Mitosis And Meiosis: Significance And Variation

Mitosis: Definition and Stages

Give a description of mitosis and meiosis and discuss their significance [This must an illustrated and annotated flow diagram]

Describe the impact of one geneticist on our understanding of genetics. [pick one key scientist and explain there findings]

Describe the main sources of variation, including crossing over and gene and chromosome mutations. [Must use Annotated diagrams]

Mitosis is a process of division of eukaryotic cell in which a mother cell is divided into two daughter cells. It takes place only in eukaryotic cells where, division of the chromosomes takes place and is followed by division of nucleus and division of cytoplasm (cytokinesis) occurs along with the cell membrane resulting into two new genetically identical cells. Stapes involved in mitosys are interphase, prophase, metaphase, anaphase and telophase. Interphase is the preparatory stage for mitosis. The process actually begins with prophase (Tiang, He and Pawlowski  2012).

Interphase is the metabolic phase or “daily living” of a cell where it metabolises, grows and copies its DNA and gets prepared for mitosis. It is the longest period of a cell cycle. Three stages are involved in interphase, those are:

G₁ or Gap 1, during which protein synthesis takes place and the cell grows.

Synthesis, in which DNA replication takes place.

G₂ or Gap 2, where the mitochondria divide and the cell grows until mitosis begins.

G₀ or Gap zero is a stage where the cell either has an extended Gap 1 or it does not divide ever after.

Figure: 1

(Source: Phschool.com, 2016)

Figure: 2

(Source: Woods, 2016)

Prophase is the first stage of mitosis that follows G₂ interphase. In this stage the chromosomes starts to condense and forms long thread like structure. Each chromosome has two chromatids that are joined at centromere. The centrosome of the cell consists of two centrioles and it coordinates the microtubules. Prior to mitosis, the centrosome gets duplicated by the cell. Centrosomes form microtubules and moves opposite to each other.

Figure3: Prophase

(Source: Phschool.com, 2016)

At the beginning of metaphase nuclear membrane disintegrates and disappears allowing the microtubules to enter in nuclear space. In case of fungi and some protists spindle forms inside the nucleus or microtubules penetrate the intact nuclear membrane. A kinetochore is a protein based binding agent that forms during prophase (Tiang, He and Pawlowski 2012).

Figure4: Metaphase

(Source: Phschool.com, 2016)

The microtubules from the opposite centrosome interact with corresponding microtubules and forms mitotic spindle. Now the chromosomal kinetochore attaches with microtubules and two centrosomes pull the chromosomes towards opposite sides. In this stage the chromosomes align along the midline of the cell. In this stage the chromosomes becomes clearly visible and is known as metaphase checkpoint.

Significance of Mitosis

During anaphase, the chromatids of each chromosome get separated and moves towards the opposite poles along with the spindle fibres. The polar microtubules push each other and results the cell to elongate. The chromosomes reach to the poles of each cell and the nuclear membrane starts to form around the chromosomes (Nasmyth 2001).

Figure 5: Anaphase

(Source: Phschool.com, 2016)

Telophase is the last stage of mitosis where the polar microtubules grow and the cell elongates even more. The reformation of nuclear membrane gets completed using the residues of old envelope. Now each nucleus is having identical sets of chromosomes.

Figure 6: Telophase

(Source: Phschool.com, 2016)

The cell is now separated by forming a cleavage containing a contractile ring, which gradually separates two cells. Each cell contains one nucleus (Sawin et al. 1992).

Figure 7: Mitosis

(Source: cycle et al., 2009)

Mitosis increases the number of cells in any organism which is the basis of growth and development. Replacement of old cells takes place by mitosis process, which is also helpful in healing any injury. With the help of mitosis, some organisms like starfish, lizard etc. can regenerate their body part. This is the means of reproduction for organism like hydra (Tiang, He and Pawlowski 2012).

Meiosis is the process of producing four daughter cells from one mother cell and each with half of the chromosomes of the mother cell. It occurs in the gametes of sexually reproducing eukaryots. It consists of two steps: meiosis 1 and meiosis 2. Prior to meiosis, there is a synthesis phase, where the replication of genetic material takes place to form two sister chromatids. Both meiosis 1 and 2 consist of four phases. Those are prophase 1 & 2, metaphase 1&2, anaphase 1&2 and telophase 1&2 (Nasmyth 2001).

During prophase 1 the chromosomes align to form tetrads. In this stage the sister chromatids twists and crosses. During this regional exchange crossing over or exchange of genetic material may occur.

