Inheritance, Variation and Evolution Flashcards
What is sexual reproduction?
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
What are gametes?
Sex cells (sperm cells and egg cells in animals, pollen and egg cells in flowering plants).
Haploid (half the number of chromosomes)
What is meiosis?
Form of cell division involved in the formation of gametes (non-identical haploid cells) in reproductive organs
Chromosome number is halved
Involves two divisions
What must occur prior to meiosis?
Interphase - copies of genetic information are made during this process
What happens during the fist stage of meiosis?
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 plled is random, creating variation)
Chromosome number is halved
What happens during the second stage of meiosis?
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
Why is meiosis important for sexual reproduction? (2)
It increasaes genetic variation
It ensures that the zygote formed at fertilisation is diploid
Descrive fertilisation and its resulting outcome
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 embtyo develops, cells differentiate
What is the advantage of secual reproduction?
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
Describe the disadvantages of sexual reproduction (2)
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
What is asexual reproduction?
Type of reproduction
Involves mitosis only
Produces genetically identical offspring known as daughter cells
Describe the advantages of asexual reproduction (3)
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
What is the disadvantage of asexual reproduction?
No genetic variation (except from spontaneous mutations) reducting the probability of a species being able to adapt to environmental change
Describe the circumastances in which malarial parasites reproduce sexually and asexually
Sexual reproduction in the mosquito
Asexual reproduction in the human host
Describe the circumstances in which fungi reproduce sexually and asexually
asexual reproduction by spores
Sexual reproduction to give variation
Describe the circumstances in which plants reproduce sexually and asexually
Sexual reproduction to produce seeds.
Asexual reproduction by runners (e.g. strawberry plants) or bulb division (e.g. daffodils)
What is DNA?
A double-stranded polymer of nucleotides, wound to form a double helix.
The genetic material of the cell found in its nucleus
Define genome
The entire genetic material of an organism
Why is understanding the human genome important?
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
What is a chromosome?
A long, coiled molecule of DNA that carries genetic information in the form of genes
How many chromosomes do human body cells have?
46 chromosomes (23 pairs)
How many chromosomes do human gametes have?
23 chromosomes
Define gene
A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
What are the monomers of DNA?
Nucleotides
What are DNA nucleotides made up of?
Common sugar
Phosphate group
One of four bases: A, T, C or G
State the full names of the four bases found in nucleotides
Adenine
Thymine
Cytosine
Guanine
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
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) nad function of protein formed
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
What is protein synthesis?
The formation of a protein from a gene
What are the two stages of protein synthesis?
- Transcription
- Translation
What does transcription involve?
The formation of mRNA from a DNA template
Outline transcription
- DNA double helix unwinds
- RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand
- RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand
- mRNA formation complete. mRNA detaches and leaves the nucleus
What does translation involve?
A ribosome joins amino acids in a specific order dictated by mRNA to form a protein
Outline translation
- mRNA attaches to a ribosome
- 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)
- A polypeptide chain is formed from the sequence of amino acids which join together
Wha is a 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
Describe the effect of a gene mutation 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
What is non-coding DNA?
DNA which does not code for a protein but instead controls gene expression
Describe the effect of a gene mutation in non-coding DNA
Gene expression may be altered, affecting protein production and the resulting phenotype
What are alleles?
Different versions of the same gene
What is a dominant allele?
A version of a gene where only one copy is needed for it to be expressed
What is a recessive allele?
A version of a gene where two copies are needed for it to be expressed
What is meant when an organism is homozygous?
When an organism has two copes of the same allele (two recessive or two dominant)
What is meant when an organism is heterozygous?
When an organism has two different versions of the same gene (one dominant and one recessive)
What is the genotype?
The genes present for a trait
What is the phenotype?
The visible characteristic
How are dominant alleles represented in a punnett square?
They are represented using uppercase letters
How are recessive alleles represented in a punnett square?
