. Termination

The process ending

There are three stop codons.

When one reaches site A, a release factor (protein) fills site A.

The release factor does not carry amino acid. It causes the bond linking the the tRNA to the P site to hydrolyse. Releasing the polypeptide from the ribosome.

The ribosome then separates form the mRNA and splits into it’s subunits.

You must be able to draw and label a diagram showing the structure of a peptide bond between two amino acids.

Page 205 fig. 7.11