Cell Divison By Meiosis Flashcards
Where does cell division by meiosis take place and why does it occur here
It only takes place in sex organs because cell division by meiosis is only ever used to produce gametes.
What is the cell at the beginning of meiosis compared to at the end
At the beginning of cell division by meiosis we start with a diploid cell and at the end we have 4 haploid gametes.
What do the cells therefore contain
Individual chromosome and not pairs
Why is it important that the gametes are haploid cells
As during fertilisation they fuse together to produce a fertilised egg or zygote and because the gametes are haploid the zygote will have the diploid number of chromosomes.
What is a feature of all the gametes produced by meiosis
All the gametes produced by meiosis are genetically different
What do diploid cells have
They contain chromosomes in pairs
What is a chromosome pair called
A homologous pair of
What is a feature of the chromosomes in a homologous pair
They have exactly the same genes. However they have different alleles which are inherited from each parent.
What happens before division by meiosis and what are sister chromatids joined at
All the chromosomes are copied in interphase. Where sister chromatids are joined at the centromere.
What is crossing over
During Prophase 1 the two chromosomes in a homologous pair come together. And the chromatids of the two non-sister chromatids of different homologous pairs wrap around each other.
What are the attached pair of chromosomes called and what are the points of attachment called
They are called a bivalent with the points of attachment being called chiasmata.
What can parts of the chromatids do during crossing over
They can then break of and exchange between the homologous chromosomes, so the maternal and paternal chromosomes have exchanged DNA.
What are the chromosomes produced by crossing over called
These are called recombinant chromosomes.
Because during crossing over DNA is exchanged what else is exchanged
Because DNA is exchanged they have also exchanged alleles.
Because chiasmata can form at multiple points what does this mean and what is this exchange a major source of
A large number of alleles can be exchanged and this exchange of alleles is a major source of genetic variation in meiosis.
What does meiosis involve
2 rounds of nuclear division
What occurs in meiosis 1
Homologous chromosomes are separated from each other
What occurs in Meiosis 2
sister chromatids are separated from each other
What occurs in interphase before meiosis and what are the chromosomes visible as
The cell will have copied the chromosomes and the organelles. The chromosomes are not visible as distinct structures during interphase.
What occurs during prophase 1 (1)
The chromosomes condense and become visible
The homologous chromosomes link together forming chiasmata.
Crossing over will then take place - exchanging alleles between the homologous chromosomes
What occurs during prophase 1 (2)
The nuclear membrane also breaks down and centrioles move to opposite poles of the cell.
Spindle fibres start to assemble into the spindle apparatus.
What occurs in Metaphase 1
The pairs of homologous chromosomes are lined up on the equator of the spindle apparatus.
What occurs during Anaphase 1
The spindle fibres shorten and the homologous chromosomes move towards opposite poles.
In order for this to happen, the chiasmata between homologous chromosomes break.
What occurs during telophase 1
The chromosomes have now reached the poles of the cell
The nuclear membranes reform and the chromosomes uncoil back to their chromatin state
What is a feature of the cells produced in Telophase 1
These are haploid cells as they don;t contain pairs of homologous chromosomes.
What occurs in prophase 2
The chromosomes condense and become visible again
The nuclear membranes reform breaks down and the spindle fibres begin to develop
What occurs in metaphase 2
The chromosomes are lined up on the equator of the spindle apparatus
What occurs during anaphase 2
The centromere of each chromosome divides
The spindle fibres shorten
The chromatids are now pulled towards opposite poles of the cell
What occurs during Telophase 2
The chromatids have reached the poles of the cell
They are now called chromosomes
The nuclear membranes reform reforms and the chromosomes uncoil back to their chromatin state
Each cell undergoes cytokinesis, producing 2 haploid cells.
Why is Mitosis 1 reduction division
Because the chromosomes number halves it is called reduction division
What is independent assortment
When homologous chromosome pairs line up on the spindle, we can’t predict whether the paternal or maternal chromosomes will end up in which gamete.
What stages of meiosis does independent assortment occur
It occurs during Metaphase 1 and 2
What is the formula for the number of genetically different gametes produced by independent assortment
2^n where n is the number of homologous chromosome pairs line
What occurs during fertilisation which introduces genetic variation further
Male and female gametes fuse randomly with each other, and this random fusion of gametes introduces another level of genetic variation in the offspring.
