Topic 4B: Diversity and Selection Flashcards
What are gametes?
Gametes are the sex cells, which have a haploid number of chromosomes
What is the process of fertilisation?
1) A random process where a haploid sperm fuses with a haploid egg, making a zygote with the normal diploid number of chromosomes.
2) Half of the chromosomes of the zygote are from the father, and half are from the mother.
What is the process of meiosis?
1) During interphase the DNA unravels and replicates forming chromatids.
2) The DNA then condenses to form chromosomes.
3) (Meiosis I - First division) Chromosomes arrange themselves into homologous pairs at the equator, before being separated by spindles forming two separate cells.
4) (Meiosis II - Second division)
The sister chromatids are separated at the centromere by spindles, forming four genetically different haploid cells.
Genetic variation: Crossing over of chromatids
1) During meiosis I, homologous pairs twist around each other and bits of the chromatids swap over.
2) Therefore, the four daughter cells formed from meiosis II contain chromatids with different alleles.
Genetic variation: Independent segregation
1) During meiosis I, it’s completely random as to which chromosome from each pair is passed on to which daughter cell.
2) Therefore, the four daughter cells produced have completely different combinations of maternal and paternal chromosomes.
What are gene mutations?
When there is a change in the base sequence of DNA of chromosomes.
What are the different types of mutation?
- Substitution
* Deletion
What are the effects of mutation?
- If a mutation occurs, the sequence of bases changes, and therefore the sequence of amino acids changes and a new protein may be formed.
- Because the code is degenerate, some amino acids are coded for by more than one DNA triplet. Therefore, not all substitution mutations lead to a change in the amino acid produced.
What are mutagenic agents?
Something which increases the rate of mutations.
What are chromosome mutations?
Variations in the number of chromosomes or parts of chromosomes caused by mutations in the gametes during meiosis.
What is genetic diversity?
The number of different alleles of genes in a species/population.
How is genetic diversity increased?
- Mutations in DNA forming new alleles.
- Gene flow - different alleles being introduced into a population when individuals from another population migrate into it and reproduce.
Genetic diversity: Genetic bottlenecks
1) A genetic bottleneck is an event which causes a big reduction in a population, which reduces the number of different alleles in a gene pool, and so reduces genetic diversity.
2) The survivors then reproduce and a larger population is produced from a few individuals.
Genetic diversity: Founder effect
1) A few organisms from a population start a new colony.
2) Therefore, the frequency of alleles in the new colony compared to the initial gene pool could be very different.
3) When this new population reproduces the genetic diversity is massively decreased.
What is the process of natural selection?
1) Some organisms have beneficial adaptations, meaning they are more likely to survive, reproduce and pass on their genes.
2) Therefore a greater population of the next generation inherits the beneficial allele.
3) They are then more likely to survive, reproduce and pass on their genes.
4) Therefore, the frequency of the beneficial allele in the population increases from generation to generation.
5) This leads to evolution as the advantageous alleles become more common in a population.
