topic 3 and 10 Flashcards
1
Q
A
2
Q
define a gene
A
a heritable factor that consists of a length of DNA and influences a specific characteristics
3
Q
where are genes located?
A
a gene occupies a specific position on one type of chromosome- this is called a locus
4
Q
define an allele
A
alternative forms of the same gene
5
Q
alleles have the same —— and only one allele can occupy the —–
A
locus; the locus of the gene on a chromosome
6
Q
how do alleles differ from each other?
A
alleles differ from each other by one or a few bases only
7
Q
how many chromosomes do humans have?
A
46 chromosomes; 23 pairs of chromosomes, where half come from the mother and half come from the father
8
Q
how are new alleles formed?
A
new alleles are formed by gene mutation, or random changes to the base sequence of DNA
9
Q
what is a genome?
A
the whole of the genetic information of an organism
10
Q
what does the genome of humans consist of?
A
the 46 molecules that form the chromosomes in the nucleus plus the DNA molecule in the mitochondrion
11
Q
what does the genome of plants consist of?
A
the DNA molecules of chromosomes in the nucleus plus the DNA molecules in the mitochondrion and the chloroplast
12
Q
what does the genome of prokaryotes consist of?
A
the DNA in the circular chromosome, plus any plasmids that are present
13
Q
what was done during the human genome project?
A
the entire base sequence of human genes was sequenced
14
Q
what is the cause of sickle cell anaemia ?
A
- a mutation of the gene that codes for the beta-globin polypeptide in haemoglobin
- normal allele= Hb^A, 6th codon of gene is GAG
- sickle cell allele= Hb^s, 6th codon of gene is GTG
- mutation only inherited by offspring if it occurs in a cell of the very or testis that develops into egg or sperm
- when Hb^s allele is transcribed, the mRNA has GUG as its 6th codon instead of GAG, so when mRNA is translated the 6th AA in the polypeptide is valine instead of glutamic acid
- this causes haemoglobin molecules to stick together in tissues with low oxygen concentrations
- bundles of haemoglobin are rigid so distort the RBCs into a sickle shape
15
Q
give the consequences of sickle cell anaemia
A
- sickle cells become trapped in blood capillaries, blocking them and reducing blood flow
- both haemoglobin and plasma membrane damaged and life of a RBC shortened to as little as 4 days
- RBCs not replaced at a fast enough rate= anaemia develops
16
Q
describe prokaryotic genetic material
A
prokaryotes have one chromosome consisting of a circular DNA molecule, which is naked as it is not associated with proteins. There is a single copy of each gene
17
Q
what types of organisms contain plasmids?
A
some prokaryotes but NOT eukaryotes
18
Q
what are plasmids?
A
small extra DNA molecules that are commonly found in prokaryotes- they are usually small, circular and naked, containing few genes that may be useful to the cell but not those needed for its basic life processes
19
Q
give an example of a gene that may be in a plasmid
A
genes for antibiotic resistance
20
Q
what is the problem with plasmids?
A
they are not always replicated at the same time as the chromosomes of a prokaryotic cell or at the same rate.
- may not be passed on to both cells formed by cell division
- may be multiple copies of a plasmid in a cell
21
Q
what is different about plasmids?
A
copies of plasmids can be transferred from one cell to another, allowing spread through a population. they can also cross the species barrier.
22
Q
how did John Cairns measure the length of DNA?
A
by autoradiography
23
Q
describe eukaryotic genetic material
A
eukaryotic chromosomes are linear DNA molecules associated with histone proteins
different chromosomes carry different genes
24
Q
how many types of chromosome are there in humans?
A
23
25
what are homologous chromosomes?
chromosomes that carry the same sequence of genes but not necessarily the same alleles of those genes
26
compare the genome sizes of T2 Phage, E.Coli, Drosophila melanogaster, Homo sapiens and Paris Japonica
T2 Phage- virus- 0.18 m base pairs
E.coli- gut bacterium- 5 m bp
Drosophila melanogaster- fruit fly- 140 m bp
Homo sapiens- human- 3,000 m bp
Paris Japonica- woodland plant- 150,000 m bp
27
describe haploid nuclei and give an example
have one chromosome of each pair (eg 23 in humans) human sex cells or gametes
28
describe diploid nuclei
have pairs of homologous chromosomes
- have two copies of each gene, apart from genes on sex chromosomes
29
give an advantage of having diploid nuclei
the effects of harmful recessive mutations can be avoided if a dominant allele is also present
30
explain the importance of chromosome numbers
the number of chromosomes is a characteristic feature of members of a species
31
compare the diploid chromosome numbers of Homo sapiens, pan troglodytes, Canis familiaris, oryza sativa, parascaris equorum
parascaris equorum- horse threadworm- 4
oryza sativa- rice- 24
Homo sapiens- 46
pan troglodytes- chimpanzee- 48
canis familiaris-dog- 78
32
what is sex determined by?
