A IDENTITAS
Hari/Tanggal : Kamis/ 26 Juli 2012
Waktu :
09.00-12.00
B KEGIATAN
Metabolism of biomolecules/genetics material
transmission
C RINGKASAN KEGIATAN
Hari
ini kami belajar tentang aliran informasi genetik, yang memebahas masalah DNA
sampai terbentuk protein. Dosen pengajarnya seorang profesor, beliau
mengajarkan masalah basic dari peristiwa transkripsi yang rumit, yang
melibatkan banyak preotein dan enzim pada organisme prokariotik maupun
eukariotik. Kami sempat mengunjungi laboratorium rekayasa genetik beliau.
Beliau merekayasa Arabidopsys Thaliana
dan Tobacco. Ternyata Arabidopsys adalah tanaman sejenis
rumput yang memiliki sedikit sequen DNA yang telah dipelajari dan diketahui sequences DNAnya,
dimana hasil penelitian terhadap Arabidopsys dapat diaplikasikan pada tanamana
agricultural yang lain
An overview of the flow of information
through the cell
The
flow of information in an eucaryotic cell:
Step 1: Selected site on the DNA are
transcribed into pre-mRNA
Step 2: Pre-mRNA are processed into mRNA
(messenger RNA)
Step 3: The mRNA are transported out of
the nucleus
Step 4: mRNA are translated into
polypeptides by ribosomes that move along the mRNA
Genes can be expressed with different
efficiencies
Gene A is transcribed and translated
much more efficiently than gene B.
This allows the amount of protein A in
the cell to be much greater than that
of protein B.
RNA
is a polymer composed of alternating units of ribonucleotides
connected
through a phosphodiester bond.
1.
Pentose (five carbon) sugar:
RNA contains ribose, which have a
hydroxyl group on the 2’ carbon of the
ribose sugar
2.
Nitrogen bases: A,U,G,C
The
base uracil in place of thymine
3.
Phosphate group
From DNA to RNA
(transcription)
Transcription:
The synthesis of an RNA from a DNA template.
Because its nucleotide sequence is
complementary to that of the gene from which it is transcribed, the mRNA
retains the same information as the gene it self.
The use of messenger RNA allows the cell
to separate information storage from information utilization
While the gene remains stored away in
the nucleus as part of a huge, stationary DNA molecule, its information can be
imparted to a much smaller, mobile nucleic acid that pass into the cytoplasm.
Once in the cytoplasma, the mRNA can serve as a template for synthesis of a
large number of polypeptide chain.
Materials
involved in transcription:
• Substrate: NTPs (ATP, UTP, GTP, CTP)
• Template: DNA
• DNA-dependent RNA polymerase: Enzyme
that are able to incorporate nucleotides, one at a time, into a strand of RNA
according to a DNA template. 1 in prokaryotes, 3 in eukaryotes.
• Other protein factors
General
properties of DNA-dependent RNA polymerases
(i) Polarity: RNA polymerase reads the
DNA template in the 3’→5’ direction while synthesizing RNA in the 5’→3’
direction
(ii) DNA template: Either strand of a
DNA double helix can serve as a
template for RNA synthesis
RNA Polymerase direction is determined
by the orientation of the promoter
sequence.
The promoter region in higher
eukaryotes:
• The TATA box is located approximately
30 base pairs from the mRNA
start site.
• Two or more promoter-proximal elements
are found 100 and 200 bp
upstream of the mRNA start site. The CCAAT
box and the GC-rich box
are shown here.
• Other upstream elements include the
sequences GCCACACCC and
ATGCAAAT.
Transcription
Factors and RNA polymerases bind to the promoter region of a gene :
Three stages of RNA synthesis:
• Initiation:
RNA polymerase binds to the promoter
sequence in duplex DNA, then melts duplex DNA near the transcription start site
to form a transcription “bubble” before catalyzes the phosphodiester linkage of
two initial NTPs.
