Meiosis I : Reductional Cell Division: Stages, Overview & Importance

Meiosis I : Reductional Cell Division: Stages, Overview & Importance

Edited By Irshad Anwar | Updated on Nov 09, 2024 08:30 AM IST

Meiosis specialises in contributing to sexual reproduction in eukaryotic organisms: plants, animals, and fungi. Unlike mitosis, it brings about a reduction in the number of chromosomes by half to create gametes-sperm and eggs-genetically unique cells. Meiosis I is a topic of the chapter Cell Cycle and Cell Division in Biology.

Meiosis I

Meiosis I is a reductional cell division that halves the number of chromosomes. It changes a diploid to a haploid state so that gametes can then combine during reproduction without excessive chromosome numbers. The reduction is accomplished in two successive cell divisions: meiosis I and meiosis II.

Whereas mitosis produces two identical daughter cells, meiosis I produce two cells that differ from each other. The parent cell after prophase I, in which homologous chromosomes pair up, and anaphase I, during which the paired chromosomes separate. This way, by allowing for more genetic diversity through crossing over, all the resulting gametes will have a unique combination of genetic material, being thus a driver for evolutionary adaptation.

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Meiosis I Stages

Meiosis I is studied under Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

  • It is the longest phase of meiosis I.

  • It is divided into five substages: leptotene, zygotene, pachytene, diplotene and diakinesis.

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Leptotene

  • The chromatin begins to condense to form chromosomes.

  • Chromosomes appear as thin and long threads.

  • By the end of this phase, chromosomes become visible under a microscope.

Zygotene

  • The pairing of the homologous chromosomes initiates in this phase.

  • The pairing of homologous chromosomes is called synapsis.

  • The synapsed homologous chromosomes appear in the form of bivalent chromosomes or tetrad of chromatids.

  • In the tetrad, two similar chromatids of the same chromosome are called sister chromatids and those of two homologous chromosomes are termed non-sister chromatids.

  • A filamentous ladder-like nucleoproteins complex, called a synaptonemal complex appears between the homologous chromosomes. It holds the homologous chromosomes together.

Pachytene

  • The exchange of parts between non-sister chromatids occurs during this phase.

  • It is called the crossing over.

  • Crossing over occurs through breakage and reunion of chromatid segments.

  • Breakage is called nicking. It is assisted by an enzyme endonuclease.

  • Reunion is termed annealing. It is aided by an enzyme ligase.

Diplotene

  • This phase involves pulling away the synapsed homologous chromosomes.

  • The point of attachment of the homologous chromosomes where crossing over occurs is called chiasma.

  • Homologous chromosomes remain attached only at chiasma.

  • There can be more than one chiasmata.

Diakinesis

  • It marks the terminalization of chiasma.

  • The nuclear membrane and nucleolus degenerate.

  • Chromosome recondenses and tetrad moves to the metaphase plate.

  • Spindle fibres begin to form.

  • When the diakinesis of prophase-I is completed than cell enters into metaphase-I.


Prophase I

Metaphase I

  • During this phase, bivalents arrange themselves on the metaphase plate.

  • Hence, a fully formed spindle and equatorial alignment of the chromosomes are seen during this phase.

  • The alignment of homologous chromosomes is independent of each other.

  • This is responsible for generating genetic variability.

Anaphase I

  • The homologous chromosomes of each bivalent separate from each other.

  • The separated homologous chromosomes move to opposite poles

  • Therefore, in this phase, the chromosomes separate and not the chromatids.

  • So, each chromosome will still have two sister chromatids.

  • Hence, anaphase I involve a reduction in the number of chromosomes.


Anaphase I

Telophase I

  • Two daughter nuclei are formed but the chromosome number is half the chromosome number of the mother cell.

  • This phase is not necessarily complete wholly.

  • The spindle disappears, but new nuclear envelopes need not form before the onset of meiosis II.

Cytokinesis I

  • It may or may not follow the telophase I.

  • When it occurs, it forms the dyad of cells.

Interkinesis

  • Following cytokinesis I, the cells enter interkinesis.

  • It is also known as intermeiotic interphase.

