The initial RNA

transcript is spliced into

mature mRNA

RNAtranscriptwith capand tail

Exons spliced together

Introns removed

TranscriptionAddition of cap and tail

Tail

DNA

mRNA

Cap

Exon Exon ExonIntron Intron

Coding sequenceNucleus

Cytoplasm

Eukaryotic cells modify RNA after transcription

• Enzymes in the nucleus modify pre-mRNA before the genetic messages are sent to the cytoplasm

• At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the ______.– This 5’ cap helps protect mRNA from hydrolytic enzymes

and functions as an “attach here” signal for ribosomes

5’ cap

• At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the __________.

• The poly(A) tail also seems to help with the export of mRNA from the nucleus. It also inhibits hydrolysis and helps ribosome attachment to the mRNA.

• The mRNA molecule also includes nontranslated leader and trailer segments.

poly(A) tail

RNA Splicing• Most eukaryotic genes and their RNA transcripts have

long noncoding stretches of nucleotides.– Noncoding segments, introns, lie between coding regions.

The final mRNA transcript includes coding regions, exons, that are translated into amino acid sequences, plus the leader and trailer sequences.

“intervening sequences”

“expressed sequences”

• RNA splicing removes introns and joins exons to create an mRNA molecule with a continuous coding sequence

• RNA splicing appears to have several functions.– First, at least some introns contain sequences that

control gene activity in some way.– Splicing itself may regulate the passage of mRNA from

the nucleus to the cytoplasm.– One clear benefit of split genes is to enable a one gene

to encode for more than one polypeptide.

• Alternative RNA splicing gives rise to two or more different polypeptides, depending on which segments are treated as exons.

From nucleic acid

to protein:

Translation

The mechanics of translation: In the cytoplasm, three players come together…

• The mRNA molecule which carries the information in its codons.

• Another type of RNA molecule, transfer RNA (______), matches up the right amino acid with the right mRNA codon.

• The ribosome – composed of two subunits – is the structure which helps the mRNA and tRNA match up properly. It also catalyzes the synthesis of the new peptide (covalent) bonds between the amino acids. The ribosome is made of ________ and ______.

tRNA

proteins rRNA

Anticodon

Amino acid attachment site

RNA polynucleotide chain

Hydrogen bond

tRNA

tRNA

How tRNA anti-codons match up with mRNA codons

Ser

Let’s look at the genetic code again… What tells translation to start and stop?

mRNA Codon Chart

Small ribosomalsubunit

Startcodon

P site

mRNA

A site

Large ribosomalsubunit

Initiator tRNA

Met Met

21

Translation initiation

Polypeptide

A site

1 Codon recognitionCodons

Aminoacid

Anticodon

P site

mRNA

Translation,

cont.

Polypeptide

A site

1 Codon recognitionCodons

Aminoacid

Anticodon

P site

mRNA

2 Peptide bondformation

Polypeptide

A site

1 Codon recognitionCodons

Aminoacid

Anticodon

P site

mRNA

2 Peptide bondformation

3 Translocation

Newpeptidebond

1

Polypeptide

A site

Codon recognitionCodons

Aminoacid

Anticodon

P site

mRNA

2 Peptide bondformation

3 Translocation

Newpeptidebond

Stopcodon

mRNAmovement

Each amino acidattaches to its propertRNA with the help ofa specific enzyme and ATP.

2

mRNA is transcribedfrom a DNA template.1

RNA polymerase

Amino acid

DNA Transcription

mRNA

tRNAATP

Translation

Enzyme

3

The mRNA, the firsttRNA, and the ribo-somal sub-units come together.

InitiatortRNA

Largeribosomalsubunit

Anticodon

Initiation ofpolypeptide synthesis

Smallribosomalsubunit

mRNA

Start Codon

New peptidebond formingGrowing

polypeptide

4

A succession of tRNAsadd their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time.

Elongation

Codons

mRNA

Polypeptide

5

The ribosome recognizes a stop codon. The poly-peptide is terminatedand released.

Termination

Stop codon

Protein Synthesis,

from Transcription to Translation

mRNA is transcribedfrom a DNA template.

RNA polymerase

Each amino acidattaches to its propertRNA with the help of aspecific enzyme and ATP.

Amino acid

DNA Transcription

mRNA

tRNAATP

Translation

Enzyme

The mRNA, the firsttRNA, and the ribosomalsub-units come together.

InitiatortRNA

Largeribosomalsubunit

Anticodon

Initiation ofpolypeptide synthesis

Smallribosomalsubunit

mRNA

Start Codon

1

2

3

New peptidebond formingGrowing

polypeptide

4

A succession of tRNAsadd their amino acidsto the polypeptide chainas the mRNA is movedthrough the ribosome,one codon at a time.

Elongation

Codons

mRNA

Polypeptide

5

The ribosome recognizes a stop codon. The polypeptide is terminated and released.

Termination

Stop codon

Let’s review translation…• http://highered.mcgraw-hill.com/sites/0072437316/student_view

0/chapter15/animations.html#

(Click on Protein Synthesis Link)• http://www.stolaf.edu/people/giannini/biological%20anamations.

html• http://www.biostudio.com/d_%20Transcription.htm• http://www.dnalc.org/resources/3d/15-translation-basic.html

(realistic animation, real-time)

• http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html

• http://www.biostudio.com/demo_freeman_protein_synthesis.htm