AQA GCSE Single Science Biology: Inheritance, Variation and Evolution

What’s the difference between sexual and asexual reproduction?

• Sexual reproduction involves the fusion of male and female gametes to produce genetically varied offspring.

• Asexual reproduction involves only one parent and produces clones

Name some examples of organisms that can reproduce sexually and asexually

• Malarial parasites reproduce sexually in the mosquito, but asexually in a human host

• Fungi release spores both sexually and asexually

• Strawberry plants produce runners (asexual) and seeds (sexual)

• Daffodils and other plants produce bulbs (asexual) and seeds (sexual)

What are the advantages of sexual and asexual reproduction

Sexual

• Produces variation in offspring which gives survival advantage due to natural selection

• Has potential for selective breeding

Asexual

• Only requires one parent

• Time and energy efficient

• Can produce many identical offspring in favourable conditions

Explain the process of meiosis

• Genetic material is duplicated

• Similar chromosomes pair up

• Sections of DNA are swapped

• Cell divides for the first time

• Cell divides again

• Four gametes are produced, each one genetically different due to shuffling of genetic material

What is the structure of DNA?

• It is a polymer made up of two strands forming a double helix

• The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases.

• The DNA polymer is made of these repeating nucleotide units.

Define the following terms: gene, genome, nucleotide

• Gene: section of DNA on a chromosome

• Genome: entire set of genetic material in an organism

• Nucleotide: repeating units consisting of a sugar, phosphate group and base

Why is understanding the human genome important?

• Allows scientists to identify genes linked to disease

• Helps us understand inherited diseases better so we can develop effective treatments

• To trace the migration of certain populations

Explain the process of protein synthesis

• A section of DNA unwinds and a template of the DNA called mRNA moves out of the nucleus and onto a ribosome

• tRNA molecules on one end bind to a codon on the mRNA strand and on the other, the corresponding amino acid that the gene codes for.

• The tRNA molecules soon leave so the chain can fold to form proteins.

What are some examples of proteins?

• Enzymes

• Hormones

• Structural proteins

How does the structure of DNA affect the protein made?

• The order of bases in a gene decide the order of amino acids in a protein

What do non-coding parts of DNA do?

Control whether or not a gene is expressed

What are the three types of mutations?

• Insertions- when a new base is inserted

• Deletions- when a random base is deleted

• Substitutions- when a random base is changed to a different one

What are mutations?

• Random, continuous changes to genes

• Most mutations have very little to no effect on the protein

Describe possible effects of mutations on proteins

• If the shape of an enzyme’s active site is changed, its substrate may no longer be able to bind to it

• Structural proteins such as collagen may lose their strength

Define the following terms: gamete, chromosome, allele

• Gamete: sex cells (sperm in males, ova in females)

• Chromosome: a DNA structure within the nucleus that codes for all the characteristics of an organism.

• Allele: different form of same gene

Define the following terms: dominant, recessive, homozygous, heterozygous

• Dominant: allele that is always expressed

• Recessive: allele that is only expressed if the dominant allele is not present

• Homozygous: alleles are identical for the same characteristic

• Heterozygous: alleles are different for the same characteristic

Define the following terms: genotype, phenotype, fertilisation

• Genotype: the collection of alleles that determine characteristics

• Phenotype: The visible characteristics of an organism which occur as a result of its genes

• Fertilisation: the fusion of the nucleus of a male gamete with the nucleus of a female gamete.

What characteristics are controlled by a single gene?

• Red-green colour blindness in humans

• Fur colour in mice

Give an example of a disorder caused by a dominant allele and a disorder caused by a recessive allele

• Polydactyly (having extra fingers or toes) is caused by a dominant allele

• Cystic fibrosis (a disorder of cell membranes causing a build up of sticky mucus in air passages and the pancreas) is caused by a recessive allele

The human body cells contain 23 pairs of chromosomes. What do they control?

• 22 chromosomes control characteristics

• The last pair determines sex- XX is female and XY male

What can cause variation in a population? Name examples

• Genes (eye colour)

• Environmental causes (scars)

• A combination of both (weight)

How does genetic variation occur?

As a result of random mutation- most have no effect, and while some may influence phenotype, very few determine phenotype as most characteristics are determined by more than one gene

Define evolution

A change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.

What does the theory of evolution state?

All species of living things have evolved from simple life forms that first developed more than three billion years ago.

What is speciation?

If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring

What is selective breeding?

The process by which humans breed plants and animals for particular genetic characteristics.

Explain the process of selective breeding

• Select two organisms with desired characteristics

• Select the offspring with the desired characteristics and breed them with another

• Continue this process until all the offspring have the desired characteristics

Give examples of desired characteristics that organisms may be bred for

• Disease resistance in food crops.

