Transcription

Primary structure of RNA

sequence of nucleotides

Secondary structure of RNA

Folds into hairpin allowing hydrogen bonding with itself

All cells

mRNA, rRNA, tRNA

only in eukaryotes

pre-mRNA,snRNA,snoRNA,miRNA,siRNA,piRNA

only in prokaryotes

CRISPR RNA

Transcription

transfer of genetic information from DNA by the synthesis of a complementary RNA molecule copied by the DNA template

Requirements for transcription

DNA, RNA polymerase, NTPs, no primers

template strand

antisense

non-template strand

sense

Transcription unit

stretch of DNA that encodes an RNA molecule and the sequences necessary for its transcription

Components of a transcription unit

promoter, RNA-coding region, terminator

Promotor

binding site for RNA polymerase and the transcription initiation apparatus

RNA-coding region

a sequence of DNA nucleotides that is copied into a RNA molecule

Terminator

a sequence of nucleotides that signals where transcription is to end

Formation of phosphodiester bond

NTP-NMP+NTP = NTP- NMP + PP

Initiation

NTP+NTP = NTP-NMP + PP

elongation

NTP-(NMP) + NTP = NTP-(NMP) + PP

DNA

Unwinds at front of transcription bubble then rewinds

Pribnow box

similar to the tata box

sigma factor

recognized the promotor and directs RNA polymerase to it

Transcription initiation site

Where transcription starts(+1, the first base to be transcribed), sigma factor dissociates after initiation

Holoenzyme

opens bubble, binds to promotor tightly and unwinds DNA, also known as RNA polymerase II

Termination in bacteria

depends on hairpin loop a secondary structure at the terminator site

Hairpin loop

forms after inverted repeats in the gene are transcribed into the RNA molecule, destabilizing the DNA-RNA pairing

RHO dependent termination

RHO protein, a helicase break that DNA-RNA pairing

RHO independent termination

weak DNA-RNA pairing at the terminator site is enough to destabilize the interaction, 6 Adenine nucleotides

RNA Polymerase II

maintains transcription bubble during elongation

Termination in Eukaryotes

Rat1 exonuclease at cleavage site

Rat 1

a 5’ to 3’ RNA exonuclease that binds to the unprotected 5’ end of the trailing, non-coding RNA fragment and begins to degrade it until it catches up with the RNA polymerase, which is still transcribing DNA

Clustered regularly interspaced short palindromic repeats (CRISPR)

DNA arrays consisting of a number of palindromic sequences separated by spacer sequences.

Spacers

derived from invading DNA molecules such as bacteriophages or plasmids. form adaptive immunity.

3 steps of CRISPR

1. Acquisition

2. Expression

3. Interference

Acquisition

Foreign DNA enters the cell, and it is identified, processed, and inserted into the CRISPR array as a new spacer

Expression

The entire array is transcribed into a long CRISPR precursor RNA, then cleaved by a CAS protein into CRISPR RNAs, each one containing one spacer homologous to a foreign DNA

Interference

If the same foreign DNA enters the cell again, the effector complex recognizes it by its base-pair complementarity to the crRNA, and the CAS protein cleaves it with its endonuclease activity.

Effector complex

crRNA combined with a CAS protein

In which types of cells is telomerase not active?

Somatic human cells