sex chromosomes
33
what are autosomes?
chromosomes that do not determine sex
34
describe the 2 chromosomes that determine sex in humans
- the X chromosome is relatively large and has its centromere near the middle
- the Y chromosome is much smaller and has its centromere near the end
35
why must all humans have the X chromosome?
it has many genes that are essential in both males and females
36
what chromosome gene causes a foetus to develop as a male?
SRY or TDF- initiates the development of male features, including testes and testosterone production
37
when is the sex of a human determined and why?
at fertilisation. this is because when sperm are formed, half contain the X chromosome and half the Y chromosome.
38
what is a karyogram?
a karyogram shows the chromosomes of an organism in homologous pairs of decreasing length
39
when can we get the clearest view of chromosomes?
during metaphase, in cells that are in mitosis
40
how do we obtain a karyogram?
- dividing cells stained and placed on microscope slide
- burst
- chromosomes become spread
- micrograph taken
41
describe meiosis
when one diploid nucleus divides to produce 4 haploid nuclei
42
what are the 2 divisions in meiosis known as
meiosis I and meiosis II
43
define a diploid nucleus
has two chromosomes of each type
44
chromosomes of the same type are known as....
homologous chromosomes
45
define a haploid nucleus
has one chromosome of each type
46
meiosis is also known as...because...
a reduction division because it involves a halving of the chromosome number
47
the halving of the chromosome number allows
a sexual life cycle with fusion of gametes
48
what does sexual reproduction involve in eukaryotic organisms?
fertilisation (the fusion of gametes)
49
when in the sexual life cycle does meiosis occur?
any stage, but in animals it happens during the process of creating gametes
50
main events in meiosis
x2:
- prophase
- metaphase
- anaphase
- telophase
51
prophase I
-Chromosomes condense
- nuclear membrane dissolves
- homologous chromosomes form bivalents (synapsis)
- crossing over occurs
52
metaphase I
Spindle fibres from opposing centrioles connect to bivalents (at centromeres) and align them along the middle of the cell
53
orientation of paternal and maternal chromosomes on either side of the equator is...
random and independent of other homologous pairs
54
anaphase I
- homologous pairs separated by spindle fibres
- one chromosome of each pair moves to each pole
55
telophase I
- chromosomes decondense
- nuclear membrane may reform
- cell divides (cytokinesis) to form two haploid daughter cells.
56
prophase II
- Chromosomes condense
- nuclear membrane dissolves
- centrioles move to opposite poles
57
metaphase II
Spindle fibres from opposing centrioles attach to chromosomes (at centromere) and align them along the cell equator
58
anaphase II
centromeres separate and chromatids are moved to opposite poles
59
telophase II
- chromatids reach opposite poles
- nuclear envelope forms
- cytokinesis occurs
60
what is meiosis preceded by?
interphase- DNA is replicated to produce chromosomes consisting of two sister chromatids
61
diploid human cell
46 chromosomes in 23 pairs
62
n
=23=haploid no of chromosomes
63
what three things promote genetic variation in meiosis?
- random orientation
- crossing over
- fusion of gametes from different parents
64
random orientation
in metaphase I, the orientation of bivalents is random and the orientation of one bivalent does not influence the orientation of the others.
- this generates genetic variation among genes that are on different chromosome types
65
for every additional bivalent, the no of possible chromosome combinations in a cell produced by meiosis...
doubles
66
crossing over
in prophase I:
- new gene combinations are formed on chromatids (recombination)
67
gametes fuse to form a
zygote
68
define crossing over
the exchange of DNA material between non-sister homologous chromatids
69
what are chiasmata?
connection points where non-sister chromatids continue to adhere after crossing over has occurred
70
what causes Down syndrome?
