ch19 - control of gene expression = transcription

what is gene expression

the process in which information in a gene is used to synthesise a protein via transcription and translation

define transcription and translation

transcription - when a section of a base sequence of DNA is copied into a complementary mRNA strand

translation - when the mature mRNA strand is decoded at a ribosome to synthesize a protein

what are the 4 processes that control gene expression

1. transcriptional

2. post transcriptional

3. translational

4. post translational

what occurs at each stage

1. transcriptional - controlling synthesis of mRNA by turning a gene on or off which determines if mRNA will be produced

2. post transcriptional - editing primary mRNA created into mature mRNA by removing introns (non coding regions of dna)

3. translational - rate at which mRNA is translated into a polypeptide regulated by various mechanisms

4. post translational - modifying the polypeptide to make a protein

what occurs during transcription

when the DNA base sequence of a gene is copied into mRNA

1. dna helicase breaks the hydrogen bonds between the complimentary bases causing double helix to unwind and the two strands to separate

2. only the antisense strand acts as a template as it goes 3’ 5’ - which will enable the mrna strand to be 5’ 3’

3. fre rna nucleotides found in nucleus will pair up with the exposed bases along the antisense strand by complimentary base pairing forming hydrogen bonds

4. rna polymerase will join all adjacent nucleotides together - so they form phosphodiester bonds - continuous chain of phosphodiester bonds creates the sugar phosphate backbone

what occurs during transcriptional control

when genes are turned on (expressed) or off (not expressed)

what is meant by a gene being turned on or off

turned on = gene is expressed (transcription has occurred)

turned off = gene is NOT expressed (transcription NOT occured)

why may transcription occur or not occur

depends on whether RNA polymerase can bind to the gene

> if RNA polymerase CANNOT bind to the dna base sequence of a gene then mRNA cannot be made

what 3 factors contribute to gene being expressed or not - eukaryotes

chromatin structure

epigenetics

transcription factors

what are histones

proteins associated with DNA - DNA wraps around histones to form chromatin

why is it that DNA is attracted to histones

DNA is negatively charged due to the -ve charged phosphate group (in nucleotides) forming the sugar phosphate backbone

Histones are positively charged

» DNA wraps around histones because their opposite charges attract

name the 2 types of chromatin

heterochromatin

euchromatin

what is heterochromatin

DNA tightly wound around histones

can transcription occur in heterochromatin + explain why

no transcription occurs - DNA is tightly wound so RNA polymerase cannot access and bind to the gene

what is euchromatin

DNA loosely wound around histones

can transcription occur in euchromatin + explain why

transcription occurs - DNA loosely wound so RNA polymerase can access and bind to the gene enabling transcription

what is epigenetics

histone modifications that change chromatin structure (into euchromatin or heterochromatin) altering gene expression - does NOT involve any changes to DNA base sequence

what are histone modifications

processes which alter how tightly or loosely DNA is wound around histones 

describe the 2 histone modifications for Heterochromatin to Euchromatin 

Acetylation = adding acetyl group to histones

Phosphorylation = adding phosphate group to histones 

» both of these processes decrease positive charge of histones = DNA becomes less attracted and loosely wound around histones = increased transcription

describe the histone modifications for Euchromatin to Heterochromatin

Methylation = adding methyl group to histones 

» increases hydrophobic nature of histones = DNA becomes more attracted to histones and tightly wound around histones = decreased transcription

what are transcriptional factors

proteins that bind to DNA to promote or prevent transcription of a gene into mRNA 

name the transcription factor which promotes transcription + their mode of action

activator transcription factors - help RNA polymerase to attach to a gene

name the transcription factor which prevents transcription + their mode of action

repressor transcription factors - blocks RNA polymerase from attaching to a gene