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topic 3 and 10 Flashcards

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1
Q
A
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2
Q

define a gene

A

a heritable factor that consists of a length of DNA and influences a specific characteristics

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3
Q

where are genes located?

A

a gene occupies a specific position on one type of chromosome- this is called a locus

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4
Q

define an allele

A

alternative forms of the same gene

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5
Q

alleles have the same —— and only one allele can occupy the —–

A

locus; the locus of the gene on a chromosome

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6
Q

how do alleles differ from each other?

A

alleles differ from each other by one or a few bases only

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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

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8
Q

how are new alleles formed?

A

new alleles are formed by gene mutation, or random changes to the base sequence of DNA

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9
Q

what is a genome?

A

the whole of the genetic information of an organism

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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

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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

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12
Q

what does the genome of prokaryotes consist of?

A

the DNA in the circular chromosome, plus any plasmids that are present

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13
Q

what was done during the human genome project?

A

the entire base sequence of human genes was sequenced

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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
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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
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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

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17
Q

what types of organisms contain plasmids?

A

some prokaryotes but NOT eukaryotes

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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

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19
Q

give an example of a gene that may be in a plasmid

A

genes for antibiotic resistance

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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

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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.

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22
Q

how did John Cairns measure the length of DNA?

A

by autoradiography

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23
Q

describe eukaryotic genetic material

A

eukaryotic chromosomes are linear DNA molecules associated with histone proteins
different chromosomes carry different genes

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24
Q

how many types of chromosome are there in humans?

A

23

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25
Q

what are homologous chromosomes?

A

chromosomes that carry the same sequence of genes but not necessarily the same alleles of those genes

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26
Q

compare the genome sizes of T2 Phage, E.Coli, Drosophila melanogaster, Homo sapiens and Paris Japonica

A

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

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27
Q

describe haploid nuclei and give an example

A

have one chromosome of each pair (eg 23 in humans) human sex cells or gametes

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28
Q

describe diploid nuclei

A

have pairs of homologous chromosomes
- have two copies of each gene, apart from genes on sex chromosomes

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29
Q

give an advantage of having diploid nuclei

A

the effects of harmful recessive mutations can be avoided if a dominant allele is also present

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30
Q

explain the importance of chromosome numbers

A

the number of chromosomes is a characteristic feature of members of a species

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31
Q

compare the diploid chromosome numbers of Homo sapiens, pan troglodytes, Canis familiaris, oryza sativa, parascaris equorum

A

parascaris equorum- horse threadworm- 4
oryza sativa- rice- 24
Homo sapiens- 46
pan troglodytes- chimpanzee- 48
canis familiaris-dog- 78

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32
Q

what is sex determined by?

A

sex chromosomes

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33
Q

what are autosomes?

A

chromosomes that do not determine sex

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34
Q

describe the 2 chromosomes that determine sex in humans

A
  • 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
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35
Q

why must all humans have the X chromosome?

A

it has many genes that are essential in both males and females

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36
Q

what chromosome gene causes a foetus to develop as a male?

A

SRY or TDF- initiates the development of male features, including testes and testosterone production

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37
Q

when is the sex of a human determined and why?

A

at fertilisation. this is because when sperm are formed, half contain the X chromosome and half the Y chromosome.

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38
Q

what is a karyogram?

A

a karyogram shows the chromosomes of an organism in homologous pairs of decreasing length

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39
Q

when can we get the clearest view of chromosomes?

A

during metaphase, in cells that are in mitosis

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40
Q

how do we obtain a karyogram?

A
  • dividing cells stained and placed on microscope slide
  • burst
  • chromosomes become spread
  • micrograph taken
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41
Q

describe meiosis

A

when one diploid nucleus divides to produce 4 haploid nuclei

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42
Q

what are the 2 divisions in meiosis known as

A

meiosis I and meiosis II

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43
Q

define a diploid nucleus

A

has two chromosomes of each type

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44
Q

chromosomes of the same type are known as….

A

homologous chromosomes

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45
Q

define a haploid nucleus

A

has one chromosome of each type

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46
Q

meiosis is also known as…because…

A

a reduction division because it involves a halving of the chromosome number

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47
Q

the halving of the chromosome number allows

A

a sexual life cycle with fusion of gametes

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48
Q

what does sexual reproduction involve in eukaryotic organisms?

A

fertilisation (the fusion of gametes)

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49
Q

when in the sexual life cycle does meiosis occur?