non-disjunction- this can cause other chromosome abnormalities as well
71
define non-disjunction
when homologous chromosomes fail to separate at anaphase - both of the chromosomes move to one pole and neither to the other pole, resulting in a gamete that either has an extra chromosome or is deficient in a chromosome (fertilization= individual w either 45/47 chromosomes)
72
Describe:
- Down syndrome
- trisomy 21: non-disjunction event leads an individual with three of chromosome 21 rather than 2
- hearing loss, heart and vision disorders, mental and growth retardation
73
give 3 other common non-disjunction disorders
trisomy 18 and 13
Klinefelter's syndrome (caused by having the sex chromosomes XXY)
Turner's syndrome (caused by having only one sex chromosome, X)
74
State two methods used to obtain cells for karyotype analysis
- amniocentesis : passing a needle through the mother's abdomen wall, using ultrasound to guide the needle which withdraws a sample of amniotic fluid containing metal cells from the amniotic sac
- chorionic villus sampling; sampling tool enters the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops
75
describe the risks associated with chorionic villus sampling and amniocentesis
amniocentesis- risk of miscarriage is 1%
Chronic villus sampling- risk is 2%
76
chiasmata formation between non-sister chromatids can result in
an exchange of alleles
77
why does independent assortment of genes occur?
due to the random orientation of pairs of homologous chromosomes in meiosis I
- the pole to which each chromosome in a pair moves depends on which way the pair is facing - this is random
- the direction in which one pair is facing does not affect the direction in which any other pairs are facing
78
bivalent =
tetrad, composed of 4 chromatids - the combination of homologous chromosomes when they align beside each other
79
describe the process of crossing over w a diagram
1. at one stage in prophase I all of the chromatids of two homologous chromosomes become tightly paired up together (synapsis)
2. the DNA molecule of one of the chromatids is cut. A second cut is made at exactly the same point in the DNA of a non-sister chromatid
3. the DNA of each chromatid is joined up to the DNA of the non-sister chromatid. this has the effect of swapping sections of DNA between the chromatids
4. in the later stages of prophase I the tight pairing of the homologous chromosomes ends, but the sister chromatids remain tightly connected. The X shaped structure after crossing over is called a chiasma
80
give 4 ways in which meiosis I differs from mitosis and meiosis II
- sister chromatids remain associated w each other
- homologous chromosomes behave in a coordinated fashion in prophase
- homologous chromosomes exchange DNA leading to genetic recombination
- meiosis I is a reduction division
81
how did Mendel discover the principles of inheritance?
with experiments in which large numbers of pea plants were crossed
82
what did Mendel discover?
When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants
When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1
83
gametes are haploid so...
contain one allele of each gene, and one chromosome of each type
84
what is the name of the single cell produced when two gametes fuse?
a zygote
85
define segregation of alleles
the separation of alleles into different nuclei
86
define a dominant allele
an allele that masks the effects of a recessive allele
87
define co-dominant alleles
alleles that have joint effects
88
what is the usual reason for dominance of one allele?
this allele codes for a protein that is active and carries out a function, whereas the recessive allele codes for a non-functional protein
89
what is a genetic disease and what is it caused by?
a genetic disease is an illness that is caused by a gene, and many genetic diseases in humans are due to recessive alleles of autosomal genes
90
what is a carrier of a genetic disease?
an individual that only has one recessive allele for a genetic disease and one dominant allele, who therefore does not shows symptoms of the disease but can pass on the recessive allele to their offspring.
91
give 2 causes of genetic diseases other than by recessive alleles
- dominant/co-dominant alleles
- sex-linked diseases
92
give an example of a disease caused by co-dominant alleles
sickle cell anaemia
93
define sex linkage
an inheritance pattern where the ratios are different in males and females
94
many genetic diseases have been identified in humans but most are ----------
very rare
95
what do radiation and mutagenic chemicals do?
they increase the mutation rate and can cause genetic disease and cancer
96
define a mutation
a random change to the base sequence of a gene
97
give two types of factors that will increase the mutation rate
- radiation (if it has enough energy to cause chemical changes in DNA) eg radioactive isotopes such as gamma rays and alpha particles, UV radiation and X rays
- chemical substances
98
what is the origin of genetic diseases?
mutations in body cells are eliminated when the individual dies but mutations in cells that develop into gametes can be passed onto offspring.
99
blood group A
IAIA or IAi
100
blood group B
IBIB or IBi
101
blood group AB
IAIB
102
blood group O
ii
103
describe how alleles of blood group will affect red blood cells
all 3 alleles cause the production of a glycoprotein in the membrane of RBCs:
IA:
- alters the glycoprotein by addition of acetyl-galactosamine.
- people with IA blood have anti-B antibodies in their plasma
IB:
- alters the glycoprotein by addition of galactose
- people with IB blood have anti-A antibodies in their plasma
IAIB:
- alters glycoprotein by addition of acetyl-galactosamine AND galactose
- neither anti-A nor anti-B antibodies will be produced by people with THIS genotype
ii:
- glycoprotein not altered
- both anti-A and anti-B antibodies in blood plasma
104
what blood group is a universal donor?
