13.1: Transcription: DNA to RNA

central dogma

used to describe the sequential flow of genetic information from DNA to RNA to protein = gene expression

gene expression

information encoded in genes used to synthesize a functional product

gene expression processes

1. transcription

2. translation

types of RNA

messenger, ribosomal and transfer

mRNA

carries the genetic information from DNA to the ribosome, where proteins are synthesized.

rRNA

structural component of ribosomes, essential for protein synthesis.

tRNA

brings amino acids to the ribosome, facilitating the assembly of proteins according to the mRNA sequence.

coding strand

the DNA strand that carries the same sequence as the mRNA

template strand

serves as the template for mRNA synthesis during transcription.

transcription

formation of a specific RNA sequence from a specific DNA sequence

where does transcription occur

pro = cytoplasm eu = nucleus

3 steps of transcription

1. Initiation, 2. Elongation, 3. Termination.

initiation

transcription preinitiation complex assembles around promoter of gene. Proteins help RNA polymerase find and bind to promoter and unwinds DNA

• proteins in pro = sigma factors eu = transcription factors

elongation

RNA polymerase moves along DNA template strand from 3’ to 5’ direction and produces RNA transcript by adding nucleotides complementary to DNA template -  elongating RNA molecule in 5' to 3' direction

termination

when RNA polymerase reaches termination site the RNA transcript and polymerase are released from template

promoters in pro and eu

pro = several genes share a promoter eu = one promoter per gene

• tells RNA polymerase where to start transcription

intron

noncoding regions that are spliced out of RNA in nucleus - only occurs in eu

exons

remaining sequences once introns are removed that makes up mRNA that reaches ribosomes - only in eu

bacterial RNA polymerase

5 polypeptide subunits - 2 identical alpha subunits, beta and beta-prime subunits and omega subunit - sigma factors associate with core enzyme which then initiates transcription. - only 1 type

Eu RNA polymerase

3 different RNA polymerases - 1, 2 and 3 - structurally similar to pro RNA polymerase but transcribes different classes of RNA

• 1 transcribes most rRNA

• 3 transcribes tRNA some sRNA

• 2 transcribes mRNA

pre-mRNA

intermediate RNA containing introns and exons - processed to form mRNA before translation

RNA processing

only in Eu 2 modifications to protect mRNA when leaving nucleus

• 5’ cap for binding to ribosome

• 3’ poly A tail - polyadenylation signal causes enzyme to cut pre-mRNA and another enzyme then adds long string of adenine = signals to transport molecules that mRNA is ready to leave nucleus

RNA splicing

in Eu - snRNPs and proteins which together recognise and removes introns and splices exons together