3.8.1 Control of Gene Expression Flashcards
What is a mutation
A change to the sequence of DNA bases or to the amount of DNA in a chromosome
What are some causes of DNA mutation
Ionising radiation (alpha particles, x rays, uv rays) chemicals (nicotine and other mutagenic agents)
What is a substitution
When one DNA base (eg T) is replaced with another (eg C)
Explain how a substitution can be a “silent” mutation
If the mutation changes a codon to a different codon that codes for the same amino acid (due to the degnerate nature of the code) then the mutation will not affect the polypeptide produced
Why are insertion or deletion mutations usually quite damaging
They cause a frame shift that means all codons following are affected
How can epigenetic changes affect the functioning of the genome?
Methylation of DNA can prevent transcription. Acetylation of histones causes DNA to be loosely associated so allows more transcription
What is gene expression controlled by ?
Altering the rate of transcription of genes
What are transcription factors ?
Proteins that bind to DNA and activate or deactivate genes increasing or decreasing the rate of transcription. They often bind to promoter regions of the DNA which is a section of DNA just upstream of the gene where RNA polymerase binds
Describe how oestrogen can activate transcription
It is a steroid hormone and can diffuse across the phospholipid bilayer. It binds to an oestrogen recetpor inside the cell causing a conformational change that reveals a DNA binding site. The hormone-receptor complex moves to the nucleus and binds to the promoter region of a gene activating transcription
What can epigenetic control determine ?
Whether certain genes are expressed, altering the phenotype
How does increased methylation of DNA repress a gene?
A methyl group is attached to the DNA coding for a gene(cytosine base). Increased methylation changes the DNA structure so that chromatin packs more tightly making it harder for transcription factors and RNA polymerase to bind. Hence there is reduced transcription
How does modification of histones affect gene expression ?
When histones are acetylated, the chromatin is less condensed so transcription proteins can bind to the DNA allowing genes to be transcribed. When acetyl groups are removed, the chromatin is highly condensed so genes in the DNA cannot be transcribed
What are histones ?
Proteins that DNA wraps around to form chromatin which makes up chromosomes
what is heterochromatin
tightly packaged chromatin associated with silenced genes
what is euchromatin
loosely packaged chromating associated with actively transcribed genes
How are epigenetic changes to a cells gene expression passed on
When a cell divides and replicates the changes are passed on to the daughter cells
What are stem cells ?
unspecialised cells that can develop into other types of cells
What is differentiation ?
the process by which a cell becomes specialised
What is totipotency ?
the ability of a stem cell to produce all cell types, including all the specialised cells in an organism and extra embryonic cells
What is pluripotency ?
the ability of a stem cell to produce all the specialised cells in an organism but not extra embryonic cells because the genes for these cell types have become inactivated
Where might you find a multipotent stem cell
In the adult body. For example bone marrow cells are multipotent and can differentiate into many types of blood cell
Where are totipotent and unipotent stem cells present ?
In the first few cell divisions of an embryo
What is an IPSC?
Induced Pluripotent Stem Cell. An adult cell can be induced to go back to a pluripoten state (this is done by artificially activating a few genes)
How do stem cells become specialised ?
different genes in their DNA become activated or inactivated. This may occur through epigenetic mechanisms