Transcript Central dogma: from genome to proteins

Central dogma: from genome
to proteins I: Transcription
Haixu Tang
School of Informatics
The flow of genetic information
Transcription: DNA  RNA
• Copy a particular portion of its DNA
nucleotide sequence a gene into an RNA
nucleotide sequence.
• The information in RNA, although copied
into another chemical form, is still written
in essentially the same language as it is in
DNA the language of a nucleotide
sequence.
Genes can be expressed with
different efficiencies
The structure of RNA
RNA (single strand) can fold into
specific structure
DNA transcription produces a single-stranded RNA
molecule, complementary to one strand of DNA
RNA polymerase uses DNA as
template
DNA polymerase vs. RNA
polymerase
• RNA polymerase is more error prone: 10-4
vs 10-7
• A modest proofreading mechanism
• RNA stores information only temporarily
Principal Types of RNAs
Produced in Cells
mRNAs
messenger RNAs, code for proteins
rRNAs
ribosomal RNAs, form the basic structure of the
ribosome and catalyze protein synthesis
tRNAs
transfer RNAs, central to protein synthesis as adaptors
between mRNA and amino acids
snRNAs
small nuclear RNAs, function in a variety of nuclear
processes, including the splicing of pre-mRNA
snoRNAs
small nucleolar RNAs, used to process and chemically
modify rRNAs
Other noncoding RNAs
function in diverse cellular processes, including
telomere synthesis, X-chromosome inactivation, and
the transport of proteins into the ER
The transcription cycle
Initiating signal
Directions of transcription
Types of RNA polymerases in
eukaryotic cells
TYPE OF POLYMERASE
GENES TRANSCRIBED
RNA polymerase I
5.8S, 18S, and 28S rRNA genes
RNA polymerase II
all protein-coding genes, plus snoRNA genes and some
snRNA genes
RNA polymerase III
tRNA genes, 5S rRNA genes, some snRNA genes and
genes for other small RNAs
Several important differences between the
bacterial and eucaryotic RNA polymerases.
• .While bacterial RNA polymerase (with s factor as one of
its subunits) is able to initiate transcription on a DNA
template in vitro without the help of additional proteins,
eucaryotic RNA polymerases cannot. They require the
help of a large set of proteins called general transcription
factors, which must assemble at the promoter with the
polymerase before the polymerase can begin
transcription.
• Eucaryotic transcription initiation must deal with the
packing of DNA into nucleosomes and higher order
forms of chromatin structure, features absent from
bacterial chromosomes.
Initiation of
transcription of
a eucaryotic
gene by RNA
polymerase II
Consensus sequences found in the vicinity of
eucaryotic RNA polymerase II start points
3D structure of TBP (TATAbinding protein) bound to DNA
Transcription initiation
Supercoiling of DNA
Transcription in Eukaryotic cells
RNA processing: pre-RNA 
mature RNA
• 5’ Cap
• Poly-A
• Splicing
• Editing
• Coupled with elongation
5’ Cap of RNA
Splicing
Splicing reactions and Lariats
Splicing signals
Alternative splicing
The RNA
splicing
mechanism
Variation in intron and exon
lengths
Splicing errors
Additional factors for correct
splicing site recognition
• RNA factory concept
• Exon definition hypothesis
The "RNA factory" concept
The exon definition hypothesis
Three major type of splicing
mechanism
Abnormal
splicing
cause
disease
Self splicing
PolyA addition
Exportation of mature RNA