d2.2 gene expression

gene expresion is

the mechanism by which information in genes has effects on the phenotype

role of promoters, enhancers and transcription factors in regulation of transcription

• promoter→sequence of dna found upstream from starting point and it indicates the starting point of transcription. serves as the binding site for transcription factors to bind to, which initiate the process of transcription

• enhancer region (upstream from the gene)→allows certain activator proteins to attach to it and encourage transcription. dna bends into a loop such that the promoter region is opposite to the group of transcription factors. when the transcription factors are connected to both enhancer and promoter regions, transcription starts

• transcription factors are→proteins that bind to promoter regions and help regulate the transcription of dna

Control of the degradation of mRNA as a means of regulating translation

• After an mRNA molecule is produced in the nucleus, it is available for ribosomes to use for different periods of time, anywhere from minutes to days

• Some mRNA molecules are more stable due to their protective cap at the 5’ end, and poly A tail at the 3’ end

• decapping complex removes the cap and deadenylase complex removes adenine nucleotides in poly a tail

• These mRNA molecules can be broken down by nucleases back into nucleotide bases

• Hence, by controlling when these mRNA molecules are broken down, the rate of translation can be controlled as mRNA molecules are needed in order for translation to occur.

epigenesis is

development of patterns of differentiation in the cells of a multicellular organism. DNA base sequences are not altered by epigenetic changes, so phenotype but not genotype is altered.

epigenetic tags are dna methylation/histone modification

genome

all the genetic information received from parent (except rbc and gametes)

note: no cell expresses all of the genes in its genome as they specialise and only express the genes they need to perform their special function

transcriptome

all the RNA that a cell makes

proteome

all the proteins that a cell, tissue or an organism can produce

methylation of promoter

• In a promoter, the cytosine can be methylated and a methyl group is added to the cytosine

• This addition of a methyl group silences the DNA code at that point.

• As a result, the RNA polymerase is unable to start the transcription process, preventing the production of proteins

methylation of amino acids in histones

• when a methyl group is added onto the histone, 

• the loops of dna coil more tightly around the histones so they are compressed and not accessible or loops of dna loosen to separate histones to make dna accessible

2 places methylation can be at

promoter/histones

Inheritance of epigenetic tags

• There is a possibility of phenotypic changes in a cell or organism being passed on to daughter cells or offspring without changes in the nucleotide sequence of DNA.

• This can happen if epigenetic tags, such as DNA methylation or histone modification, remain in place during mitosis or meiosis.

environmental effects on gene expression in cells and organisms

methyl tags on dna can be added by air pollution for a mother and the effects can be passed on to the children which could result in health issues like asthma and heart disease

What is the consequences of removing most but not all epigenetic tags from gametes, using tigons and ligers as example.

• Usually, primordial germ cells have epigenetic tags removed, then the DNA in developing sex cells is remethylated to produce a viable zygote

• Imprinted genes have been silenced by methylation in only one of two copies. (imprinted gene is silenced – remaining copy expressed)

• In ligers, imprinted genes for growth function differently in each species – genes from father that affect growth are switched on while in tigons, the genes from father that affect growth are not switched on

monozygotic twin studies

• environment affects methylation so twins can have diff characteristics if they grow up in diff environment though their genotype is the same

External factors impacting the pattern of gene expression

• Insulin and lactose

• insulin → when glucose is present in blood, insulin transcription starts and transcription factor bind to enhancer region to start transcription

• lactose → when lactose is present it binds to the lac repressor, inhibiting its action. rna polymerase can bind to promoter to synthesise lactase. when lactose is not present, lac repressor can bind to operator to prevent rna polymerase from transcription, so cell saves resources by not synthesising glucose all the time