subject guide notes

D2.2.1—Gene expression as the mechanism by which information in genes has effects on the phenotype

is the process by which genetic info is used to produce RNA & proteins

cells produced by fertilisation & during early embryonic development are unspecialised - hv fewer genes than differentiated cells

differentiation & maturation in cells = not all genes are expressed in the same level, in all cells

  • difference in gene expression allows them to exhibit diverse structures & functions required for specialised roles

stages in gene expression: transcription, translation, then the produced protein carries out functions either in or out of the cell

D2.2.2—Regulation of transcription by proteins that bind to specific base sequences in DNA

promoter is a non-coding region of DNA to which RNA polymerase binds to begin transcription

  • is non-coding cuz doesnt code for proteins

transcription factors can activate gene expression - encourage RNA polymerase to bind to promoter

  • others can repress gene expression - block RNA polymerase from binding to promoter

  • availability & activity of transcription factors within cell influences how easily RNA polymerase binds to promoter & transcribes gene

enhancers are non-coding regions of DNA, which regulate transcription of the gene - are important in regulating when & to what extent a gene is expressed

D2.2.3—Control of the degradation of mRNA as a means of regulating translation

longer mRNA strand persists (remains intact for translation) = higher the likelihood it’ll be translated multiple times, increasing the protein product

mRNA can persist for different time ranges, until it’s degraded by nucleases

lifespan of mRNA depends on different factors, such as:

  • chemical modification (addition of guanine cap to 5’ end or poly-A tail to 3’ end)

  • presence of stabilising proteins that can interfere with activity of nucleases, blocking active sites & preventing them from binding to & degrading mRNA

  • presence of nucleases - cells with high nuclease levels may experience faster mRNA degradation

  • cellular stress

D2.2.4—Epigenesis as the development of patterns of differentiation in the cells of a multicellular organism

presence of environmental factors can result in modifications to gene expression patterns, without changes to DNA sequence

  • so DNA base sequences arent altered by epigenetic changes

  • this means the phenotype of the organism is altered

D2.2.5—Differences between the genome, transcriptome and proteome of individual cells

no cell expresses all of its genes

pattern of gene expression in a cell, determines how it differentiates

all somatic cells in body have the same genome (complete set of DNA)

epigenetic changes influence gene expression in certain cells or tissues

  • results in cells hving distinct sets of RNA transcripts (transcriptomes) cuz not all genes are expressed at the same time

differences in transcriptomes leads to different cells hving unique proteomes, cuz proteins are synthesized based on the info encoded in RNA transcripts

D2.2.6—Methylation of the promoter and histones in nucleosomes as examples of epigenetic tags

methylation of cytosine in DNA of a promoter, prevents transcription factors from binding

  • so it represses transcription

  • results in decreased gene expression

heterochromatin is less accessible to RNA polymerase = reduced transcription

euchromatin is more accessible to RNA polymerase = increases transcription

acetylation decreases overall charge of histone protein, reducing the electrostatic attraction between histone & DNA

  • DNA becomes less tightly wrapped around the proteins

  • it is easier for RNA polymerase to access DNA, meaning increased gene expression

removal of acetyl group reverses process = reduced gene expression

methylation of amino acids in histone tails - depending on the location of the amino acid methylated & how many methyl groups are added, transcription can be repressed or activated

D2.2.7—Epigenetic inheritance through heritable changes to gene expression

in order for epigenetic inheritance to occur during sexual reproduction, epigenetic changes such as DNA methylation or histone modification must occur in germline cells & be maintained during meiosis (spermatogenesis & oogenesis) & passed on to offspring

D2.2.8—Examples of environmental effects on gene expression in cells and organisms

air pollution

exposure to air pollution can affect DNA methylation patterns of genes involved in inflammation & immune response pathways

  • can disrupt normal cellular processes

there can also be alteration to the methyl tags on DNA in response to air pollution

diet

different foods can affect gene expression

studies in mice show hat exposure to high folic acid diet can lead to increased methylation of specific genes related to coat colour, affecting offspring’s phenotype

  • during pregnancy, mice exposed to diet high in folic acid were more likely to give birth to offspring with higher levels of methylation & brown coats

temperature

changes in temp affect gene expression

in plants, temperature changes can activate or repress certain genes involved in developmental stages

sex of many reptiles, such as turtles, is determined by temperature at which their eggs are incubated

  • warm temps = more females

  • cool temps = more males

  • increase in global temperatures due to climate change disrupts this delicate balance & can skew sex ratios among turtle populations

  • imbalance can impact the reproductive success & long-term survival

D2.2.9—Consequences of removal of most but not all epigenetic tags from the ovum and sperm

epigenetic tags (such as methyl groups) are molecular markers or modifications which help regulate gene expression

after fertilisation, majority of epigenetic tags are removed from parental DNA

  • rests epigenetic environment of genome by eliminating genetic modifications that occurred during parent’s lifetimes

  • allows for development of new epigenetic modifications, so that cells can differentiate into different cell types

D2.2.10—Monozygotic twin studies

if twins brought up together & share same environment, they’ll share similar overall patterns of epigenetic tags during this time

D2.2.11—External factors impacting the pattern of gene expression

lac operon is a cluster of 3 genes found in bacterial DNA which codes for proteins involved in digestion of lactose

  • included the gene encoding lactase, the enzyme that hydrolyses lactose

  • when lactose is absent, the lac repressor binds to region called operator, preventing attachment of RNA polymerase to lac operon promoter & repressing transcription of gene

  • when lactose is present, it bind to repressor, detaching it from promoter & allowing RNA polymerase to bind & transcribe the lac operon

tryptophan operator is a cluster of 5 genes found in bacterial DNA that’s needed for synthesis of amino acid tryptophan

when tryptophan is absent, RNA polymerase is able to bind to tryptophan operon & transcribe the genes

when tryptophan is absent, it binds to repressor protein, causing repressor protein to undergo conformational change, allowing it to bind to operator region of tryptophan operon

  • this inhibits transcription of tryptophan operon

oestradiol - check kognity