1. Basics of RNA structure and modeling
Dr. MAHA SMAISM

2. Translation - making proteins
Nuclear
membrane
Transcription
RNA Processing
Translation
DNA
Pre-mRNA
mRNA
Ribosome
Protein
Eukaryotic Cell

3. RNA types & functions Types of RNAs Primary Function(s)
mRNA - messenger
Transfers genetic information from genes to ribosomes to synthesize protein
rRNA - ribosomal
Provides structural framework for ribosomes & catalytic role
t-RNA - transfer
Transfers a.a to mRNA for synthesis of protein.
hnRNA - heterogeneous nuclear
precursors for mRNAs & other RNAs
scRNA - small cytoplasmic
involved in selection of protein for export ,signal recognition particle (SRP)
snRNA - small nuclear
snoRNA - small nucleolar
mRNA processing, poly A addition <catalytic>
rRNA processing/maturation/methylation
regulatory RNAs RNA
regulation of transcription and translation, other??

4. Major Types of RNA Three types of RNA: A. messenger RNA (mRNA)
B. transfer RNA (tRNA)
C. ribosome RNA (rRNA)
Remember: all produced in the nucleus!

5. rRNA Ribosomes are the sites of protein synthesis
- they consist of ribosomal RNA (65%) and proteins (35%)
- they have two subunits, a large one and a small one

6. rRNA functions Structural
rRNA is the major structural component of ribosomes
BUT - its role is not just structural, also:
Catalytic
RNA in the ribosome has peptidyltransferase activity
Enzymatic activity responsible for peptide bond formation between amino acids in growing peptide chain

7. Ribosomes
Large
subunit P
Site A
Site
mRNA A U G C
Small subunit

8. Transfer RNA Transfer RNA
- Consists of a single RNA strand that is only about 80 nucleotides long
translates the genetic code from the mRNA and brings specific amino acids to the ribosome for protein synthesis
Each amino acid is recognized by one or more specific tRNA
Each carries a specific amino acid on one end and has an anticodon on the other end
tRNA has a tertiary structure that is L-shaped

9. Transfer RNA (tRNA) methionine amino acid attachment site anticodon U

10. Introns are pulled out and exons come together.
End product is a mature RNA molecule that leaves the nucleus to the cytoplasm.
pre-RNA molecule
intron
exon
exon
intron
splicesome
exon
Mature RNA molecule

12. Messenger RNA (mRNA) Primary structure of a protein A U G C aa1 aa2
start
codon
codon 2
codon 3
codon 4
codon 5
codon 6
codon 7
codon 1
methionine
glycine
serine
isoleucine
alanine
stop
codon
protein
Primary structure of a protein
aa1
aa2
aa3
aa4
aa5
aa6
peptide bonds

13. The Genetic Code
The genetic code is found in the sequence of nucleotides in mRNA that is transcripted from the DNA
A codon is a triplet of bases along the mRNA that codes for a specific amino acid
Each of the 20 amino acids needed to build a protein , has at least 2 codons
There are also codons that signal the “start” and “end” of a polypeptide chain

14. mRNA Codons and Associated Amino Acids

21. Translation and tRNA Activation
Once the DNA has been transcribed to mRNA, the codons must be tranlated to the amino acid sequence of the protein
The first step in translation is activation of the tRNA
Each tRNA has a triplet called an anticodon that complements a codon on mRNA
A synthetase uses ATP hydrolysis to attach an amino acid to a specific tRNA

22. Initiation and Translocation
Initiation of protein synthesis occurs when a mRNA attaches to a ribosome
On the mRNA, the start codon (AUG) binds to a tRNA with methionine
The second codon attaches to a tRNA with the next amino acid
A peptide bond forms between the adjacent amino acids at the first and second codons
The first tRNA detaches from the ribosome and the ribosome shifts to the adjacent codon on the mRNA (this process is called translocation)
A third codon can now attach where the second one was before translocation

23. Termination
After a polypeptide with all the amino acids for a protein is synthesized, the ribosome reaches the the “stop” codon: UGA, UAA, or UAG
There is no tRNA with an anticodon for the “stop” codons
Therefore, protein synthesis ends (termination)
The polypeptide is released from the ribosome and the protein can take on it’s 3-D structure
(some proteins begin folding while still being synthesized, while others do not fold up until after being released from the ribosome)