Mutasiyalar haqqında

93

Ara keçidlər bir sonrakı və bir öncəki canlının xüsusiyyətlərini ehtiva etdikləri üçün yeni canlı növlərindən ibarətdirlər (bu yazımızı oxumaqda fayda var). Əslində, ara keçidlərin atası olan canlılar da onlardan əvvəlki canlılarla arakeçidlər arasında bir ara keçiddirlər. Eyni misalla izah etsəm, sizin ulu babanızla sizin aranızda yaşamış bütün fərdlər ara keçiddir və bir-birlərindən fərqləndikləri qədər ortaq xüsusiyyətlərə də sahibdirlər. Hər nə qədər bənzəsələr də yeni bir fərddirlər. Eynilə ara keçidlər də belədir.

 

Hər bir canlını özündən bir öncəki canlıdan kiçik fərqliliklər ayırır, lakin bu birdən-birə, bir nəsil müddətində deyil, uzun zaman ərzində baş verir. Yəni yeni bir növ bir ara keçiddir və bir anda siçanın filə çevrilməsi və ya dənizdə yaşayan canlının quruya çıxıb qaçmağa başlaması bir anda baş vermir. Növləşmə Bioloji Növ Tərifi daxilində sonradan “ortaq əcdad” olaraq xatırlanacaq olan bir qrup canlının müxtəlif mexanizmlər daxilində bir-birləriylə cütləşə bilməyəcək qədər dəyişmələri nəticəsində, öz içlərində cütləşə bilən ancaq digər canlı qruplarıyla məhsuldar döllər verə bilməyən, iki və ya daha çox yeni canlı qrupunun meydana gəlməsi deməkdir. Bu növyaranma mexanizmlərinə Allopatrik növləşmə (fiziki baryerlərlə ayrılma), Simpatrik növləşmə (mühit xüsusiyyətlərinə görə fərqliləşmə – çox xromosomluluq (polyploidy) deyilən yanlış kopiyalanma kimi genetik bir faktordan qaynaqlanır), Peripatrik növləşmə (köç etmək), Parapatrik növləşmə (heterozigot fitness kimi səbəblərdən qaynaqlanan populyasiyaların qarışmaması) daxildir.

 

 

Daha spesifik olaraq yeni növlərin əmələ gəlməsində isə Təbii Seçmə, Süni Seçmə, Cinsi Seçmə, Qohum Seçməsi, Qrup Seçməsi, Gen Axışı, Genetik Sürüklənmə, Mutasiyalar, Xromosomal Gen Dəyişikliyi, Transpozonlar, Plazmidlər, Üfiqi Gen Köçürülməsi kimi 20-dən çox təkamül mexanizmi məsuldur. Lakin nədənsə bu mexanizmlərdən xəbərsizmiş kimi sadəcə mutasiyalar üzərində durulur və “Baxın, təkamülçülər hər şeyin təsadüf olduğunu deyir!”, “Təkamül mutasiya deməkdir!” kimi boş iddialarla öz fikirlərincə Təkamül nəzəriyyəsini “çürüdürlər”. Bilməniz lazımdır ki, mutasiyalar təkamül prosesinin çox kiçik bir hissəsini ehtiva edir və təkamül nəzəriyyəsi sadəcə mutasiyalara bağlı deyil, mutasiyalardan daha funksional mexanizmlər vardır.

 

 

Mutasiyalar təkamülə səbəb olmurlar, sadəcə, çeşidlilik yaradan mexanizmlərdən biridir. Digər mexanizmlər isə Genetik sürüklənmə və Gen axışıdır. Mutasiyaların qısavədəli ən böyük təsiri müşahidə olunan fakt isə variyasiyadır. Mutasiyalar, xromosom rekombinasiyaları və krosingover(Krosingover-meyoz bölünmə zamanı xromosom hissələrinin dəyişməsidir) variyasiyanın səbəbləridir. Variyasiya isə öz növbəsində təkamül üçün xammal sayılır. Mutasiyalar neytral (70-90%), zərərli (8-25%) və xeyirli (2-5%) olmaq üzrə üç yerə bölünür və fərdin mühitə uyğunlaşma müvəffəqiyyətinə təsir edir. Bir çox mutasiya neytral olsa belə zamanla bu neytral mutasiyalar yığılaraq fərdlərdə dəyişiklərə səbəb olur. Yararlı mutasiyaların adətən, 5-10%-lik təsiri olsa da faydalarının sərhəddi yoxdur, zərəli mutasiyaların bir sərhəddi var: ölüm. Yararlı mutasiyalar 1600%-lik təsirə malik ola bilsə də zərərli mutasiyalarda bu maksimum 100%-dir ki, bu da ölümlə nəticələnir. Mutasiyaların populyasiya daxilində təsiri və yayılması Herdi-Veynberq tənliyi ilə riyazi olaraq modellənir.

