What type of cell division leads to identical cells being formed?
Mitosis
What type of cell division leads to non-identical cells being formed?
Meiosis
What does sexual reproduction, in animals, involve?
The joining (fusing) of male and female gametes (sperm and egg)
What does sexual reproduction, in plants, involve?
The joining (fusing) of gametes (pollen and egg cells)
What does sexual reproduction lead to and why?
Variety in the offspring as there is mixing of genetic information
What is asexual reproduction?
One parent (no fusion or mixing of genetic information) leading to genetically identical offspring
What does meiosis do to the number of chromosomes in the gamete?
It halves it (46 to 23)
What happens to the number of chromosomes during fertilisation?
It doubles (male and female gametes both have 23 chromosomes, combining to make 46)
What key stages occur when a cell divides to form gametes (meiosis)?
- Copies of the genetic information are made
- The cell divides twice forming four gametes (each with a single set of chromosomes)
- All the gametes are genetically different from each other
What is the genetic material in the nucleus of a cell composed of?
DNA
What is the structure of DNA?
A polymer made up of two strands forming a double helix
What is DNA contained in?
Chromosomes
What is a gene?
A small section of DNA on a chromosome
What does a gene code for?
A particular sequence of amino acids, which make specific proteins
What is the genome of an organism?
The entire genetic material of that organism
Why is having the human genome sequence important?
It will be of benefit to medicine
Why is understanding the human genome important?
- Genes linked to different types of disease can be searched for
- Inherited disorders can be understood / treated
- Migration patterns of the past can be traced
Explain the term: gamete
A sex cell (e.g. sperm or egg)
Explain the term: chromosome
A long molecule of DNA, found in the nucleus of a cell, which carries genes
Explain the term: gene
A short section of DNA, found on a chromosome, carrying the instructions needed to make a protein (and so controls the development of a characteristic)
Explain the term: allele
An alternative version of a gene
Explain the term: dominant
The allele for the characteristic that’s shown by an organism if two different alleles are present for that characteristic
Explain the term: recessive
An allele whose characteristic only appears in an organism if there are two copies present
Explain the term: homozygous
Where an organism has two alleles for a particular gene that are the same
Explain the term: heterozygous
Where an organism has two alleles for a particular gene that are different
Explain the term: genotype
the combination of alleles
Explain the term: phenotype
The characteristics an organism has, e.g. tall
Give an example of characteristics controlled by a single gene
Fur colour in mice / red-green colour blindness in humans
Which allele is always expressed?
The dominant (even if only one copy is present)
Complete a Punnett square for the % chance of having a boy or girl
What is polydactyly?
An inherited disorder (extra fingers or toes) caused by a dominant allele
What is cystic fibrosis?
An inherited disorder (cell membrane issues) caused by a recessive allele
How many pairs of chromosomes are found in ordinary human body cells?
23 pairs
22 pairs of chromosomes control characteristic, but what does the 23rd pair control?
The gene which determines sex (XX in female and XY in male)
What is variation?
Differences in the characteristics of individuals in a population
What can cause variation?
- The genes inherited (genetic cause)
- The conditions in which the organism has developed (environmental cause)
- A combination of genes and environment
How do variants in a species arise?
Mutations: most have no effect on phenotype, some influence phenotype, and very few determine phenotype
If a mutation leads to a new phenotype, suited to an environmental change, what might occur?
Relatively rapid change in the species
What is evolution?
A change in the inherited characteristics of a population over time which may result in the formation of a new species
What is the driving force of evolution?
Natural selection
What is the theory of evolution by natural selection?
All species of living things have evolved from simple life forms, that first developed more than three billions years ago
Variants that give rise to phenotypes best suited to their environment causes what?
Evolution, occurring through natural selection
How do new species form?
If two populations of one species become so phenotypically different they cannot interbreed (to produce fertile offspring) then they have formed two new species
What is selective breeding?
Selective breeding is artificial selection – humans breed plants and animals for particular genetic characteristics
What does selective breeding involve?
Choosing parents with the desired characteristics from a mixed population, which are bred, and offspring with desired characteristics are bred
What characteristics might be chosen during selective breeding
- Disease resistance in food crops
- Animals which produce more milk / meat
- Domestic dogs of a gentle nature
- Large / unusual flowers
What can selective breeding lead to?
Inbreeding – some breeds are prone to diseases or inherited defects
What is genetic engineering?
A process involving modifying the genome or an organism by introducing a gene from another organism: giving a desired characteristic
How have plant crops been genetically engineered?
To be resistant to disease / produce bigger and better fruits
How have bacterial cells been genetically engineered?
To produce useful substances such as human insulin (treating diabetes)
How does genetic engineering occur?
Genes from chromosomes are ‘cut out’ and transferred to cells of other organisms
What are genetically modified (GM) crops?
Crops that have had their genes modified: resistance to insect attack / herbicides for example (generally show yield increase)
What concerns are there about GM crops?
The effect on populations of wild flowers and insects / human health (not being fully explored)
Higher Q. How are vectors used in genetic engineering?
Enzymes isolate the required gene, which is inserted into a vector (usually a bacterial plasmid / virus)
Higher Q. How are genes transferred during genetic engineering?
The bacterial plasmid / virus vector inserts the gene into the required cells
Higher Q. Why are genes transferred at an early stage of the organism’s development during genetic engineering?
So they develop with the desired characteristic
What evidence is there for evolution?
Fossils and antibiotic resistance in bacteria
What are fossils?
The ‘remains’ of an organism from millions of years ago, found in rocks
How may fossils be formed?
- From parts of organisms that have not decayed because one or more the conditions for decay was absent
- Parts of the organism are replaced by minerals as they decay
- Preserved traces (footprints / burrows etc…)
Why are there very few traces of early life forms?
They were soft-bodies (fossil record incomplete)
What do fossils show?
How much / how little different organisms have changed as life developed on Earth
What is extinction?
When there are no remaining individuals of a species still alive
What may cause extinction of a species?
- Environmental changes
- New predators
- New diseases
- Catastrophic events
- Unable to compete
Why can bacteria evolve rapidly?
They reproduce at a fast rate
How do new strains of bacteria occur?
Mutations: some strains may be resistant to antibiotics so are not killed and the resistant strains rises
What is MRSA resistant to?
Antibiotics
How can antibiotic resistance be reduced?
- Reduce prescription of antibiotics, especially when inappropriate (e.g. for a viral infection)
- Patients should complete course of antibiotics so all bacteria are killed
- Restriction of agricultural antibiotic use
What issues are there with antibiotic development?
It is costly and slow – it is unlikely to keep up with the emergence of new resistant strains
What did Carl Linnaeus develop?
A classification systems, based on structures and characteristics
How did Carl Linnaeus characterise living organisms?
Kingdom > phylum > class > order > family > genus > species
How are organisms named?
The binomial system of genus and species
What caused new models of classification to be proposed?
Evidence of internal structures (microscopes) and biochemical processes were better understood
What is the ‘three-domain system’?
Carl Woese divided organisms into:
- Archaea (primitive bacteria)
- Bacteria (true bacteria)
- Eukaryota (protists, fungi, plants and animals)
How can the relation of organisms be shown?
Evolutionary trees – current classification data for living organisms and fossil data for extinct organisms is used