15 - Identical Twins are Not Flashcards
1
Q
Identify various mechanisms for regulation of gene expression as summarized in Figure 14.6.
A
Transcription
- regulation of transcription intiation
- Repression of transcription
- Combinatorial gene regulation
- Coordinated regulation of transcription of genes with related functions
Posttranscription
- variations in pre-mRNA processing
- removal of masking proteins
- expression of small RNAs
- variations in rate of mRNAbreakdown
Translation
- variations in rate of initiation of protein synthesis
Posttranslation
- Variations in rate of protein processing
- removal of masking segments
- variations in rate of protein breakdown
2
Q
Identify characteristics of genetic mosaics.
A
- When the X chromosome is active in particular cells and inactive in others and vice versa.
- this happens in females because the two X chromosomes carry different allels of a gene, where one alllele will be active in cell lines in which one X chromosome is active, and the other allele will be active in cell lines in which the other X chromosome is active.
- examples of this include:
- orange and black patches of fur in calico cats
- or not as noticable, human females with patchy distributions of sweat glands.
3
Q
Identify role of histones in DNA packaging and expression.
A
- Histones - a class of small, positively charged (basic) proteins that are complexed with DNA in the chromosomes of eukaryotes.
- they link to DNA by an attraction.
- DNA packaging - they pack DNA molecules into the narrow confines of the cell nucleus
- when DNA is packed tightly into heterochromatin, it shuts off the genes in that block, just like how the X chromosome is shut off.
4
Q
reasons why, if identical twin women have sons with identical twin men, the sons will not be identical.
A
the sexual recombination will be different because person make different gametes
5
Q
definition or explanation of “heritability” with respect to human disease risk.
A
- if there is a 50% heritability of schizophrenia then that means that over 50% of the variablitity in schizophrenia seem to be intributable to genetic variation. so if you have the same genes as your twin with schizophrenia your risk is muxh higer
6
Q
characteristics that would, and would not, be different between monozygotic (“identical”) twins.
A
Differences
- Gametes
- Gene expression
- mutations/environmental mutagens
- x inactivation
Sameness
- same alleles
7
Q
process of random X inactivation leading to genetic mosaicism.
A
- occurss during embryonic development
- which of the two X chromosomes becomes inactive in a particular embryonic cell line is a random event
- Once one of the X chromosmes is inactivated in a cell, that same X chromosome is inactivated in all descendants of that cell
- Thus, within one female, one of the X chromosomes is active in particular cells and inactive in others and vice versa.
8
Q
role of Xist RNA in X inactivation.
A
- the reason that the X chromosome becomes inactive is because it expresses the Xist RNA (X inactive specific Transcript RNA)
- it coats the chromosome and keeps the DNA away from the transcription factors
- it directly associated with the DNA, it is not complementary base paring
- it is stable throughout the organism’s life
9
Q
role of Tsix RNA in regulation of Xist RNA expression.
A
- another gene, Tsix shuts off the exression of Xist when it is expressed. it is antigonistic.
- it is just RNA.
- this is to regulate the Xist RNA in inactivating the X chromosome
10
Q
meaning of antisense.
A
- example is that Tsix is antisence to Xist gene. It acts on it ans is transcribed the opposite way.
11
Q
structure of nucleosomes.
A
- DNA wound around core of 2 molecules each of H2A, H2B, H3, H
12
Q
function of nucleosomes in chromatic structure.
A
- this is the part where the DNA winds for almost two truns
- they compact DNA by a factor of about 7; a length of DNA becomes about one seventh of the original length
13
Q
role of nucleosomes in chromatin remodeling.
A
- An activator binds to a regulatory sequence upstream of the gene’s promoter and recruits a necleosome remodelling complex.
- the multiprotein complex uses the eneregy of ATP hydrolysis to slide the nucleosme along the DNA to expose the promote,
- or to restructure the nucleosome wothout moving it to allow transcription factors to bind.
14
Q
relationship between acetylation/deacetylation of histone tails on chromatin structure.
A
- an activator binds to a regulatory sequence upstream of the gene’s promoter and recruits an enzyme that acetylates (adds acytel groups: CH3 COO) lysine amino acids in the tails of histones in the nucleosome, where the promoter is located.
- Acetylation removes the positively charged amino group of the lysine, which makes the histone less attractive to the negatively charged DNA.
- As a result the histones loosen their asociation with DNA, an the promoter becomes accessible.
- this can be reversed by decetylation enzymes that remove the acetyl groups from the histones.
15
Q
relationship between methylation/demethylation of cytosine on chromatin structure or protein binding.
A
- DNA methylation - where enzymes add a methyl group (CH3 ) to cytosyne bases in the DNA.
- Methylated cytosines in promoter regions can regulate transcription through a process called silencing, in which transcription of genes controlled by those promoters is greatly reduced.
- Demethylation - the removal of a methyl group
- genes encoding the blood protein hemoglobin are methylated and inactive in most vertibrate boy cells. In the cell lines giving rise to red blood cells, enzymes remove the methyl groups from the promoters of the hemoglobin genes, which can then attract transcription factors and RNA polymerase.
