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

the process by which information from a gene is used to synthesize a functional gene product, typically proteins.

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 and requires several components - DNA template for base pairing, 4 ribonucleoside triphosphates (GTP, ATP, CTP and UTP), RNA polymerase and salts and pH buffer to create specific environment for polymerase

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 which includes a transcription factor binding site, binding site for RNA polymerase and transcription start site between the 2. 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 (U replaces T) to DNA template to 3’ end of growing RNA -  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 of DNA that are transcribed into mRNA but are spliced out of pre-mRNA in nucleus - occurs only in eu

exons

remaining sequences that makes up mRNA that reaches ribosomes - only in eu

bacterial RNA polymerase

Consists of 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

An intermediate RNA molecule that contains both exons and introns, which is processed to form mature mRNA before being translated into protein.

RNA processing

only in Eu 2 modifications -

• 5’ cap added to 5’ end which is modified guanine nucleotide and facilitates binding of mRNA to ribosome

• poly A tail added to 3’ end - polyadenylation signal which directs an enzyme to cut pre-mRNA and another enzyme then adds long string of adenine nucleotides = poly A tail helps signal to transport molecules that mRNA is ready to leave nucleus

• = mods helps protect the ends of the transcript from degradation as moves out of nucleus and 3' tail also signals to transport molecules that mR

RNA splicing

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