Meymunlarda 48, insanlarda isə 46 xromosom olması necə izah olunur?

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Təkamül nəzəriyyəsinin səhv və boşluqlarla dolu olduğunu deyən insanlar çox zaman özlərini belə arqumentasiya edirlər: insanabənzər meymunlarda – şimpanze, qorilla, oranqutanlarda – insana nisbətən 2 xromosom çoxdur. Təkamül prosesində necə oldu ki, insanlarda 2 xromosom itib? İndi belə hallar olurmu? Necə olur ki, insanlar mutant olduqlarını heç ehtimal belə etmirlər? Bəs, onların törəyib artmaları nəyə görə baş verir?

Birinci, xatırlatmaq istərdim ki, xromosom –  DNT və zülallardan əmələ gəlmiş hüceyrəvi strukturdur. İnsanda 23 cüt xromosom var – bunlardan 23-ü anadan, qalan 23-ü isə atadan bizə keçmişdir. Cəmi 46. Şimpanzelərdə isə – 24+24=48. Xromosomların tam dəsti “kariotip” adlanır. . Hər bir xromosomda bizim DNT-miz qatlanmış şəkildə yerləşir. Əslində xromosomların sayı heç o qədər də vacib deyil, vacib olan xromosomları təşkil edən genlərdir. Eyni gen dəsti başqa sayda xromosoma yerləşdirilə bilər.

(Müqayisə: birinci, insanın xromosomları, ikinci isə şimpanzenin)

 

1980-ci ildə “Science” jurnalında Minneapolis Universitetinin genetik araşdırmaçıları tərəfindən məqalə yayımlanıb. Araşdırma çərçivəsində həmin vaxt üçün “xromosom rənglənməsinin” qabaqcıl metodu istifadə edilirdi (xromosomlarda müxtəlif ölçülərdə və parlaqlıqda köndələn zolaqlar yaranır, bununla yanaşı hər bir xromosom öz xətlər dəsti ilə seçilir). Məlum oldu ki, insan və şimpanzenin xromosom zolaqları nisbətən identikdir. Yaxşı, şimpanzelərdə olan artıq xromosom nədir bəs? Hər şey çox sadədir: əgər qarşı-qarşıya insanın ikinci və şimpanzenin 12 və 13-cü xromosomlarını (bir-birlərinə birləşdirərək) qoysaq, nəticədə görə bilərik ki, onlar bir-birləri ilə tamamilə oxşardırlar.

Bundan sonra, 1991-ci ildə, alimlər ikinci xromosomu daha diqqətlə araşdırmağa başladılar və orada axtardıqlarının – DNT ardıcıllığını gördülər. Bu xüsusiyyət telomerlər – xromosomların sonluqları üçün xarakterdir. Və bir il daha sonra həmin xromosomda ikinci sentromedin (- hüceyrənin düzgün bölünməsini təmin edən sahə. Sentromerlər xromosomları çiyinlər adlanan iki yerə ayırır: hər bir xromosomda yalnız bir aktiv sentromer olur) izlərini tapdılar. Tamamilə aydın oldu ki, görünən bir xromosom əvəzində, əvvəllər iki xromosom olub. Nəticədə, bizim əcdadlarımızda olan iki xromosom, bir xromosomla əvəz olunub.

Bu hadisə nə vaxt baş verib? İndi, paleagenetiklərin qazıntılar nəticəsində aldığımız sümüklərdəki genomu bərpa edə bildiklərinə görə, bilirik ki, neandertallarda və denis insanında bir neçə on min il əvvəl, bizdə olduğu kimi artıq 46 xromosom olmuşdur. Müasir məlumatlara görə birləşmə daha əvvəl baş verib, təxminən 2,5 – 4,5 milyon il əvvəl. Daha dəqiq tarixi əldə etmək üçün Homo heydelbergensis və Homo erectus-un genomu, həm də indi yaşayan insanabənzər meymunların lazım olan xromosomlarını yenidən qurmaq lazımdır.

Mutasiyaların genişlənməsinə görə xromosomların birləşməsi günümüzdə də baş verir, – buna “robertson translokasyası” deyirlər (qısa olaraq – ROB).

Məqaləyə əlavə edilən şəklə baxın. Çox güman düşünəcəksiniz ki, o çox zaman iki “çiyin” kimidir. Bəzən bu çiyinlər eyni uzunluqdadır – belə xromosomları metasentrik adlandırırlar. Əgər çiyinlər eyni deyillərsə, – submetasentrik adlanırlar. Və əgər çiyinlərdən biri görünməyəcək dərəcədə qısadırsa, – xromosom akrosentrikdir.

Beləliklə, ROB olarkən iki akrosentrik xromosom sentromer hissəsində qırılır və onların uzun çiyinləri birləşərək yeni – tək xromosom yaradırlar. Qısa çiyinlər də birləşirlər və kiçik xromosom yaradırlar. Lakin çox hallarda bir neçə hüceyrə bölgüsündən sonra itirlər. – Bu anda bir xromosom azalır. Buna baxmayaraq kiçik xromosom öz tərkibində çox az genetik material daşıyır, bu da heç bir həyata təsiri olmadan yox ola bilər. Hər şey yaxşıdır, lakin orqanizmdə tək xromosom dəstəsi yaranır (46 əvəzinə, 22+23=45).

Robertson translokasyası – o qədər də nadir hadisə deyil. 45 xromosom hər min yeni doğulmuş uşaqda rast gəlinir. İnsanda ROB bu xromosomları, akrosentrik xromosomlara təsir edə bilər – 13, 14, 15, 21 və 22. ROB daşıyıcılarının çox tamamilə sağlam insanlardır, və heç bir ehtimalları belə yoxdur ki, xromosomlarından biri yoxdur. O vaxta kimi ki, uşaq dünyaya gətirməyə çalışırlar. Lakin ola bilər ki, heç bir problem yaranmasın, – bunun da nəticəsi kimi, mutasiya nəsildən-nəsilə heç bir yaranma ehtimalı olmadan ötürüləcəkdir.

Araşdırma müəlliflərinin sözlərinə görə ROB daşıyıcıları və sadə insanlar arasında olan reproduktiv baryer, teoretik olaraq 44 xromosomlu insanların izolyasiya olunmuş populyasiyasına gətirib çıxara bilər (yəni bir-birləri ilə nəsilartırma prosesində iştirak etsələr). Bu da yeni Homo sapiens yarım-nəslinin meydana gəlməsinə şərait yarada bilər.

 

Mənbə və əlavə araşdırma işləri:

  1. Eklund A, Simola KO, Ryynänen M. Translocation t(13;14) in nine generations with a case of translocation homozygosity // Clin Genet. 1988;33:83—6.
  2. Jieping Song et al. A family with Robertsonian translocation: a potential mechanism of speciation in humans // Molecular Cytogenetics 2016 9:48 DOI: 10.1186/s13039-016-0255-7
  3. Stankiewicz P. One pedigree we all may have come from — did Adam and Eve have the chromosome 2 fusion? // Molecular Cytogenetics 2016 9:72 DOI: 10.1186/s13039-016-0283-3
  4. Yunis JJ, Sawyer JR, Dunham K. The striking resemblance of high-resolution G-banded chromosomes of man and chimpanzee // Science. 1980 Jun 6;208(4448):1145-8
  5. Avarello R et al. Evidence for an ancestral alphoid domain on the long arm of human chromosome 2 // Hum Genet. 1992 May;89(2):247-9 https://www.ncbi.nlm.nih.gov/pubmed/1587535

 

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