Lecture 14 Flashcards
Technologies facilitate genetic discovery and therapy
Can sequence genome easily
-ethical discussion
-cost has reduced so much that it could be used more clinically and easily
Cost per Raw Megabase of DNA : sudden drop in cost, as new technology has been introduced
-Technologies facilitating genetic discoveries
What are the 4x major categories of Genetically determined Diseases?
- Single gene disorders (variation/difference/mutation which leads to gene/phenotye)
- Chromosomal disorders (contains over 100 genes. duplication more/loss of chromosome can lead to major change in cell and development. can lead to death before birth)- only small proportion make it alive
- Multifactorial genetic disorders: Complex disroders, Multiple genes, interact with environment, pathogens etc
- Somatic disorders (somatic cells arent in germline, not testes or ovaries, not involved with passing genetics to offspring) - include genetic changes in cancer
Single Gene-Disorder
Beta-Thalassaemia
many organ systems effected
-mutation in single gene can have very broad reaching effects. different effects in different organs. can effect single organ (blood) leading to secondary effects in body
single gene =/= single tight phenotype
-can lead to both primary and secondary effects
Chromosomal Disroders
100s of genes effected
Trisomy 21 (Downsyndrome)(3x copies)
large numbers of genes effective
more or less copies
-can often lead to embryionic nafality (not compatable with life)
-small number chromosomal disorders do allow human to survive
-also Trisomy 18, 13
Error in early cell division
Increased likelihood of down syndrome offspring with maternal age (rapid after mid 30s) (genetic disease that manifests itself late in maternal life)
Cause I: Non-disjunction, Metaphase I
during first stage of meiosis in ovaries there is a halt in metaphase 1, later reactivated. Longer gap, more likely for non-dysjunction (moving apart of chromosomes not occur evenly. one daughter cell gets 2x copies, other gets none)
Cause II: Translocations
-
1. Genetic disorder is discovered decades before understanding of how it arises
2. Genetic chromosomal disorders are often incompatable with life- only very small number are compatible (chromosome 13, 18, 21)
3. Mechanisms can range from very simple (Non-dysjunction in Meiosis, 2x copies to one daughter cell and none to the other) or much more complex (Robertsonian Translocation)
Translocations results in Down syndrome
Chromosome 21 is very assymetrical (centromere in centre) - one arm contains nearly all genetic information, other arm contains hardly any
“Translocation”= cutting and rejoining
1. the short arm of acrocentric chromosome
Robertsonian Translocation:
2. is exchanged with long arm of another
3. creating a large metacentric chromosome (2x large arms= is essntially 2x copies of chromsomes)
4. and a fragment that often fails to segregate and is lost
Multifactorial Genetic Disorders
Result from interaction of multiple genes(rather than just chromosme)
-each of the genes alone may have minor effect, but together they synergise (often also interaction with environment and infection); some may have a major effect but many have relatively minor effect
Most common and Least understood of genetic diseases
-Diabetes Mellitus
-Hypertension
common chronic diseases
-Coronary Artery Disease
-Schizophrenia
-Cleft lip, cleft palate, most congenital heart disease
All results from complex synergy of multiple genes + environment, nutrition, infection influences the likelihood of getting disease
Somatic cell Genetic Disroder
-very different to germline disorders, where change in genome of germ cell can be passed on to offspring
Arise only in specific somatic cells (in contrast to other genetic disorders where the abnormality is found in the DNA of all cells of the body, including germ cells)
Are not transmitted to subsequent generations
Paradigm is CANCER; (not all tumours nescessarily involve somatic disorder) development of malignancy often a consequence of mutations in genes that control growth
Individual genes: APC. Grey lines= random fragments
Mutation in half of the bases = mutation of one copy
Whole genes: Chromosome 11 (1300 genes) - number of different alleles. chromosome 11 completely lost. remains of chromosome left over by normal cells of tumour. Germ cell can still survive and may make cancer tumour survive better
Mutation
Any permenant heritable change in the sequence of genome DNA
Any alteration of DNA from its natural state: may be disease-causing or a benign normal variant
Polymorphism
The occurance together in a population of two or more alternative genotypes, each at a frequency greater than that which could be maintained by recurrent mutation alone
-recurrent mutation alone is not maintaining multiple copies in the population. there has to be some other evolutionary reason for them to be maintained
Natural variations in genomic DNA sequence, that usually have no obvious adverse effects on the individual and occur with fairly high frequency in the general population
-Neutral, no particular advantage, just making us differnet. But may make some people better suited to certain environment (potential advantage)
Single nucleotide polymorphisms (SNPs) in human (‘1 per 100-300bp) being catealogued as extension of Human Genome Project (differences in population. almost always netural. doesnt give predisposition to disease. Good markers as linked to change in gene that may be associated with disease)
SNPS can be flags, if close enough to changed gene which leads to disease, can be used to identify/trace this disease causing gene sequencethrough population/family
5x Types of Mutation
3x bases = code each aa.
