gene expression in eukaryotes
DESCRIPTION
genitics and molecular biologyTRANSCRIPT
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Gene Expression in Eukaryotes
M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar
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Outline Central dogma in Eukaryotes
Nature of Genes in Eukaryotes
Initiation and Elongation of Transcription
RNA Processing
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Eukaryotic Transcription
Transcription occurs in the nucleus in eukaryotes, nucleoid in bacteria
Translation occurs on ribosomes in the cytoplasmmRNA is transported out of nucleus through the
nuclear pores
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In Eukaryotes (cells where the DNA is
sequestered in a separate nucleus) the exons
must be spliced (many eukaryotes genes contain
no introns! Particularly true in ´lower´ organisms).
mRNA (messenger RNA) contains the assembled
copy of the gene. The mRNA acts as a
messenger to carry the information stored in the
DNA in the nucleus to the cytoplasm where the
ribosomes can make it into protein.
Eukaryotic Central Dogma
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~6 to 12% of human DNA encodes proteins (higher fraction in nematode)
~90% of human DNA is non-coding
~10% of human DNA codes for UTR
Eukaryotic Genome - Facts
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Untranslated regions (UTRs)
• Introns (can be genes within introns of another
gene!)
• Intergenic regions:
- Repetitive elements
- Pseudogenes: Dead genes that may
(or may not) have been retroposed back in
the genome as a single-exon “gene”
Non-Coding Eukaryotic DNA
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Coding and Non-coding Sequences
In bacteria, the RNA made is translated to a proteinIn eukaryotic cells, the primary transcript is made of
coding sequences called exons and non-coding sequences called introns
It is the exons that make up the mRNA that gets translated to a protein
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Eukaryotic Gene
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Eukaryotic Nuclear Genes
Genes transcribed by RNA Pol II
• Upstream Enhancer elements.
• Upstream Promoter elements.
• GC box (-90 nt) (20 bp), CAAT box (-75 nt) (22 bp)
• TATA promoter (-30 nt - 70%, 15 nt consensus
(Bucher et al., 1990)
• Transcription initiation.
• Transcript region, interrupted by introns. Translation
Initiation (Kozak signal 12 bp consensus: 6 bp prior
to initiation codon)
• polyA signal (AATAAA, 99%)
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• Transcript region is interrupted by introns
Each intron (on DNA):
starts with a donor site consensus (G100T100A62A68G84T63..) – GU on RNA
has a branch site near 3’ end of intron (one not very conserved consensus UACUAAC)
ends with an acceptor site consensus. (12Py..NC65A100G100)
Introns
……AG GUAAGU ………A ……(Py) …..NCAG GU
Donor 5’ splice site Acceptor 3’ splice site
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The exons of the transcript region are
composed of:
5’ UTR with a mean length of 769 bp
AUG (or other start codon)
Remainder of coding region
Stop Codon
3’ UTR with a mean length of 457 bp
Exons
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Eukaryotic Promoter
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Polymerases also use transcription factorsBind in a specified order, either to promoter or each other
RNA polymerase II must be phosphorylated before it can start synthesizing RNA
Sequences of Eukaryotic promoter
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RNA polymerase I- makes precursors for ribosomal RNAs (except for smallest subunit)
RNA polymerase II- mRNA and snRNAs (involved in RNA processing)
RNA polymerase III- variety of RNAs: smallest rRNA subunit, tRNA precursors
Each uses a different promoter (DNA sequencesthat direct polymerase to begin tran-scribing there)
Promoters are “upstream” from coding sequence
Eukaryotic RNA polymerases
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Initiation in EukaryotesTRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
Eukaryotic promoters
T A T A AA A
A T A T T T T
TATA box Start point TemplateDNA strand
53
35
Several transcriptionfactors
Additional transcriptionfactors
Transcriptionfactors
53
35
1
2
3
Promoter
53
355
RNA polymerase IITranscription factors
RNA transcript
Transcription initiation complex
Several transcription factors must bind to promoter sequences upstream of the gene
Then RNA polymerase can bind
TATA box
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Requirements for initiation of Transcription
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Transcription Factors – Order of their binding
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General transcription factors
Eukaryotic Transcriptioninitiation
TATA binding protein (TBP)/TFIID binds toTATA box (-25)
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Role of Enhancers in Initiation
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Overall Transcription Process