Topic 4B

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Diploid body cells and Gametes

  • Normal body cells have the diploid number (2n) of chromosomes, this means that each cell has 2 of each chromosome, 1 from mum and 1 from dad
  • The chromosoms are the same size and contain the same genes, although they could have different versions of the gene called alleles.
  • These pairs of matching chromosomes are called homologus pairs
  • Humans have 23 homologus pairs which menas they have 46 chromosomes in total. This means the diploid number in humans is 46
  • Gametes are the sperm cells in men and egg cells in women.
  • Gametes have a haploid (n) number and in humans this is 23 and they only conain one copy of each chromosome in a homolgous pairs.
  • During fertilisation the 2 gametes join together to form a zygote, which divides and develops.
  • Random fertilisation produces zygotes with different combinations of chromosomes to both parents.
  • This mixing of genetic material in sexual reproduction increases genetic diversity
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  • Meiosis is a type of cell division and takes place in the reproductive organs.
  • Before meiosis starts, the DNA unravels and replicates so there are 2 copies of each chromosome called chromatids
  • The DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids. The chromatids are joined in the middle by a centromere
  • Meiosis I (first division)- the chromosomes arrange themselves into homologus pairs
  • These homologus pairs are them seperated, halving the chromosome number
  • Meiosis II (second division)- the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided)
  • 4 haploid  cells that are genetically different from each other are produced 

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Creating genetic variation in gametes

  • During meiosis I, homologus chromosomes come together and pair up. The chromatids twist around each other and bits of chromatids swap over, this means that the chromatids still contain the same genes but now have a different combination of alleles. Also it meas that each of the 4 daughter cells formed from meiosis II contain chromatids with different alleles.
  • Each homologus pair of chromosome is made from 1 chromosome from your mother and father. So when the homologus pairs are separated in meiosis 1 it is completely random which chromosome from each pair ends up in the daughter cell. So the 4 daughter cells produced by meiosis have completely different combinations of those from the mum and dad. This is called independent segregation of the chromosomes and this "shuffling" leads to genetic diversity
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Life Cycles

  • Image result for life cycles meiosis (
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Outcomes of mitosis and meiosis

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Gene mutations and effects

  • Gene mutations involve a change in the DNA base sequence of chromosomes. The order of bases in a gene determines the order of amino acids in a particular protein. If a mutation occurs in a gene, the sequence of amino acids it codes for could be altered.
  • The degenrate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet. This means that not all substitution mutations will result in a change to the amino acid sequence of the protein. Deletions will always change the number of bases present which will cause a frameshift in the base triplets
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Chromosome mutations

  • In humans, when meiosis works properly, all 4 daughter cells will end up with 23 whole chromosomes- one from each homologous pair (1-23). But sometimes meiosis goes wrong and the cells produced contain variations in the numbers of whole chromosomes or parts of chromosomes. For example 2 cells might have 23 whole chromosomes, one each of 1-23,  but the other 2 might get muddled up, with 1 having 2 chromosome 6 and the other no chromosome 6. This is called chromosome mutation and is caused by errors during meiosis. These mutations lead to inherited conditioons because the errors present in the gametes. One type of chromosome mutation is called chromosome non-disjunction and its a failure of chromosomes to separate properly
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What is genetic diversity

  • Genetic diversity is the number of different alleles of genes in a species or population. A large number of different alleles in a population means a large variety of different characteristics and a high genetic diversity. Genetic diversity is important, if a population has a low genetic diversity, it might not be able to adapt to a change in the environment and the whole population could be wiped out by a single event. Diversity is increased by, mutations in the DNA forming new alleles which can be both positive and negative and it is increased by migration from another population, introducing new alleles, this is called gene flow. Genetic diversity is what allows natural selection to occur as some characteristics are more advantageous that others.
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Gentic bottlenecks and Founder effect

  • A genetic bottleneck is an event that cause a large reduction in population. This reduces the number of alleles in the gene pool and reduces genetic diversity. The survivors reproduce and a larger population is created from few individuals.
  • The founder effect describes what happen when just a few organisms from a population start a new colony and there are only a small number of different alleles in the intial gene pool. The frequency of each allele in the new colony might be very different to the frequency of those alleles in the original population. The founder effect can occur as a result of migration leading to geogrpahical separation or if a new colony is separated from the original population for another reason.
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Process of natural selection

  • Randomly occuring mutations can result in new alleles forming. This can be harmful or advantageous. Not all individuals are as likely to reproduce as each other, so there is different reproductive success. Individuals with an allele that increases their chance of survival are more likely to survive. This mean more of the next generation inherits the beneficial allele. They are more likely to survive and pass on their genes. So the frequency of the beneficial allele increases and eventually leads to evolution.
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Types of adaptation

  • Behavioural- the way in which an organism acts that increase its chance of survival. For example possums"playing dead".
  • Physiological adaptations- these are process inside an organisms body that increase its chance of survival. For example hibernation in brown bears which lowers their rate of metabolism.
  • Anatomical adaptations- these are structual features of an organism's body that increase its chance of survival. For example thick layers of blubber in whales and pengunis.
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Types of selection

  • Directional- is where individuals with alleles for an extreme characteristic are more likely to survive and reproduce. This could be due to the environment.
  • Stabilising selection- is where individuals towards the middle of the range are more likely to survive. It occurs when the environment isn't changing and it reduce the range of possible characteristics.
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