Dr Mohammed Hussein MBChB MSC DCH UK MRCPCH Transcription and RNA Processing Transcription What is Transcription The First Stage in the expression of genetic information ID: 912322
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Slide1
Molecular Biology
Lec.2
Dr. Mohammed Hussein
M.B.Ch.B, MSC, DCH (UK), MRCPCH
Slide2Slide3Transcription and RNA
Processing
Slide4Transcription
Slide5What is Transcription?The First Stage
in the expression of genetic information.Is the transcription of the information in the base sequence of a dsDNA molecule to form the base sequence of a ssRNA.
Slide6Slide7Types of RNA
Ribosomal RNA (rRNA), which is the most abundant type of RNA in the cell. It is used as a structural component of the ribosome. Transfer RNA (tRNA), which is the second most abundant type of RNA. Its function is to carry amino acids to the ribosome, where they will be linked together during protein synthesis.
Messenger RNA
(mRNA), which carries the information specifying the amino acid sequence of a protein to the ribosome.
Heterogeneous
nuclear RNA
(hnRNA or pre-mRNA), which is found only in the nucleus of eukaryotic cells. It represents precursors of
mRNA.
Small
nuclear RNA
(
snRNA
), which is also only found in the nucleus of eukaryotes. One of its major functions is to participate in splicing (removal of introns) mRNA.Ribozymes, which are RNA molecules with enzymatic activity. They are found in both prokaryotes and eukaryotes.
1. Ribosomal RNA (rRNA)
2. Transfer RNA (tRNA)
3. Messenger RNA (mRNA)
4. Heterogeneous nuclear RNA (hnRNA or pre-mRNA)
5. Small nuclear RNA (snRNA)
6. Ribozymes
Slide8The 3 main types of RNA
Slide9RNA PolymerasesAre enzymes that synthesis RNAIn eukaryotic cells, there are 3 types
RNA polymerase I synthesizes rRNARNA polymerase II synthesizes mRNA
RNA
polymerase III
synthesizes tRNA
Slide10Gene
Physically a gene consists of a sequence of DNA that encodes
a specific protein.
It is now estimated that the human genome
contains
20 000–25 000
genes
Slide11Slide12Slide13Slide14Exons & Introns
Exon = Expressed
Intron= Interval
Slide15Transcription
Slide16Promoter & Terminator
Slide17RNA polymerase locates genes in DNA (template strand) by searching for promoter regions
3՜
5՜
Slide18RNA polymerase moves along the template strand in the 3՜ to 5՜ direction as it synthesizes the RNA product in the 5՜ to 3՜ direction using NTPs (nucleoside triphosphates) as ATP,GTP,………etc.
3՜
5՜
Slide19The coding (antitemplate) strand is not used during transcription. It is identical in sequence to the RNA molecule, except that RNA contains Uracil
instead of the Thymine found in DNA.
Coding (antitemplate) strand
Template strand
Slide20Slide21Sample QuestionsDuring RNA synthesis, the
DNA template sequence TAGC would be transcribed to produce ………………………. sequences?ATCG
GCTA
CGTA
AUCG
GCUA
Slide22Our DNA template is 5՜ TAGC 3՜This will transcribed to 3՜AUCG
5՜ in mRNAAs all base sequences should be written in 5՜ to 3՜So the sequences should write as 5՜ GCUA 3՜
Or just
GCUA
Answer
Slide23Sample QuestionsDuring RNA synthesis, the
DNA template sequence TAGC would be transcribed to produce ………………………. sequences?ATCG
GCTA
CGTA
AUCG
GCUA
Slide24Sample QuestionsThe base sequence of a gene
is CTGCGC. What is the base sequence of the mRNA produced upon
transcription?
GCGCTG
CUGCGC
GCGCUG
CAGCGC
GUCGCG
Slide25Answer Our DNA coding strand is 5
՜ CTGCGC 3՜So the template strand must be 3՜ GACGCG 5՜
This will transcribed to
5՜
CUGCGC
3՜
in mRNA
NOTE:
The mRNA
sequence must be
identical to the code strand sequences
(except
U for T)
Slide26Production
of Messenger
RNA
Production of
pre-mRNA (
precursors of
mRNA)
Processing
of
pre-mRNA to mature mRNA
Slide27Production of pre-mRNA
Slide28Slide29Processing of
Pre-Messenger RNA
7-methylguanosine
cap is added to the 5
՜
end while the RNA molecule is still being synthesized. The cap structure serves as a ribosome-binding site and also helps to protect the mRNA chain from degradation.
Poly-A
tail is attached to the 3՜ end
. In this process, an endonuclease cuts the molecule on the 3՜ side of the sequence AAUAAA (poly-A addition signal), then poly-A polymerase adds the poly-A tail (about 200 As) to the new 3՜ end. The poly-A tail protects the message against rapid degradation and aids in its transport to the cytoplasm.
Introns
are removed
from hnRNA by splicing, accomplished by spliceosomes, which are complexes of
snRNA
and protein.
Slide30Slide31