  • During this phase, there is no duplication or replication of DNA as the chromosomes are already duplicated.

Significance of Meiosis I

Meiosis I is essential in providing ways in which genetic diversity can take place. Through crossing over processes during prophase I and independent assortment that takes place in metaphase I leads to specific sets of genetic materials that form the gametes. Such genetic diversity is quite important for the survival as well as adaptation of species.

Since it allows for greater variability in the offspring; this way, they could become better adapted to changing environmental conditions. Meiosis I also provides for the decreased number of chromosomes from diploid to haploid so that when such gametes fuse during fertilisation. The correct number of chromosomes characteristic of the species is present in the zygote. Thus, meiosis I is recognised as a process of great importance in maintaining genetic integrity and therefore in promoting evolutionary success among sexually reproducing organisms.

Also Read:


Recommended Video for Meiosis I



Frequently Asked Questions (FAQs)

1. What is meiosis I and how does it differ from meiosis II?

Meiosis I is a division where homologous chromosomes are paired and crossed over in such a way that they are separated into two daughter cells with half the number of the original chromosomes. On the other hand, Meiosis II is similar to mitosis but it deals with haploid cells; its end products are four genetically different daughter cells.

2. What are the stages of prophase I in meiosis?

Stages of prophase I include leptotene, zygotene, pachytene, diplotene and diakinesis.

3. How does crossing over contribute to genetic diversity?

Crossing over, which occurs in prophase I of meiosis, involves the exchange of DNA segments between homologous chromosomes. New combinations are now formed due to crossing over on the chromosomes. This results in more genetic variation in the gametes and offspring.

4. Why is meiosis called reductional division?

Meiosis is called reductional division since meiosis reduces by half the number of chromosomes. In meiosis I, homologous chromosomes separate. This results in two daughter cells with half the number of chromosomes of the parent cell.

5. What is the significance of homologous chromosomes in meiosis I?

Homologous chromosomes pair up during meiosis I, facilitating crossing over and ensuring that each resulting gamete receives a unique combination of genetic material from both parents. This process promotes genetic diversity among offspring, which is crucial for adaptation and evolutionary success.

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Directions for question :

M/s Deloitte Touche Tohmatsu Limited, one of the top four audit and accounting firms in the world with headquarters at London, UK, and with an operational presence in 153 countries, hires Management Trainees (MT) from all the premier management institutes of India thrice every year, in the months of January, May and September.

Each new group of Management Trainees (MT) have to go through a four month rigorous training schedule, after which they have to pass through a test consisting of a written assessment and a case-analysis. The top hundred ranked Management Trainees (MT) based on the performance in the test are confirmed as Management Executives (ME). The rest are given the opportunity of undergoing the training for four months one more time along with the next batch of Management Trainees (MT) and then passing through the subsequent test consisting of the written assessment and case-analysis. The Management Trainee (MT) who fails to get confirmed as a Management Executive (ME) the second time is fired.

The scatter-graph below depicts the number of Management Trainees (MT) at Deloitte taking the tests from January 2020 till May 2022, and the vis-à-vis hired Management Trainees (MT) at Deloitte who were fired :

It is also known that for the month of September 2019 at Deloitte, 96 hired Management Trainees (MT) failed to be confirmed as a Management Executive (ME) the first time, and that 36 hired Management Trainees (MT) were fired. 

Question :

In which test did the minimum number of Management Trainees (MT) get confirmed as a Management Executive (ME) in the second attempt ?

Option: 1

September 2020

 

 


Option: 2

May 2021


Option: 3

January 2021

 


Option: 4

January 2022 


Directions for question:

Two friends Moloy and Niloy passed out from the Purulia Institute of Science and Technology with B.Tech degrees in Mechanical Engineering, but even after a year placement was hard to find. So they decided to take the challenge head-on, came down to Kolkata, rented a garage space on Park Street, and having an affinity towards making people enjoy good food, started their firm named 'B.Tech Bread-Omlette Wala'. 

