Topic 3 - Genetics Flashcards
(113 cards)
1
Q
gene
A
- heritable factor consisting of a length of DNA
- influences a specific characteristic
2
Q
locus
A
the specific position a gene occupies on a chromosome
3
Q
allele
A
- the various specific forms of a gene
- basically they are alternative forms of the same gene, with the same locus
- only one allele can occupy the locus of the gene on a chromosome
- most cells have 2 copies of each chromosome, so it’s possible for 2 different alleles to be present
4
Q
differences between alleles
A
only by one or a few bases
5
Q
single nucleotide polymorphism
A
- pronounced snips
- position in a gene where the base may be different for each allele
6
Q
mutation
A
- random changes
- most significant type is base substitution
- new alleles are formed via gene mutation
- almost all mutations are either neutral or harmful
- mutations can be passed onto offspring, causing genetic disease
7
Q
base substitution
A
type of mutation where one base in the gene sequence is replaced by a different base
8
Q
sickle cell anemia
A
- genetic disease
- caused by a base substitution mutation of the gene coding for the alpha-globin polypeptide in hemoglobin
- homogeneous sufferers develop severe anemia
- heterogeneous sufferers develop mild anemia
9
Q
effect of base substitution in sickle cell anemia
A
- the mutated gene is Hb^S while most humans have Hb^A
- when Hb^S is transcribed, the mRNA has GUG, not GAG, in its 6th codon
- when translated, the 6th amino acid is valine instead of glutamic acid
10
Q
effect of sickle cells on the body
A
- causes damage to tissues by getting stuck in blood capillaries
- this causes blockages, reducing blood flow
11
Q
what happens during the blood circulation process of a sickle cell anemia patient?
A
- the change causes hemoglobin molecules to stick together in low oxygen conditions
- the bundles of hemoglobin molecules are rigid enough for RBCs to distort into a sickle shape
- upon return to high oxygen conditions (in the lungs), the hemoglobin bundles break up and return to their normal shape
- the hemoglobin and the plasma membrane are damaged
- the life of a RBC can be as little as 4 days
- the body can’t replace RBCs at a rapid enough rate
12
Q
genome
A
- the whole of the genetic information of an organism
- essentially the entire base sequence of each of its DNA molecules
13
Q
genome makeup in humans
A
- the 46 molecules forming the chromosomes in the nucleus
- the DNA molecule in the mitochondrion
14
Q
genome makeup in plants
A
- the DNA molecules of chromosomes in the nucleus
- DNA molecule in the mitochondrion
- DNA molecule in the chloroplast
15
Q
genome makeup in prokaryotes
A
- much smaller than multicellular organisms
- consists of the DNA in the circular chromosome & plasmids
16
Q
satellite DNA
A
- DNA that isn’t transcribed
- but they still affect gene expression
17
Q
DNA in bacteria
A
- circular DNA
- only one chromosome
- so there’s usually only one copy of each gene
- two are briefly present after the DNA replication stage of cell division
- not associated with any proteins
- can be described as ‘naked’
18
Q
plasmids
A
- small extra DNA molecules
- commonly found in prokaryotes but rare in eukaryotes
- usually small, circular, and naked
- contains genes that are useful but not essential (e.g. antibiotic resistance)
19
Q
replication of plasmid
A
- they’re not always replicated at the same time as the chromosomes
- so there may be multiple copies of plasmids in a cell
- sometimes a plasmid is not passed to both cells in cell division
- they can be transferred from one cell to another
- it can even be transferred across species (e.g. if a plasmid released upon the death of a prokaryote is absorbed by a cell of a different species)
20
Q
Sanger technique for genome sequencing
A
- a DNA sample is chopped up and single stranded copies are made with DNA polymerase
- before the whole sequence is replicated, small quantities of a non-standard nucleotide are added to the reaction mixture
- this is done separately with each of the 4 possible DNA bases
- then each sample is separated with gel electrophoresis
21
Q
process of genome sequencing
A
- colored fluorescent markers are used to mark the DNA copies
- each of the 4 samples is distinguished by a particular color
- the samples are mixed together and all the DNA copies are separated in 1 lane of a gel according to the no of nucleotides
- a laser scans along the lane to cause fluorescence
- an optical detector detects the colors of fluorescence
- a computer deduces the base sequence from the sequence of colors detected
22
Q
eukaryote chromosomes
A
- chromosomes are composed of DNA and protein
- DNA is linear and long
- associated with histone proteins
- histone is globular and wider than DNA
- 1 DNA molecule per chromosome but lots of histone molecules in a chromosome
- DNA is wound around histone and straight when not in contact with histone
- gives the appearance of a string of beads during interphase
23
Q
differences between chromosomes
A
- differ in length and position of centromere
- the centromere can be positioned at any point in a chromosome
- there are at least 2 different types of chromosomes in every eukaryote
- there are 23 different types of chromosomes in humans
- each type of chromosome carries a specific sequence of genes along the DNA molecule
- in many chromosomes there are 1000+ genes
