genetics + genetic disorders Flashcards
what is the phenotype?
outward, physical manifestation of organism
what is the genotype?
full hereditary information of organism, although not all of the information may be expressed
a DNA nucleotide contains (3)
deoxyribose sugar, a base (T, G, A or C) and a phosphate group
describe the structure of a DNA molecule
- double helix
- strands run antiparallel (that is one runs 5’ to 3’ from the top of the helix in the picture, while the other runs 3’ to 5’.)
- sugar phosphate backbone
- right handed
what are the complimentary base pairs
adenine always pairs with thymine (t replaced with uracil in RNA molecules) (A-T/U)
-cytosine pairs with guanine (C-G)
describe the structure of the nucleus
- contains most of the DNA within the cell
- site of DNA replication + transcription for protein synthesis
describe a chromosome
- the structure that a single piece of DNA molecule is packaged into in the nucleus
- has shorter p arms, a centromere and longer q arms
- humans have 46
what is chromatin, and what two forms can it take?
- chromatin is the name of the mixture of DNA, proteins and RNA that package DNA within the nucleus
- heterochromatin is the condensed form
- euchromatin is the extended/loose and more functional form
describe the structures of transcription/replication
-both require 2 separate strands of DNA so that enzymes can access the DNA template
-cells must have means of opening up chromatin fibres and/or removing histones for access
= histones can be enzymatically modified or can be displaced by chromatin remodelling complexes (both reversible)
describe the process of DNA replication
-a semiconservative process
-DNA molecule ‘unzips’ by breaking hydrogen bonds
-new nucleotides are then paired with the two strands so one strand is old and one new
-replicated in the 5’ to 3’ direction; nucleotides added by DNA polymerase (3’ end of strand)
-okazaki fragments are formed on the lagging strong which are joined together by DNA ligase to form one continuous strand
at 5’ end, telomerase replicates these ends of the lagging strand
during which mitotic phase are chromosomes most compacted and visible?
metaphase, where DNA is packed about 10,000 tighter than in cells of other states
describe the function and purpose of exons and introns
- exons are coding regions of DNA
- introns are non-coding regions
- during transcription, introns are spliced out and axons form mRNA
describe a codon
- a set of 3 bases
- each codon specifies a particular amino acid
describe degeneracy/redundancy of the genetic code
- no. of codons > no. of amino acids
- all amino acids but 2 are coded for by more than one codon
- codons which refer to the same amino acid are called synonyms and tend to be very similar.
what is the role of RNA?
-to participate in protein synthesis (mRNA)
describe the function of mRNA
- like all DNA it is (obviously) transcribed from DNA
- it carries information required for protein synthesis
what does the process of alternative splicing entail?
Alternative splicing is a process by which the exons of the RNA produced by transcription of a gene (a primary gene transcript or pre-mRNA) are reconnected in multiple ways during RNA splicing. The resulting different mRNAs may be translated into different protein isoforms; thus, a single gene may code for multiple proteins.
describe the function of tRNA
- major role is to translate mRNA sequence into amino acid sequence.
- acts as an adapter molecule between the coded amino acid and the mRNA
describe the function of rRNA
- it is a component of ribosomes
- rRNA molecules are produced in the nucleus then transported to the cytoplasm to combine with proteins to form a ribosome
how is a polysome created?
several ribosomes can translate mRNA at one time creating what is known as a polysome
describe the structure of a eukaryotic chromosome
- linear
- telomeres at tips
- centromere
- hetero and euchromatin regions
in a telomere, what are okazaki fragments?
- okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are complementary to the lagging template strand, together forming short double-stranded DNA sections.
- each okazaki fragment has to be initiated by an RNA primer that will be removed from the fragment before it gets ligated to the next piece of DNA
what are the stages of the cell cycle?
- interphase (G1, growth; S, DNA replication; G2, further growth and preparing to divide)
- mitosis (propase, metaphase, anaphase, telophase)
describe prophase
- chromosomes condense
- nuclear membrane dissolves
- spindle fibres form from the centriole
describe metaphase
- chromosomes align at cell equator
- they are attached by fibres to each centriole
- stage at which chromosome is maximally condensed
describe anaphase
- sister chromatids separate at the centromere of the chromosome
- chromatids move to opposite poles of the cell
describe telophase
- new nuclear membranes form
- each cell now contains 46 chromosomes (diploid)
- the cytoplasm separates, causing formation of 2 new daughter cells
let’s talk about what a centromere is/does/hopes and dreams
- is a constricted region joining sister chromatids
- is the site of the kinetochore, the protein complex that binds microtubules, required during telophase
tandemly repeated DNA sequences include satellite and minisatellite DNA; describe both of these.