They started with three items on the menu. One was the French Toast which could be prepared in 3 minutes. The second was the Egg Tortillas which took 15 minutes to prepare. Any one of Moloy and Niloy could prepare any one of them at a time. The third was the Egg Bhurji with French Fries. This however was prepared on an automated fryer which could prepare 3 servings at a time and took 5 minutes irrespective of the number of servings equal to or below 3. The fryer did not need anyone to attend to it, and the time to put in the raw ingredients could be neglected. So one could tend to the preparation of other items while the Egg Bhurji with French Fries were being prepared. 

They wanted to serve the orders as early as possible after the order was given. The individual items in any order were served as and when all the items were ready, and the order was then considered closed. None of the items on the menu were prepared in advance in anticipation of future orders. 

On the first day, 3 groups of customers came in and ordered at 6.00 pm, 6.10 pm, and 6.13 pm. The first order was for a plate of Egg Tortillas, two plates of French Toast, and three plates of Egg Bhurji with French Fries. The second order was for a plate of French Toast and two plates of Egg Bhurji with French Fries. The third order was for a plate of Egg Tortilla and a plate of Egg Bhurji with French Fries. 

On the backdrop of the above information answer the questions given :

Question: 

Assuming that the next customer's order could only be attended to when the previous customer's order was closed, at what time would the first customer's order be considered closed ?

 

Option: 1

6.15 pm

 

 


Option: 2

6.17 pm

 


Option: 3

6.18 pm

 


Option: 4

6.20 pm


Directions for question :

Six sticks of equal lengths were kept in the vertical position in an empty flower-vase, to be arranged at the six corners of a regular hexagon. The two ends of each of the sticks were of different colours. 

The top ends of the sticks were one of each of the following colours – Red, Cyan, Pink, Brown, Black and Green. The bottom ends were one of each of the following colours – Blue, Yellow, White, Orange, Purple and Grey. Both the sets of colours mentioned were in no particular order.

It was also known that :

a) The stick with the red colour was opposite to the stick with the blue colour

b) There were exactly two sticks whose both ends had colours whose names started with the same letter

c) The stick with the grey colour was adjacent to the stick with the white colour

d) The stick with the cyan colour was adjacent to both the sticks with the brown colour and the one with the blue colour

e) The stick with the purple colour was adjacent to both the sticks with the grey colour and the one with the green colour

f) The stick with the white colour was opposite to the stick with the green colour

Question :

What was the colour of the bottom end of the stick having brown colour at the top end ?

Option: 1

 White 

 


Option: 2

Yellow 


Option: 3

Black

 


Option: 4

Grey


Directions for question:

Two friends Moloy and Niloy passed out from the Purulia Institute of Science and Technology with B.Tech degrees in Mechanical Engineering, but even after a year placement was hard to find. So they decided to take the challenge head-on, came down to Kolkata, rented a garage space on Park Street, and having an affinity towards making people enjoy good food, started their firm named 'B.Tech Bread-Omlette Wala'. 

They started with three items on the menu. One was the French Toast which could be prepared in 3 minutes. The second was the Egg Tortillas which took 15 minutes to prepare. Any one of Moloy and Niloy could prepare any one of them at a time. The third was the Egg Bhurji with French Fries. This however was prepared on an automated fryer which could prepare 3 servings at a time and took 5 minutes irrespective of the number of servings equal to or below 3. The fryer did not need anyone to attend to it, and the time to put in the raw ingredients could be neglected. So one could tend to the preparation of other items while the Egg Bhurji with French Fries were being prepared. 

They wanted to serve the orders as early as possible after the order was given. The individual items in any order were served as and when all the items were ready, and the order was then considered closed. None of the items on the menu were prepared in advance in anticipation of future orders. 

On the first day, 3 groups of customers came in and ordered at 6.00 pm, 6.10 pm, and 6.13 pm. The first order was for a plate of Egg Tortillas, two plates of French Toast, and three plates of Egg Bhurji with French Fries. The second order was for a plate of French Toast and two plates of Egg Bhurji with French Fries. The third order was for a plate of Egg Tortilla and a plate of Egg Bhurji with French Fries. 

On the backdrop of the above information answer the questions given :

Question: 

Assuming that the next customer's order could only be attended to when the previous customer's order was closed, at what time would the third customer's order be considered closed ?

