Transcription Flashcards
Who proposed the central dogma of biology and when?
1956 Francis Crick proposed a fundamental concept in genetics
Central Dogma
flow of heredity information DNA provides the code that eventually leads to proteins- DNA does not directly code for proteins DNA ---> RNA ---> protein DNA- RNA= transcription RNA- protein= translation
Transcription
DNA is converted to RNA
What are the 3 stages of transcription?
- Initiation
- Elongation
- Termination
Describe the basic process of transcription?
DNA is converted to mRNA
Uses one strand of DNA (called the template strand) to make single-stranded RNA
The new RNA molecule is synthesised via RNA polymerase
Direction of transcription is 5’-to-3’
Antiparallel to the template strand
The complementary DNA strand to the template strand is the coding strand (older term no longer used: non-template strand)
What is the direction of transcription?
Direction of transcription is 5’-to-3’
Antiparallel to the template strand
RNA polymerase
Pairs template-strand nucleotides with RNA- specific complementary nucleotides
catalyses the formation of phosphodiester bond to join the RNA nucleotide (NTP)
adds the nucleotides to the 3’ end of the growing RNA strand
synthesizes the new RNA strand from the template DNA strand
Complementary and antiparallel nucleotide base-pairing to the template strand
Attaches to a promoter- that is upstream of the gene to be transcribed
Ends transcription at the termination sequence,- which is downstream of the gene to be transcribed (3’ to the end of the coding region)
coding strand
The complementary DNA strand to the template strand
(older term no longer used: non-template strand)
Same direction (5’-to-3’) and sequence as mRNA, only has T’s instead of U’s
Promoters
are regions on the DNA template strand that RNA polymerase recognises and initiates transcription (“this is the start of the gene this is where we need to begin”)
are not transcribed but rather direct the RNA polymerase to the correct starting place, the start of transcription
upstream of the gene to be transcribed
Where is DNA transcribed?
The DNA is transcribed between the start of transcription region and the termination sequence
nucleus
start of transcription
the DNA location where transcription of a sequence begins
termination sequences
Transcription ends
tells RNA polymerase to stop
is downstream of the gene to be transcribed (3’ to the end of the coding region)
Introns
are regions that are transcribed but do not code for specific proteins (ie non-coding) and so will get removed before the mRNA is translated
Exons
are the regions of mRNA that do code for proteins and will remain in the mRNA for translation
What are the 4 mains kinds of RNA?
Messenger RNA (mRNA): transcription, carries genetic code
Ribosomal RNA (rRNA): translation, part of ribosomes
Transfer RNA (tRNA): translation, carries amino acids
Small nuclear RNA (snRNA): mRNA processing and intron removal
Micro RNA (miRNA) and small interfering RNA (siRNA): regulatory RNA
What are some Features of RNA ?
Contains 4 ribonucleotides: (made of ribose sugar instead of deoxy)
Uracil (U) instead of Thymine (T)
Adenine, cytosine, guanine same as DNA
Hydroxyl (OH) group on 2’ carbon in ribose
Usually single stranded
- Can form short double-stranded hairpin structures
- Helps in tertiary protein folding
RNA polymerase core
(Bacteria)
Protein complex composed of 5 subunits
2 alpha subunits, 2 beta subunits, 1 omega subunit
Binds to a 6th polypeptide called sigma subunit
sigma subunit
sixth polypeptide that when rna polymerase core attaches to it switches polymerase to the active form- holoenzymee
Describe bacterial promoters
are double stranded regulatory DNA sequences that bind transcription proteins (RNA polymerase)
In bacteria, 2 promoter regions
although they are double stranded they are written in a single shot hand form that gives the 5’- 3’ sequence of the coding strand
What are the 2 promoter regionsin bacteria?
Pribnow box sequence (aka -10 consensus sequence) (minus means upstream from gene)
5’-TATAAT-3’
-35 consensus sequence
5’-TTGACA-3’
closed promoter complex
initial stage of transcription when the RNA polymerase core enzyme and sigma subunit (together form holoenzyme) bind to the promoter consensus sequences,
open promoter complex
forms when DNA unwinds near the transcription start site to
Outline bacterial Transcription Initiation
- The RNA polymerase core enzyme and sigma subunit bind to the promoter consensus sequences, forming a closed promoter complex
- DNA unwinds near the transcription start site to form the open promoter complex
- The holoenzyme (RNA polymerase + sigma subunit) progresses downstream to initiate RNA synthesis at the +1 nucleotide on the template strand
sigma subunit dissociates shortly after transcription initiation and the core enzyme continues transcription
Outline bacterial Transcription Elongation
The core enzyme of the RNA polymerase synthesises RNA until it reaches the termination sequence
As the RNA polymerase progresses, DNA unwinds to allow the template strand to direct RNA assembly
- Progresses at ~40bp/s in the 5’-to-3’ direction
The DNA immediately returns to a helix after RNA polymerase passes by