- Satellite DNA: consists of very large arrays of tandemly repeating, non-coding DNA. Main component of functional centromeres, and form the main structural constituent of heterochromatin.
- A minisatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from 10–60 base pairs) are typically repeated 5-50 times.
chromatin is formed of histone proteins and DNA; describe this process
- DNA is -ve, histones have +ve charge
- the DNA traps around a core of histone proteins in a ‘beads on a string’ formation, a formation called a nucleosome
- further wrapping of nucleosomes forms a ‘solenoid’ structure
what is the purpose of packaging DNA?
- negative DNA is neutralised
- DNA takes up less space
- inactive DNA can be folded into inaccessible locations until required
describe a metacentric chromosome
x-shaped chromosomes, with the centromere in the middle so that the two arms of the chromosomes are almost equal
when is a chromosome submetacentric?
when the length of the p and q arms are unequal
when is a chromosome acrocentric?
if the p (short) arm is so short that it is hard to observe, but still present,
what is a karyotype?
Karyotype is a test to identify and evaluate the size, shape, and number of chromosomes in a sample of body cells
what is fluorescent in situ hybridisation (FISH)?
it is a cytogenetic technique that uses fluorescent probes that bind to only those parts of the chromosome with a high degree of sequence complementarity and is used to detect and localize the presence or absence of specific DNA sequences on chromosomes.
name some types of FISH probes (4)
- unique sequence probes
- centromeric probes (good for determining chromosome number)
- telomeric probes (useful for detecting subtelomeric rearrangements)
- whole chromosome probes (used for detecting translocations and rearrangements)
describe the process of meiosis
- cell division in germ cells
- diploid cells in ovaries and testes divide to form haploid cells
- chromosomes are passed on as recombined/re-arranged copies to create genetic diversity
- meiosis 1 follows the same steps as simple mitosis
- meiosis 2 produces gametes
describe oogenesis
- is the process of egg formation
- begins in early embryonic life
- takes a very long time
- gametes are 1 ovum + 3 polar bodies
describe spermatogenesis
-is the process of sperm production
-commences at puberty
-duration is short at 60-65 days
4 spermatids produced
-more mitoses occur the in oogenesis so there is a higher chance of mutation
describe fertilisation
- two haploid cells form one diploid cell called a zygote
- sperm determine whether XX or XY
- mitochondrial DNA comes only from mother via the egg (maternal inheritance)
what is x-inactivation and when does it occur?
- X-inactivation is a process by which one of the copies of the X chromosome present in female mammals is randomly inactivated.
- occurs when the zygote has developed into an early embryo
- prevents females (XX) from having twice the amount of genetic information as males
define non-disjunction
non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.
what are the features of trisomy 21 (down syndrome)?
- Characteristic facial dysmorphologies
- IQ less than 50
- Average life expectancy (50-60 years)
- Alzheimer’s disease in later life
what are the features of trisomy 13 (patau syndrome)?
- Incidence: 1 in 5000
- Multiple dysmorphic features and mental retardation
- About 5% die within first month, very few survive beyond first year
what are the features of trisomy 18 (edwards syndrome)?
- Severe developmental problems
- most patients die within first year, many within first month
what are the features of 45, X (turner syndrome)?
- Incidence at conception much greater, about 97% result in spontaneous loss
- Females of short stature and infertile
- Neck webbing and widely spaced nipples
- Intelligence and lifespan is normal
what are the features of 45, XXY (klinefelter syndrome)?
- Tall stature, long limbs
- Male but infertile, small testes, about 50% gynaecomastia
- Mild learning difficulties
what are types of structural abnormalities that could occur?
- Balanced or unbalanced rearrangements
- Translocations (Reciprocal: involving breaks in two chromosomes with formation of two new derivative chromosomes; Robertsonian: fusion of two afrocentric chromosomes)
- Deletions
- Insertions
- Inversions
describe balanced translocation
an even exchange of material with no genetic information extra or missing, and ideally full functionality
describe unbalanced translocation
where the exchange of chromosome material is unequal resulting in extra or missing genes
describe a robertsonian translocation
Unique type of translocation in that two (acrocentric) chromosomes are fused but no genetic information is lost
describe the process of deletion
breaks in the chromosome; a piece of genetic material is simply deleted
there are two types of inversion; name and describe them
paracentric: the inverted piece of DNA comes entirely from an arm of the chromosome
pericentric: the inverted piece of DNA includes the centromere of the chromosome
(both are balanced rearrangements)
mutations can be either coding or non-coding. what are the 4 types of coding mutation?
