Lecture 11 - Transcription

What is the central dogma of molecular biology?

DNA → RNA → Protein = gene expression

Where does transcription and translation fall on the central dogma of biology?

DNA → TRANSCRIPTION → RNA → TRANSLATION → Protein = gene expression

Collinearity

DNA base sequence that determines protein amino acid sequence

What are two evolutionary implications of the order of central dogma?

1. Phenotype can NOT directly influence genotype within an individual

2. Acquired phenotypic characteristics CANNOT be inherited

when does transcription and translation occur in prokaryotes

Occurs simultaneously

Where does transcription take place in eukaryotic cells

Nucleus (including RNA processing)

Where does translation take place in eukaryotic cells

Cytoplasm

What is the product of transcribed DNA?

mRNA

What 5 things are required for transcription

1. DNA template (one strand)

2. 4 ribonucleotides (ATP, CTP, GTP, UTP)

3. RNA polymerase (5’ → 3’ chain elongation)

4. Recognition site or PROMOTER on the template instead of primers

What does uracil replace in RNA

Thymine

What produces thymine?

The methylation of uracil

What is the evolutionary thoery behind Uracil?

Believed to be the original base until thymine replaced uracil in the heritable code because it’s more stable and more efficient for DNA replication (RNA stability)

Promoter

A specific sequence tha t promotes polymerization. Marks where RNA polymerase starts

What are the 4 stages of transcription

1. Promoter recognition

2. Chain initiation

3. Chain elongation

4. Termination (different in eukaryotes and prokaryotes)

Promoter recognition

Promoters (specific DNA sequences) are recognized by RNA polymerase binding. These sequences are conserved

What is the significance of the binding position of polymerase?

Determines the start of mRNA

Transcription bubble

A portion of unwound DNA that is created during promoter recognition

Chain initiation

RNA polymerase unzips the double helix. It is bound then the first few phosphodiester bonds are made

RNA polymerase in prokaryotic chain initiation

Has a complex of proteins and o factor that are added for initiation. Sigma factor released after 8-9 bases are transcribed

RNA polymerase in eukaryotic chain initiation

MUST have TATA binding protein and several other transcription factors attached at the promoter region to initiate transcription

Chain elongation

RNA polymerase moves down DNA with transient transcription bubble

Chain termination in prokaryotes

Termination sequence is Rho independent (G + C rich to form hairpin structure followed by A + U rich region to self terminate)

Rho dependent

Termination sequence is 50-90 bases long (rich in C bases and LOW in G bases). Rho releases RNA transcript when polymerase encounters sequence

Chain termination in eukaryotes

1000-2000 bases downstream from the last nucleotide that will be part of the protein coding message. ACTUAL signals for termination are not fully known

mRNA cleaved 11-30 bases down from a conserved sequence AAUAAA

mRNA processing definition

A process in eukaryotes that is required before becoming a mature mRNA ready for translation

When and where does mRNA processing take place

Begins once RNA polymerase is 50-60 nucleotides into the sequence

Occurs in the nucleus

What are the steps of mRNA processing

1. Early in the transcription process, 7 methyl guanosine cap is added to 5’ end of pre-mRNA to protect it from 5’ degradation

2. Pol-A tail is added to 3’ end of mRNA to protect 3’ degradation. Happens after transcription termination

3. Spliceosomes cut out intron regions and join exon regions to form mature mRNA

Intron

A DNA region within a gene that is NOT translated into protein

Exon

Region of gene that IS translated

pre-mRNA

Non coding sections that are transcribed

Splicing

The remobal of introns in pre-mRNA

What is the nucleotide sequence for eukaryotic nuclear introns

exon(GU…intron…AG)exon

UTR

5’ and 3’ untranslated regions that are included in the mature mRNA with the 5’ 7-methyl guanosine cap and 3’ poly A tail