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- 1where is the genetic information found?The chromosomes contain the genetic information
- 2Describe what happens when cells divide by mitosisWhen a body cell divides by mitosis:
■ copies of the genetic material are made
■ then the cell divides once to form two genetically identical body cells. - 3When does mitosis occur and what is its functionMitosis occurs during growth or to produce replacement cells.
- 4What type of cell division creates gametesThe type of cell division in which a cell divides to form gametes is called meiosis.
- 5What happens when a cell divides to form gametesWhen a cell divides to form gametes:
■ copies of the genetic information are made
■ then the cell divides twice to form four gametes, each with a single set of chromosomes. - 6Describe what happens after fertilisationWhen gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual then develops by this cell repeatedly dividing by mitosis.
- 7Describe the difference between cell differentiation in plants and animalsMost types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.
- 8where are stem cells found and what are their abilities and functionsCells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of cells, eg nerve cells
Human stem cells have the ability to develop into any kind of human cell. Treatment with stem cells may be able to help conditions such as paralysis. - 9Why does sexual reproduction leed to variationSexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.
- 10What are sex chromosomesIn human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY).
- 11what is an alleleSome characteristics are controlled by a single gene. Each gene may have different forms called alleles.
- 12What is a dominant alleleAn allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.
- 13What is a recessive alleleAn allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.
- 14What is a chromosomeChromosomes are made up of large molecules of DNA (deoxyribo nucleic acid) which has a double helix structure.
- 15What is a geneA gene is a small section of DNA.
Each gene codes for a particular combination of amino acids which makes a specific protein. - 16what is DNA fingerprintingEach person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting.
- 17Give an example of diease on a dominant allelePolydactyly – having extra fingers or toes – is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder.
- 18Give an example of diease on a recessive alleleCystic fibrosis (a disorder of cell membranes) must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder.
- 19What is embryo screeningEmbryos can be screened for the alleles that cause these and other genetic disorders.
- 20What are fossilsFossils are the ‘remains’ of organisms from many years ago, and are found in rocks.
- 21How are fossils formedFossils may be formed in various ways:
■ from the hard parts of animals that do not decay easily
■ 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 other materials as they decay
■ as preserved traces of organisms, eg footprints, burrows and rootlet traces. - 22Why is there little evidence of organisms from the early earthMany 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.
- 23What can we learn from fossilsWe can learn from fossils how much or how little organisms have changed as life developed on Earth.
- 24what may cause extinction■ changes to the environment
■ new predators
■ new diseases
■ new, more successful, competitors
■ a single catastrophic event, eg massive volcanic eruptions or asteroids
■ through the cyclical nature of speciation. - 25How do new species arise■ isolation – two populations of a species become separated, eg geographically
■ genetic variation – each population has a wide range of alleles that control their characteristics
■ natural selection – in each population, the alleles that control the characteristics which help the organism to survive are selected
■ speciation – the populations become so different that successful interbreeding is no longer possible
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