‘Several questions about origin of life have been answered, but much remains to be studied’’ said Stanley Miller and Harold Urey way back in 1959 after reporting laboratory synthesis of amino acids in primitive earth conditions. Many advances down the line yet the scientists have long been grappling with a fundamental question – which genetic material was first to be formed on the primitive earth, DNA or RNA, or a bit of both? There is evidence now to suggest that DNA na RNA both may have co-existed in the primordial soup from where the life forms may have evolved with respective genetic materials.
Fundisho kuu la biolojia ya molekuli linasema hivyo DNA hufanya RNA hufanya protini. Protini are responsible for majority, if not all the reactions taking place in an organism. The entire functionality of an organism is majorly dependent upon their presence and interaction of protini molecules. According to central dogma, protini are produced by the information contained in DNA which is converted to functional protini via a messenger called RNA. However, it is possible that protini themselves can survive independently without any DNA or RNA, as is the case with prions (misfolded protini molecules that do not contain DNA or RNA), but can survive on their own.
Kwa hivyo, kunaweza kuwa na hali tatu za asili ya maisha.
A) If the protini or its building blocks were able to form abiotically during the atmosphere that existed billions of years ago in primordial soup, protini can be termed as the basis of asili ya maisha. Ushahidi wa majaribio kwa niaba yake unatoka kwa jaribio maarufu la Stanley Miller1, 2, ambayo ilionyesha kwamba wakati mchanganyiko wa methane, amonia, maji na hidrojeni huchanganywa pamoja na kuzunguka kabla ya kutokwa kwa umeme, mchanganyiko wa amino asidi huundwa. Hii ilithibitishwa tena miaka saba baadaye3 in 1959 by Stanley Miller and Harold Urey stating that the presence of reducing atmosphere in primordial earth gave rise to synthesis of kikaboni compounds in the presence of above-mentioned gases plus smaller amounts of carbon monoxide and carbon dioxide. The relevance of Miller-Urey experiments was questioned by the scientific fraternity for a number of years, who thought that the gas mixture used in their research was too reducing with respect to the conditions that existed on primordial Earth. A number of theories pointed towards a neutral atmosphere containing an excess of CO2 with N2 and water vapor4. Hata hivyo, hali ya kutoegemea upande wowote pia imetambuliwa kuwa mazingira yanayokubalika kwa usanisi wa asidi ya amino5. Kwa kuongeza, kwa protini to act as origins of life, they need to self-replicating leading to a combination of different protini to cater to different reactions taking place in an organism.
B) Kama supu primordial ilitoa masharti kwa ajili ya ujenzi wa vitalu DNA na / au RNA to be formed, then either of these could have been the genetic material. The research until now favoured RNA to be the genetic material for the origin of life forms due to their capability of folding upon itself, existing as a single strand and acting as an enzyme6, capable of making more RNA molecules. A number of self-replicating RNA enzymes7 have been discovered over the years suggesting RNA to be the starting genetic material. This was further strengthened by the research performed by John Sutherland’s group that led to the formation of two bases of RNA in an environment similar to primordial soup by including phosphate in the mixture8. Uundaji wa vizuizi vya ujenzi vya RNA pia umeonyeshwa kwa kuiga angahewa ya kupunguza (iliyo na amonia, monoksidi kaboni na maji), sawa na ile iliyotumiwa katika majaribio ya Miller-Urey na kupitisha utokaji wa umeme na leza zenye nguvu nyingi kupitia kwao.9. If RNA is to be believed to be the originator, then when and how did DNA and proteins come into being? Did DNA develop as a genetic material later because of the unstable nature of RNA and proteins followed suit. Answers to all these questions still remain unanswered.
C) Hali ya tatu kwamba DNA na RNA zinaweza kuwepo pamoja katika supu ya awali ambayo ilisababisha asili ya uhai ilitokana na tafiti zilizochapishwa mnamo 3.rd June 2020 by John Sutherland’s group from the MRC Laboratory at Cambridge, UK. The researchers simulated the conditions that existed on a primordial Earth billions of years ago, with shallow ponds in the lab. They first dissolved chemicals that form RNA in water, followed by drying and heating them and then subjecting them to UV radiation that simulated sun’s rays existing in primordial time. This not only led to the synthesis of the two building blocks of RNA lakini pia ya DNA, suggesting that both nucleic acids co-existed at the time of origin of life10.
Kulingana na ujuzi wa kisasa uliopo leo na kuheshimu fundisho kuu la biolojia ya molekuli, inaonekana kuwa sawa kwamba DNA na RNA zilikuwepo ambazo zilisababisha asili ya maisha na uundaji wa protini zilikuja/zilizotokea baadaye.