- silent/synonymous
- missense
- nonsense
- frameshift (deletion/insertion)
what are the two types of point mutation?
- transitions (i.e. purine to purine)
- transversions (i.e. purine to pyrimidine)
what is Polymerase Chain Reaction? (PCR)
a technique used to amplify a single copy or a few copies of a piece of DNA, generating thousands to millions of copies of a particular DNA sequence. It is an easy and cheap tool to amplify a focused segment of DNA, useful for such purposes as the diagnosis and monitoring of genetic diseases
describe the process of gel electrophoresis
- Separate DNA fragments by size
- Apply an electric field
- DNA is negatively charged
- Separate through agarose gel matrix
- Visualise DNA fragments
what is Amplification Refractory Mutation System (ARMS) used for?
- for detecting known point mutations
- a specific primer will only permit amplification to take place when its [3’ terminal nucleotide] matches with its target sequence
describe the action of restriction endonucleases
- Enzymes from bacterial cells
- Protective mechanism
- Degrade DNA of invading viruses
- Recognise specific DNA sequences
- Always cut DNA at the same site
describe RFLP analysis
In RFLP analysis, the DNA sample is broken into pieces and (digested) by restriction enzymes and the resulting restriction fragments are separated according to their lengths by gel electrophoresis.
in a family pedigree diagram, which shapes are used to detail male/female?
male - square
female - circle
what is a punnet square?
the cheeky wee box thingy that can be used to determine an individual’s chance of being either a carrier or affected by autosomal and sex-related conditions?
in autosomal dominant conditions, what is the chance of a child being affected if one parent is affected and the other is not?
1 in 2 chance of being affected (50%)
no ‘skipped generations’ as no carriers
describe the characteristics of sex-linked inheritance
genes on the x chromosome are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome in most cases. In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other. This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes.
if the father is the carrier of a sex-linked condition, what are the chances of inheritance if the mother is unaffected?
0% chance of sons being since the genes of the x chromosome from the father are not being passed down
all of the daughters will be carriers since they will have one affected x (father) and one unaffected (mother)
if the mother is the carrier of a sex-linked condition, what are the chances of inheritance if the father is unaffected?
50% chance sons will have the condition
50% chance daughters will be a carrier (daughter would only be affected if mother was a carrier and father was affected)
examples of autosomal disorders and whether they are dominant or recessive
- sickle cell anaemia (recessive + carriers are immune to malaria)
- cystic fibrosis (recessive + carriers have no symptoms)
- achondroplasia (dominant)
- huntington’s disease/chorea (dominant)
give examples of x-linked conditions
- duchenne muscular dystrophy
- haemophilia
describe the mechanisms of non-mendelian inheritance in terms of inheritance pattern
- incomplete penetrance (environmental factor + genetic modifiers)
- genomic imprinting (variants from parents)
- extranuclear inheritance (eg. mitochondria mutations)
- anticipation (e.g. triplet repeat expansion)
define penetrance
the frequency with which a trait is manifested by individuals carrying the gene.
define the ‘wild type’
a strain, gene, or characteristic which prevails among individuals in natural conditions, as distinct from an atypical mutant type.
define a genetic modifier
genes that have small quantitative effects on the level of expression of another gene.
(may involve polymorphism)
list some environmental factors
Lifestyle, Diet, Smoking, Alcohol, Drugs,
Stress, Air pollution, Chemicals,
Infection, etc.
define genomic imprinting
genomic imprinting is the epigenetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner. If the allele inherited from the father is imprinted, it is thereby silenced, and only the allele from the mother is expressed.
