1
Q
What is sexual reproduction?
A
• Type of reproduction.
• Involves the production of gametes by meiosis.
• A gamete from each parent fuses to form a zygote.
• Genetic information from each gamete is mixed so the resulting zygote is unique.
2
Q
What are gamets?
A
• Sex cells (sperm cells and egg cells in animals, pollen and egg cells in flowering plants).
• Haploid (half the number of chromosomes).
3
Q
What is meiosis?
A
• Form of cell division involved in the formation of gametes (non-identical haploid cells) in reproductive organs.
• Chromosome number is halved.
• Involves two divisions.
4
Q
What must happen before meiosis?
A
Interphase - copies of genetic information are made during this process.
5
Q
What happens during the first stage of meiosis?
A
• Chromosome pairs line up along the cell equator.
• The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation).
• Chromosome number is halved.
6
Q
What’s happens during the second stage of meiosis?
A
• Chromosomes line up along the cell equator.
• The chromatids are separated and move to opposite poles of the cell.
• Four unique haploid gametes are produced.
7
Q
Why is meiosis important for sexual reproduction?
A
• It increases genetic variation.
• It ensures that the zygote formed at fertilisation is diploid.
8
Q
Describe fertilisation and its resulting outcome
A
Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells).
As the embryo develops, cells differentiate.
9
Q
What’s the advantage of sexual reproduction ?
A
It creates genetic variation in offspring, increasing the probability of a species adapting to and surviving environmental changes.
Natural selection can be speeded up by humans in selective breeding to increase food production.
10
Q
What are the disadvantages of sexual reproduction?
A
• Two parents are required. This makes reproduction difficult in endangered populations or in species which exhibit solitary lifestyles.
• More time and energy is required so fewer offspring are produced.
11
Q
What is asexual reproduction?
A
• Type of reproduction.
• Involves mitosis only.
• Produces genetically identical offspring known as daughter cells.
12
Q
What are the advantages of asexual reproduction?
A
• Only one parent is required.
• Lots of offspring can be produced in a short period of time, enabling the rapid colonisation of an area and reducing competition from other species.
• Requires less energy and time as do not need a mate.
13
Q
What are the disadvantages of asexual reproduction?
A
No genetic variation (except from spontaneous mutations) reducing the probability of a species being able to adapt to environmental change.
14
Q
Describe the circumstances in which malarial parasites reproduce sexually and asexually
A
Sexual reproduction in the mosquito.
Asexual reproduction in the human host.
15
Q
Describe the circumstances in which fungi reproduce sexually and asexually
A
Asexual reproduction by spores.
Sexual reproduction to give variation.
16
Q
Describe the circumstances in which plants reproduce sexually and asexually
A
Sexual reproduction to produce seeds.
Asexual reproduction by runners (e.g. strawberry plants) or bulb division (e.g. daffodils).
17
Q
What is DNA?
A
A double-stranded polymer of nucleotides, wound to form a double helix.
The genetic material of the cell found in its nucleus.
18
Q
Define genome
A
The entire genetic material of an organism.
19
Q
Why is understanding the human genome important?
A
The whole human genome has been studied and is important for the development of medicine in the future.
• Searching for genes linked to different types of disease.
• Understanding and treating inherited disorders.
• Tracing human migration patterns from the past.
20
Q
What is a chromosome?
A
A long, coiled molecule of DNA that carries genetic information in the form of genes.
21
Q
How many chromosomes do human body cells have?
A
46 chromosomes
23 pairs of chromosomes
22
Q
How many chromosomes do human gamers have?
A
23 chromosomes
23
Q
Define a gene
A
A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein.
24
Q
What are the monomers of DNA?
A
Nucleotides
25
What are the nucleotides made up of?
• Common sugar
• Phosphate group
• One of four bases: A, T, C or G
26
State the full names of the four basses found in nucleotides?
• Adenine
• Thymine
• Cytosine
• Guanine
27
Describe how nucleotides interact to form a molecule of DNA?
• Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand.
• Base connected to each sugar.
• Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds.
28
Explain how a gene codes for a protein
• A sequence of three bases in a gene forms a triplet.
• Each triplet codes for an amino acid.
• The order of amino acids determines the structure (i.e. how it will fold) and function of protein formed.
29
Why is the folding of amino acids important in proteins such as enzymes?
The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate.
30
What is protein synthesis?
The formation of a protein from a gene.
31
What is protein synthesis?
The formation of a protein from a gene.
32
What are the two satges of protein synthesis?