A

any stage, but in animals it happens during the process of creating gametes

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50
Q

main events in meiosis

A

x2:
- prophase
- metaphase
- anaphase
- telophase

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51
Q

prophase I

A

-Chromosomes condense
- nuclear membrane dissolves
- homologous chromosomes form bivalents (synapsis)
- crossing over occurs

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52
Q

metaphase I

A

Spindle fibres from opposing centrioles connect to bivalents (at centromeres) and align them along the middle of the cell

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53
Q

orientation of paternal and maternal chromosomes on either side of the equator is…

A

random and independent of other homologous pairs

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54
Q

anaphase I

A
  • homologous pairs separated by spindle fibres
  • one chromosome of each pair moves to each pole
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55
Q

telophase I

A
  • chromosomes decondense
  • nuclear membrane may reform
  • cell divides (cytokinesis) to form two haploid daughter cells.
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56
Q

prophase II

A
  • Chromosomes condense
  • nuclear membrane dissolves
  • centrioles move to opposite poles
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57
Q

metaphase II

A

Spindle fibres from opposing centrioles attach to chromosomes (at centromere) and align them along the cell equator

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58
Q

anaphase II

A

centromeres separate and chromatids are moved to opposite poles

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59
Q

telophase II

A
  • chromatids reach opposite poles
  • nuclear envelope forms
  • cytokinesis occurs
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60
Q

what is meiosis preceded by?

A

interphase- DNA is replicated to produce chromosomes consisting of two sister chromatids

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61
Q

diploid human cell

A

46 chromosomes in 23 pairs

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62
Q

n

A

=23=haploid no of chromosomes

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63
Q

what three things promote genetic variation in meiosis?

A
  • random orientation
  • crossing over
  • fusion of gametes from different parents
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64
Q

random orientation

A

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

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65
Q

for every additional bivalent, the no of possible chromosome combinations in a cell produced by meiosis…

A

doubles

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66
Q

crossing over

A

in prophase I:
- new gene combinations are formed on chromatids (recombination)

67
Q

gametes fuse to form a

A

zygote

68
Q

define crossing over

A

the exchange of DNA material between non-sister homologous chromatids

69
Q

what are chiasmata?

A

connection points where non-sister chromatids continue to adhere after crossing over has occurred

70
Q

what causes Down syndrome?

A

non-disjunction- this can cause other chromosome abnormalities as well

71
Q

define non-disjunction

A

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
Q

Describe:
- Down syndrome

A
  • 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
Q

give 3 other common non-disjunction disorders

A

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
Q

State two methods used to obtain cells for karyotype analysis

A
  • 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
Q

describe the risks associated with chorionic villus sampling and amniocentesis

A

amniocentesis- risk of miscarriage is 1%
Chronic villus sampling- risk is 2%

76
Q

chiasmata formation between non-sister chromatids can result in

A

an exchange of alleles

77
Q

why does independent assortment of genes occur?

A

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
Q

bivalent =

A

tetrad, composed of 4 chromatids - the combination of homologous chromosomes when they align beside each other

79
Q

describe the process of crossing over w a diagram

A
  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
Q

give 4 ways in which meiosis I differs from mitosis and meiosis II

A
  • 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
Q

how did Mendel discover the principles of inheritance?

A

with experiments in which large numbers of pea plants were crossed

82
Q

what did Mendel discover?

A

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
Q

gametes are haploid so…

A

contain one allele of each gene, and one chromosome of each type

84
Q

what is the name of the single cell produced when two gametes fuse?

A

a zygote

85
Q

define segregation of alleles

A

the separation of alleles into different nuclei

86
Q

define a dominant allele

A

an allele that masks the effects of a recessive allele

87
Q

define co-dominant alleles

A

alleles that have joint effects

88
Q

what is the usual reason for dominance of one allele?

A

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
Q

what is a genetic disease and what is it caused by?

A

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
Q

what is a carrier of a genetic disease?

A

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
Q

give 2 causes of genetic diseases other than by recessive alleles

A
  • dominant/co-dominant alleles
  • sex-linked diseases
92
Q

give an example of a disease caused by co-dominant alleles

A

sickle cell anaemia

93
Q

define sex linkage

A

an inheritance pattern where the ratios are different in males and females

94
Q

many genetic diseases have been identified in humans but most are ———-

A

very rare

95
Q

what do radiation and mutagenic chemicals do?

A

they increase the mutation rate and can cause genetic disease and cancer

96
Q

define a mutation

A

a random change to the base sequence of a gene

97
Q

give two types of factors that will increase the mutation rate

A
  • 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
Q

what is the origin of genetic diseases?

A

mutations in body cells are eliminated when the individual dies but mutations in cells that develop into gametes can be passed onto offspring.

99
Q

blood group A

A

IAIA or IAi

100
Q

blood group B

A

IBIB or IBi

101
Q

blood group AB

A

IAIB

102
Q

blood group O

A

ii

103
Q

describe how alleles of blood group will affect red blood cells

A

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
Q

what blood group is a universal donor?

A

O (because its glycoprotein is unaltered)

105
Q

what blood group is a universal recipient?

A

AB (because it does not have any antibodies in its plasma)

106
Q

what 2 things will happen if blood of the wrong group is given to someone?

A
  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
Q

describe cystic fibrosis

A
  • 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
Q

describe Huntington’s disease

A
  • 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
Q

describe red-green colour blindness

A
  • 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
Q

describe haemophilia

A
  • inability to make Factor VIII, one of the proteins involved in the clotting of blood
  • recessive allele located on the X chromosome
111
Q

give an example of the consequences of nuclear bombing and accidents at nuclear power stations

A
  • hiroshima and nagaasaki
  • nuclear accidents at Three Mile island and Chernobyl
112
Q

unlinked genes ——- ———- as a result of meiosis

A

segregate independently

113
Q

what is segregation?

