differentiation and variation

what is a stem cell?

a cell that can divide by mitosis an unlimited number of times and has the potential to remain a stem cell or differentiate into a specialised cell

what is potency?

the ability of stem cells to differentiate into more specialised cell types

what are the two main types of potency?

totipotency and pluripotency

what is totipotency?

stem cells that can differentiate into any cell type found in an embryo as well as extra-embryonic cells (cells that make up the placenta and umbilical cord)

what is pluripotency?

stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into cells forming the placenta and umbilical cord

what is multipotency?

adult stem cells that can differentiate into a limited range of cell types

what are two sources of human stem cells for use in medicine and research?

embryonic and adult stem cells

how can embryonic stem cells be used in medicine and research?

huge potential in therapeutic treatment of many diseases e.g. replacing damaged nerve tissue to treat spinal cord injuries
embryos used for research are often the unwanted embryos from IVF

how can adult stem cells be used in medicine and research?

bone marrow can be used to treat leukaemia

what are three benefits of using stem cells?

potential to treat wide variety of diseases
organs developed from patient’s own stem cells reduces risk of organ rejection
adult stem cells are already used successfully

what are three risks of using stem cells?

stem cells cultured in the lab could become infected with a virus which could be transmitted to patient
risk of cultured stem cells accumulating mutations → develop into cancer cells
low number of stem cell donors + stem cell from donor can be rejected by patient’s immune system

what is differential gene expression?

a process where only certain genes in the DNA of the stem cell are activated and get expressed in order for stem cells to become specialised

what is the process of differential gene expression?

some genes in a stem cell are activated, whilst others are inactivated
mRNA is transcribed from active genes only
this mRNA is translated to form proteins
these proteins modify the cell, determining the structure and processes, and the cell becomes increasingly specialised
this process of specialisation is irreversible

what are transcription factors?

proteins that control the transcription of genes by binding to a specific region of DNA in eukaryotes, ensuring that genes are being expressed in the correct cells at the correct time and to the right level

what are the two types of transcription factors?

activators and repressors

what are activators?

they increase the rate of transcription by helping RNA polymerase bind to the DNA at the start of a gene and to begin transcription of that gene

what are repressors?

they decrease the rate of transcription by stopping RNA polymerase from binding to the DNA at the start of a gene, inhibiting transcription of that gene

where do some transcription factors bind to?

the promotor region of a gene

how is gene expression controlled in prokaryotes?

the binding of transcription factors to operons

what is an operon?

a section of DNA that includes structural genes transcribed together, control elements like a promotor region and an operator region, and regulatory genes that code for activators of repressors

what is the lac operon?

controls the production of the enzyme lactase in bacteria

what is the function of lactase?

breaks down lactose so that it can be used as an energy source in the bacterial cell
is an inducible enzyme (only synthesised when lactose is present), helping bacteria avoid wasting energy and materials

what is the structure of the lac operon?

promotor for structural genes, operator, structural genes lacZ (codes for lactase), lacY (codes for permease which allows lactose into cell), lacA (codes for transacetylase)

what is located to the left of the lac operon?

promotor for regulatory gene and regulatory gene lacI (codes for lac repressor protein)

what are two features of the lac repressor protein?

has two binding sites that allow it to bind to the operator in the lac operon and to lactose

what happens when the lac repressor protein binds to the operator?

prevents the transcription of structural genes as RNA polymerase cannot attach to promotor

what happens when the lac repressor protein binds to lactose?

the shape of the repressor protein distorts and it can no longer bind to operator

what happens when lactose is absent in the medium the bacterium is growing in?

the regulatory gene is transcribed and translated to produce lac repressor protein
lac repressor protein binds to operator region
RNA polymerase cannot bind to promotor region
no transcription of structural genes, so no lactase enzyme synthesised

what happens when lactose is present in the medium the bacterium is growing in?

uptake of lactose by bacterium
lactose binds to second binding site of lac repressor protein, distorting its shape so it cannot bind to operator region
RNA polymerase binds to promotor region and transcription occurs
mRNA from all three structural genes is translated
lactase is produced and lactose is broken down

what is epigenetics?

the control of gene expression by factors other than an individual’s DNA sequence

how is chromatin formed in eukaryotic cells?

nuclear DNA wraps around proteins called histones to form chromatin

what are two ways chromatin can be chemically modified?

methylation of DNA (chemical addition of CH3 groups) and histone modification via acetylation of amino acid tails
these are called epigenetic tags

what is the epigenome?

all the epigenetic tags in an organism

how can the epigenome undergo change?

due to environmental factors e.g. smoking, stress, exercise, diet
internal signalling from the body’s own cells can also cause modification

what does the chemical modification of DNA and histones control?

controls how tightly the DNA is wound around them as the intermolecular bonding between histones and DNA changes

what does DNA methylation cause?

the inactivation of genes

how does DNA methylation cause the inactivation of genes?

methyl groups added to cytosine bases suppresses the transcription of the affected gene by inhibiting the binding of transcription factors and enzymes needed for transcription

what does acetylation of histones affect?

gene expression

how does acetylation of histones affect gene expression?

acetyl groups added to lysine amino acids on histone proteins which removes the positive ion from lysine and removes a bond between the histone protein and DNA, causing the DNA to be less tightly wrapped
RNA polymerase and transcription factors can bind more easily to DNA and gene expression can occur
gene is activated

how can epigenetic changes be inherited?

through cell division

how can epigenetic changes be inherited through cell division?

methyl groups can be present in DNA on gamete cells
epigenetic changes in response to an environmental factor may be beneficial to also occur in daughter cells, so they are better adapted for the environmental factor

what is phenotype variation?

the difference in phenotypes between organisms of the same species

what can the phenotype of an organism be determined by?

genotype and environment

what is genetic variation?

small differences in DNA base sequence between individual organisms within a species population

what are the two types of characteristics determined by genes?

monogenic and polygenic

what are monogenetic characteristics?

characteristics controlled by a single gene, showing discontinuous variation (blood group)

what are polygenic characteristics?

characteristics controlled by several genes, showing continuous variation (height, mass, skin colour)

what are two examples of how the environment can affect phenotypic variation?

diet in animals and growing conditions for plants

what are two types of variation in the phenotypes of organisms?

continuous and discontinuous variation

how is discontinuous variation represented?

qualitative differences in the phenotypes of individuals within a population
these differences fall into discrete categories, and are easy to identify (blood group)

how is continuous variation represented?

quantitative differences in the phenotypes of individuals within a population
a range of values exist between two extremes which the phenotype will fall (height)

what are causes of continuous variation?

phenotypes that are affected by multiple different genes or by multiple alleles for the same gene at many different loci (polygenic inheritance) and the environment

what are polygenes?

a large number of genes that have a combined effect on the phenotype due to an additive effect