Walakini, mwandishi anapenda kukisia hali nyingine ambapo macromolecules zote tatu muhimu za kibaolojia, yaani. DNA, RNA na protini zilikuwepo pamoja kwenye supu ya awali. Hali ya fujo iliyokuwepo katika supu ya awali inayohusisha asili ya kemikali ya uso wa dunia, milipuko ya volkeno na uwepo wa gesi kama vile amonia, methane, monoksidi kaboni, dioksidi kaboni pamoja na maji inaweza kuwa bora kwa macromolecules zote kuundwa. Dokezo la hili limetolewa na utafiti uliofanywa na Ferus et al., ambapo nucleobases ziliundwa katika angahewa sawa ya kupunguza.9 kutumika katika majaribio ya Miller-Urey. Ikiwa tutaamini nadharia hii, basi wakati wa mageuzi, viumbe tofauti vilipitisha nyenzo moja au nyingine ya maumbile, ambayo ilipendelea kuwepo kwao kusonga mbele.
Hata hivyo, tunapojaribu kuelewa asili ya viumbe, utafiti zaidi unahitajika ili kujibu maswali ya msingi na yanayofaa kuhusu jinsi uhai ulivyotokea na kuenezwa. Hili lingehitaji mkabala wa "nje ya kisanduku" bila kutegemea ubaguzi wowote unaoletwa katika fikra zetu na itikadi za sasa zinazofuatwa katika sayansi.
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Marejeo:
1. Miller S., 1953. Uzalishaji wa Asidi za Amino chini ya Masharti Yanayowezekana ya Primitive Earth. Sayansi. 15 Mei 1953: Vol. 117, Toleo la 3046, ukurasa wa 528-529 DOI: https://doi.org/10.1126/science.117.3046.528
2. Bada JL, Lazcano A. et al 2003. Supu ya Prebiotic–Kupitia Majaribio ya Miller. Sayansi 02 Mei 2003: Vol. 300, Toleo la 5620, ukurasa wa 745-746 DOI: https://doi.org/10.1126/science.1085145
3. Miller SL na Urey HC, 1959. Mchanganyiko wa Mchanganyiko wa Kikaboni kwenye Dunia ya Mwanzo. Sayansi 31 Jul 1959: Juz. 130, Toleo la 3370, ukurasa wa 245-251. DOI: https://doi.org/10.1126/science.130.3370.245
4. Kasting JF, Howard MT. 2006. Muundo wa anga na hali ya hewa kwenye Dunia ya mapema. Philos Trans R Soc Lond B Biol Sci 361:1733–1741 (2006). Iliyochapishwa:07 Septemba 2006. DOI: https://doi.org/10.1098/rstb.2006.1902
5. Cleaves HJ, Chalmers JH, et al 2008. Tathmini upya ya usanisi wa kikaboni wa prebiotic katika angahewa za sayari zisizo na upande. Orig Life Evol Biosph 38:105–115 (2008). DOI: https://doi.org/10.1007/s11084-007-9120-3
6. Zaug, AJ, Cech TR. 1986. The intervening sequence RNA of Tetrahymena is an enzyme. Science 31 Jan 1986: Vol. 231, Issue 4737, pp. 470-475 DOI: https://doi.org/10.1126/science.3941911
7. Wochner A, Attwater J, et al 2011. Unukuzi wa Ribozime-Catalyzed wa Ribozimu Inayotumika. Sayansi 08 Apr: Vol. 332, Toleo la 6026, ukurasa wa 209-212 (2011). DOI: https://doi.org/10.1126/science.1200752
8. Powner, M., Gerland, B. & Sutherland, J., 2009. Mchanganyiko wa ribonucleotides ya pyrimidine iliyoamilishwa katika hali ya awali inayowezekana. Asili 459, 239–242 (2009). https://doi.org/10.1038/nature08013
9. Ferus M, Pietrucci F, et al 2017. Uundaji wa nucleobases katika anga ya kupunguza Miller-Urey. PNAS Aprili 25, 2017 114 (17) 4306-4311; ilichapishwa kwa mara ya kwanza Aprili 10, 2017. DOI: https://doi.org/10.1073/pnas.1700010114
10. Xu, J., Chmela, V., Green, N. et al. 2020 Selective prebiotic formation of RNA pyrimidine and DNA purine nucleosides. Nature 582, 60–66 (2020). Published: 03 June 2020. DOI: https://doi.org/10.1038/s41586-020-2330-9
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