D2.2 Gene expression

D2.2 GENE EXPRESSION

Gene expression= mechanism by which genetic info in a gene is expressed as a functional protein and involves transcription and translation

functional protein= will affect an organism’s phenotype

A phenotype= is the observable traits of the organism resulting from its genotype and environmental factors.

Genotype= combination of alleles inherited by an organism

2 alleles= in the same locus and are 2 copies/versions of a gene on homologous chromosomes

Transcription= regulated by promoter regions, enhancers and transcription factors

1. Promoter regions are located near the beginning og genes and serve as a binding site for RNA polymerase to transcribe the gene into mRNA

2. Enhancers= regularoty sequences of DNA which are located away from the gene

3. Transcription factors= regulatory proteins that bind to promoter regions and enhancers on DNA and different transcription factors can either ativate or repress transcription by interacting with each other, other proteins and RNA polymerase

Promoters, enhancers and transcription factors allow the cell to respons to the internal and external factors and control how genes are transcribed and which ones

Regulation of transcription= regulates the quantity of mRNA produced which regulates cells rate of translation

Degradation of mRNA by nuclease enzymes further regulates translation

Nucleases= break down mRNA to RNA nucleotides which can be recycled by the cell to produce new mRNA molecules

Epigenesis= process by which multicellular organisms develop from a zygote through a sequence of steps in which cells differentiate and organs form resulting from the interaction between genes and their environment during development

Epigenetics= the study of changes in organisms phenotypes caused by modification of gene expression which is how genetic info is expressed as a functional protein

Gene expression = dictates the phenotype as functional proteins expressed are what makes up the phenotype of an organism as in their physical traits and cellular phenotype

Epigenetics is done by epigenetic tags rather than changing the genetic code (genotype) itself so the phenotype is changed but the genotype is not

Epigenetic tags= chemical markers that attatch to DNA or histone proteins influencing the transcription and therefore translation of genes affecting an organism’s phenotype by activating or silencing specific genes which contrlos the functional proteins produced

1. Genome= includes all chromosomes and mithochondrial and chloroplast DNA found in a cell

No cell expresses all genes in its genome and the pattern of gene expression determines how cells differentiate and the organism’s phenotype

2. Transcriptome= refers to all the RNA molecules transcribed within a cell and represents the active genes and their expression within a cell or organism

3. Proteome= the complete set of proteins present in a cell at a given time and proteomes of different cll types in an organism will be different because gene expression varies depending on cell type. the proteome is larger than the genome due to 1. alternative splicing of mRNA exons producing multiple proteins and 2. modifications of proteins after translation increases the proteome

Methyl groups (CH3) can act as epigenetic tags by attatching to the promoter region or histones

1. methyl groups attatched to cysteine nucleotides in a promoter region= repress promoter activity and prevent RNA polymerase from transcribing the gene

2. can also attatch to amino acids of histone proteins found in nucleosomes and methylation of histone proteins can= 1. Repress transcription by DNA binding tighter to nucleosome or 2. activate transcription by unwinding DNA from nucleosome

Epigenetic tags= often remain in place during mitosis and meiosis meaning daughter cells inherit phenotypic changes and epigenetic tags of the parent without change to the actual DNA sequence and base pairs/nucleotides

Epigenetic inheritance is benefitial during growth of an organisms as daughter cells will have the same epigenetic tags as parent cell meaning the daughter cell will express the same genes and carry out same functions as the parent cell

however most epigenetic tags are erased at fertilization and not passed on to future generations

Epigenome= consists of all epigenetic tags on DNA and histone proteins

Interactions between DNA and environment can impact the epigenome, for example

1. Diet (of a mother during pregnancy)

2. Cigarette smoke (leads to lung cancer)

3. Air polution from diesel can add of remove methyl epigenetic tags from a person’s epigenmos which would change gene expression and the phenotype

Reprogramming/removal of epigenetic tags= happens at fertilization of egg and sperm and the epigenome of egg and sperm allows them to be specialized cells

however a small number of epigenetic tags are not erased at fertilization

e.g. Tigons and Ligers are sterile hybrids with lion and tiger parents: 1. Tigon is offspring from male tiger and female lion and 2. Liger is offspring from male lion and female tiger

the difference in their phenotype (physical appearance/size) is due to the difference in imprinted genes

Gene imprinting= process by which only 1 of 2 inherited genes (alleles) for a trait is expressed and the other allele is silenced by epigenetic tags during egg and sperm formation

Monozygotic twins= identical twins that are clones with the same genome and very similar epigenmoes resulting from fertilisation of 1 same egg

Their epigenomes will be very similar as they develop within the same environment in the same uterus but become increasingly different as they develop later in life due to different chemical epigenetic tags resulting from their environments resulting in different gene expression

Gene expression= can be impacted by factors outside a cell like

1. presence of biochemical molecules which the cell metabolizes (e.g. presence of lactose sugar effect in E-coli bacteria gene expression)

2. hormones forming hormone-receptor complexes within the cytoplasm of cells which then move into the nucleus binding to genes and regulating transcription of DNA

The Lac Operone

E-coli bacteria regulate production of enzymes involved in lactose metabolism by the lac operon which regulates transcription of DNA for the enzymes required for lactose metabolism based on presence or absence of lactose

lac operon consists of a promoter, an operator and 3 genes and when lactose is absent, the lac repressor will bind to the operator and preventing RNA polymerase from transcribing the gene blocking transcription

when lactose is present the lac repressor unbinds from the operator and transcription proceeds at a slow rate as the 3 genes can now be transcribed by RNA polymerase and the mRNA can be translated into proteins involved in lactose metabolism

Testosterone regulates gene expression

it’s a steroid hormone which diffuses directly through plasma membrane of cell where it binds to testosterone receptor forming a hormone-receptor complex

the hormone-receptor complex then moves into the nucleus through the pores and interacts with specific DNA sequences known as ANDROGEN RESPONSE ELEMENTS which are present in regulatory sections of target genes

the testosterone hormone-receptor complex may enhance or repress transcription of these target genes producing different proteins depending on this