O (because its glycoprotein is unaltered)
105
what blood group is a universal recipient?
AB (because it does not have any antibodies in its plasma)
106
what 2 things will happen if blood of the wrong group is given to someone?
1. agglutination, causing RBCs to clump together to facilitate phagocytosis
2. white blood cells will perforate RBC's membrane, causing toxins/toxic substances to come out and causing the person to feel unwell
107
describe cystic fibrosis
- due to recessive allele of the CFTR gene, located on chromosome 7
- gene product is a chloride ion channel that is involved in the secretion of sweat, mucus and digestive juices
- recessive alleles= channel does not function properly
- sweat produced contains excessive amounts of NaCl, but digestive juices and mucus w insufficient NaCl produced
- not enough water moves by osmosis into secretions making them viscous
- sticky mucus builds up in lungs causing infections
- pancreatic duct blocked so digestive enzymes do not reach small intestine
108
describe Huntington's disease
- due to dominant allele of the HTT gene, located on chromosome 4
- gene product is a protein called huntingtin (function unknown)
- allele results in degenerative changes in the brain
- symptoms start between 30 and 50, people have already started having kids by then
- causes changes in behaviour, thinking and emotions
109
describe red-green colour blindness
- caused by a recessive allele of a gene for of the photoreceptor proteins, which are made by cone cells in the retina of the eye and detect specific wavelength ranges of visible light
- males; if their X chromosome carries the allele then it will be expressed in the phenotype
- females; both X chromosomes must carry the allele in order for it to be expressed
110
describe haemophilia
- inability to make Factor VIII, one of the proteins involved in the clotting of blood
- recessive allele located on the X chromosome
111
give an example of the consequences of nuclear bombing and accidents at nuclear power stations
- hiroshima and nagaasaki
- nuclear accidents at Three Mile island and Chernobyl
112
unlinked genes ------- ---------- as a result of meiosis
segregate independently
113
what is segregation?
the separation of the two alleles of every gene that occurs during meiosis
114
what is independent assortment?
the alleles of one gene segregate independently of the alleles of other genes
115
genes found on different chromosomes are
genes which are on the same chromosome are
the exception is
unlinked and segregate independently
linked and do not segregate independently
linked genes that are far apart on the chromosome (due to crossing over)
116
look at how to form a punnet square for dihybrid traits
p446
117
describe the difference between a monohybrid and a dihybrid trait
monohybrid cross: the cross happening in the F1 generation offspring of parents differing in one trait only, ie the inheritance of a single trait.
dihybrid cross: the cross happening in the F1 generation offspring of parents differing in two traits, the simultaneous study of the inheritance of two different traits.
118
when are gene loci said to be linked?
if they are on the same chromosome and hence don’t independently assort (unless synapsis occurs)
119
define a locus
a locus is a specific, fixed position on a chromosome where a particular gene or genetic marker is located.
120
define homologous chromosomes
two chromosomes that have the same sequence of genes.
121
why are homologous chromosomes not usually identical to each other?
for at least some of the genes on them, the alleles will be different
122
what is the name of non-sex chromosomes?
autosomes- these are common to both males and females
123
what are the two types of linkage?
autosomal gene linkage, when the genes are on the same autosome
sex linkage, when the genes are located on the X chromosome
124
variation can either be
discrete - no in-between categories
continuous - eg height
125
discrete variation
Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
126
continuous variation
Polygenic traits (characteristics controlled by more than two gene loci) tend to exhibit continuous variation, with an individual’s phenotype existing somewhere along a continuous spectrum of potential phenotypes
127
describe how the phenotypes of polygenic characteristics tend to show continuous variation
Increasing the number of loci responsible for a particular trait increases the number of possible phenotypes
This results in a phenotypic distribution that follows a normal distribution curve
128
monohybrid inheritance =
linked genes = two potential gamete combinations
129
dihybrid inheritance =
unlinked genes = 4 potential gamete combinations
130
why do offspring with unlinked genes have an equal possibility of inheriting any potential phenotypic combination?
due to the random segregation of alleles via independent assortment
131
Offspring with linked genes will only express the phenotypic combinations present in either parent unless
Consequently, the ‘unlinked’ recombinant phenotypes occur ----- ------- than the ‘linked’ parental phenotypes
crossing over occurs
less frequently
132
revise the use of chi-squared tests
133
Null hypothesis (H0):
Alternative hypothesis (H1):
There is no significant difference between observed and expected frequencies (i.e. genes are unlinked)
There is a significant difference between observed and expected frequencies (i.e. genes are linked)
134
Morgan’s discovery of non-Mendelian ratios in Drosophila.
