Topic 3: Voice of the Genome Part 2 Flashcards
what is a locus
the position that a gene holds on a chromosome
define homologous chromosomes
they are 2 chromosomes that have genes occupying the same locus and are a similar size and shape
outline meiosis
cell starts with 46 chromosomes
1. DNA replicates to give 2 identical copies of each chromosome, which is 2 chromatids
2. DNA condenses and the chromosomes arrange into homologous pairs
3. 1st division - homologous pairs are separated and 2 cells have formed
4. 2nd division - sister chromatid pairs are separated and 4 genetically different, gamete daughter cells form; each with 23 chromosomes (haploid)
how does meiosis produce genetically non-identical cells?
- crossing over
it occurs before the first division
homologous chromosomes pair up
the non-sister chromosomes of these can cross over (forming chiasmata) and get entangled a
this can cause a chromosome section of one chromosome to break and rejoin with the chromatid of the other chromosome
they have the same genes, but allele combinations are now different so the gametes produced will all have different alleles
- independent assortment
allele combinations vary in daughter cells because homologous pairs line up randomly in the equator in meiosis
so the maternal and paternal chromosome combinations in each cell vary; increasing genetic variation in gametes
what are linked genes
genes are linked if they have loci on the same chromosome; as they’re on the same chromosome, they’ll stay tother during independent assortment and the alleles will be passed on the the offspring together
unless they are split up by crossing over
the closer together the loci of 2 genes on a chromosome, the less likely they are to be split up by crossing over and they are said to be more closely linked
what are sex linked genes
genes/a characteristic is sex linked if the locus of the allele that codes for it is on a sex chromosome
female - XX male - XY
Y is smaller and carries fewer genes so males often have only one allele for sex-linked genes and so will express the characteristic even if its recessive
males are more likely to show recessive phenotypes for sex-linked genes
X-liked disorders - colour blindness and haemophilia
what is a stem cell
an undifferentiated cell that can give rise to specialised cells and is capable of unlimited division
what is a totipotent cell
a cell that has the ability to differentiate into any and all cells of an organism, it is the least differentiated cell type eg. embryonic stem cell
what is a pluripotent cell
a cell with the ability to differentiate into most of the cells in an organism; some differentiation options may no longer be available because it has already started along the cell specialisation pathway
eg. bone marrow
how do stem cells become specialised
- stimuli activate and inactivate certain genes
- mRNA is only transcribed from active genes
- this mRNA is then translated into proteins
- these proteins modify the cell and determine structure and function
- the changes that these proteins cause are what makes the cell increasingly specialised
- the changes are difficult to reversible so specialisation/differentiation is irreversible
how can gene expression be controlled
by altering the rate of transcription in genes which is done by transcription factors; activators and repressors
what is an operon
a functioning section of DNA that contains a cluster of structural genes that get transcribed together, control elements, and sometimes a regulatory gene
structural genes - code for useful proteins eg. enzymes
control elements - includes a promoter and an operator
regulatory gene - codes for an activator or repressor
what is a promoter
the section of DNA where transcription starts
what is a terminator
the DNA section where transcription ends
what is a transcription factor
a molecule that regulates transcription rate
activator - increases transcription rate by helping RNA polymerase to bind to the DNA at the start of a gene so that transcription starts
repressor - decreases transcription rate by preventing RNA from binding to DNA at the start of a gene so that transcription occurs
what is the lac operon
it is found in E.coli bc E.coli usually respires with glucose but can use lactose
the lac operon carries the genes that produce the necessary enzymes to respire lactose
it has 3 structural genes; lacZ, lacY and lacA, these produce proteins that help bacteria to digest lactose, including beta-galactosidase and lactose permease
how does the lac operon work when lactose isn’t present vs is present
lactose not present:
the lac repressor is produced by the regulatory gene, the repressor binds to the operator site
- transcription of lactase is blocked because RNA polymerase now can’t bind to the promoter
lactose present:
lactose binds to the repressor to change the repressor’s shape, making it unable to bind to the operator site
RNA polymerase will now be able to begin the structural gene transcription and produce lactase
how are cells organised
cells have proteins on their surface called adhesion molecules, which help them locate and stick to ‘like minded’ cells
similar specialised cells –> tissue
tissues –> organ
organs –> organ system
organ systems –> organism
how can stem cells be used in treatment
they could be used to repair damaged nerve tissue in spinal cord injuries
they could be used to repair damaged heart tissue for heart disease/heart attack caused damage
what are adult stem cells and how are they obtained
they’re obtained from an adults body tissues, eg. bone marrow
relatively simple operation to obtain with very little risk but a lot of discomfort
donor is anaesthetised and a needle is inserted into the centre of a bone to remove a small quantity of bone marrow