- genes are arranged in a standard sequence to allow parts of chromosomes to be swapped during mitosis
24
Q
homologous chromosomes
A
- chromosomes that have the same sequence of genes
- but not necessarily the same alleles
- this allows members of a species to interbreed
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haploid nucleus
- has one chromosome of each type
- in humans, a haploid nucleus contains 23 chromosomes
- gametes have haploid nuclei
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diploid nucleus
- have pairs of homologous chromosomes
- has 2 full sets of the chromosomes found in its species
- so they have 2 copies of every gene (except sex chromosome genes)
- so in humans, it contains 46 chromosomes
- zygotes have diploid nuclei
- most cells are diploid
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hybrid vigour
phenomenon in which organisms are often more vigorous if they have 2 different alleles of genes
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advantages of diploid cells over haploid cells
- harmful recessive mutations can be avoided if a dominant allele is present
- hybrid vigour
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significance of number of chromosomes
- organisms with a differing number of chromosomes are unlikely to be able to interbreed
- the number of chromosomes can change during evolution
- can decrease if chromosomes become fused together
- can increase if splits occur
- chromosome numbers can also double via certain mechanisms
- but these are rare and chromosome numbers are unlikely to change
30
sex chromosomes
- x chromosome is large and has a centromere around the middle
- y chromosome is small and has centromere near the end
- all humans have one x chromosome as it has genes essential to both genders
- y chromosomes only have a small number of genes and are not needed for female development
- and one of the y chromosome genes (SRY or TDF) cause a fetus to develop as a male
- it stimulates the development of male features (testes, testosterone production)
- so a fetus with xy chromosomes will develop as a male
- as females pass on x chromosomes only, all offspring will inherit an x chromosome from their mother
- the gender of a human is determined at the moment of fertilization
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autosome
chromosomes that don't determine sex
32
how to observe chromosomes
- differences can't be spotted with a light microscope during interphase due to limited resolution
- can only be spotted in mitosis/meiosis when supercoiling occurs
- so stains that bind either DNA or proteins can be used to see them
- if dividing cells are stained and placed on a microscope slide, they can be burst by pressing on the cover slip
- this will cause the chromosomes to spread
- as most cells are diploid, chromosomes are usually in homologous pairs
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karyogram
- image of the chromosomes of an organism
| - arranged in homologous pairs of decreasing length
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karyotype
property of an organism (i.e. number and type of chromosomes an organism has in its nuclei)
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uses of karyotypes
- to deduce whether an individual is male or female
| - to diagnose down syndrome
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diagnosis of down syndrome using karyotypes
- fetuses with down syndrome will have 3 copies of chromosome 21 instead of 2
- this is called trisomy 21
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features of down syndrome
- hearing loss
- heart disorders
- vision disorders
- mental/growth retardation
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meiosis
- one of the 2 ways an eukaryote can divide
- in animals, it occurs during the process of creating gametes
- one diploid nucleus divides to produce 4 haploid nuclei
- divided into 2 stages: meiosis I and meiosis II
- meiosis involves halving the chromosome number
- so it's also known as reduction division
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meiosis I
- the diploid nucleus divides to form 2 haploid nuclei
- the halving of the chromosome number occurs at this stage
- while the 2 nuclei produced here are haploid, each chromosome still consists of 2 chromatids
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meiosis II
- the chromatids of chromosomes in the 2 nuclei separate at this stage
- this produces 4 haploid nuclei with a single chromatid for each chromosome
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chromatid
- two identical copies of DNA
- they are attached at the centromere
- typically each chromosome consists of a single chromatid
42
chromatin
- the DNA complex and histone
| - appears as DNA coiled around histone proteins
43
significance of division of chromosome number in meiosis
- asexual reproduction results in genetically identical offspring to the parent (same chromosomes)
- sexual reproduction results in variations in chromosomes between the offspring and their parents
- in eukaryotes, sexual reproduction involves fertilization
- fertilization involves the merging of 2 sex cells
- therefore it doubles the number of chromosomes every time it occurs
- the result: a doubling of chromosome number every generation
- this is prevented by the halving of chromosome number during meiosis
44
when is DNA replicated for meiosis?