1. Transcription
2. Translation
33
What does transcription involve?
The formation of mRNA from a DNA template.
34
Outline transcription
1. DNA double helix unwinds.
2. RNA polymerase binds to a specific base sequence of non-coding
DNA in front of a gene and moves along the DNA strand.
3. RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand.
4. mRNA formation complete. mRNA detaches and leaves the nucleus.
35
What does translation evolve ?
A ribosome joins amino acids in a specific order dictated by mRNA to form a protein.
36
Outline translation
1. mRNA attaches to a ribosome.
2. Ribosome reads the mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule (carrier molecule).
3. A polypeptide chain is formed from the sequence of amino acids which join together.
37
What is mutation?
A random change in the base sequence of DNA which results mostly in no change to the protein coded for, or genetic variants of the protein (slight alteration but appearance and function remain).
Mutations occur continuously.
38
Describe the effect of a gene mutating in coding DNA?
• If a mutation changes the amino acid sequence, protein structure and function may change (an enzyme may no longer fit its substrate binding site or a structural protein may lose its strength).
• If a mutation does not change amino acid sequence, there is no effect on protein structure or function.
39
What is non-coding DNA?
DNA which does not code for a protein but instead controls gene expression.
40
Describe the effect of a gene mutation in non-coding DNA
Gene expression may be altered, affecting protein production and the resulting phenotype.
41
What are alleles?
Different versions of the gene
42
What is a dominant allele?
A version of a gene where only one copy is needed for it to be expressed.
43
What is a recessive allele?
A version of a gene where two copies are needed for it to be expressed.
44
What is meant when an organisms is homozygous?
When an organism has two copies of the same allele (two recessive or two dominant).
45
What is meant when an organisms is heterozygous?
When an organism has two different versions of the same gene (one dominant and one recessive).
46
What is the genotype?
The genes present for a trait
47
What is a phenotype?
The visible characteristics
48
How are dominant allleles represented in a punnet square?
They are represented using uppercase letters
49
How are recessive allleles represented in a punnet square?
They use the lowercase version of the same letter as the dominant allele.
50
Draw a Punnett square for a cross between a homozygous recessive blue eyed female (bb) with a heterozygous brown eyed male (Bb)
B b 50% brown and 50%blue
b Bb Bb
b Bb bb
51
Draw a Punnett square for a cross between a homozygous dominant red flower (RR) with a homozygous recessive white flower (rr)
R. R 100% red flowers
r. Rr. Rr
r. Rr. Rr
52
How are recessive allleles represented in a punnet square?
They use the lowercase version of the same letter as the dominant allele.
53
PKU is a recessive condition. Two heterozygous parents (Pp) have offspring. Using a punnett square, predict the proportion of offspring that will have PKU.
75% chance of normal phenotype
25% chance of PKU phenotype
F E M A L E
M P p
A P PP Pp
L P PP. pp
E
54
What is the problem with single gene crosses?
Most characteristics are controlled by multiple alleles rather than just one.
55
What is an inherited disorder?
A disorder caused by the inheritance of certain alleles.
56
Give 2 examples of inherited disorders
• Polydactyly (having extra fingers or toes) - caused by a dominant allele.
• Cystic fibrosis (a disorder of cell membranes) - caused by a recessive allele.
57
How are embryos screened for inherited disorders?
During IVF, one cell is removed (from an 8 cell embryo) and tested for disorder-causing alleles. If the cell doesn't have any indicator alleles, then the originating embryo is implanted into the uterus.
58
What are the ethical issues concerning embryo screening?
• It could lead to beliefs in society that being disabled or having a disorder is less human or associated with inferiority.
• The destruction of embryos with inherited disorders is seen by some as murder as these would go on to become human beings.
• It could be viewed as part of the concept of designer babies as it may be for the parents convenience or wishes rather than the child's wellbeing.
59
What are the economic issues concerning embryo screening?
• Costs of hospital treatment and medication will need to be considered if it is known that a child will have an inherited disorder and financial support explored if necessary.
60
What are the social issues concerning embryo screening?
• Social care for children with inherited disorders may need to be considered if parents are unable to provide care.
• If an embryo is found to have an inherited disorder and is terminated, this can prevent a child and its parents from potential suffering in the future due to the disorder.
61
What is gene therapy?
The insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele.
62
What are the ethical issues concerning gene therapy?
• Some people believe that it is going against and 'playing God'.
• The introduced genes could enter sex cells and so be passed to future generations.
63
What are sex chromosomes?