• Elongation:
Polymerase advance the 3’ to 5’ of the
template strand, melting the duplex DNA and adding NTPs to growing RNA.
• Termination:
Termination occurs when the RNA
polymerase encounter a specific termination sequence.
Transcription
in procaryotes:
- Initiation
- Elongation
- Termination
Differences
between Prokaryotic and Eukaryotic Gene Expression
• Eukaryotic genes contain introns;
prokaryotic genes do not.
• An eukaryotic mRNA code for 1 gene; A
prokaryotic mRNA code for several related genes at one time.
• Eukaryotic mRNA must be moved to
cytoplasm; prokaryotic mRNA is already in the cytoplasm and translation starts
before transcription is complete.
Transcription
initiation in eukaryotes requires many proteins
• RNA polymerase II is required for the
synthesis of mRNA.
• Unlike the E. coli RNA polymerase
holoenzyme, RNA polymerase II require a number of additional proteins called
“general transcription factors” in order to specifically bind to a promoter and
initiate transcription.
• Eukaryotic promoters are composed of a
variety of different cis sequence elements which recruit some of these
trans-acting factors through DNA-protein interactions.
• Protein-protein interactions also
occur and account for many of the multi-component complexes found at eukaryotic
promoters.
Eukaryotic
RNA polymerases consist of many subunits
Post-transcriptional modifications:
In eukaryotic cells, a genetic process
for the conversion of a primary transcript RNA to mature RNA
Processing
of the human globin mRNA
1st
Step: RNA capping
the 5’ end of the new RNA molecule is modified
by addition of a cap that consists of a modified guanine nucleotide
The
structure of the cap at the 5’ end of eucaryotic mRNA molecules
The methylguanosine cap at the 5’ end of
an mRNA serves several functions:
1. It prevents the 5’ end of the mRNA
from being digested by exonucleases.
2. It aids in transport of the mRNA out
of the nucleus.
3. It plays an important role in the
initiation of mRNA translation.
4. It signifies the 5’ end of eucaryotic
mRNAs, and this landmark helps the cell to distinguish mRNAs from the other
types of RNA molecules present in the cell.
2nd
Step : RNA splicing
The intron sequences are removed from
the newly synthesized RNA
Structure of two human genes showing the
arrangement of exons and introns
The pre-RNA splicing reaction
1. a specific adenine nucleotide in the
intron sequence (indicated in red) attacks the 5’ splice site and cuts the
sugar phosphate backbone of the RNA at this point.
2. The cut 5’ end of the intron becomes
covalently linked to the adenine nucleotide, thereby creating a loop in the RNA
molecule.
3. The released free 3’-OH end of the
exon sequence then reacts with the start of the next exon sequence, joining the
two exons together and releasing the intron sequence in the shape of a lariat.
4. The two exon sequences become joined
into a continuous coding sequence; the released intron sequence is eventually
degraded.
The RNA splicing reaction is performed
on the C-terminal domain of RNA polymerase II (CTD)
- key steps in RNA splicing are
performed by RNA molecules rather than proteins
- These RNA molecules are relatively
short (less than 200 bp each), and there are five of them (U1, U2, U4, U5, and
U6) involved in the major form of pre-mRNA splicing, know as SnRNAs (small
nuclear RNAs).
- Each SnRNA is complexed with at least
seven protein subunits to form a snRNP (small nuclear ribonucleo protein).
These snRNPs form the core of the spliceosome, the large assembly of RNA and
protein molecules that performs pre-mRNA splicing in the cell.
3rd
Step : RNA 3’ polyadenylation
Consensus nucleotide sequences that
direct cleavage and polyadenylation to form 3’ end of a eucaryotic mRNA
Poly(A) tail invariably begins
approximately 20 nucleotides downstream from the sequence AAUAAA, which serve
as a recognition site for the assembly of a complex of proteins that carry out
the processing reactions at the 3’ end of the mRNA.