In metaphase 1, the chromosomes align across the equatorial plane in such a way that centromeres lay at either side of the plain.

In anaphase 1 the chromosomes are pulled towards the opposite ends of the cell. The homolog pair moves in opposite direction. The chromatids of the chromosome stay together.

In telophase 1, the chromosomes contain a pair of chromatids reach at the poles of cell. New nuclear membranes are formed surrounding each haploid sets of chromosome.

Meiosis: Definition and Stages

In meiosis 2 again the above mentioned phases occur resulting in formation of four daughter cells (Nei 1975).

Significance of Meiosis:

Meiosis maintains the number of chromosomes in the diploid cell after fertilization. It helps to incorporate new characteristics among the offspring by means of crossing over. In meiosis four daughter cells are produced and it leads to a genetic variation in daughter cells (Darlington 1939).

Figure 8: Meiosis

(Source: Ib.bioninja.com.au, 2016)

2.1  Gregor Johann Mendel is known as The Father of Genetics for his findings on three basic principles of genetics. He did some experiments on pea trees with different characteristics.

Experiment 1:

A purebred tall plant was crossed with a purebred short plant. All the offspring of the first generation or F₁ generation were tall. The same experiment was performed with other pairs of contrasting characteristics, which showed only one of the two characteristics (Mendel 1996).

Figure 9: Mendel’s Law of Dominance

(Source: Pixgood.com, 2016)

This phenomenon was described by Mendel’s Law of Dominance. He stated that, the dominating phenotypes or physical characteristics appear in the first generation. The other phenotype, that was not found in the first generation are the recessive characteristics. Form this experiment. Mendel concluded that the contrasting hereditary information from two different forms of same species can co-exist at the same time in a single individual. But, only the dominating character appears in the phenotype (Mendel Corcos and Monaghan 1993).

Experiment-2

From the first experiment, it has been proven that hybridizing a tall and a short pea will produce all tall pea plants in the first generation. Mendel let seven F₁ generation plant to self-hybridize, producing a second or F₂ generation. F₂ generation showed a ratio of 3:1 of tall and short pea plants.

From this result he concluded that, hereditary information of an individual species came from two different genotypes, each of them derived from each parents. Each trait was either a dominant form (denoted by “A”) or a recessive form (“a”). These two units gave four possible forms of combination and those are “AA”, “Aa”, “aA” and “aa”. As “Aa” and “aA” shows the same phenotype, there were three combinations (Bateson and Mendel 2013). Since “A” is dominated over “a”, both “AA” and “Aa” shows the same physical characteristics. The “aa” is the only genotype that shows recessive characteristic. Paired units are separated randomly during formation of gamete. So, each gamete contains one unit only. This law is known as Law of Segregation (Mendel 2008).

Figure 10: Mendel’s Law of Segregation

(Source: Pixgood.com, 2016)

Experiment-3

In this experiment two different traits of pea plant for two characteristics are selected. One is the colour of seed and another is the shape of seed. The two plants have green wrinkled seeds (yyrr) and yellow round seeds (YYRR) respectively. As the plants are homozygous, the gametes for green wrinkled seeded plant is “yr” and the gamete for yellow round seeded plants are “YR”. Offspring of the F₁ generation will be “YyRr” that is yellow round seeded. For the F₂ generation, four types of gamete can be formed after the heterozygote “YyRr” is self- hybridized as follows: “YR”, “Yr”, “yR” and “yr”. These gametes give 16 genotype combinations. The phenotypes will be round yellow, round green, wrinkled yellow and wrinkled green and the phenotype ratio will be 9:3:3:1 respectively (Mendel Corcos and Monaghan 1993).

Significance of Meiosis

Figure 11: Mendel’s Law of Independent Assortment

(Source: "Mendel's Law of Independent Assortment", 2016)

From this experiment Mendel stated that genes do not get influenced by each other with respect to the sorting of alleles into gametes. Every gene has the equal chance to contain every possible combination of alleles. It is known as Mendel’s Law of Independent Assortment.

3: Genetic variation occurs due to the variation of alleles of gene in a gene pool. It can occur both among and within populations. Genetic variation mainly occurs due to gene mutation, chromosome mutation and crossing over.