A pair of chromosomes that determine sex:
• Males have an X and a Y chromosome
• Females have two X chromosomes
64
What are the ethical issues concerning gene therapy?
• Some people believe that it is going against and 'playing God'.
• The introduced genes could enter sex cells and so be passed to future generations.
65
Why does the inheritance of a Y chromosome mean that an embryo develops into a male?
Testes development in an embryo is stimulated by a gene present on the Y chromosome.
66
A couple have a child. Using a punnett square, determine the probability of having offspring that is female.
F E M A L E 50% chance of fem-
X. X Ale
M. X XX. XX
A
L Y XY. XY
E
67
What is a sex-linked characteristic?
A characteristic that is coded for by an allele found on a sex chromosome.
68
Why are the majority of genes found on the X chromosome rather than the Y chromosome?
The X chromosome is bigger than the Y chromosome so more genes are carried on it.
69
Why are men more likely to show the phenotype for a recessive sex-linked trait than women?
• Many genes are found on the X chromosome that have no counterpart on the Y chromosome.
• Women (XX) have two alleles for each sex-linked gene whereas men
(XY) often only have one allele .. only one recessive allele is required to produce the recessive phenotype in males.
70
Haemophilia is a recessive X-linked condition. A carrier female and a normal male have a son. What is the probability of the child having haemophilia?
50% chance of haemophilia (X"Y)
Parents Female. Male
Genotype XHXh. XHY
Gametes XH. Xh. XH. Y
Genotype
Offspring’s XHXH. XHXh. XHY. XhY
Genotype
71
What is variation?
Differences in the characteristics of individuals in a population is called variation.
72
What are the two causes of variation within a species?
• Genetics
• Environment
• A mixture of both of the above
73
What is genetic information?
• Variations in the genotypes of organisms of the same species due to the presence of different alleles.
• Creates differences in phenotypes.
74
What creates genetic variation in a species?
• Spontaneous mutations
• Sexual reproduction
75
What is a mutation?
A random change to the base sequence in DNA which results in genetic variants.
They occur continuously.
76
What are the three types of gene mutations?
• Insertion
• Deletion
• Substitution
77
How may a gene mutation affect an organisms phenotype?
• Neutral mutation does not change the sequence of amino acids.
Protein structure and function same. No effect on phenotype.
• Mutation may cause a minor change in an organism's phenotype e.g change in eye colour.
• Mutation may completely change the sequence of amino acids. This may result in a non-functional protein. Severe changes to phenotype.
78
What is the consequence of a new phenotype caused by a mutation being suited to an environmental change?
There will be a rapid change in the species
79
What is evolution?
• A gradual change in the inherited traits within a population over time.
• Occurs due to natural selection which may result in the formation of a new species.
80
Outline the theory of natural selection
All species of living things have evolved from simple life forms that first developed more than 3 billion years ago.
1. Genetic variation exists due to spontaneous mutations.
2. Selection pressures (e.g. competition, disease) exist.
3. Random mutation gives an organism a selective advantage.
4. Organism is better adapted to the environment and survives.
5. Organism reproduces, passing on its beneficial alleles.
6. Frequency of advantageous alleles increase.
81
How do two populations become different species?
When their phenotypes become different to the extent that they can no longer interbreed to produce fertile offspring.
82
What is selective breeding?
The process by which humans artificially select organisms with desirable characteristics and breed them to produce offspring with similar phenotypes.
82
How do two populations become different species?
When their phenotypes become different to the extent that they can no longer interbreed to produce fertile offspring.
83
Outline the main steps involved in selective breeding
1. Identify a desired characteristic e.g. disease resistance.
2. Select parent organisms that show the desired traits and breed them together.
3. Select offspring with the desired traits and breed them together.
4. Process repeated until all offspring have the desired traits.
84
Give examples of characteristics selected for it selective breeding
• Disease resistance in crops
• Higher milk or meat production in animals
• Gentle nature in domestic dogs
• Large flowers
85
What is the main advantage of selecting breeding?
Creates organisms with desirable features:
• Crops produce a higher yield of grain
• Cows produce a greater supply of milk
• Plants produce larger fruit
• Domesticated animals
86
Other than in agriculture where else is selective breeding useful?
• In medical research
• In sports e.g. horse racing
87
Outline the disadvantages of selective breeding?
• Reduction in the gene pool (which becomes especially harmful if sudden environmental change occurs).
• Inbreeding results in genetic disorders.
Development of other physical problems eg respiratory problems in bulldogs.
• Potential to unknowingly select harmful recessive alleles.
88
89
What is genetic engineering?
