Crossing-Over

Question 1

A homologous pair of chromosomes before crossing-over

Answer 1

It takes place in Meiosis 1 of Prophase
Homologous chromosomes come to lie close together
Homologous chromosomes (consisting of 4 chromatids in total) involved in crossing over is referred to as a bivalent

Question 2

A pair of homologous chromosomes after crossing-over

Answer 2

The chromosomes separate in such a way that each has one original chromatid and one chromatid with some genetic material from its homologous partner

Question 3

Chromatids interwinding

Answer 3

One chromatid of each chromosome overlaps with a chromatid of its homologue (or homologous partner)
The points of crossing-over are called chiasmata (singular chiasma)

Question 4

What is Crossing-Over used for

Answer 4

Crossing-Over results in an exchange of genetic material so that each chromosome has genetic material from both parents
The cells (usually gametes) produced will therefore be different from each other
This, therefore results in variation in the offspring

Question 5

Why the cells produced during meiosis are genetically different

Answer 5

Firstly, Crossing-Over during the Prophase 1 results in the exchange of genetic material
Secondly, during Metaphase 1, the chromosomes arrange themselves along the equator of the cell randomly

Question 6

The importance of Meiosis

Answer 6

Leads to the formation of haploid gametes in some organisms and haploid spores
The halving effect of meiosis neutralizes the doubling effect of fertilization, thus maintaining a constant chromosome number
Crossing-Over introduces genetic variation. Genetic variation results in offspring that are better adapted to a particular environment ensuring a better chance of survival