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Pregunta	Respuesta
where is the genetic information found?	The chromosomes contain the genetic information
Describe what happens when cells divide by mitosis	When 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.
When does mitosis occur and what is its function	Mitosis occurs during growth or to produce replacement cells.
What type of cell division creates gametes	The type of cell division in which a cell divides to form gametes is called meiosis.
What happens when a cell divides to form gametes	When 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.
Describe what happens after fertilisation	When 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.
Describe the difference between cell differentiation in plants and animals	Most 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.
where are stem cells found and what are their abilities and functions	Cells 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.
Why does sexual reproduction leed to variation	Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.
What are sex chromosomes	In 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).
what is an allele	Some characteristics are controlled by a single gene. Each gene may have different forms called alleles.
What is a dominant allele	An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.
What is a recessive allele	An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.
What is a chromosome	Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid) which has a double helix structure.
What is a gene	A gene is a small section of DNA. Each gene codes for a particular combination of amino acids which makes a specific protein.
what is DNA fingerprinting	Each person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting.
Give an example of diease on a dominant allele	Polydactyly – 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.
Give an example of diease on a recessive allele	Cystic 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.
What is embryo screening	Embryos can be screened for the alleles that cause these and other genetic disorders.
What are fossils	Fossils are the ‘remains’ of organisms from many years ago, and are found in rocks.
How are fossils formed	Fossils 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.
Why is there little evidence of organisms from the early earth	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.
What can we learn from fossils	We can learn from fossils how much or how little organisms have changed as life developed on Earth.
what 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.
How 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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	Creado por david doran hace alrededor de 8 años