• The modification of the genome of an organism by the insertion of a desired gene from another organism - genes from chromosomes of humans and other organisms can be 'cut out' and transferred to cells of other organisms.
• Enables the formation of an organism with beneficial characteristics.
90
Give an example of uses for genetically modified plants?
.disease resistance
.produce larger fruits
91
What is a use for genetically modified modified bacteria cells?
To produce human insulin to treat diabetes mellitus.
92
Describe the benefits of genetic engineering
• Increased crop yields for growing population e.g. herbicide-resistance, disease-resistance.
• Useful in medicine e.g. insulin-producing bacteria, anti-thrombin in goat milk, possibility to overcome some inherited disorders (being explored in medical research).
• GM crops produce scarce resources e.g. GM golden rice produces beta-carotene (source of vitamin A in the body).
93
Describe the risks of genetic engineering
• Long-term effects of consumption of GM crops unknown.
• Negative environmental impacts e.g. reduction in biodiversity, impact on food chain, contamination of non-GM crops forming 'superweeds'.
• Late-onset health problems in GM animals.
• GM seeds are expensive. LEDCs may be unable to afford them or may become dependent on businesses that sell them.
94
What is the name for crops that have had their genes modified?
Genetically modified (GM) crops e.g. those modified to be resistant to insect attack and herbicides.
95
What is Bacillus thuringiensis (Bt)?
• Insect larvae are harmful to crops.
• Bt is a bacterium which secretes a toxin that kills insect larvae.
96
How is genetic engineering used to protect crops against insects?
• The gene for toxin production in Bt can be isolated and inserted into the DNA of crops.
• Bt crops now secrete the toxin which kills any insect larvae that feed on it.
97
What are the benefits of Bt crops?
• Increased crop yields (fewer crops damaged).
• Lessens the need for artificial insecticides.
• Bt toxin is specific to certain insect larvae so is not harmful to other organisms that ingest it.
98
What are the risks of Bt crops?
• Long term effects of consumption of Bt crops unknown.
• Insect larvae may become resistant to the Bt toxin.
• Killing insect larvae reduces biodiversity.
99
Describe the process of genetic engineering
1. DNA is cut at specific base sequences by restriction enzymes to create sticky ends.
2. Vector DNA cut using the same restriction enzymes to create complementary sticky ends.
3. Ligase enzymes join the sticky ends of the DNA and vector DNA
forming recombinant DNA.
4. Recombinant DNA mixed with and 'taken up' by target cells.
100
What is a vector?
A structure that delivers the desired gene into the recipient cell e.g. plasmids, viruses.
101
How can plants be cloned?
• Taking plant cuttings
• Tissue culture
102
What is tissue culture?
Using small groups of cells from part of a plant to grow identical new plants.
103
Describe how plants are grown using tissue culture
1. Select a plant that shows desired characteristics.
2. Cut multiple small sample pieces from meristem tissue.
3. Grow in a petri dish containing growth medium.
4. Transfer to compost for further growth.
104
What must be ensured when preparing tissue cultures?
Ensure aseptic conditions to prevent contamination by microorganisms.
105
What does the growth medium contain?
Nutrients and growth hormones
106
What are the advantages of growing plants by tissue culture?
• Fast and simple process.
• Requires little space.
• Enables the growth of many plant clones with the same desirable characteristics.
• Useful in the preservation of endangered plant species.
107
What are the disadvantages of growing plants by tissue culture?
• Reduction in the gene pool.
• Plant clones often have a low survival rate.
• Could unknowingly increase the presence of harmful recessive alleles.
108
Describe the plant cutting methods of plant cloning
Older but simpler method than tissue culture.
Gardeners use this method to produce many identical new plants from a parent plant.
109
Detail the process of the plant cuttings method of plant cloning
1. A branch is cut off from the parent plant.
2. The lower leaves of the branch are removed and the stem is planted.
3. Plant hormones are used to encourage new root development.
4. A plastic bag is used to cover the new plant to keep it warm and moist.
5. New roots and a new plant is formed after a few weeks.
110
What does embryo transplanting involve?
• Pre-specialised cells from a developing animal embryo are split apart.
• The resulting separate but identical embryos are transplanted into host mothers.
111
Describe how a dual cell cloning is performed
1. The nucleus is removed from an unfertilised egg cell.
2. The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.
3. An electric shock stimulates the nucleated egg cell to divide and it forms an embryo.
4. The embryo cells contain the same genetic information as the adult body cell.
5. When the embryo is a ball of cells, it is inserted into the uterus of an adult female to continue developing.
112
Outline the theory of evolution by natural selection
Individuals of a species show a wide range of variation for a characteristic.