- DNA replication occurs during interphase (before meiosis)
- the chromosomes are supercoiled in the early stages of mitosis so they are visible
- and it's clear that by then, they already consist of 2 sister chromatids
- initially the two chromatids are genetically identical
- DNA replication doesn't occur at all in meiosis, which explains why the chromosome number is halved and why there is only a single chromatid for each chromosome in the final products
45
early stage of meiosis I
- homologous chromosomes pair up with each other
- as DNA replication has already occurred, there are 2 sister chromatids for each chromosome
- so there are 4 DNA molecules for each pair of homologous chromosomes
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synapsis
pairing process of homologous chromosomes in meiosis
47
bivalent
a pair of homologous chromosomes
48
crossing over
- occurs after synapsis
- the two homologous chromosomes exchange genetic material in the same position on chromatids
- occurs at random positions anywhere along the chromosomes
- at least one crossover will occur for each bivalent
- as chromatids are homologous but not identical, some alleles are likely to differ
- chromatids with new allele combinations are produced
49
significance of the orientation of bivalents on meiosis
- while bivalents form, spindle microtubules grow from the poles of the cell
- after the breakdown of the nuclear membrane, the spindle microtubules attach to the centromeres of the chromosomes
- the attachments here are different from mitosis
- each chromosome is only attached to a single pole
- so the two homologous chromosomes in a bivalent are attached to different poles
- the pole a chromosome is attached to is determined by its orientation (i.e. its position and where it's facing)
- the orientation of bivalents is random
- the orientation of one bivalent will not affect other bivalents
- this is all meiosis I!
50
differences between mitosis and meiosis
- in mitosis, the centromere divides and the 2 chromatids move to opposite poles
- in meiosis, the centromere doesn't divide so the whole chromosome moves to a pole
- initially the bivalent is held together by chiasmata, but these slide to the ends to allow the chromosomes to separate
- the separation of chromosomes results in the halving of the cells
- so meiosis I is where the reduction division occurs
51
disjunction
separation of homologous chromosomes in meiosis I
52
methods used to obtain cells from a fetus
- amniocentesis
| - chorionic villus sampling
53
amniocentesis
- technique used to obtain fetus cells for karyotyping
- involves passing a needle through the mother's abdomen wall, using ultrasound to guide the needle
- the needle withdraws a sample of amniotic fluid containing fetal cells
- has a 1% chance of miscarriage
54
chorionic villus sampling
- a sampling tool enters through the vagina to obtain cells from the chorion
- the chorion is one of the membranes from which the placenta develops
- has a 2% chance of miscarriage
55
differences between mitosis and meiosis
- meiosis involves halving the number of chromosomes
| - meiosis produces genetically variant offspring while mitosis produces genetically identical offspring
56
prophase I
- cell has diploid number of chromosomes
| - synapsis and crossovers occur here
57
metaphase I
- spindle microtubules move homologous pairs to the equator
| - orientation of bivalents is random and independent of other homologous pairs
58
anaphase I
- homologous pairs separate
| - one chromosome of each pair moves to each pole
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telophase I
- chromosomes uncoil
- interphase follows (but in this interphase, no DNA replication occurs)
- reduction of chromosome number complete
- two separate haploid nuclei have formed
- cytokinesis occurs
60
prophase II
- the chromosomes supercoil
| - they still consist of 2 chromatids
61
metaphase II
- spindle microtubules attach
62
anaphase II
- centromeres separate
| - chromatids move to opposite poles
63
telophase II
- chromatids reach opposite poles
- a nuclear envelope forms
- cytokinesis occurs
64
what causes genetic variation?
- humans have 2 copies of each gene
- sometimes they're the same allele, sometimes differing alleles
- each allele has an equal chance of being passed on in a gamete
- crossing over and random orientation also promote genetic diversity
- random orientation promotes genetic variation among genes on different chromosome types
- crossing over promotes genetic variation by allowing reshuffling of linked genes on the same chromosome
65
how does fertilization promote genetic variation?
