4. Meiosis causes genetic diversity Flashcards
Mitosis vs Meiosis
Mitosis – 1 single parent cell divides to produce 2 genetically identical daughter cells. Mitosis is important in growth and repair of tissues as well as asexual reproduction.
Meiosis – Occurs in ovaries and testes. 1 parent cell divides to produce 4 gametes (sex cells). Each gamete is different and is a means of passing DNA from one generation to the next.
Pictures showing Meiosis
Diploid vs Haploid
Normal body cells have the diploid number of chromosomes (2n) – meaning each cell contains 2 of each chromosome - 1 maternal & 1 paternal.
Gametes (sex cells) have a haploid number of chromosomes (n) – only 1 copy of each chromosome.
During fertilisation the diploid number of chromosomes is restored in the zygote which then divides by mitosis.
Fertilisation results in new combinations of alleles leading to increased genetic variation.
Chromatids, chromosomes, centromeres and homologous pairs
Remember you have 2 chromosomes that carry the same genes (but may have different alleles), they are called Homologous chromosomes (OR BIVALENTS) and are the same size.
Before meiosis starts – The DNA unravels and replicates so that each chromosome consists of 2 copies of the DNA - these copies are called chromatids. The 2 sister chromatids are identical and joined together by a centromere.
Match the following
So why is meiosis important?
It reduces the number of chromosomes in a cell before reproduction. Fusion of gametes in fertilisation will restore the diploid number of chromosomes.
It results in new combinations of alleles increasing genetic diversity and the ability of a species to survive a changing environment.
Meiosis pictures
Meiosis - Two divisions
Meiosis 1 - separates homologous chromosomes.
Results in new combinations of alleles through crossing over and independent segregation.
Reduction division gives 2 haploid cells
Meiosis 2 – separation of chromatids –
resulting in 4 haploid daughter cells.
Meiosis results in genetic variation
In meiosis:
Crossing over, DURING PROPHASE 1
Independent segregation or random assortment DURING METAPHASE 1
…result in new combinations of alleles.
This in turn results in gametes which are genetically different from each other.
Which results in variation between individuals of the same species.
Meiosis and crossing over – prophase 1
Meiosis and independent segregation / random assortment – metaphase 1
Random fertilisation
In sexual reproduction any egg can be fertilised by any sperm which again results in a huge number of different combinations of alleles and genetic variation.
Random fertilisation produces a zygote with different combinations of chromosomes AND SO DIFFERENT COMBINATIONS OF ALLELES to both parents.
This further increases genetic diversity within a species
The human cell contains 23 pairs of homologous chromosomes
- What is the possible number of different combinations of these chromosomes in a single gamete?
- In fertilisation a sperm fuses with an egg – what is the possible combinations of chromosomes in the zygote?
- 2²³ 2. 2²³x 2²³ = 2⁴⁶
Detailed picture of mitosis vs meiosis
When are cells classed as Haploid and diploid?
Before meiosis, cells are classed as diploid
After meiosis 1, the cells are classed as haploid.
After meiosis 2, the cells are still classed as haploid
As diploid is 2x copies of each chromosome, one from mother and one father, initially the parent cell is diploid 2n
After the first division, there is only one copy of that chromosome, n, even though the amount of genetic material might seem the same because it is in the form of sister chromatids joined (duplicated chromosomes)
After the second division, the new cells are still classed as haploid, n, because they still only contain one set of the chromosomes each but now in an unduplicated form.
