B6 Flashcards
What does meiosis lead to?
Non-identical cells being formed
What does mitosis lead to?
Identical cells being formed
What does sexual reproduction involve?
The joining(fusion) of male and female gametes:
- Sperm and egg cells in animals
- Pollen and egg cells in flowering plants
What does sexual reproduction involve?
- In sexual reproduction there is mixing of genetic information which leads to variety in the offspring
- The formation of gametes involves meiosis
What does asexual reproduction involve?
- Asexual reproduction involves only one parent and no fusion of gametes
- There is no mixing of genetic information. This leads to genetically identical offspring (clones)
- Only mitosis is involved
How does meiosis halve the number of chromosomes in gametes?
- As a cell divides to form gametes: A copy of all of the genetic information is made
- The cell divides twice to form four gametes, each with a single set of chromosomes haploid
- This means the chromosome number has halved
How does fertilisation restore the full number of chromosomes?
1) After two gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself.
2) Mitosis repeats many times to produce lots of new cells in an embryo.
3) As the embryo develops, these cells then start to differentiate into the different types of specialised cell that make up a whole organism.
What is the structure of DNA?
- The genetic material in the nucleus of a cell is composed of a chemical called DNA which stands for deoxyrybonucleic acid
- DNA is a polymer made up of two strands forming a double helix
- The DNA is contained in structures called chromosomes
What is a gene in the DNA?
- A gene is a small section of DNA on a chromosome
- Each gene codes for a particular sequences of amino acids, to make a specific protein
What is the genome in DNA?
- The genome of an organism is the entire genetic material of that organism
- The whole human genome has now been studied and this will have great importance for medicine in the future
What is the importance of the human genome?
It enables us to:
- Search for genes linked to different types of disease
- Understand inherited disorders and their treatment
- Trace human migration patterns from the past
What is a gamete?
- A gamete is a sex cell (in animals: sperm and ovum; in plants: pollen nucleus and ovum)
- Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells - we say they have a haploid nucleus.
What is a chromosome?
- Chromosomes carry genetic information in a molecule called DNA
- A type of cell division called mitosis ensures that when a cell divides each new cell produced has the same genetic information
- DNA exists in a cell’s nucleus within structures called chromosomes
What is a gene?
- A gene is a small section of DNA on a chromosome, which codes for a particular sequence of amino acids, to make a specific protein
- It is the unit of heredity, and may be copied and passed on to the next generation
What is an allele?
- Alleles are different versions of the same gene
- For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour
- For any gene, a person may have the same two alleles, known as homozygous or two different ones, known as heterozygous
What is dominant and recessive alleles?
- Alleles can be dominant or recessive
- A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
- A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype
What are homozygous and heterozygous alleles?
- A homozygote is an individual who has identical alleles for a particular gene
- Heterozygous alleles are both different for the same characteristic, for example Aa
- A heterozygote is an individual who has different alleles for a particular gene
What is a genotype and a phenotype?
- The genotype is the collection of alleles that determine an organism’s characteristics
- When these interact with the environment they are expressed as a phenotype
What are most characteristics a result of?
Multiple genes reacting, rather than a single gene
Note:
AQA says that students should be able to:
- Understand the concept of probability in predicting the results of a single gene cross, but recall that most phenotype features are a result of multiple genes rather than single gene inheritance
- Be able to use direct proportion and simple ratios to express the outcome of a genetic cross.
- Be able to complete a Punnett square diagram and extract and interpret information from genetic crosses and family trees.
- Be able to construct a genetic cross by Punnett square diagram and use it to make predictions using the theory of probability.
(Check page 71 for all of this)
What are inherited disorders caused by?
The inheritance of certain alleles
Give examples of disorders
- Polydactyly (having extra fingers or toes) is caused by a dominant allele
- Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele
What are some judgements about the economic issues concerning embryo screening
Insurance companies could make these individual pay higher premiums
What are some judgements about the social concerning embryo screening
Child will need extra care meaning less time spent with other children
What are some judgements about the ethical issues concerning embryo screening
Banned due to religious beliefs and not morally right to kill a foetus
How many chromosomes are in the normal human body?
23 human chromosomes
What do the 23 chromosomes do in the human body?
22 pairs control characteristics only, but one of the pairs carries the genes that determine sex
What are the sex chromosomes in male and females?
- In females the sex chromosomes are the same (XX)
- In males the chromosomes are different (XY)
Note:
AQA says that students should:
- Be able to carry out a genetic cross to show sex inheritance.
- Understand and use direct proportion and simple ratios in genetic crosses.
What do genetic and environmental variation do?
Genetic and environmental variation combine together to produce different phenotypes.
What is continuous variation?
- Continuous variation is often caused by a mixture of genes and environment
- For example, a girl may inherit the genes to become really tall, but if she doesn’t eat nutritious food then she won’t be able to grow as tall as she might
What is discontinuous variation?
- Discontinuous variation is usually caused by one gene and is not affected by the environment
- A person’s eye colour does not change with age
There are differences in characteristics of individual in a population, this is called variation. What are the differences due to?
- The genes they have inherited (genetic causes)
- The conditions in which they have developed (environmental causes)
- A combination of genes and the environment
What is there ususally in genetics?
An extensive genetic variation within a population of a species
How do all variants rise in genetics?
From mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.