 

 

Hər nə qədər mutasiyaların 90%-nin zərəli olduğu fikri ilə yanaşı çox nadir olaraq baş verdiyini eşitsək də bu ifadə doğru və tam deyil. Doğru ifadə “Nukleotid sayına uyğun baş verən mutasiya sayı çox azdır” formasındadır. İnsanda milyonlarca hüceyrə və bu hüceyrələrin daxilində nukleotidlər (3200000000 ədəd) vardır. İnsanlarda tək nukleotid üzərində mutasiya baş verməsi üçün hüceyrənin 4000000 dəfə mitoz bölünməsi lazımdır. Halbuki, bir insanda bu say 10 katrilyondur. Hesablamar göstərir ki, bir insanda ortalama ömür müddəti boyunca 100 milyon – 10 milyard arası sayda mutasiya baş verir. Bunların çoxu gen düzəltmə mexanizmləri vasitəsi ilə düzəldilsə də bəzi mutasiyalar gözdən qaçır və dəyişikliklərə səbəb olur.

 

Dəyişiklərin nəsildən-nəsilə ötürülməsini isə kəpənək efekti kimi ələ ala bilərik. Bir nukleotiddə baş vermiş və gen mexanizmləri tərəfindən gözdən qaçırılmış mutasiya digər baş verəcək mutasiyalarla birlikdə uzun zaman dilimində dəyişikliklərə səbəb olur. Yəni ani bir mutasiyaya uğradığımız anda 5 başımız, 8 ayağımız çıxmayacaq. Valideynlərimizdən 56-160 mutasiya qədər fərqlənirik və bu, heç də baş vermiş kimi görünmür. Əgər mutasiyaların çoxu zərərli olsaydı, bu gün insan deyə canlı növü yaşamazdı. Faydalı mutasiya olaraq isə tetraxromasiyaya malik olaraq digər insanlardan daha çox rəng çalarını görməyi arzu edə bilərsiniz.

 

Bəs mutasiyaların təsirlərini zamanda geriyə doğru baxaraq necə təyin edirik? Məsələn, bizi şimpanzelərlə, qorilllarla, neandertallarla ortaq atamızdan ayıran mutasiyalar hansılar idi? Bunun üçün molekulyar saatadlı metod istifadə edilir. Molekulyar saat müəyyən edilmiş bir protein ailəsinin (məsələn, ribosom) içində sabit nisbətdə amin turşusu yaranmasından yola çıxır və iki növ arasındakı dissosiasiya dərəcəsinimüəyyən etmək üçün istifadə edilən molekulyar təkamül analizi üsuludur. Zülalları yaradan amin turşularının genlərdəki kodlarına baxaraq baş vermiş mutasiyalardan qaynaqlanan genetik fərqliliklərə əsaslanaraq bir növün digər növdən nə zaman ayrıldığı, bu növlərin nə qədər fərqləndiyi və oxşar olduğu müəyyən edilir.

 

 

Misal üçün, genlər alfa-qlobin zülalını (hemoqlobinin bir komponenti) kodlayır və hər milyard ildə hər bağ cütü (DNT və RNT üzərində olan və bir-birlərinə hidrogen rabitəsi ilə bağlanan nukleobağlar) 56 dəyişikliyə məruz qalır. Bu nisbəti və genetik kodlamadakı dəyşiklikləri nəzərə alaraq növlərin nə zaman ortaq atadan təkamül etdiklərini, yəni ayrıldıqlarını deyə bilərik. Təsəvvür edin ki, hər hansı iki növdə olan DNT zəncirində ümumilikdə 4, hər bir növün DNT zəncirində isə 2 fərqli  bağ var və bu genlərin üzərində aparılmış genetik araşdırmalar nəticəsində bilirik ki, bir bağın dəyişmə müddəti 25 milyon illik zaman dilimində baş verir. Bu o deməkdir ki, bu DNT-lər 100 milyon illik təkamül prosesi nəticəsində formalaşıb, fərqliləşiblər və onların ortaq atası 50 milyon il öncə yaşayıb.

 

Müəllif; Ilkin Cəfərov

93 ŞƏRH

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