RNA and Transcription

RNA structures, cellular types of RNA, requirements for transcription, transcription units, RNA synthesis, consensus dequences, transcription in bacteria, transcription in eukaryotes, CRISPR RNA

The central dogma of molecular genetics

The flow of genetic information in cells

The primary structure of RNA

The primary structure of a nucleic acid is the sequence of nucleotides

RNA forms secondary structures

All cellular RNA types are generated by transcription: In all cells

Messenger RNA (mRNA)

Ribosomal RNA (rRNA)

Transfer RNA (tRNA)

All cellular RNA types are generated by transcription: Only in eukaryotes:

Pre-messenger RNA (pre-mRNA)

Small nuclear RNA (snRNA)

Small nucleolar RNA (snoRNA)

MicroRNA (miRNA)

Small interfering RNA (siRNA)

Piwi-interfering RNA (piRNA)

All cellular RNA types are generated by transcription: Only in prokaryotes:

CRISPR RNA (crRNA)

All cellular RNA types are generated by transcription - ETC.

Note: RNA replication occurs only with the genomes of some RNA virues

Transcription

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

Requirements:

• Template: DNA (a gene)

• Enzyme: RNA polymerase

• Free NTPs (nucleoside triphosphate)

• No primers

RNA is transcribed from the template DNA strand

Template strand (the transcribed strand): antisense.

Non-template strand: sense.

A transcription unit

A transcription unit is a stretch of DNA that encodes an RNA molecule and the sequences necessary for its transcription.

A transcription unit: Components

• promoter: the binding site for RNA polymerase and the transcription initiation apparatus

• RNA-coding region: a sequence of DNA nucleotides that is copied into an RNA molecule (the gene)

• terminator: a sequence of nucleotides that signals where transcription is to end (it is part of the gene)

Visual transcription unit

The gene is only the transcribed portions of the transcription unit. The final gene product must be a function RNA moleucle or protein.

Formation of the phosphodiester bonds during RNA synthesis

Initiation and elongation of RNA synthesis in transcription

Initiation:

• NTP + NTP → NTP-NMP + PPi (*the (-) showcases a bond)

Elongation:

• NTP-(NMP)n + NTP → NTP-(NMP)n+1 + PPi

Notice: the initial nucleotide of the chain (5’) remained as a nucleoside triphosphate (NTP)