What is the result of crossing over during meiosis?

Meiosis is required in the maintenance of chromosome number as well as bring about an increase in genetic diversity. In this BiologyWise post, we explain the process of crossing over and why is it important.

Crossovers and Genetic Mapping

The process of crossing over was used in genetic mapping to understand the order of genes on a chromosome, and to determine the distance between them. This works on the basis that if two genes are present far apart on the chromosome, the frequency of crossing over between the two will be greater.

All individuals produce beings similar to themselves by the process of reproduction. We can classify reproduction into two basic types, sexual and asexual. All prokaryotes and some eukaryotes reproduce by asexual methods. These include processes like budding, binary fission, fragmentation, vegetative propagation, parthenogenesis,etc.

Most eukaryotes reproduce by sexual reproduction. Here, the genetic material of two organisms combines to give rise to a new individual. This process takes place with the help of two underlying mechanisms of meiosis: the process of gamete formation and fertilization―the fusion of the male and female gametes.

Meiosis is a specialized type of cell division that takes place only in specialized sex cells or gametes. This division reduces the chromosome number by half, it is required for the formation of haploid cells (n) from diploid cells (2n). This process is required for the maintenance of the chromosome number in individuals. Before we begin with what is crossing over, we would like to talk about meiosis in brief.

What is Meiosis?

♦ Meiosis can be defined as a reduction division that takes place in primordial germ cells. Every diploid cell will give rise to four haploid daughter cells at the end of a meiotic division. Prior to meiosis, when the cell is in the S-phase of the cell cycle, replication of the DNA takes place to generate two identical copies of each strand of the chromosome. These identical copies are called ‘sister chromatids’.

♦ During meiosis, chromosomes are usually found in pairs, there is one chromosome that is of maternal origin and the other is of paternal origin. This pair of chromosomes are known as homologous chromosomes.

♦ Meiosis can be divided in two stages: meiosis I and meiosis II. It is in the prophase of meiosis I that crossing over of the chromosomes takes place, and the homologous chromosomes are separated into two daughter cells. In meiosis II, the sister chromatids are pulled apart from each other to give rise to four haploid daughter cells. Meiosis reduces the number of chromosomes into half, which double once again in the process of fertilization and give rise to a new diploid zygote.

What is Crossing Over?

♦ Crossing over is simply the exchange of genetic material between two homologous chromosomes to give rise to recombinant chromosomes. In prophase I, homologous chromosomes align lengthwise or pair with each other, and exchange of genetic material between the two chromosomes takes place, which is known as crossing over. The pairing of the homologous chromosomes is known as synapsis, and the point at which these chromosomes pair with each other is known as a chiasma (pl. chiasmata).

♦ The process of crossing over or recombination is initiated by the protein Spo 11. This protein forms a complex with other proteins like RAD50 and MRE11A, and NBS 1 is required for breaking of the double stranded DNA. Certain exonucleases are required to bring about digestion of the 5′ ends in order to generate single stranded 3′ tails. MRE11A has also been seen to possess certain exonuclease as well as endonuclease activities.

♦ The DNA recombinases like DMC1 and RAD51 then take over. These two proteins bind with a couple of other proteins and are required for the invasion of the non-sister chromatid. DMC1 is required to find the allelic sequences on the non-sister chromatid. RAD51 helps to bring about strand invasion of the non-sister chromatid in an ATP dependent manner as well as in the search for allelic sequences.

♦ Next, the 3′ end of the invading strand is used as a primer for the synthesis of the complementary DNA on the non-sister chromatid that has been invaded, annealing the invading strand to it. As the synthesis of the complementary DNA sequence continues, it displaces the original complementary strand.

♦ The displaced complementary DNA strand then anneals itself to the strand that was originally complementary to the invading strand. The structure that is thus formed is known as a Holliday junction.

♦ These interlocked strands are then nicked and ligated with the help of certain endonucleases and ligase. It must be noted that the creation of the single stranded 3′ tails only takes place in the non-coding parts of the DNA or in the junk DNA.

Why is Crossing Over Important?

♦ Crossing over helps to bring about random shuffling of genetic material during the process of gamete formation. This results in formation of gametes that will give rise to individuals that are genetically distinct from their parents and siblings.

♦ This genetic variation is required to increase the ability of a population to survive. A greater genetic diversity would reduce the chances of inheritance of deleterious traits in the population, and therefore, help increase the fitness of the individuals of a population.

♦ An increased genetic variation would also mean a greater variation in susceptibility to diseases. So, if there were to be an epidemic of a disease, this variability would prevent the whole population from being wiped out.

♦ Another benefit of genetic variation is that some traits that would increase an individual’s ability to survive may be introduced in the population.

Crossing-over is a normal part of meiosis in which homologous chromosomes can exchange alleles. Mutations happen when two chromosomes don't line up correctly.

Genetic diversity occurs because certain physical characteristics, like eye color, are variable; this variability is the result of alternate DNA sequences that code for the same physical characteristic. These sequences are commonly referred to as alleles. The various alleles associated with a specific trait are only slightly different from one another, and they are always found at the same location (or locus) within an organism's DNA. For example, no matter whether a person has blue eyes, brown eyes, or green eyes, the alleles for eye color are found in the same area of the same chromosome in all humans. The unique combination of alleles that all sexually reproducing organisms receive from their parents is the direct result of recombination during meiosis.

What happens during recombination?

Genetic recombination is a complex process that involves alignment of two homologous DNA strands, precise breakage of each strand, equal exchange of DNA segments between the two strands, and sealing of the resultant recombined DNA molecules through the action of enzymes called ligases. Despite the complexity of this process, recombination events occur with remarkable accuracy and precision in the vast majority of instances.

When recombination occurs during meiosis, the cell's homologous chromosomes line up extremely close to one another. Then, the DNA strand within each chromosome breaks in the exact same location, leaving two free ends. Each end then crosses over into the other chromosome and forms a connection called a chiasma. During this process, it is common for large sections of DNA containing many different genes to cross from one chromosome to another. Finally, as prophase I draws to a close and metaphase I begins, the crossing-over process concludes, and the homologous chromosomes prepare to separate. When the homologous chromosomes are later pulled apart during anaphase I, each chromosome carries new, unique allele combinations that are a direct result of recombination.

Does recombination occur in cells other than gametes?

Beyond its role in meiosis, recombination is important to somatic cells in eukaryotes because it can be used to help repair broken DNA, even when the break involves both strands of the double helix. These breaks are known as double-stranded breaks, or DSBs. When DSBs happen, a homologous chromosome can serve as the template for synthesis of whatever portion of the genetic material has been lost as a result of the break. Then, once synthesized, this new DNA can be incorporated into the broken DNA strand, thereby repairing it. In effect, this is a form of recombination, because the broken-off area is replaced with new material from a homologous chromosome. Recombination can also be used in a similar way to repair smaller, single-stranded breaks. In general, recombination can occur any time homologous chromosomes pair up, whether they are freely floating in tandem or lined up on the metaphase plate during meiosis.

Recombination isn't limited to eukaryotes, however. A special type of recombination called conjugation occurs in many prokaryotes, and it has been particularly well studied and characterized in E. coli bacteria. During conjugation, genetic material from one bacterium is transferred to another bacterium, and it is then recombined in the recipient cell. Recombination also plays important roles in DNA repair in prokaryotic organisms, just as it does in eukaryotic organisms.