- the start of a life for an individual
- allows alleles from 2 different individuals to be combined into a single individual
- the combination of alleles is unlikely to have existed before
- fusion of gametes therefore promotes genetic variation in a species
- and variation is essential for evolution
66
non-disjunction
- when homologous chromosomes fail to separate at anaphase
| - results in an individual with either 45 or 47 chromosomes
67
problems that arise due to non-disjunction
- an abnormal number of chromosomes often lead to a syndrome
- Down Syndrome is due to a non-disjunction event
- some trisomies are so serious that offspring don't survive
68
Mendel's theory of inheritance
- males and females contribute equally to offspring
- inheritance is discrete (not a blending of 2 individuals)
- some characters display a stronger tendency to be expressed than other characters
69
gametes
- also called sex cells
- male gamete + female gamete => zygote
- the male gamete is normally smaller
- the female gamete normally displays little to no movement
- has a haploid nucleus
- so only one allele of each gene
- so male and female parents make equal genetic contribution despite the differences in gamete sizes
70
zygote
- result of fusion of gametes
- diploid nucleus
- as they contain 2 chromosomes of each type
- can contain more than one allele of a gene
71
segregation
- separation of alleles into different nuclei
| - the two alleles of each gene will separate into different haploid daughter nuclei during mitosis
72
dominant allele
- expressed in preference
- will mask the effects of recessive alleles
- when represented by a symbol, it's capitalized
73
co-dominance
- occurs when 2 different types of dominant alleles are present
- the result is a joint/hybrid effect
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recessive allele
- only expressed in homozygous state
75
why are some alleles dominant over others?
- dominant alleles code for a protein that is active and carries out a function
- the recessive allele codes for a non-functional protein
76
genetic determinant of blood types
- the gene symbol is I
| - there are 3 alleles: I^A, I^B, and i
77
why are I^A and I^B co-dominant but i recessive?
- all 3 alleles cause the production of a glycoprotein in the membrane of RBCs
- I^A causes an alteration by the addition of acetyl-galactosamine -- thus, they have anti-A antibodies
- I^B causes an alteration by the addition of galactose -- thus, they have anti-B antibodies
- heterozygous I^A I^B causes the addition of both acetyl-galactosamine and galactose, resulting in neither anti-A nor anti-B antibodies
- i causes the production of basic glycoprotein (no alterations)
- it's recessive because it doesn't cause the production of a special glycoprotein
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genetic disease
illness caused by a gene (typically recessive)
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carriers
- individuals that carry an allele for a genetic disease yet don't show symptoms of that disease
- specific to genetic diseases caused by recessive genes
- as it's impossible to be a carrier for a genetic disease caused by a dominant allele...
80
sex linkage
- inheritance pattern that differs between genders
- such as red-green color-blindness and hemophilia
- almost all due to genes on the X chromosome
- as there are very few genes on the Y chromosome
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cause of cystic fibrosis
recessive allele of the CFTR gene located on chromosome 7
82
effect of cystic fibrosis
- gene product is a chloride ion channel involved in the secretion of sweat, mucus, and digestive juices
- this allele causes the chloride channels not to function properly
- so sweat containing excessive NaCl is produced but digestive juices/mucus don't have enough NaCl
- so not enough water moves via osmosis into them, causing viscosity
- sticky mucus builds up in the lungs, causing infections
- the pancreatic duct will be blocked, so digestive enzymes can't reach the small intestine
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cause of Huntington's disease
a dominant allele of the HTT gene located on chromosome 4
84
effect of Huntington's disease
- gene product is the protein huntingtin
- its function is unknown
- this allele causes degenerative changes in the brain
- symptoms start between ages of 30-50
- life expectancy after the start of symptoms is 20 years
- usually cause of death is heart failure, pneumonia, or some other infectious disease
85
cause of red-green color blindness
- recessive allele of a gene in the X chromosome
- codes for one of the photoreceptor proteins
- these proteins are made by cone cells
- used to detect specific wavelengths of visible light
86
cause of hemophilia
- recessive allele of a gene located on the X chromosome
| - females are generally carriers, it's mostly males that suffer hemophilia
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effect of hemophilia
- life-threatening genetic disease in which blood can't clot wounds
- due to an inability to make Factor VIII (one of the proteins involved in blood clotting)
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hemophilia treatment
by infusing purified Factor VIII from donor blood
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causes of mutation
- radiation can increase the mutation rate if it has enough energy to cause chemical changes in DNA
- chemical substances can cause chemical changes to DNA
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examples of mutagenic factors
- gamma rays & alpha particles from radio isotopes
- short-wave UV radiation
- X-rays
- benzoapyrene & nitrosamines in tobacco smoke
- mustard gas
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gel electrophoresis
- technique used to separate proteins or fragments of DNA according to their size
- involves separating charged molecules in an electric field according to their size & charge