How often do mutations occur?
- Mutations occur continuously
- Very rarely a mutation will lead to a new phenotype
- If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species
What is 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 is the theory of evolution?
The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago
How does evolution occurs through natural selection of variants that give rise to phenotypes best suited to their environment?
- Natural selection is a process where organisms that are better adapted to an environment will survive and reproduce
- This means that the advantageous alleles of this variant organism are passed on to offspring
- Over many generations, the process of natural selection leads to evolution occurring
If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring what do they form?
Two new species
What is selective breeding?
- Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics
- Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals
What does selective breeding involve?
- Selective breeding involves choosing parents with the desired characteristic from a mixed population
- They are bred together
- From the offspring those with the desired characteristic are bred together
- This continues over many generations until all the offspring show the desired characteristic
What are examples of characteristics chosen for usefulness or appearance?
• Disease resistance in food crops.
• Animals which produce more meat or milk.
• Domestic dogs with a gentle nature.
• Large or unusual flowers.
What can selective breeding lead to?
Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects
What is the impact of selective breeding of food plants and domesticated animals?
- Selective breeding leads to future generations of selectively bred plants and animals, all sharing very similar alleles which will reduce variation
- Genes and their different alleles within a population are known as its gene pool
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
How have plant crops been genetically engineered?
Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits
How have bacterial cells been genetically engineered?
Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes
What are the potential benefits of genetic engineering?
- Genetic modification is a faster and more efficient way of getting the same results as selective breeding
- Improve crop yields or crop quality, which is important in developing countries. This may help reduce hunger around the world
- Introduce herbicide resistance, which results in less herbicides being used, as weeds are quickly and selectively killed.
- Insect and pest resistance can be developed and inserted into the plants. The plant produces toxins, which would discourage insects from eating the crop
- Sterile insects could be created such as a mosquito. They would breed, which would lead to infertile offspring. This may help with spread of diseases, such as malaria, dengue fever and the Zika virus
What are the potential risks of genetic engineering?
- Transfer of the selected gene into other species. What benefits one plant may harm another
- Some people believe it is not ethical to interfere with nature in this way. Also, GM crop seeds are often more expensive and so people in developing countries cannot afford them
- GM crops could be harmful, for example toxins from the crops have been detected in some people’s blood
- GM crops could cause allergic reactions in people.
- Pollen produced by the plants could be toxic and harm insects that transfer it between plants
What can happen in genetic engineering?
Genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms
What are crops called when they have been genetically engineered?
- Crops that have had their genes modified in this way are called genetically modified (GM) crops
- GM crops include ones that are resistant to insect attack or to herbicides
- GM crops generally show increased yields
What are the concerns about GM crops?
- Concerns about GM crops include the effect on populations of wild flowers and insects
- Some people feel the effects of eating GM crops on human health have not been fully explored
What is modern medical research discovering about genetic modification?
Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
What are the main stages in genetic engineering?
- Enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus
- The vector is used to insert the gene into the required cells and genetic engineering,
- Genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.
What is the evidence for evolution?
- Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes
- There is also further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria
- The theory of evolution by natural selection is now widely accepted
What are fossils?
Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks
How may fossils be formed?
Fossils may be formed:
- From parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
- When parts of the organism are replaced by minerals as they decay
- As preserved traces of organisms, such as footprints, burrows and rootlet traces.
What were many early forms of bodies like?
- 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
- This is why scientists cannot be certain about how life began on Earth
What are we able to learn about fossils?
We can learn from fossils how much or how little different organisms have changed as life developed on Earth
Note:
AQA says that students be able to extract and interpret information from charts, graphs and tables such as evolutionary trees.
When do extinctions occur?
When there are no remaining individuals of a species still alive
What are the factors that contribute to the extinction of a species?
Several factors can cause a species to become extinct. They include:
- New diseases
- New predators
- New, more successful competitors
- Changes to the environment over geological time, such as climate change
- A single catastrophic event, such as a massive volcanic eruption or a collision between an asteroid and the Earth
Why can bacteria evolve rapidly?
Bacteria can evolve rapidly because they reproduce at a fast rate
What do mutations of bacterials pathogens produce?
- They 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
What is MRSA resistant to?
Antibiotics
What is done to reduce the rate of development of antibiotic resistant strains?
- 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
Traditionally, how have living things been classified into groups?
Depending on their structure and characteristics in a system developed by Carl Linnaeus
How did Linnaeus classify living things into?
- Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species.
- Organisms are named by the binomial system of genus and species.
Note:
Students should be able to use information given to show understanding of the Linnaean system.
What is the impact of developments in biology on classification systems?
- Species that are more closely related are likely to have fewer differences in the sequence of their DNA bases
- Therefore, we are more likely to classify species into the same group if they have the same genetic variation in their genome
As evidence of internal structures what happened?
As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed.
What did Carl Woese develop?
Due to evidence available from chemical analysis there is now a ‘three-domain system’ developed by Carl Woese
What are the organisms divided into with the Carl Woese system?
• Archaea (primitive bacteria usually living in extreme environments)
• Bacteria (true bacteria)
• Eukaryota (which includes protists, fungi, plants and animals).
What is evolutionary trees?
- Evolutionary trees are a method used by scientists to show how they believe organisms are related
- They use current classification data for living organisms and fossil data for extinct organisms