They use the lowercase version of the same letter as the dominant allele
Draw a punnett square for a cross between a homozygous recessive blue eyed female (bb) with a heterozygous brown eyed male (Bb)
. B b
b Bb bb
b Bb bb
50% brown eyes Bb
50% blue eyes bb
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
What is the problem with single gene crosses?
Most characteristics are controlled by multiple alleles rather than just one
What is an inherited disorder?
A disorder caused by the inheritance of certain alleles
Give 2 examples of inherited disorders
polydactyly (having extra fingerse or toes) - caused by a dominant allele.
Cystic fibrosis (a disorder of cell membranes) - caused by a recessive allele
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 celldoesn’t have any indicator alleles, then the originating embryo is implanted into the uterus
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 ingeriority.
The desctruction 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
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.
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
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
What are the ethical issues concerning gene therapy?
Some people beliebe that it is going against and ‘playing god’
The introduced genes could enter sex cells and so be passed to future generations
What are sex chromosomes?
A pair of chromsomes that determine sex:
Males have an X and a Y chromosome
Females have two X chromosomes
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
A couple have a child. Using a punnett square, determine the probability of having offspring that is female
50% chance of female (XX)
What is a sex-linked characteristic?
A characteristic that is coded for by an allele found on a sex chromosome
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
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
Haemophilia is a recessive X-linked condition. A carrier gemale and a normal male have a son. What is the probability of the child having haemophilia?
50% chance of haemophilia (X^h Y)
What is variation?
Differences in the characteristics of individuals in a population is called variation
What are the two causes of variation within a species?
Genetics
Environment
A mixture of both of the above
What is genetic variation?
Variations in the genotypes of organisms of the same species due to the presence of different alleles
Creates differences in phenotypes
What creates genetic variation in a species
Spontaneous mutatioin
Sexual reproduction
What is a mutation?
A random change to the base sequence iin DNA which results in genetic variants. They occur continuously
State the three types of gene mutation
insertion
deletion
substitution
How may a gene mutation affect an organism’s phenotype? (3)
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
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
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
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
2. Selection pressures (e.g. competition, disease) exist
3. random mutation gives an rganism 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
How do populations become different species?
When their phenotypes become different to the extent that they can no longer interbreed to produce fertile offspring
What is selective breeding?
The process by which humans artificially select organisms with desirable characteristics and breed them to produce offspring with similar phenotypes
Outline the main steps involved in selective breeding
- Identify a desired characteristic e.g. disease resistance
- Select parent organisms that show the desired traits and breed them together
- select offspring with the desired traits and breed them together
- Process repeated until all offspring have the desired traits
Give examples of characteristics selected for in seelctive breeding
disease resistance in crops
higher milk or meat production in animals
gentle nature in domestic dogs
large flowers
What is the main advantage of selective breeding?
Creates organisms with desirable features:
crops produce a higher yield of grain
cows produce a greater supply of milk
plants roduce larger fruit
domesticated animals
Other than in agriculture, where else is selective breeding useful?
In medical research
In sports e.g. horse racing
Outline the disadvantage of selective breeding (4)
reduction in the gene pool (which becomes especially harmful if sudden environmental change occurs)
Inbreeding results in genetic disorders
Development of other physical problems e.g. respiratory problems in bulldogs
Potential to unknowingly select harmful recessive alleles
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
Enable the formation of an organism with beneficial characteristics
Give an example of uses for genetically modified plants
Disease resistance
Produce larger fruits
What is a use for genetically modified bacteria cells?
To produce human insulin to treat diabetes mellitus
Describe benefits of genetic engineering (3)
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)
Describe the risks of genetic engineering (4)
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 dependant on business that sell them
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
What is Bacillus thuringiensis (Bt)?
Insect larvae are harmful to crops
Bt is a bacterium which secretes a toxin that kills insect larvae
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
What are the benefits of Bt crops? (3)
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
What are the risks of Bt crops? (3)
Long term effects of consumption of Bt crops unknown
Insect larvae may become resistant to the Bt toxin
Killing insect larvae reduces biodiversity
Describe the process of genetic engineering
- DNA is cut at specific base sequences by restriction enzymes to create sticky ends.