Crossing over is referred to the exchange of genetic material of homologous chromosomes to form a new recombinant chromosome during sexual reproduction. It occurs during the prophase-1 stage of meiosis. At this stage, the chromosomes replicate into two sister chromatids. The homologous chromosomes of a pair come together causing a breakage at the corresponding points of the non sister chromatids. Since the homologous chromosomes break at corresponding points, the broken sections have the corresponding alleles which are then exchanged within the chromosomes to form new recombined chromosomes (Burgoyne 1982).

Figure 12: Crossing Over

(Source: Desertbruchid.net, 2016)

A gene mutation is a permanent change in the sequence of the gene forming DNA. The effect of mutation can range from a single DNA base pair to a large number of chromosomes including a multiple genes. Gene mutation can occur mainly in two ways:

The mutations that inherited from parents are called hereditary mutations. When a sperm and an egg cell get fertilized, the zygote receives DNA from both parent cells. If the DNA undergoes a mutation, then the child will also have the mutation of each cell (Handel and Schimenti 2010).

Acquired or somatic mutation is a type of mutation that can occur during the life of a person and are present in certain cells of the body. It can occur from any biological reason or from environmental reasons. Somatic mutation cannot be transmitted to the next generation.

Figure: 13

(Source: Bailey, 2016)

A chromosome mutation is a sudden change that can occur in chromosome. Chromosome mutations can change the structure or number of chromosomes in the cell. It can change a large segment if DNA on a chromosome (Cooper and Krawczak 1993).

Chromosomal mutation can occur by structural changes such as translocation of chromosome fragments, deletion or breakage of chromosome, inversion, duplication or improper division of a chromosome (Orr 2010).

Figure: 14

(Source: Biology | Mrs. McComas, 2016)

References:

Bateson, W. and Mendel, G., 2013. Mendel's principles of heredity. Courier Corporation.

Burgoyne, P.S., 1982. Genetic homology and crossing over in the X and Y chromosomes of mammals. Human genetics, 61(2), pp.85-90.

Sources of Variation

Cooper, D.N. and Krawczak, M., 1993. Human gene mutation. Bios Scientific Pub Ltd.

Darlington, C.D., 1939. The evolution of genetic systems. The Evolution of Genetic Systems.

Handel, M.A. and Schimenti, J.C., 2010. Genetics of mammalian meiosis: regulation, dynamics and impact on fertility. Nature Reviews Genetics, 11(2), pp.124-136.

Mendel, G., 2008. Experiments in plant hybridisation. Cosimo, Inc..

Mendel, G., Corcos, A.F. and Monaghan, F.V., 1993. Gregor Mendel's Experiments on plant hybrids: a guided study. Rutgers University Press.

Mendel, O.V.G., 1996. the first Geneticist (p. 363). Oxford: Oxford University Press.

Moore, K.L., Persaud, T.V.N. and Torchia, M.G., 2015. The developing human: clinically oriented embryology. Elsevier Health Sciences.

Nasmyth, K., 2001. Disseminating the genome: joining, resolving, and separating sister chromatids during mitosis and meiosis. Annual review of genetics, 35(1), pp.673-745.

Nei, M., 1975. Molecular population genetics and evolution. North-Holland Publishing Company..

Nigro, J.M., Baker, S.J., Preisinger, A.C., Jessup, J.M., Hostetter, R., Cleary, K., Bigner, S.H., Davidson, N., Baylin, S. and Devilee, P., 1989. Mutations in the p53 gene occur in diverse human tumour types. Nature,342(6250), pp.705-708.

Orr, H.A., 2010. The population genetics of beneficial mutations.Philosophical Transactions of the Royal Society of London B: Biological Sciences, 365(1544), pp.1195-1201.

Paridaen, J.T., Wilsch-Bräuninger, M. and Huttner, W.B., 2013. Asymmetric inheritance of centrosome-associated primary cilium membrane directs ciliogenesis after cell division. Cell, 155(2), pp.333-344.

Plomin, R., DeFries, J.C., Knopik, V.S. and Neiderheiser, J., 2013.Behavioral genetics. Palgrave Macmillan.

Sawin, K.E., LeGuellec, K., Philippe, M. and Mitchison, T.J., 1992. Mitotic spindle organization by a plus-end-directed microtubule motor. Nature,359(6395), pp.540-543.

Tiang, C.L., He, Y. and Pawlowski, W.P., 2012. Chromosome organization and dynamics during interphase, mitosis, and meiosis in plants. Plant physiology, 158(1), pp.26-34.        

Vogel, F. and Motulsky, A.G., 2013. Vogel and Motulsky's Human Genetics: Problems and Approaches. Springer Science & Business Media.

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