There are many evidences that support the endosymbiotic theory to be established. Endosymbiosis meaning shared internal life informs how eukaryotic features evolved from the prokaryotic cells. (Reece, C. et al 2008). Mitochondria being a b-proteobacteria (aerobic bacteria)and chloroplast being cynobacteria (photosynthetic bacteria) started life as prokaryotes living within a larger cell (Archibald 438-46).
According to the theory, the cell wall for the larger bacteria disappeared and started to ingest the smaller bacteria cells. In 1967, Lynn Margulis suggested a hypothesis that billions of years ago, the chance of organisms that were acquiring their energy from organic substance manufactured by cells must have ingested aerobic bacteria in which some of the bacteria cells started life within the cell (Tortora, G. et al. Eds. 2007). Due to symbiotic relationship of different organisms interacting and gaining something constructive in an environment without causing harm to each other, the bacteria began to evolve and produced energy that could be used by the cell as the cell also supplied nutrients (Oliver. 2003).
There are evidences that support the endosymbiotic theory and be the likelihood of mitochondria and chloroplast having similarities with prokaryotes in detailed comparison and it is as followed:
DNA
Even though chloroplast and mitochondria are present in the eukaryotic cell, the configurations of their DNA are different. The DNA molecule in the eukaryotic cell is multiple linear and the chromosomes are grouped in a nucleus but the mitochondria and the chloroplast have one single loop DNA genome and a circular chromosome which is similar to a prokaryotic cell. (IUPUI Department of Biology. 2002)
Size
The size of the eukaryotic cell is 50- 500 microns but the size of both mitochondria and chloroplast are 1- 10 microns which is the same as a prokaryotic cell. In comparison with the prokaryotic cell, the eukaryotic cell is big. These similarities indicates how both chloroplast and the mitochondria were originates of the prokaryotes. (IUPUI Department of Biology. 2002)
Ribosomes
The ribosomal unit in the mitochondria and the chloroplast are the same as the prokaryotes and it is 70S but the eukaryotic is 80S. (IUPUI Department of Biology. 2002)
Replication and Reproduction
Mitochondria and chloroplast replicate and reproduce by binary fission (1 cell splits into 2) which is the same as the prokaryotic cell but the eukaryotic undergoes that process by mitosis. (IUPUI Department of Biology. 2002)
Electron transport chain
The electron transport chain are found in the plasma membrane for the mitochondria and the chloroplast which is also the same as the prokaryotes but there is none found in the plasma membrane of the eukaryotes. (IUPUI Department of Biology. 2002)
Appearance on earth
According to the IUPUI Department of Biology (2002), the first establishment of the prokaryotic for anaerobic bacteria was approximately 3.8 billion years ago, photosynthetic bacteria was 3.2 billion years ago and the aerobic bacteria was 2.5 billion years but the eukaryotic, mitochondria and chloroplast are the same. Reece, C. et al (2008), shows that all three were established 2.1 billion years ago.
All these evidence from the comparison and fossil records indicates that mitochondria and chloroplast evolved as prokaryotes but have integrated and is now dependant of eukaryotic cell. For instance, because of the synthesis of ATP, mitochondria depend on the eukaryotic cell for ribosome for the making of protein that will be contributed to the mitochondria. (Reece, C. et al. 2008)
Reference List
Archibald, J. M. "Secondary Endosymbiosis." Encyclopedia of Microbiology. Ed. Schaechter Moselio. Oxford: Academic Press, 2009. 438-46.
OLIVER. 2003., The Student's Guide To Research Ethics. [online]. Open University Press. Available from:<http://lib.myilibrary.com?ID=94751> 2 November 2010
IUPUI DEPARTMENT OF BIOLOGY(2002): The endosymbiotic theory [online]
available at:http://www.biology.iupui.edu/biocourses/n100/2k2endosymb.html [accessed 2 november 2010]
Tortora,G et al. (2007).Microbiology:An Introduction,ninth edition, Pearson International Edition
Reece, C. et al. (2008). Biology,Pearson International Edition
Describe the role of helicase in the DNA replication
DNA (deoxyribonucleic Acid) replication is an essential process which ensures that an accurate copy of the original nucleotide sequence is duplicated in preparation for the makings of a daughter cell (Fanning et al. 156-77).
The process of DNA replication is semi-conservative because when the DNA copies an identical form of itself, one strand of the parental duplex is conserved under an accurate reproduction of hereditary information and the complimentary DNA strand is then biosynthesised using each of the two parental strands as templates(Schempp 119-45).
A DNA molecule consists of a phosphate binding to a deoxyribose sugar making it the backbone of the DNA. They are linked together by a strong covalent bond. The bases in the middle are connected to their complimentary strand and they are categorised into two groups which are the purines (A-T, Adenosine- Thymine) and the pyrimidines (G-C, Guanine- Cytosine). This is called the base paring rules (Reece, C. et al 2008)
When a DNA molecule copies or duplicate, each strand becomes a template for instructing nucleotides into new complimentary strands. These nucleotides are lined up alongside the templates according to their base pair.
A DNA molecule begins replication at special sites called origins of replication. The process at each origin is bidirectional until the duplication of the whole molecule. Each end of a replication bubble is known as a replication fork; this region is 'Y-shaped' where the unwinding of the parental strands takes place. A number of proteins contribute in the unwinding of the parental strands.
DNA requires different functions of different proteins such as helicase, Topoisomerase, SSB (Single-Stranded Binding), DNA polymerase and DNA ligase. (Reece, C. et al 2008)
Helicase is an enzyme as well as a motor protein that begins the replication process by untwisting the double helix and separating the two parental DNA strands. The process occurs by catalysing the interference of the hydrogen bonds that secures the nucleic acids together with energy obtained from ATP hydrolysis making them into template strands. When they separate, the two annealed nucleic acid moves along a nucleic acid phosphordiester backbone (Matson, Bean, and George 13-22).
There are other enzymes that help helicase undergo its role and so therefore after the separation, SSB (Single- Strand Binding) proteins connect to the unpaired DNA strand which stabilises them. Untwisting of the double helix causes a strong twist and strain ahead of the replication fork. Topoisomerase is an enzyme which helps helicase in many ways. It prevents the strain by breaking, rotating and rejoining. (Reece, C. et al 2008)
Helicase performs strand separation process by binding to a region on a single stranded nucleic acid. After this process it then translocate at different direction 5'-3' or 3'-5' prime making it anti-parallel. (Biochemical Society Transaction 2005).
