Download DNA Replication - Texas Tech University

Transcription
AHMP 5406
Objectives:
1.
Describe the general process of DNA transcription
2.
Discuss the function of RNA polymerases
3.
Differentiate between messenger RNA, small nuclear RNA,
ribosomal RNA, and transfer RNA
4.
Explain the purpose of the promoter and terminator
5.
Compare and contrast the three types of RNA polymerase in
eukaryotic nuclei: RNA polymerase I, RNA polymerase II, and RNA
polymerase III
6. Discuss RNA capping & RNA splicing
7. Explain the selective exportation of mRNAs from the nucleus
8. Discuss ribosomal RNA and its function
What is Transcription ?
DNA does not direct protein
synthesis directly
RNA is used as
intermediate
Transcription = Copying
DNA into RNA
RNA
Ribonucleic Acids

Contain ribose instead of
deoxyribose (DNA)
Polymers formed by A, G, C, and
U



U = uracil
Complementary to Thymine
But sometimes binds to G
Single stranded

Can bind to itself and form 3D
structures
DNA to RNA
Transcribed RNA is
complementary to 1 DNA
strand
Transcribe RNA =
transcript
Transcripts can only reach
a few thousand bases in
length
Unidirectional

5’-3’
RNA Types
Coding

Messenger RNA (mRNA)
Non-coding

Ribosomal RNA (rRNA)

Transfer RNA (tRNA)

Small nuclear RNA (snRNA)

Small nucleolar RNA (snoRNA)
mRNA
Codes for proteins
Can be modified by splicing, 5’ and 3’
additions
Non-Coding RNAs
rRNA

Form basic structure of ribosome
tRNA

Adaptors b/w mRNA and AA
snRNA


Several nuclear processes
Assist in splicing
snoRNA

Process and chemically modify rRNA
DNA sequences used as signals
Sequences signal
transcription to start and
stop
Promoters – start signals

s factor binds to promoter
in bacteria
Terminators – stop
signals

In bacteria A-T region
causes fold
RNA polymerases
RNA pols perform transcription
Immediate release of transcript allows
many RNA to be synth. in a short time
More error prone than DNA pols
They have limited error correction abilities
Types of RNA polymerases
RNA pol I



Transcribe 5.8S, 18S and 28S genes
S = Svedberg units, sedimentation coefficient related
to centrifugation
The bigger the number the bigger the molecule
RNA pol II


Transcribe protein-coding genes, snoRNA genes
Some snRNA genes
RNA pol III

Transcribe tRNA genes and some snRNA genes
Transcription in Euks.
Requires general transcription
factors to initiate transcription
GTFs help position RNA pol II
correctly

TFIIA, TFIIB, TFIID, TFIIE, TFIIF,
TFIIH
Recognize specific sites on DNA
RNA pol start points
TATA Box promoter




Recognized by subunit of TFIID
TBP – TATA Box Binding protein
approx 25 bp upstream of initiation point
Causes conformational change in DNA
Other necessary proteins for
initiation of transcription
Transcriptional activators

Attract RNA pol II to start
site
Mediators

Allow communication
between activator and
GTFs
Chromatin remodeling
proteins

Modify DNA conformation
Elongation Factors
Increase affinity of RNA Pol II to DNA
Assist in moving through chromatin
structure
mRNA modifications
5’ capping
Splicing = removing
introns
3’ Polyadenylation
5’ Capping
When RNA Pol II at 25 bases
5’ cap is added to transcript
Modified guanine
Three enzymes


Phosphatase = removes
phosphate from 5’ end
Guanyl transferase = adds GMP
Reversed linkage 5’-5’

Methyltransferase = adds methyl
group to guanosine
Splicing
Removes introns (or
sometimes shuffling)
Sequences signal where to
cut
Spliceosome performs
cutting

Complex of snRNAs (Us) and
protein
Self Splicing Mechanisms
Group I

Bind free G nucleotide to
specific site
Group II

Uses reactive A nucleotide
Unusual mechanisms
Poly A tail
Cleavage stimulating factor F
(CstF)
Cleavage and polyadenylation
specificity factor (CPSF)
Export of mRNA
Goes through nuclear pore complexes
Mature mRNA are modified and protein bound
Signal (passport) for transport of mRNA outside
of nucleus
5’ cap proceeds first
rRNAs
Transcribed by RNA Pol I
rRNAs are made from a
larger precursor

Sections are modified,
cleaved and assembled into
ribosomes in nucleolus
Subnuclear structures
Sites where snRNA
processing machinery is
assembled, stored and
recycled



Cajal bodies
GEMS (Gemini of coiledbodies)
Interchromatin granule
structures