- Vector DNA cut using the same restriction enzymes to create complementary sticky ends
- Ligase enzymes join the sticky ends of the DNA and vector DNA forming recombinant DNA
- Recombinant DNA mixed with and ‘taken up’ by target cells
What is a vector?
A structure that delivers the desired gene into the recipient cell e.g. plasmids, viruses
How can plants be cloned? (2)
Taking plant cuttings
Tissue culture
What is tissue culture?
Using small groups of cells from a part of a plant to grow identical new plants
Describe how plants are grown using tissue culture
- Select a plant that shows desired characteristics.
- Cut multiple small sample pieces from meristem tissue.
- Grow in a petri dish containing growth medium
- Transfer to compost for further growth
What must be ensured when preparing tissue cultures?
Ensure aseptic conditions to prevent contamination by microorganisms
What does the growth medium contain?
Nutrients and growth hormones
What are the advantages of growing plants by tissue culture? (4)
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
What are the disadvantages of growing plants by tissue culture? (2)
Reduction in the gene pool
Plant clones often have a low survival rate
Could unknowingly increase the presence of harmful recessive alleles
Describe the plant cuttings method of plant cloning
older but simpler method than tissue culture
Gardeners use this method to produce many identical new plants from a parent plant
Detail the process of the plant cuttings method of plant cloning
- A branch is cut off from the parent plant
- The lower leaves of the branch are removed and the stem is planted
- Plant hormones are used to encourage new root development.
- A plastic bag is used to cover the new plant to keep it warm and moist.
- New roots and a new plant is formed after a few weeks
What does embryo transplanting involve?
Pre-specialised cells from a developing animal embryo are split apart.
The resultin separate but identical embryos are transplanted into host mothers
Describe how adult cell cloning is performed
- The nucleus is removed from an unfertilised egg cell
- The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell
- An electric shock stimulates the nucleated egg cell to divide and it forms an embryo
- he embryo cells contain the same genetic information as the adult body cell
- When the embryo is a ball of cells, it is inserted into the uterus of an adult female to continue developing
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
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
What was Jean-Baptiste Lamarck’s theory of inheritance
That changes during the lifetime of an organism can be inherited
What is speciation?
The formation of a new species, when two populations become so varied that they cannot interbreed to produce fertile offspring
What is the definition of a species?
A group of organisms with similar characteristics which are able to interbreed to produce fertile offspring
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
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
How did Mendel study inheritance?
Through carry breeding experiments on plants and analysing the ratio of characteristics in offspring
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
State two kinds of evidence used to show evolution
fossils
antibiotic resistance in bacteria
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
Why are there few traces of early life-forms left behind?
They are mostly soft-bodied
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
What do branches in evolutionary trees indicate?
Where speciation has occured
What is extinction?
Where there are no individuals of a species still alive?
State the factors that may lead to extinctions
new disease
predation
competition
changes to the environment
catastrophic events
What enables bacteria to evolve quickly?
The fast rate of their reproduction
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
Wg=hy are resistant strains of bacteria dangerous?
People have no immunity to them and there is no effective treatment
State an example of a resistant strain of bacteria
MRSA
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
Why is it difficult to keep up with emerging resistance strains?
Developing antibiotics have a high cost and take a long time to develop
What are the classes of organisms as determined by Carl Linnaeus?
Kingdom, phylum, class, order, family, genus, species
Which features are living creatures traditionally classified by?
By their structure and characteristics
What is the binomial system of naming organisms?
Genus name followed by species name
Why were new classification models proposed?
Developments in microscoy allows better examination of internal structures.
Improvement in understanding of biochemical processes
State the three domains
archaea
Eukarya
Bacteria
Which organisms belong in the domain Archaea?
Bacteria, usually living in extreme environments
Which organisms belong in the domain bacteria?
bacteria
which kingdoms belong in domain Eukarya?
plants
animals
fungi
protists
How are evolutionary trees created?
By examining the DNA of different species nd analysing how similar the sequences are
