Differences between Prokaryotes and Eukaryotes

Both prokaryotes and eukaryotic genomes are DNA genomes. Prokaryotic genome is usually found on the cytoplasm and consists of only one chromosome, while the eukaryotic one consists of multiple chromosomes and most of the eukaryotic genome is found on the nucleus. Prokaryotic genome is less complex compared to the eukaryotic one which generally consists of longer genes. The coding sequences compart nearly the 90% of the prokaryotic genome and only the 3% of the eukaryotic genome. The prokaryotic genome tends to be circular, while the eukaryotic genomes are generally linear. The prokaryotic genomes are always haploid, but the eukaryotes can be either diploid or haploid. Compared to the eukaryotic genomes, the prokaryotic genomes have a high rate of protein coding genes. Furthermore, the eukaryotic genome has hundreds of rRNA genes compared to the prokaryotic ones that have 1-10 rRNA genes. Eukaryotic genomes have a higher rate of repeated sequences, end telomeres and introns compared to the prokaryotic genomes that rarely consist of those parts. Also origin of replications usually are numerous on eukaryotes but found only once in prokaryotes.[1]. Both eukaryotic and prokaryotic gene structures include similar sequence elements necessary for gene expression. They both involve regulatory elements such as promoters, silenceres and enhancers usually on their ends and contain ORI. Also the reading strands of both eukaryotic and prokaryotic strands is on the 5′-3′ direction. Eukaryotic genes differ from prokaryotic genes because introns are absent in prokaryotic genes and in prokaryotic genes there is no such a phenomenon such as alternative splicing. Also eukaryotic genes consist of more regulatory regions compared to the prokaryotic ones. In prokaryotes the genes can sometimes organize themselves in a polycistronic operon. Riboswitches also are some regulatory regions of the prokaryotic genes that lack on the eukaryotic genes. The main difference of the RNA polymerase II assisted transcription initiation is that in prokaryotes only DNA containing promoter and polymerase II are needed, while in eukaryotes several transcription initiation factors and proteins more are needed, such as TF, TBP, TFIID, TAF[2].



2 – https://www.ncbi.nlm.nih.gov/books/NBK9935/

Cell Division

In this experiment, observing phases of cell division for mitotic and meiotic was our purpose. We used microscope and the slides having samples. We used microscope and placed slides carefully according to lab rules.
There are 2 types of cell division, mitosis and meiosis. Mitotic cell division is realized for repairing, development and growth by multicellular organisms, but it is also reproduction for unicellular organisms. After mitotic division, cell transforms identical 2 daughter cells. Meiosis has more different purpose for organism. It is used for producing sex cells and these cells have same variations after some events such as crossing over and separation of homologs. These gametes or spores, having half number of chromosomes or haploid, combine and form zygote, having 2n chromosomes or diploid. By this way, meiotic cell division helps that organisms protect specific number of chromosomes and also have varieties.1
Interphase is common phase for both divisions. With interphase, cell cycle alternates. Interphase includes 3 stages, G1 (gap 1), S (synthesis), G2 (gap 1). In all stages, production of protein, endoplasmic reticulum, cytoplasmic … causes cell growth. 2
Cell division:
General cell division or M phase for mitotic phase has 5 stages; prophase, prometaphase, metaphase, anaphase, and telophase. 2, 3, 4
In prophase, chromosomes become visible because of condensing. The centrioles are formed for moving to opposite ends of cell or poles. The nuclear membrane disappears and mitotic spindles are formed also. These spindles are produced by centrioles in animal cells or some proteins in plant cells.2, 3, and 4
In prometaphase, the spindles or kinetochore microtubules are used for attaching to centromere of each sister chromatid on kinetochores, specialized proteins.2, 3, and 4
After prometaphase, cell division is at metaphase and sister chromosomes are arranged at metaphase plate by pushes of spindles. 3, 4
Chromosomes are separated to sister chromosomes from centromeres at anaphase. Each one goes to different poles.2
Nuclear envelopes is formed at telophase and karyokinesis is completed.2, 3
Finally, cytoplasm is divided (cytokinesis) by cleavage furrow in animals or cell plate in plants.2, 3, 4
Differences between mitotic and meiotic cell divisions:
Meiotic division have some differences despite a common interphase with mitosis. Firstly, it has 2 parts. In meiosis I, homologous chromosomes form four-structural chromosomes, tetrads and sometimes they can make exchanging parts of their chromosomes, crossing over at prophase as difference than mitosis. Besides in meiosis I, homologous chromosomes are separated, not sister chromatids in mitosis. These changes can make some variations for daughter cells.5, 6
At end of meiosis I, number of chromosome decreases half. After meiosis I, meiosis II follows it and processes of phases of meiosis are same with mitosis. At end of this two-part division, 4 daughter cells having half number of chromosome are produced.5, 6
At nature, meiosis helps that organism has a certain number of chromosome. 3, 6
Materials and Method:
In this laboratory, we observed cell division by using different types of cell, onion and lily anther. Also dyed cells are used for observation to be seen easily. We used prepared slides before and examined them under microscope by starting 4x lenses. They were used for performing from 4x to 10x and 40x. During experiment, we used experimental materials carefully and we tidied up.
After observation and investigation of images, we determined positions and density of blackspots enough to obtain phases.
We observed different types of cells at different phase of cell divisions, mitosis and meiosis. We could obtain phases under microspore by using preparing slides, having pollens of lily anther, onion cells.
The properties of onion and lily anther for this experiment are that onion meristem cells in root often divide and lily anther can produce a lot of pollen than another plants.
We determined meiotic phases and tetrads structures at pollen cells and mitotic phases at onion cells. We saw that cells dividing can be at different stages. Prophase, metaphase, anaphase, telophase and cytokinesis, having cell plate at onion samples were observed (especially at onion root images under 40x and 10x like showing at results, figure 2 ). When we saw spots or distributions, we thought that this is prophase because these points have been formed by chromosomes condensation. Then because of these blackspots could be dense at centre, we determined that the stage is metaphase. At some cell, we obtained that the blackspots goes to poles and then it made us think that it is anaphase. Finally we saw cell plate at the middle of the cells and this helped that we obtain cytokinesis. Pollens of lily anther had some phases, but we couldn’t see phases too clearly. Because of that, we could only obtain last stages of meiosis. We determined some tetrad structures and some phases. The reason we can’t see clearly could be dyes. At some places, dyed was so dense and also we can think that stain wasn’t effective enough or we couldn’t focus cell division with microscope well.

[1] Robert. S Hine, ed. (2008). Oxford Dictionary Biology (6th ed.). New York: Oxford University Press. p. 113. ISBN 978-0-19-920462-5.
[2]The Cell Cycle & Mitosis Tutorial The Biology Project – Cell Biology. University of Arizona.
[3] “How Cells Divide”. PBS. Public Broadcasting Service. Retrieved 6 December 2012.
[4]Gardner J.Eldon (1972). Principles of Genetics (4th edition). p. 57-65
[5]Moody Amous Paul (1967). Genetics of Man. p.27-31
[6]Moody Amous Paul (1967). Genetics of Man. p.31-37