Genetics 1 Flashcards
what does hereditary / inheritance refer to
what is happening to traits passed on or within multiple generations of a given family
what is the best way to follow inheritance patterns
family pedigree analysis
family pedigree standardised symbols:
what do the roman numerals down lhs mean
generation number (ie I will be at the top row, fourth row is 4th generation so IV etc)
family pedigree standardised symbols:
what do diagonal lines across a shape mean
individual has already died (so we don’t know if they’re affected or not)
family pedigree standardised symbols:
what do circles represent
females
family pedigree standardised symbols:
what do squares represent
males
family pedigree standardised symbols:
what does a plain circle or square represent
healthy individuals
family pedigree standardised symbols:
what does a filled in (ie in black) circle or square represent
diseased individuals
family pedigree standardised symbols:
what do dots or vertical stripes inside shapes / half shaded shapes represent
carriers of the disease (carry the defective gene but dont have the disease)
family pedigree standardised symbols:
how are twins symbolised
lines from parents make a triangle formation
family pedigree standardised symbols:
how are IDENTICAL twins symbolised
same triangle BUT they are also joined by a horizontal line between them
in what 5 ways can genetic information be passed from one generation to the next
1) human karyotype
2) chromosome structure
3) double helix
4) nucleobases and nucleotides
5) the genetic code
human karyotype:
what do chromosomes carry in eukaryotic cells
genetic information
human karyotype:
what is the normal karyotype for 1) males and 2) females
1) XY
2) XX
human karyotype:
how is genetic information passed on via this route
one chromosome in each of the 23 pairs is derived from mother and other from father
the dna of every chromosome records info that can be translated by the cells
chromosome structure:
during what stage of cell growth can we best observe condesnsed tightly packed DNA chromosomes
metaphase
chromosome structure:
what are the 3 steps of packing unwound structure of dna double helix into densely condensed chromosomes
1) dna wound around histone proteins (octamer) forming nucleosome
2) many nucleosomes look like ‘beads on a string’ = this is chromatin
3) regions of chromatin molecule interact and form chromatin fibres which are coiled and condensed to form chromosomes
the double helix:
which 2 scientists first described the double helix
watson + krick
base pairs are called watson-crick base pairs
the double helix:
what does it consist of
2 anti-parallel polynucleotide chains of specific chemical composition, held together by hydrogen bonding between pairs of nucleobases
the double helix:
what do nucleobases make up
the 4 letter code which encodes the genetic information
nucleobases and nucleotides:
what are the 2 types of nucleobases and what are they grouped based on
1) purine
2) pyrimidines
3) chemical structure
nucleobases and nucleotides:
list the purine bases
adenine
guanine
nucleobases and nucleotides:
list the pyrimidine bases
cytosine
thymine
uracil
nucleobases and nucleotides:
what is the structure of a nucleotide
a deoxyribose sugar with 1 phosphate and 1 nucleobase attached to it
nucleobases and nucleotides:
how are single nucleotides with different bases linked together into 1 sequence
phosphate group forms bonds with 2 ribose sugars
nucleobases and nucleotides:
how is the double helix formed
2 strands of antiparallel sequences of nucleotides bind to each other through hydrogen bonding
nucleobases and nucleotides:
what is the hydrogen bond pairing between the bases
adenine and thymine (or uracil in rna) = 2 H bonds
guanine and cytosine = 3 H bonds
(forms backup bc same info encoded)
the genetic code:
what can letters of nucleobases encoded in DNA be translated into
proteins (ie Val = valine)
the genetic code:
what does a codon translation table show
how cells interpret free nucleobases as a signal for each amino acid AND where to start / stop reading sequences
the genetic code:
what does the genetic code being redundant mean
several triplet codons code for the same amino acid
mutations can be silent (where a triplet changes so genetic code changes but new triplet still codes for the same amino acid)
what is the basis of inherited disease
MUTAGENESIS
mutations are heritable permanent changes in dna base seq which allow new changes/disease to arise because they change information encoded
list the 5 types of mutation
1) point
2) deletions / insertions
3) silent
4) missense
5) nonsense
point mutations:
what are they
happen in one base / one base pair of double helix seq
change only 1 letter of code at given position
point mutations:
what are the 2 types and what do they do
1) TRANSITION = where a purine base is substituted with another purine base OR pyrimidine substituted with another pyrimidine
2) TRANSVERSION = where a purine is substituted with a pyrimidine or vice versa
transition is more limited
deletions / insertions:
what are these mutations characterised by
loss / gain of nucleotides
deletions / insertions:
what may these mutations lead to…
FRAMESHIFT MUTATIONS (change how genetic code is read and affect all information encoded after the mutation because the cell applies a new triplet reading frame)
silent mutations:
what are these
Usually point mutations changing nucleotide seq but not coded amino acid bc of redundancy of genetic code
missense mutations:
explain these
point mutations in which a single nucleotide change results in different amino acid being encoded
nonsense mutations:
what is this
codon usually coding for an amino acid is changed to a STOP CODON
nonsense mutations:
what do they lead to
