Crossing Over: Introduction and Mechanism

UPDATED ON:- 01-01-2024

जीन विनिमय: परिचय व क्रियाविधि (Crossing Over: Introduction and Mechanism):-

जीन विनिमय (Crossing Over):- वह प्रक्रिया जिसमें समजात गुणसूत्रों की Non – sister chromatids के मध्य जीनों का आदान – प्रदान होता है, जीन विनिमय कहलाता है।

(The process in which genes are exchanged between the non-sister chromatids of homologous chromosomes is called crossing over.)

 

·     यह केवल अर्धसूत्री विभाजन की प्रोफेज – I में होने वाली प्रक्रिया है।

(This is a process occurring only in the Prophase - I of meiosis.)

क्रियाविधि (Mechanism):- इसके 5 मुख्य चरण हैं –

(It has 5 main steps –)

1. Synapsis

2. Duplication

3. Crossing Over

4. Chiasmata Formation

5. Terminalization

1. Synapsis:-

Ø  यह प्रोफेज – I की Zygotene उपअवस्था में होता है।

(It occurs in the zygotene sub-stage of prophase - I.)

Ø  इस चरण में समजात गुणसूत्र एक – दूसरे के पास आकार परस्पर 1000A° की दूरी पर व्यवस्थित हो जाते हैं। इस प्रक्रिया को Synapsis कहते हैं।

(In this step the homologous chromosomes are arranged near each other at a distance of 1000A°. This process is called Synapsis.)

Ø  Synapsis के निर्माण में 3 प्रकार का युग्मन होता है।

(3 types of coupling occurs in the formation of synapsis.)

Ø  Synaptonemal complex:- Synapsis बनने के पश्चात 1000A° की जगह में एक तन्तुनुमा जटिल बनाता है। इसकी खोज M. J. Moses ने क्रेफिश की Spermatocyte में की थी।

(After formation of synapsis, it forms a fibrous complex in the space of 1000A°. It was discovered by M. J. Moses in the Spermatocyte of Crayfish.)

2. Duplication:-

Ø  यह प्रोफेज – I की Pachytene उपअवस्था में होता है।

(It occurs in the Pachytene sub-stage of prophase - I.)

Ø  इस चरण में प्रत्येक समजात गुणसूत्र के विपाटन से 2 sister chromatids बनते हैं। इसमें Centromere का विपाटन नहीं होता है।

(In this step, each homologous chromosome splits to form 2 sister chromatids. There is no splitting of the centromere.)

3. Crossing Over:-

Ø  यह प्रोफेज – I की Pachytene उपअवस्था में होता है।

(It occurs in the Pachytene sub-stage of prophase - I.)

Ø  इस चरण में आन्तरिक Non – sister Chromatids जीन विनिमय करते हैं। इस प्रक्रिया में Recombinase enzyme भाग लेता है।

(In this step, crossing over takes place between the internal non-sister chromatids. Recombinase enzyme takes part in this process.)

Ø  आन्तरिक Non – sister chromatids खण्डों में टूट जाते हैं। अब इन खण्डों का आदान – प्रदान हो जाता है।

(The internal non-sister chromatids are broken into segments. Now these segments are exchanged.)

4. Chiasmata Formation:-

Ø  यह प्रोफेज – I की Diplotene उपअवस्था में होता है।

(It occurs in the diplotene sub-stage of prophase - I.)

Ø  Non – sister chromatids के खण्डों का आदान – प्रदान होने के पश्चात इन्हें जोड़ दिया जाता है। इसके फलस्वरूप Diplotene उपअवस्था में एक X – आकृति की संरचना दिखाई देती है, जिसे Chiasma कहते हैं।

(After the exchange of segments between non-sister chromatids, they are joined. As a result, an X-shaped structure appears in the diplotene sub-stage, which is called Chiasma.)

Ø  Chiasmata की संख्या समजात गुणसूत्रों की लंबाई पर निर्भर करती है। सामान्यतया इनकी संख्या 4 से अधिक नहीं होती है।

(The number of Chiasmata depends on the length of the homologous chromosomes. Usually their number does not exceed 4.)

Ø  स्थिति के आधार पर Chiasma 2 प्रकार के होते हैं:-

(There are 2 types of Chiasma depending on the position:-)

i. Terminal Chiasmata:- गुणसूत्र के शीर्ष पर उपस्थित होता है।

(Present at the top of the chromosome.)

ii. Intercalary Chiasmata:- गुणसूत्र के मध्य में उपस्थित होता है।

(It is present in the middle of the chromosome.)

5. Terminalization:-

Ø  यह प्रोफेज – I की Diakinesis उपअवस्था में होता है।

(It occurs in the Diakinesis sub-stage of Prophase - I.)

Ø  जीन विनिमय की प्रक्रिया पूर्ण हो जाने के पश्चात chromatids एक – दूसरे से दूर हो जाते हैं तथा chiasmata विलुप्त हो जाते हैं। इस प्रक्रिया को Terminalization कहते हैं।

(After the process of crossing over is complete, the chromatids move away from each other and the chiasmata get disappeared. This process is called Terminalization.)