Genetic diversity can arise as a result of mutation or during meiosis Flashcards
What is meiosis?
A type of cell division that produces 4 genetically different haploid daughter cells.
What happens before meiosis?
Before meiosis, the DNA unravels and replicates so there are two copies of each chromosome called chromatids. DNA then condenses to form double-armed chromosomes which are formed from two sister chromatids held together by a centromere.
Explain the process of meiosis.
- DNA unravels and replicates to form 2 chromatids. DNA then condenses to form double-armed chromosomes which are formed from the two sister chromatids held together by a centromere.
- In meiosis 1, the cell divides to result in 2 cells each containing 2 sister chromatids.
- In meiosis 2, the sister chromatids separate and the cells divide again to form 4 haploid daughter cells each containing 2 chromatids
What are the two mechanisms in meiosis that cause variation?
- Independent segregation of homologous chromosomes
- Crossing over between homologous chromosomes.
When in meiosis does independent segregation and crossing over take place in?
Meiosis 1 (first round of division)
Explain independent segregation, how it causes variation and how it can be calculated mathematically.
In meiosis 1 the homologous pairs of chromosomes line up opposite each other at the equator of the cell. The side the maternal and paternal chromosomes from each homologous pair are at is random. These pairs are then separated so one of each homologous pair ends up in the daughter cell to form a haploid cell.
This creates a large number of possible combinations of chromosomes in the daughter cells produced which can be calculated by 2n where n = the number of homologous pairs of chromosomes.
Explain crossing over and how it causes variation.
In meiosis 1, the homologous pairs of chromosomes line up opposite each other at the equator. Parts of the chromatids can be twisted around each other. This puts tension on the chromatids, causing pairs of the chromatid to break. The broken parts of the chromatid recombine with another chromatid, usually on the other chromosome of the pair. This results in new combinations of alleles.
Compare mitosis and meiosis.
Mitosis
- One nuclear division
- End results are diploid cells which are two sets of chromosomes
- Creates genetically identical cells
Meiosis
- Two nuclear divisions
- End results are haploid cell (one set of chromosomes)
- Creates genetic variation
Explain random fertilisation, how it causes variation and how it can be calculated mathematically.
Random fertilisation is when a random egg and sperm will fuse together which therefore increases variation. It occurs before crossing over and it can be calculated mathematically as (2n)^2
What is one event that occurs in division 2 but not in division 1 of meiosis?
Separation of sister chromatids.
What are chromosomal mutations and what is process they caused by?
A random change in the number of chromosomes by non-disjunction in meiosis.
What is non-disjunction and state the two effects it can have?
When the chromosomes in meiosis 1 or chromatids in meiosis 2 do not split equally during anaphase.
- Changes in whole sets of chromosomes (polyploidy)
- Changes in the number of individual chromosomes.(aneuploidy)
State and explain the two effect of non-disjunction.
Polyploidy
- This is a change in a whole set of chromosomes.
- It occurs when organisms have three or more sets of chromosomes rather than 2.
- This mainly occurs in plants.
How does the process of polyploidy occur in meiosis 1?
- Each homologous pair is doubled due to DNA replication in interphase.
- Non-disjunction in meiosis 1 means all chromosomes fail to separate equally.
- After meiosis 1, all the chromosomes are in one cell and there are none in the other.
- The two cells divide normally in meiosis 2. The cell containing the chromosomes divides into two diploid cells and the cell containing no chromosomes divides into two cells containing no chromosomes.
- In fertilisation, if the diploid cell fuses with the regular haploid cell, it results in a triploid (polyploidy)
How does the process of polyploidy occur in meiosis 2?
- Each homologous pair is doubled due to DNA replication in interphase.
- Normal division in meiosis 1 so all chromosomes separate equally into two cells.
- The non-disjunction in meiosis 2 causes one of the cells to fail to separate equally.
- The failed cell divides into two cells, one with no chromosomes and one containing the chromosomes (diploid).
- The other cell will divide into the two cells equally to produce haploid cells.
