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