A

the separation of the two alleles of every gene that occurs during meiosis

114
Q

what is independent assortment?

A

the alleles of one gene segregate independently of the alleles of other genes

115
Q

genes found on different chromosomes are

genes which are on the same chromosome are

the exception is

A

unlinked and segregate independently

linked and do not segregate independently

linked genes that are far apart on the chromosome (due to crossing over)

116
Q

look at how to form a punnet square for dihybrid traits

A

p446

117
Q

describe the difference between a monohybrid and a dihybrid trait

A

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
Q

when are gene loci said to be linked?

A

if they are on the same chromosome and hence don’t independently assort (unless synapsis occurs)

119
Q

define a locus

A

a locus is a specific, fixed position on a chromosome where a particular gene or genetic marker is located.

120
Q

define homologous chromosomes

A

two chromosomes that have the same sequence of genes.

121
Q

why are homologous chromosomes not usually identical to each other?

A

for at least some of the genes on them, the alleles will be different

122
Q

what is the name of non-sex chromosomes?

A

autosomes- these are common to both males and females

123
Q

what are the two types of linkage?

A

autosomal gene linkage, when the genes are on the same autosome
sex linkage, when the genes are located on the X chromosome

124
Q

variation can either be

A

discrete - no in-between categories
continuous - eg height

125
Q

discrete variation

A

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
Q

continuous variation

A

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
Q

describe how the phenotypes of polygenic characteristics tend to show continuous variation

A

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
Q

monohybrid inheritance =

A

linked genes = two potential gamete combinations

129
Q

dihybrid inheritance =

A

unlinked genes = 4 potential gamete combinations

130
Q

why do offspring with unlinked genes have an equal possibility of inheriting any potential phenotypic combination?

A

due to the random segregation of alleles via independent assortment

131
Q

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

A

crossing over occurs

less frequently

132
Q

revise the use of chi-squared tests

A
133
Q

Null hypothesis (H0):
Alternative hypothesis (H1):

A

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
Q

Morgan’s discovery of non-Mendelian ratios in Drosophila.

A

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
Q

Polygenic traits such as human height may also be influenced by

A

environmental factors, such as nutrition, disease, activity

136
Q

How are linked genes often shown?

A

As vertical pairs

137
Q

The frequency of recombinant phenotypes within a population will typically be —– than that of non-recombinant phenotypes; why?

A

lower

crossing over is a random process and chiasmata do not form at the same locations with every meiotic division

138
Q

The relative frequency of recombinant phenotypes will be dependent on; why?

A

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
Q

how can recombinant phenotypes be identified?

A

performing a test cross (crossing with a homozygous recessive for both traits); LOOK AT THIS

140
Q

what does a gene pool consist of?

A

all the genes and their different alleles, present in an interbreeding population

141
Q

how does a gene pool fit into the definition for a species?

A

species are groups of potentially interbreeding populations, with a common gene pool that is reproductively isolated from other species.

142
Q

how would it be possible for multiple gene pools to exist for the same species?

A

if some populations are geographically isolated

143
Q

what does evolution require?

A

that allele frequency’s change with time in populations

144
Q

define evolution

A

the cumulative change in the heritable characteristics of a population over time

145
Q

give 4 reasons for evolution

A
  • 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
Q

give 3 types of natural selection

A
  • directional
  • stabilising
  • disruptive
147
Q

describe stabilising selection

A

selection pressures act to remove extreme varieties
- eg average birth weights of human babies are favoured over low or high birth weight

148
Q

describe disruptive selection

A

selection pressures act to remove intermediate varieties, favouring the extremes

149
Q

describe directional selection

A

the population changes as one extreme of a range of variation is better adapted.

150
Q

state the 3 categories of reproductive isolation

A
  • temporal
  • behavioural
  • geographic
151
Q

define speciation

A

the formation of a new species by the splitting of an existing population

152
Q

state the difference between allopatric and sympatric speciation

A

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
Q

give an example of behavioural speciation

A

when closely related individuals differ in their courtship behaviour, so are only successful in attracting members of their own population

154
Q

give an example of temporal speciation

A

populations may mate or flower at different seasons or times of day

155
Q

different populations have

A

different allele frequencies

156
Q

state the 2 theories about the pace of evolutionary change

A
  • gradualism in speciation
  • punctuated equilibrium
157
Q

describe gradualism

A
  • 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
Q

describe punctuated equilibrium

A
  • 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
Q

why may rapid change be much more common in prokaryotes and insects?

A

these organisms have short generation times

160
Q

what is polyploidy?

A

a condition whereby an organism has more than two complete sets of homologous chromosomes in all cells (i.e. > diploid)

161
Q

how does polyploidy lead to sympatric speciation?

A
  • 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
Q

why is polyploidy far more common in plant species than animal species?

A

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
Q

give an example of an organism that has frequently encountered polyploidy

A

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

IB biology (60 decks)

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