Meiosis,

Mechanism of Crossover,

Significance in evolution.

By-AMIT KUMAR SAHOO15151

What is Meiosis?• Meiosis is a process where a single cell divides

twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females.• Meiosis can be divided into nine stages. These are

divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II).

We Are UNIQUE!

• One of the reasons for this genetic mix up or

uniqueness is by a process called Crossing Over

that occurs during meiosis.

What is Crossing over?• Crossing over is the exchange of segments between the

non-sister chromatids of homologous chromosome.

• The term crossing over was coined by Morgan.

Origins:There are two popular and overlapping theories explaining the origins of crossing-over, coming from the different theories on the origin of meiosis.

1. The first theory rests upon the idea that meiosis evolved as another method of DNA repair, and thus crossing-over is a novel way to replace possibly damaged sections of DNA.

2. The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity.

Mechanism of meiotic crossing over.The major steps in meiotic crossing over are1 ) synapsis2) duplication of chromosome3) crossing over and4) terminalisation.

-Synapsis is the intimate pairing between the two homologous chromosomes. -Synapsis is followed by duplication of chromosome (in pachytene). -Crossing over or exchange of segments between the non-sister chromatids of homologous chromosome occurs at the tetrad stage. -Crossing over can be divided into three major steps:1 ) breakage of chromatid segments2) their transposition (movement to the respective site) 3) fusion or joining.

The final step is terminalisation. After crossing over the non-sister chromatids starts to repel eachother.

Chiasma itself moves in a zipper fashion towards the end of tetrad. This movement of chiasma is known as terminalisation.

The Biology Underlying Mendelian Inheritance• Mendel’s Laws can be derived directly from our

understanding of Meiotic cell division or Meiosis.• The purpose of meiosis is to introduce further genetic

diversity by creating gametes, either eggcells or sperm cells, that are genetically different from the parent cells.

What is the function of crossing-over?

• In species that reproduce sexually, offspring are genetically distinct

from their parents because they inherit genetic material from both.

• Such genetic diversity is the product of meiosis, a type of cellular

division that creates reproductive cells from a parent cell.

• The paired chromosomes can exchange segments of DNA via a

mechanism called crossing-over.

Crossing-over has two main functions.

1. The first is to increase genetic recombination.

2. The second is to ensure that parental chromosomes are equitably distributed

among the reproductive cells produced by meiosis.

• Without crossing-over, the chromosomes would be distributed abruptly.

• Too many crossing over is also not good because could disrupt advantageous

gene combinations that have established themselves over evolutionary time.

Crossing over in favor of plant breeding.

• Increasing the number of crossing-over events could lead to more genetic recombination and thusnovel gene combinations, a desirable outcome in the context of plant breeding.

• Ex- Arabidopsis thaliana

Increasing genetic recombination by inhibiting mechanisms that limit crossing-over.

Mechanism of Crossing Over:• It occurs during Prophase I of Meiosis• Genetic swapping occurs between paired homologous

chromosomes in our sex cells—The Egg and SpermHomologous Chromosomes Exchanging DNA

by Crossing Over

From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over

Its Why You and I Don’t Look Alike

Crossing Over ensures a combination of the maternal

and paternal genes we inherited

BOTTOM LINE

• So, when chromosomes separated during meiosis II, some of the daughter cell receive daughter chromosome with recombined alleles.• Due to this genetic recombination offspring have a

different set of genes and alleles than there parents

Crossing over and Chiasmata• Chiasmata is the point where two homologous non-

sister chromatids exchange genetic material during crossing over during meiosis.• Chiasmata becomes visible during diplotene stage

of prophase I during meiosis.• But actual crossing over occcur during previous

pachytene stage. When each tetrad which is composed of two pairs of sister chromatids begins to split. Only point of contact is chaismata.

Crossing over and chaismata

Types of Crossing over• Single crossing over:

Chromosomal crossover (or crossing over) is the

exchange of genetic material between homologous

chromosomes that results in recombinant

chromosomes. It is one of the final phases of genetic

recombination, which occurs during prophase I of

meiosis (pachytene) during a process called synapsis.

single crossing over

Double crossing overIt refers to formation of two chiasmata between non-sister chromatids of homologous chromosomes. Two simultaneous reciprocal breakage and reunion events between the same two chromatids.

The biological significance of meiosis.

1. The conventional view that it generates by recombination

and sexual reproduction the genetic diversity on which

natural selection can act.

2. That recombination at meiosis plays an important role in the

repair of genetic defects in germ line cells.

3. That it is essential, at least in animals, for the reprogramming

of gametes which give rise to the fertilized egg.

4. That it helps maintain the immortality of the germ line

Significance of crossing over

• Crossing over is universal in occurrence, occurs in plants, animals, bacteria, viruses and moulds.• Meiotic crossing over allows a more independent

selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between sister chromatids• Helps in proving linear arrangement of genes.• Recombination does not have any influence on

the statistical probability that another offspring will have the same combination. This theory of “independent assortment” of alleles is fundamental to genetic inheritance.• Origin of new character.• Necessary for natural selection, as it increases chances

of variation.• Ex-Selection of useful recombination by geneticists has brought about green

revolution in our country.

Role of Crossing over in Evolution

• Crossing over allows genetic variants on the same chromosome to evolve independently,

which greatly increases an organism's evolutionary potential.

• (Explanation)

• If there were no crossing over, all genetic variants on a chromosome would be inherited as a

block. Image a chromosome copy which contains a good variant--let's say, flu resistance--at

one gene, and a bad variant--let's say, tapeworm susceptibility--at a different gene. Without

crossing over, the population has to choose between flu and tapeworms. Crossing over can

produce a chromosome with the good variant and without the bad one, allowing the

population to move toward a better solution. This speeds up the rate of adaptation.

References:• www.nature.com/scitable/topic/chromosomes-and-cyt

ogenetics-7• www.plantphysiol.org/content/171/4/2294.full• journals.plos.org/plosbiology/article?id=10.1371/

journal.pbio.1000501• bmcgenomics.biomedcentral.com/articles/

10.1186/1471-2164-13-53• https://www.ncbi.nlm.nih.gov/books/NBK21870/

“Sai Ram”