Those with the characteristic most suited to the environment will survive breed most successfully.
The desirable characteristic that has enabled the individuals to survive are passed onto their offspring.
113
Why was Darwin's theory of evolution not accepted initially?
• Most people believed in creationism
• Insufficient evidence to prove the theory
• The mechanism of variation and inheritance was not known at the time
114
What was Jean-Baptiste Lamarck's theory of inheritance.
That changes during the lifetime of an organism can be inherited.
115
What’s the definition of a species?
A group of organisms with similar characteristics which are able to interbreed to produce fertile offspring.
115
What is speciation?
The formation of a new species, when two populations become so varied that they cannot interbreed to produce fertile offspring.
116
Outline the process of speciation? Through geographic isolation?
• Two populations of the same species are separated geographically
• Geographic isolation prevents interbreeding and mixing of genes between the populations.
• Due to different selection pressures, different mutations occur producing different phenotypes in each population.
• Over time, the two populations may evolve so that they are not able to interbreed.
117
Why is genetic variation important in speciation?
Genetic variation produces phenotypic variation, some of which are better suited to the environment and are selected for.
118
How did Mendel study inheritance?
Through carry breeding experiments on plants and analysing the ratio of characteristics in offspring.
119
Why was Mendel's work not recognised until after his death?
He could not explain the mechanism of inheritance, as chromosomes were only discovered after his death.
It was not communicated well to other scientists and not published in a reputable scientific journal.
120
State two kinds of evidence used to show evolution.
• Fossils
• Antibiotic resistance in bacteria
121
Why was Mendel's work not recognised until after his death?
He could not explain the mechanism of inheritance, as chromosomes were only discovered after his death.
It was not communicated well to other scientists and not published in a reputable scientific journal.
122
How are fossils formed?
• Parts of organisms that have not decayed due to conditions needed for decay being absent.
• Parts of organisms that have been replaced by minerals as they decayed eg. bones.
• Traces of organisms are preserved, covered in sediment and becoming rock.
123
Why are there few traces of early life-forms left behind?
They are mostly soft-bodied
124
How do fossils act as evidence for evolution?
Scientists can identify the ages of the fossils and use them to show how organisms change over time.
125
What do branches in evolutionary trees indicate?
Where speciation has occurred.
126
What is extinction?
Where there are no individuals of a species still alive
127
State the factors that may lead to exinctions
• New disease
• Predation
• Competition
• Changes to the environment
• Catastrophic events
128
What enables bacteria to evolve quickly?
The fast rate of their reproduction.
129
Outline the process of antibiotic resistance bacteria evolving.
• Mutations occur in bacteria producing genetic variation.
• Certain strains are resistant to antibiotics and are not killed when the antibiotic is applied
• Resistant strains survive and reproduce.
• Over time, the population of the resistant strains increase.
129
Why are resistant strains of bacteria dangerous?
People have no immunity to them and there is no effective treatment.
130
What can be done to reduce the rate of development of antibiotic resistant bacteria?
• Refrain from inappropriately prescribing antibiotics eg. for viral diseases.
• Patients should complete the prescribed course of antibiotics.
• Restrict agricultural uses of antibiotics.
131
Why is it difficult to keep up with emerging resistance strains?
Developing antibiotics have a high cost and take a long time to develop.
132
What can be done to reduce the rate of development of antibiotic resistant bacteria?
• Refrain from inappropriately prescribing antibiotics eg. for viral diseases.
• Patients should complete the prescribed course of antibiotics.
• Restrict agricultural uses of antibiotics.
133
What are the classes of organisms as determined by Carl Linnaeus?
Kingdom, phylum, class, order, family, genus, species.
134
Which features are living creatures traditionally classified by?
By their structure and characteristics.
135
What is the binomial system of naming organisms?
Genus name followed by species name.
136
Why were new classification models proposed?
• Developments in microscopy allowed better examination of internal structures.
• Improvement in understanding of biochemical processes.
137
State the three domains.
• Archaea
• Eukarya
• Bacteria
138
Which organisms belong in the domain Archaea?
Bacteria, usually living in extreme environments.
139
Which organisms belong in the domain Bacteria?
Bacteria
140
Which kingdoms belong in domain Eukarya?
• Plants
• Animals
• Fungi
• Protists
141
How are evolutionary trees created?
By examining the DNA of different species and analysing how similar the sequences are.
Biology
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