describe epigenetic modifications
Heritable changes in gene function that cannot be explained by changes in DNA sequences
name the genetic mechanisms (4)
- Deletions
- Point mutations
- Imprinting errors
- Uniparental disomy (in which 2 copies of a chromosome come from the same parent, instead of 1 copy from the mother and father each)
give examples of unilateral diploidy (2)
- gynogenic (2 maternal genomes, mass of embryo, ovarian teratoma)
- androgenic (2 paternal genomes, mass of placenta, hydatidiform mole)
give examples imprinting disorders and their associated symptoms (2)
- angelman syndrome (epilepsy, severe mental retardation, awkward hair, inappropriate laughter)
- prader-willi syndrome (hypotonia, mild-moderate mental retardation, short stature, marked obesity)
mechanism of mitochondrial inheritance
only the mitochondria from the ova are inherited; mitochondria of the sperm are degraded upon fertilisation. this replicates and mitochondrial DNA from the mother are passed on :) #cellularmatriarchy
reasons why the mitochondria described as “mutation hotspots”? (3)
- lack of efficient DNA repair system
- lack of protective proteins, such as histones
- damaged by reactive oxygen species (ROS), i.e. free radicals
mitochondrial diseases affect tissues with high metabolic demand. give examples (3 major, 3 other)
- myoclonic epilepsy with ragged red fibres (MERRF)
- mitochondrial myopathy, encephalomyopathy, lactic acidosis, stroke-like symptoms (MELAS)
- chronic progressive external ophthalmoplegia (CPEO)
- leber’s hereditary optic neuropathy (LHON - loss of central vision)
- leigh’s syndrome (encephalopathy)
- diabetes mellitus and deafness (DAD)
define “anticipation”
In genetics, anticipation is a phenomenon whereby as a genetic disorder is passed on to the next generation, the symptoms of the genetic disorder become apparent at an earlier age with each generation.
triplet repeat expansion diseases are subject to anticipation. give examples of such diseases (3)
- huntington’s disease
- myotonic dystrophy
- fragile x syndrome
what is “fitness” and how can alleles affect it?
-“Fitness” means the relative ability of organisms to survive and pass on genes.
- neutral alleles mainly do not affect fitness
- deleterious alleles sometimes decrease fitness
- advantageous alleles rarely increase fitness
i’m not doing cards on how to calculate allele frequency etc.
it's too much no just n o (look at powerpoint)
what is gene flow?
- mutations, can increase the proportion of new alleles in a population
- introduction of new alleles as a result of migration (or intermarriage) leads to new gene frequency in hybrid population
what is non-random mating?
-Non-random mating leads to increase mutant alleles, thereby increasing proportion of affected homozygotes.
- assortative mating: choosing of partners due to shared characteristics
- consanguinity: marriage between close blood relatives
describe natural selection and the differences between negative and positive
- natural selection is a gradual process by which biological traits become either more or less common in a population
- negative selection: reduces reproductive fitness, decreases prevalence of traits, leads to gradual reduction of mutant allele
- positive selection: increases reproductive fitness, increases the prevalence of adaptive traits, heterozygote advantage
large populations are able to balance out fluctuations, whereas small populations are more likely to exhibit what? (2)
- genetic drift
- founder effect
what is genetic drift?
random fluctuation of one allele transmitted to high proportion of offspring by chance
what is founder effect?
the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony.
features of somatic mutations
- occur in nongermline tissues
- are nonheritible
- eg breast
features of germline mutations
- present in egg or sperm
- are heritable
- all cells affected in offspring
- cause cancer family syndromes
genetic processes associated with cancer (3)
- oncogenes
- tumour suppressor genes
- DNA damage response genes
what is the difference between a photo-oncogene and an oncogene?
proto-oncogenes are normal gene that codes for proteins to regulate cell growth and differentiation.
mutations can change this into an oncogene, which can accelerate cell division and cause cancer if stuck “on”.
what is the function of tumour suppressor genes?
- they act as the cell’s “brakes” for cell growth
- inhibit the cell cycle or promote apoptosis or both
- cancer arises when both “brakes” fail (two hit hypothesis- first in gremlin of child, second, tumour)
what is the function of DNA damage-response genes?
- the repair “mechanics” for DNA
- cancer arises when both genes fail, speeding the accumulation of mutations in other critical genes
what does mismatch repair failure lead to?
- Hereditary nonpolyposis colorectal cancer (HNPCC)
- microsatellite instability (MSI)
describe micro satellite instability with regard to the function of MMR
- MMR corrects errors that spontaneously occur during DNA replication like single base mismatches or short insertions and deletions
- MSI is the phenotypic evidence that MMR is not functioning normally.
- Cells with abnormally functioning MMR tend to accumulate errors.
