Transcription
DNA Transcription
• DNA must be copied to messenger RNA
(mRNA)
• mRNA goes from nucleus to the ribosomes in
cytoplasm
• mRNA complements known as codons
– Only 3 nucleotide “letters” long
• Remember RNA has uracil (U) instead of
thymine (T)!
Central Dogma of Molecular Biology
•
•
The flow of information in the cell
starts at DNA, which replicates to
form more DNA. Information is then
‘transcribed” into RNA, and then it is
“translated” into protein. The
proteins do most of the work in the
cell.
Information does not flow in the
other direction. This is a molecular
version of the incorrectness of
“inheritance of acquired
characteristics”. Changes in proteins
do not affect the DNA in a systematic
manner (although they can cause
random changes in DNA.
Reverse Transcription
• However, a few exceptions to the Central Dogma exist.
• Most importantly, some RNA viruses, called “retroviruses”
make a DNA copy of themselves using the enzyme reverse
transcriptase. The DNA copy incorporates into one of the
chromosomes and becomes a permanent feature of the
genome. The DNA copy inserted into the genome is called a
“provirus”. This represents a flow of information from RNA to
DNA.
• Closely related to retroviruses are “retrotransposons”,
sequences of DNA that make RNA copies of themselves,
which then get reverse-transcribed into DNA that inserts into
new locations in the genome. Unlike retroviruses,
retrotransposons always remain within the cell. They lack
genes to make the protein coat that surrounds viruses.
DNA Transcription
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Transcription -- When does it occur?
DNA Transcription
RNA differs from DNA
1. RNA has a sugar ribose
DNA has a sugar deoxyribose
2. RNA contains uracil (U)
DNA has thymine (T)
3. RNA molecule is single-stranded
DNA is double-stranded
DNA Transcription
RNA –
It doesn’t form the helix
It is shorter than DNA!!!!
DNA Transcription
Principal Points
• DNA RNA is transcription
• Catalyzed by the enzyme RNA polymerase
(does NOT require a primer)
• There are similarities and differences in
prokaryotic and eukaryotic transcription and
the mRNA product
REMEMBER: Prokaryotic cells have no nuclei, while eukaryotic cells do have true nuclei.
DNA Transcription
STEP 1 - INITIATION
RNA polymerase and cofactors bind to DNA and unwind it, creating an initiation
bubble. This is a space that grants RNA polymerase access to a single strand of the
DNA molecule.
Prokaryotes - transcription begins with the binding of RNA polymerase to the
promoter in DNA.
Eukaryotes - transcription initiation is more complex, a group of proteins called
transcription factors mediate the binding of RNA polymerase and the initiation of
transcription.
DNA Transcription
Step 2: Elongation
• One strand of DNA serves as the template for RNA synthesis, but multiple rounds of
transcription may occur so that many copies of a gene may be produced. RNA
nucleotides line up along one strand of DNA, following the base-pairing rules
• The template strand is read 3’ to 5’ - the other strand (5 to 3) is the nontemplate
strand.
• Just as in DNA synthesis, nucleotides are added to a 3’ hydroxyl (5’ to 3’)
Step 3: Termination
Termination is the final step of transcription. Termination results in the release of the
newly synthesized mRNA from the elongation complex. Single-stranded messenger RNA
peels away and DNA strands rejoin.
DNA Transcription
Transcription in Eukaryotes
• RNA pol I, located exclusively in the nucleolus, catalyzes the
synthesis of 3 of the RNAs found in ribosomes: the 28S, 18S,
and 5.8S
• RNA pol II, found only in the nucleoplasm, synthesizes mRNAs
and some snRNAs.
• RNA pol III, found only in the nucleoplasm, synthesizes the
tRNAs, 5S rRNA, and snRNAs not made by pol II
DNA Transcription
Gene regulatory regions are not necessarily transcribed!
Transcription is divided into 3 steps for both prokaryotes
and Eukaryotes—initiation, elongation, and termination.
Elongation is highly conserved.
Transcription produces genetic messages in
the form of mRNA
RNA
polymerase
RNA nucleotide
Direction of
transcription
Template
strand of DNA
Figure 10.9A
Newly made RNA
DNA Transcription
RNA polymerase
DNA of gene
Promoter
DNA
Initiation
Elongation
Terminator
DNA
Area shown
in Figure 10.9A
Termination
Growing
RNA
Completed RNA
RNA
polymerase
Figure 10.9B
DNA Transcription
1. Transcription
DNA
RNA Polymerase
pre-mRNA
DNA Transcription
Anatomy of mRNA
The mRNA molecule has 3 main parts in both prokaryotes and
eukaryotes.
Figure 5.8 shows the general structure of the mature, biologically active mRNA as
it exists in both prokaryotic and eukaryotic cells.
DNA Transcription
What happens to the mRNA?
• Carries instructions from DNA to the rest of
the ribosome.
• Tells the ribosome what kind of protein to
make
Types of RNA
Genetic information copied from DNA is
transferred to 3 types of RNA:
__________ RNA: mRNA
Copy of information in DNA that is brought to
the ribosome and translated into protein by
tRNA & rRNA.
__________ RNA: rRNA
Most of the RNA in cells is associated with
structures known as ribosomes, the protein
factories of the cells.
It is the site of translation where genetic
information brought by mRNA is translated
into actual proteins.
___________ RNA: tRNA
Brings the amino acid to the ribosome that
mRNA coded for.
From the Virtual Cell Biology Classroom on ScienceProfOnline.com
Images: Blueprint, clipart; Factory, Andreas Praefcke; Truck, PRA.
DNA Transcription
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!
DNA Transcription
Prokaryotes versus Eukaryotes – highlighted.
LOCATION
In prokaryotes (bacteria), transcription occurs in the cytoplasm. Translation of the
mRNA into proteins also occurs in the cytoplasm. In eukaryotes, transcription occurs
in the cell's nucleus, mRNA then moves to the cytoplasm for translation.
REQUIREMENTS
DNA in prokaryotes is much more accessible to RNA polymerase than DNA in
eukaryotes. Eukaryotic DNA is wrapped around proteins called histones to form
structures called nucleosomes. Eukaryotic DNA is packed to form chromatin. While
RNA polymerase interacts directly with prokaryotic DNA, other proteins mediate the
interaction between RNA polymerase and DNA in eukaryotes.
ADDITIONAL STEPS
mRNA produced as a result of transcription is not modified in prokaryotic cells.
Eukaryotic cells modify mRNA by RNA splicing, 5' end capping, and addition of a
polyA tail.
DNA Transcription
Question:
What would be the complementary RNA
strand for the following DNA sequence?
DNA 5’-GCGTATG-3’
DNA Transcription
Summary: