10.1.2 Crossing Over

Crossing over is the exchange of DNA material between non-sister homologous chromatids

• Meiosis has several mechanisms that increase the genetic diversity of gametes produced
• Both crossing over and independent assortment ( sometimes also called random orientation) result in different combinations of alleles in gametes

What are non-sister chromatids?

• In a diploid cell, each homologous pair of chromosomes consists of one chromosome that originated from the organism's father, and one from the mother
• During replication prior to meiosis, each chromosome copies to form a bivalent
• The chromatids align in prophase I, during which paternal chromatids and maternal ones can line up directly against each other
• If a pair of adjacent chromatids are originated from two different parental chromosomes, they are called non-sister chromatids
  • As such, they carry the same genes but can carry different alleles

Non-sister chromatids originate from different parents' chromosomes and align during prophase 1

Crossing over

• Crossing over is the process by which non-sister chromatids exchange alleles
• Process:
  • During prophase I, homologous chromosomes pair up and are in very close proximity to each other
  • The non-sister chromatids can get entangled and 'cross over' each other
  • The entanglement places stress on the DNA molecules
  • As a result of this molecular stress, a section of chromatid from one chromosome may break and re-join with the chromatid from the other chromosome
  • The breaking and re-joining is catalysed by endonuclease and DNA ligase enzymes respectively
• This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
• Any process that involves breaking and re-joining of DNA to create new combinations of genetic information is called recombination
  • DNA/chromosomes that have exchanged DNA in this way are referred to as recombinant
• When the DNA coils up, DNA strands at the crossing points remain attached to each other, so this causes the chromosome structure to change shape, developing an X-shaped join
• These crossing points are called chiasmata (singular: 'chiasma')
• There is usually at least one, if not more, chiasma present in each bivalent during meiosis
• Crossing over does not just occur between non-sister chromatids that are immediately adjacent to each other
  • Crossing over can occur from one chromatid to either/both chromatids of the adjacent homologous pair

The formation of chiasmata (following synapsis)

Crossing over produces new combinations of alleles on the chromosomes of the haploid cells

• So within a bivalent, or tetrad, there are 4 chromatids lying alongside each other and forming chiasmata
• Some of these chromatids will have exchanged lengths of DNA with each other at the chiasmata
• Each chromatid from a tetrad separates from the others during meiosis II
• Each chromatid goes on to form a haploid gamete
  • And so a different range of alleles will be carried in each gamete cell
  • This contributes greatly to intraspecific variation
• Back in prophase I, crossing over is more likely to occur further down the chromosome, away from the centromere
  • Because areas of DNA away from the centromere can flail around more, they are more likely to become entangled
  • Gene locus can therefore affect the genotype spread within a population, based on the likelihood of crossing over generating new allele combinations

Distinct allele combinations appear in haploid gametes following crossing over

• Because of the random nature of how chromatids align and where they break, there is an almost infinite range of combinations of how DNA can recombine during crossing over
  • This explains that even in a very large population, no two individuals will have exactly the same genotype (except identical twins)