types of RNA and GENE DR MAHA

Basics of RNA structure and modeling

Dr. MAHA SMAISM

Translation - making proteins

Nuclear
membrane

Transcription

RNA Processing

Translation

DNA

Pre-mRNA

mRNA

Ribosome

Protein

Eukaryotic Cell

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??


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!

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






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

Ribosomes

P
Site
A
Site
Large
subunit


Small subunit

mRNA

A
U
G

C
U
A

C
U
U
C
G

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

Transfer RNA (tRNA)

amino acid

attachment site

U
A

anticodon

methionine
C

pre-RNA molecule

intron
intron
exon
exon
exon


exon

exon

exon
Mature RNA molecule

exon

exon

exon

intron
intron

splicesome

splicesome

Introns are pulled out and exons come together.

End product is a mature RNA molecule that leaves the nucleus to the cytoplasm.

Messenger RNA (mRNA)

methionine

glycine

serine

isoleucine

glycine

alanine

stop
codon
protein


A

U
G
G
G
C
U
C
C
A
U
C
G
G
C
G
C
A

U
A
A


mRNA
start
codon
Primary structure of a protein

aa1
aa2
aa3
aa4
aa5
aa6
peptide bonds

codon 2

codon 3
codon 4
codon 5
codon 6
codon 7
codon 1


codon 5

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

mRNA Codons and Associated Amino Acids

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


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

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)