- Silent : single base change that does not result in aa amino acid change (triplet code degeneracy) - wobbly 3rd base, can be multiple bases in third position that can still code for the same protein/aa
- Missense: Single bas change that does result in an aa amino acid change. May or may not cause abnormal phenotype (usually doesnt really change structural function of protein)- can result in change of phenotype - cnacer predisposition syndrome, inhereited neuroogical disease
- Nonsense: single base change that changes an aa amino acid to a –> (premature) STOP codon (triplet of bases which codes for transcription to stop) (for transcription - is introduced preamturely/transcription therefore halted prematurely)
- Frameshift: Insertion of deletion of bases in anything other than multiple of 3 (changes frame. changes protein encoded. often introduce stop codon by chace downstream)
- Splice donor/Acceptor: Alteration of sequences for accurate splicing of introns ( RNA transcribed before translation, early RNA forms spliced. Sites introns removed and 2x extrons spliced together, having particular signals for splicing based on sequence of DNA. interrupted by mutation, cause skipping of exon or splice in another place = different protein made)
Sickle Cell Anemia
Missense Mutation
Wild type DNA:CTC = RNA:GAG = instructs for Glutamic acid (aa in the protein)
Missense Mutation DNA:CAC= RNA=GUG = Valine aa in protein
A substituted for a T
= Sickle shaped erythrocyte
Example of Frame shift Mutation
THE FAT CAT ATE HIS HAT
Insertion of A: THE FAA TCA TAT EHI SHA T
Deletion of A: THE FTC ATA TEH ISH AT
-still some sensible words. but not correct. alot are nonsense
“indel”= insertion or deletion, which can be more than a single base/nucleotides. Relavent to somatic mutations as well
Classes of mutations
Pathogenic: Lead to another phenotype/disease
Non-Pathogenic: Neutral. Not lead to anything
“indel”
insertion of deletion, which can be more than a single base/nucleotides
-relavent to somatic and frame shift mutations
Deciding if a DNA change is Pathogenic
If a missense alteration (single bas change), is it at a functionally important site?
- structure-function studies
- in vitro mutagenesis (putting altered gene/proteins function in other cells) (Krisper Cass echnology revolutionalised)
- evolutionary conservation
Is predicted protein truncated? (nonsense mutation premature stop codon, protein shorter and not likely to function properly, may also gunk up system and prevent cell function(dominant negative effect))
Is RNA splicing affected? (deletion/insertion of multiple bases, esp if near splice sites (introns spliced out of RNA))
Does the change segregate with disease in the family?- raises suspicion is pathogenic/leading to the disease
Somatic Mutation vs Germline Mutation
If a new mutation occurs in non-gamete-forming cells (Somatic cells), the consequence of the changes are limited to the person whom the changes occurred
If the new mutation occurs in cells that ultimately form the gametes (Germline cells- spermatocytes, spermatogonia, spermatids, oocytes), the change may be transmitted to subsequent generations