- charged molecules in the sample will move through the gel
- the gel used consists of a mesh of filaments that resist the movement of molecules in a sample
- eukaryotic DNA molecules are broken up into smaller fragments bc they're too long to move through the gel
- as all DNA molecules carry negative charges, they will move in the same direction
- but small fragments will move faster than large ones
92
use of PCR
- to copy specific DNA sequences
- a sequence is selected for copying using a primer that binds to the start of the desired sequence
- it binds via complementary base pairing
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endonuclease
- AKA restriction enzyme
| - enzyme used to cut DNA at specific base sequences
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restriction site
area that restriction endonucleases are supposed to cut through
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stages of DNA profiling
1. a DNA sample is obtained
2. sequences that have significant variation between individuals are isolated and copied using PCR
3. the copied DNA is split into fragments with restriction endonucleases
4. the fragments are separated via gel electrophoresis
5. a pattern of bands are produced -- this is the DNA profile
96
uses of DNA profile
- forensic investigations
| - paternity investigations
97
genetic modification
- carried out via gene transfer between species
- this is possible because the genetic code is universal
- they can be used to introduce new characteristics to animal species (e.g. goats that secrete milk containing spider silk protein)
98
sticky ends
- single stranded sections left by endonucleases that cut the 2 strands in DNA at different points
- they have complementary base sequences
- so they can be used to link together pieces of DNA via hydrogen bonding
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techniques for gene transfer to bacteria
- bacteria use plasmids to transfer genes
(- plasmids can encourage their replication and transfer in their host; these plasmids are the most abundant type
- there are some parallels with virus, but plasmids are not pathogenic)
- endonucleases can cut open plasmids and cut out desired genes from larger DNA molecules
- some endonucleases can cut the 2 strands of a DNA molecule at different points, leaving sticky ends
- DNA ligase joins DNA molecules by making sugar-phosphate bonds between nucleotides
- when the desired gene has been inserted in a plasmid using sticky ends, DNA ligase can seal the 'nicks' (i.e. do the final touches)
100
reverse transcriptase
- enzyme that can make DNA copies of RNA molecules
| - the copies are called cDNA
101
obtaining a copy of the gene being transferred (in gene transfer)
- it's easier to obtain mRNA transcripts than the original gene
- reverse transcriptase is often used to make the DNA needed for gene transfer
102
questions to be asked when assessing risks of genetic modification
- what is the chance of an accident or other harmful consequence?
- how harmful is that consequence?
103
claimed environmental benefits of GM crops
- pest-resistant crops = less pesticides used
- improved shelf life = reduced wastage
- reduced need for plowing and spraying crops = less fuel consumption
- herbicide-resistant crops = higher crop yields & less herbicide use
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claimed health benefits of GM crops
- higher nutritional value
- crop varieties with allergens/toxins removed = no need to worry about allergic reactions/poison
- edible vaccines can be produced
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claimed agricultural benefits of GM crops
- varieties resistant to weather damage can be produced = increase in total yields
- herbicide-resistant crops = higher yields & less herbicide use
- disease-resistant crops = higher yields
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claimed health risks of GM crops
- newly-introduced proteins could be toxic or cause allergic reactions upon consumption
- antibiotic resistance genes could spread to pathogenic bacteria
- transferred genes could mutate, causing unexpected problems
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claimed environmental risks of GM crops
- non-target organisms could be affected by toxins intended to control pests
- herbicide-resistant genes could spread = development of super-weeds
- loss in biodiversity
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claimed agricultural risks of GM crops
- seeds that are accidentally spilled can germinate to become unwanted volunteer plants; they are difficult to kill if they are herbicide-resistant
- widespread use of toxins that kill pests will lead to resistance to the toxin and cause secondary pests (that were resistant to the toxin to begin with) to flourish as well
- farmers can't save and re-sow GM seeds, so strains adapted to local conditions can't be developed
109
clone
- group of genetically identical organisms
- derived from a single original parent cell
- the smallest possible clone: a pair of identical twins
110
how do identical twins develop?
2 possibilities:
- result of human zygote dividing into 2 cells that develop into separate embryos
- result of an embryo splitting into 2 parts
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are natural methods of cloning possible?
- many plants can clone naturally
- when planted, a garlic bulb will produce a group of bulbs that are genetically identical
- female aphids can give birth to genetically identical offspring produced from diploid egg cells via mitosis
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beneficial mutation
- mutation that provides better chances of survival
- favored by natural selection
- example: people with a mutated version of LRP5 are immune to HIV
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medical use of genome sequencing
- can be used to produce new medications
| - beneficial molecules in the body are found and their gene copied to use as instructions for artificial synthesis