Breeding experiments involving fruit flies clearly demonstrated that linked genes were not independently assorted
- when cross-breeding red-eyed wild types with white-eyed mutants, he discovered a clear sex bias in phenotypic distribution
- he inferred this was caused by the gene for eye colour being located on a sex chromosome (i.e. X-linked)
Morgan also observed that the amount of crossing over between linked genes differed depending on the combination of traits
- this led to the idea that crossover frequency may be a product of the distance between two genes on a chromosome – genes with a higher crossover frequency are further apart, whereas genes with a lower crossover frequency are closer together
135
Polygenic traits such as human height may also be influenced by
environmental factors, such as nutrition, disease, activity
136
How are linked genes often shown?
As vertical pairs
137
The frequency of recombinant phenotypes within a population will typically be ----- than that of non-recombinant phenotypes; why?
lower
crossing over is a random process and chiasmata do not form at the same locations with every meiotic division
138
The relative frequency of recombinant phenotypes will be dependent on; why?
the distance between linked genes
- recombination frequency between two linked genes will be greater when the genes are further apart on the chromosome because there are more possible locations where a chiasma could form between the genes
139
how can recombinant phenotypes be identified?
performing a test cross (crossing with a homozygous recessive for both traits); LOOK AT THIS
140
what does a gene pool consist of?
all the genes and their different alleles, present in an interbreeding population
141
how does a gene pool fit into the definition for a species?
species are groups of potentially interbreeding populations, with a common gene pool that is reproductively isolated from other species.
142
how would it be possible for multiple gene pools to exist for the same species?
if some populations are geographically isolated
143
what does evolution require?
that allele frequency's change with time in populations
144
define evolution
the cumulative change in the heritable characteristics of a population over time
145
give 4 reasons for evolution
- mutations introducing new alleles
- barriers to gene flow emerging between different populations
- if a population is small, random events can significantly impact allele frequency
- selection pressures favouring the reproduction of some varieties over others
146
give 3 types of natural selection
- directional
- stabilising
- disruptive
147
describe stabilising selection
selection pressures act to remove extreme varieties
- eg average birth weights of human babies are favoured over low or high birth weight
148
describe disruptive selection
selection pressures act to remove intermediate varieties, favouring the extremes
149
describe directional selection
the population changes as one extreme of a range of variation is better adapted.
150
state the 3 categories of reproductive isolation
- temporal
- behavioural
- geographic
151
define speciation
the formation of a new species by the splitting of an existing population
152
state the difference between allopatric and sympatric speciation
allopatric; the result of geographic separation of populations//isolation of gene pools
sympatric; when speciation occurs within the same geographic area (either behavioural or temporal)
153
give an example of behavioural speciation
when closely related individuals differ in their courtship behaviour, so are only successful in attracting members of their own population
154
give an example of temporal speciation
populations may mate or flower at different seasons or times of day
155
different populations have
different allele frequencies
156
state the 2 theories about the pace of evolutionary change
- gradualism in speciation
- punctuated equilibrium
157
describe gradualism
- the idea that species slowly change through a long sequence of continuous intermediate forms
- confronted by gaps in fossil record- this as explained as imperfections in the fossil record
158
describe punctuated equilibrium
- holds that long periods of relative stability in a species are 'punctuated' by periods of rapid, abrupt evolution
- gaps in fossil record may mot be gaps at all, as there was no sequence of intermediate forms
159
why may rapid change be much more common in prokaryotes and insects?
these organisms have short generation times
160
what is polyploidy?
a condition whereby an organism has more than two complete sets of homologous chromosomes in all cells (i.e. > diploid)
161
how does polyploidy lead to sympatric speciation?
- chromosomes duplicate in preparation for meiosis but then meiosis doesn't occur
- result is a diploid gamete that when fused with a haploid gamete produces a fertile offspring
- organism is now reproductively isolated from the original population, and can only self-pollinate or mate with other polyploid plants
162
why is polyploidy far more common in plant species than animal species?
Polyploidy is far more common in plant species which lack separate sexes and are capable of self-pollination
Polyploidy is very rare in animal species due to the consequences of having extra allele copies of every gene
163
give an example of an organism that has frequently encountered polyploidy
species of angiosperms = flowering plants such as the Allium genus
many species of Allium reproduce asexually and polyploidy may confer an advantage over diploidy under certain selection pressures