premature truncation of encoded proteins (because cell stops reading information prematurely and no more amino acids are added)
leading to a shortened protein
give two general ways in which mutations may arise
1) dna replication fidelity (internal enzymes causing mutation)
2) mutagens (external stimuli)
dna replication fidelity:
how may this arise
Enzyme carrying out replication is accurate in terms of reading + copying seq ensuring identical daughter cells
dna replication fidelity:
how does DNA polymerase work
VERY LOW ERROR RATE
replicates by reading DNA seq in one strand, then inserting complementary nucleotides that can H bond giving a sequence corresponding perfectly to seq encoded in the other strand
dna replication fidelity:
what function does dna polymerase have for if a nucleotide that DOES NOT H bond is encorporated
EXONUCLEASE FUNCTION
3’ to 5’ proofreading function
removes error allowing correct nucleotide to be encorporated
mutagens:
what are the 2 types of mutagens
1) physical
2) chemical
physical mutagens:
give 2 examples
1) ionising radiation
2) non-ionising radiation
physical mutagens:
give examples of ionising radiation and explain how these lead to mutations
x-rays, gamma rays
causes strand breaks in dna or sugar/base destruction
physical mutagens:
give examples of non-ionising radiation and explain how these lead to mutations
uv light
causes NEW chemical bonds such as pyrimidine dimers
physical mutagens:
what are pyrimidine dimers
molecular lesions
form during photochemical reactions when covalent bonds form between neighbouring thymine / cyotsine bases in the same strand
chemical mutagens:
give 3 examples
1) base analogues
2) nitrous acid
3) alkylating agents
chemical mutagens:
what are base analogues
derivatives of normal bases (look similar to nucleobases) so are wrongly incorporated by dna polymerase
chemical mutagens:
how can base analogues cause mutation
cause altered base pairing
chemical mutagens:
how does nitrous acid cause mutations
carries out deamination reaction on cytosine nucleobases changing it to uracil = causes GC base pairing to AC base pairing transitions
chemical mutagens:
give an example of an alkylating agent and explain how they cause mutations
methylmethane sulfonate
Cause lesions that result in error prone repair, increasing error rate of dna polymerase
mutagens:
for safety reasons what is done to mutagenic chemicals
labelled with specific pictograms / info detailing their mutagenic effects
define allele
different forms of the same gene
we have 2 copies of each chromosome so 2 of each gene, gene may exist in 1+ form as different varieties of same genes
define dominant
a gene that, if present, expresses a certain characteristic
doesn’t matter what info on other allele is
define recessive:
gene whose character is only expressed in absence of a dominant gene
natural version of gene has to be changed on both chromosomes for recessive characteristic to cause a phenotype
define phenotype
visible appearance of a change in the genetic info
displayed / observable characteristics
define genotype
encompasses genetic components of info in an individual
what are diploid genomes
2 copies of each chromosome
what is a recessive lethal allele
pair of identical alleles
only one chromosome is affected SO individual is healthy as the healthy gene in the other chromosome is dominant so expressed
can code for dominant or recessive traits but ONLY fatal (cause death) in homozygous condition
what is the probability of offspring being a carrier when a carrier of a recessive lethal allele mates with a non affected individual
50% of offspring will be carriers but are ALL viable as non-mutated copy is dominant
what are the probabilities when both parents carry a recessive lethal allele
1) 50% healthy carriers
2) 25% healthy, non-carriers
3) 25% affected as carry 2 copies of mutated recessive allele which will be lethal (will kill)
SO OVERALL
75% healthy and 25% affected (affected die)
what is the probability of offspring being affected in
a) autosomal dominant inheritance where one parent is affected and one is unaffected
b) in autosomal recessive patterns of inheritance
a) 50%
b) 75%
what are single gene (MONOGENIC) disorders
give the 3 types / patterns of inheritance these may be
traceable to a single mutated gene
1) autosomal recessive
2) autosomal dominant
3) sex-linked
give 2 examples of autosomal recessive inheritance
cystic fibrosis = CFTR gene is affected
sickle cell disease = beta-globin gene is affected
these mutations arise on 1 of chromosomes 1-22 NOT on the sex chromosomes X and Y
give an example of autosomal dominant disorder
huntingtons
explain sex linked disorders and give an example
encoded on one of sex chromosomes (usually X as it contains more info)
amelogenesis imperfecta which affects tooth enamel
what are major monogenic recessive disorders
monogenic disease occuring at a given frequency in a population depending on size, health, external factors, genetic history
give 3 examples of major monogenic recessive disorders
1) cystic fibrosis
2) sickle cell anaemia
3) thallassaemias
what does cystic fibrosis cause and how many north europeans does it affect
recurrent lung infections
1/2000
what does sickle cell anaemia cause and how many africans does it affect
anaemia
1/50 where malaria is endemic
why are frequencies of sickle cell anaemia carriers / affected HIGHER in areas where malaria is endemic
sickle cell anaemia give a selective advantage in regions where higher rates of malaria infection by giving some protective advantage
what do thallassaemias cause and how many asians + mediterraneans does it affect
anaemia
1/50
how does selective advantage explain higher frequency in a population
bc of recessive nature individuals that are carriers reproduce at higher rate