they aren’t as flexible as embryonic stem cells, there’s a limited number of differentiation options
a patient can use their own adult stem cells if they need a stem cell transplant and there’s no rejection risk
how do you get embryonic stem cells
obtained from early embryos
embryos are created in lab, using IVF where egg cells are fertilised by sperm outside the womb
after the embryos are 4 to 5 days old, stem cells are removed and the embryo remaining is then destroyed
they can develop into all specialised cell types
what are some ethical ideas associated with embryonic stem cell use
- embryos can’t consent
many believe that at the moment of fertilisation, a genetically unique individual is formed that has the right to life, so they believe embryo destruction is wrong
- some have less objections to obtaining stem cells from an egg cell that hasn’t yet been fertilised by sperm, but has been activated artficially to start dividing; this is because these cells can’t survive more than a few days and wouldn’t form a fetus if placed in a womb
- some people think that scientists should only use adult stem cells because they don’t destroy embryos but they can’t specialise into as many cells
what do regulatory authorities do
look at research proposals to decide if they should be allowed by considering ethical issues
involve licensing and monitoring centres in embryonic stem cell research to ensure that only fully trained staff carry the research out; helps avoid unregulated research
produce guidelines and codes of practice to ensures that all scientists are working in a similar manner and results can be compared
make sure they use controlled extraction methods and have a regulated max embryo stem cell usage age
monitoring developments in scientific research and advances to ensure that any field changes are appropriately regulated with up to date guidelines
providing info and advice to govs. and professionals to promote the science in embryo research and help society realise it’s importance
genotype
the alleles that we inherit
phenotype
the physical display of our inherited genotype
recessive
a mutated disorder for which the genotype must be homozygous to display the phenotype
dominant
a characteristic that is coded for and overrides the other allele for that trait
co-dominance
when two dominant alleles are expressed in a phenotype
allele
a variation of a gene
what is polygenic inheritance
when one trait showing continuous variation is influenced by multiple genes found at various loci
what is an epigenetic change
the influence of the environment on the genome without changing the DNA base sequence
how is chromatin formed in eukaryotic cells
nuclear DNA wraps around proteins called histones to form chromatin
how is gene expression altered through epigenetics
- methylation of DNA (adding -CH3 groups)
- histone modification (via acetylation of amino acids)
both can occur in different areas of the same DNA
what can cause epigenetic changes
smoking/stress/exercise/diet/internal signalling from the body’s own cells
what is DNA methylation
an epigenetic tag that acts as a repressor and prevents gene expression
a methyl group is added to the DNA, making the DNA less accessible to transcription factors/preventing them from binding as the chromatin becomes more condensed
define tissue
the same specialised cell type grouped together to form a tissue
what is an organ
made up of many types of tissues that work together to carry out an overall function
define organ system
a group of organs with related functions, grouped together
define the term sex-linked disorder
a disorder caused by a mutated gene on the X or Y chromosome, making it more likely to occur in one gender than the other
can you reject your own stem cells
no, there’s no chance of rejection and immunosuppressant drugs don’t have to be taken
autosomal
any chromosomes that aren’t the sex chromosomes
what is histone acetylation
- acetylated histones have less condenses chromatin, so the proteins involved in transcription can bind to the DNA, allowing genes to be transcribed and so, activated
- removing acetyl groups from histones causes the chromatin to become highly condensed, the genes in the DNA can then not be transcribed as transcription proteins can’t bind to them, repressing genes
outline a transmission electron microscope (TEM)
electromagnets are used to focus a beam of electrons which is then transmitted through the specimen
more electrons are absorbed by denser areas, making them darker in the image
+ high resolution images, you can see internal structures of organelles like mitochondria
– can only be used on thin specimens
outline scanning electron microscopes (SEM)
SEM’s scan an electron beam across the specimen, knocking off electrons from the specimen which are gathered in a cathode ray tube to form an image
the images show the surface of the specimen and can be 3D
+ can be used on thick specimens
– lower resolution images then TEM’s
explain how epigenetic changes affect development
DNA is wrapped around histones and methylation affects gene transcription, altering gene expression
how could a hormone cause epigenetic changes
add methyl groups to DNA, modification of histones and therefore altering gene expression
can epigenetic changes be passed on
yes they can get passed on and the genes that were de/activated in the original cell would be the same in the daughter cells
if the epigenetic change occurred due to a change in environment the daughter cells will be equipped to deal with the changed environment in the same way
what are transcription factors
they bind to a DNA promoter sequence near the transcription start site and can be activators which increase transcription rate and help RNA polymerase to bind or repressors which do the opposite
how do stem cells specialise
THEY DIFFERENTIATE
stimuli activate or inactivate etc.