• Animals which produce more meat or milk.

• Domestic dogs with a gentle nature.

• Large or unusual flowers.

What are some drawbacks of selective breeding?

• Can lead to inbreeding- which increases the likelihood of disease or inherited defects

• Reduces genetic variation within a population- if a new disease arises, it is unlikely a resistant allele will be present

What is genetic engineering?

A process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.

Why might plant crops specifically be genetically engineered for?

• To produce bigger, better fruits

• To be resistant to disease

What have bacterial cells been genetically engineered to do?

• Produce human insulin

What are some concerns about GM crops?

• Potential effect on populations of wild flowers and insects

• Effects on human health have not fully been explored

Explain the process of genetic engineering

• Enzymes are used to isolate the required gene

• The gene is inserted into a vector (bacterial plasmid, virus)

• The vector is used to insert the gene into the required cells

• Genes are transferred to an organisms at an early stage of development so that they develop with desired characteristics

What is tissue culture?

• Using small groups of cells from part of a plant to grow identical new plants.

• This is important for preserving rare plant species or commercially in nurseries.

What are cuttings?

• A method used by gardeners to produce many identical new plants from a parent plant.

• This is simpler and done on a much smaller scale

Explain the process of embryo transplantation

• Splitting apart cells from a developing animal embryo before they become specialised

• Transplanting the identical embryos into host mothers.

Explain the process of adult cell cloning

• 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 egg cell to divide to form an embryo.

• These embryo cells contain the same genetic information as the adult skin cell.

• When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

What did Darwin propose?

• Individual organisms within a particular species show a wide range of variation for a characteristic.

• Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.

• The characteristics that have enabled these individuals to survive are then passed on to the next generation.

Why did it take a long time for Darwin’s theory to be accepted?

• the theory challenged the idea that God made all the animals and plants that live on Earth

• there was insufficient evidence at the time the theory was published to convince many scientists

• the mechanism of inheritance and variation was not known until 50 years after the theory was published.

What did Jean-Baptiste Lamarck propose?

• The changes that occur in an organism during its lifetime can be inherited.

• This was disproved through experimentation

What did Alfred Russel Wallace do?

• He worked worldwide gathering evidence for evolutionary theory.

• He is best known for his work on warning colouration in animals and his theory of speciation.

What are the steps to speciation?

• A geographical barrier separates a species

• Random mutations cause genetic variation

• If the variation is advantageous, it will be passed on to the next generation

• Due to the difference in environments, different characteristics will be advantageous and therefore different characteristics will be adopted

• If it becomes so that the two groups become so phenotypically different that they cannot interbreed to produce fertile offspring, speciation has occurred

What did Gregor Mendel discover?

The inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged.

Why were Mendel’s discoveries not recognised until after his death?

• when he presented his work to other scientists he did not communicate it well so they did not really understand it

• it was published in a scientific journal that was not well known so not many people read it

• he could not explain the science behind why characteristics were inherited

What scientific discoveries led to gene theory being developed?

• In the early 20^th Century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes.

• In the mid-20^th Century the structure of DNA was determined and the mechanism of gene function worked out.

How can fossils be formed?

• Mineral replacement- hard body parts such as bones and teeth decay slowly and are gradually replaced by minerals

• When decay conditions are absent- for example in peat bogs (too acidic) or ice (too cold)

• In preserved traces of organisms such as footprints, burrows and rootlet traces

What is evidence for evolution?

• Fossils- structures of earlier organisms are much simpler

• Resistant bacteria- number of resistant strains have increased

Why is the fossil record incomplete?

• Many early forms of life were soft-bodied, which means that they have left few traces behind.

• What traces there were have been mainly destroyed by geological activity.

When has extinction occurred?

When there are no remaining individuals of a species still alive.

How should the problem of antibiotic resistance be lessened?

• doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections

• patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains

• the agricultural use of antibiotics should be restricted.

Describe the process of antibiotic resistance

• Mutations of bacterial pathogens produce new strains.

• Some strains might be resistant to antibiotics, and so are not killed.

• They survive and reproduce, so the population of the resistant strain rises.

• The resistant strain will then spread because people are not immune to it and there is no effective treatment.

How did Linnaeus classify living things?

• Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species.

• Organisms are named by the binomial system of genus then species.

What did Carl Woese do?

• Classify living things into three domains: archaea, bacteria and eukaryota

• Archaea are primitive bacteria living in extreme environments

• Bacteria are “true bacteria”

• Eukaryota include protists, fungi, plants and animals