Reasons for Genetic Diversity Flashcards
Describe what happens in a substitution mutation
A base is changed for another
Example: Changing A to G
Describe what happens in a deletion mutation
A base is removed.
Example: Removing a T
Describe what could happen in a chromosome mutation
Changes in whole sets of chromosomes, for example where organisms have three or more sets of chromosomes instead of two (polyploidy). Changes in the number of individual chromosomes (non-disjunction)
Example: Polyploidy in plants
What can cause genetic mutation?
Mutations can be random and spontaneous as a result of errors in DNA replication. Alternatively, they can be caused by mutagenic agents such as radiation or chemicals.
State the functions of meiosis
Meiosis halves the chromosome number to create gametes, and creates genetic variation between the gametes and the somatic cells.
Describe what happens in meiosis
In meiosis, two divisions take place. The result is four daughter cells that are genetically varied, each containing a haploid number of chromosomes. First, the DNA replicates. Then meiosis I occurs, independent segregation and crossing over occur. Then, meiosis II occurs, another division resulting in 4 daughter cells.
Describe crossing over
During crossing over, the chromatids of a homologous pair will twist around each other, forming chiasmata. Sections of the chromatids will break off, and re-join resulting in chromosomes with a different combination of alleles to the original. This further increase genetic diversity.
Describe independent segregation
Homologous chromosomes line up in a random order, as the cell divides one from each pair is passed into each cell. The way they lined up originally determines the order in which the chromosomes are separated. Each resulting cell will therefore have a different combination of maternal and paternal chromosomes.
How do we calculate the number of chromosomes following meiosis (assuming no crossing over)?
2^n (^ = to the power of), where n is the haploid number
How do we calculate the number of different combinations of chromosomes following random fertilisation of two gametes?
(2^n)^2 (2 to the power of n squared, where n is the haploid number)
What is meant by the term “diploid”?
A cell with the full number of chromosomes. In humans, diploid cells have 46 chromosomes.
What is meant by the term “haploid”?
A cell with half the number of chromosomes. In humans, haploid cells have 23 chromosomes.