 

Option: 1

6.28 pm

 


Option: 2

6.35 pm

 


Option: 3

6.38 pm


Option: 4

6.45 pm


Directions for question:

Two friends Moloy and Niloy passed out from the Purulia Institute of Science and Technology with B.Tech degrees in Mechanical Engineering, but even after a year placement was hard to find. So they decided to take the challenge head-on, came down to Kolkata, rented a garage space on Park Street, and having an affinity towards making people enjoy good food, started their firm named 'B.Tech Bread-Omlette Wala'. 

They started with three items on the menu. One was the French Toast which could be prepared in 3 minutes. The second was the Egg Tortillas which took 15 minutes to prepare. Any one of Moloy and Niloy could prepare any one of them at a time. The third was the Egg Bhurji with French Fries. This however was prepared on an automated fryer which could prepare 3 servings at a time and took 5 minutes irrespective of the number of servings equal to or below 3. The fryer did not need anyone to attend to it, and the time to put in the raw ingredients could be neglected. So one could tend to the preparation of other items while the Egg Bhurji with French Fries were being prepared. 

They wanted to serve the orders as early as possible after the order was given. The individual items in any order were served as and when all the items were ready, and the order was then considered closed. None of the items on the menu were prepared in advance in anticipation of future orders. 

On the first day, 3 groups of customers came in and ordered at 6.00 pm, 6.10 pm, and 6.13 pm. The first order was for a plate of Egg Tortillas, two plates of French Toast, and three plates of Egg Bhurji with French Fries. The second order was for a plate of French Toast and two plates of Egg Bhurji with French Fries. The third order was for a plate of Egg Tortilla and a plate of Egg Bhurji with French Fries. 

On the backdrop of the above information answer the questions given :

Question: 

Suppose Moloy and Niloy had decided to process multiple orders at the same time, however strictly prioritising a first come first serve basis, when would the second customer's order be considered closed ?

Option: 1

6.20 pm

 

 


Option: 2

6.18 pm


Option: 3

6.15 pm

 


Option: 4

6.12 pm


Directions for question:

Two friends Moloy and Niloy passed out from the Purulia Institute of Science and Technology with B.Tech degrees in Mechanical Engineering, but even after a year placement was hard to find. So they decided to take the challenge head-on, came down to Kolkata, rented a garage space on Park Street, and having an affinity towards making people enjoy good food, started their firm named 'B.Tech Bread-Omlette Wala'. 

They started with three items on the menu. One was the French Toast which could be prepared in 3 minutes. The second was the Egg Tortillas which took 15 minutes to prepare. Any one of Moloy and Niloy could prepare any one of them at a time. The third was the Egg Bhurji with French Fries. This however was prepared on an automated fryer which could prepare 3 servings at a time and took 5 minutes irrespective of the number of servings equal to or below 3. The fryer did not need anyone to attend to it, and the time to put in the raw ingredients could be neglected. So one could tend to the preparation of other items while the Egg Bhurji with French Fries were being prepared. 

They wanted to serve the orders as early as possible after the order was given. The individual items in any order were served as and when all the items were ready, and the order was then considered closed. None of the items on the menu were prepared in advance in anticipation of future orders. 

On the first day, 3 groups of customers came in and ordered at 6.00 pm, 6.10 pm, and 6.13 pm. The first order was for a plate of Egg Tortillas, two plates of French Toast, and three plates of Egg Bhurji with French Fries. The second order was for a plate of French Toast and two plates of Egg Bhurji with French Fries. The third order was for a plate of Egg Tortilla and a plate of Egg Bhurji with French Fries. 

On the backdrop of the above information answer the questions given :

Question: 

Suppose Moloy and Niloy had decided to process multiple orders at the same time, however strictly prioritising a first come first serve basis, when would the third customer's order be considered closed ?

 

Option: 1

6.22 pm

 


Option: 2

6.25 pm


Option: 3

6.28 pm 


Option: 4

6.30 pm


Directions for question:

Two friends Moloy and Niloy passed out from the Purulia Institute of Science and Technology with B.Tech degrees in Mechanical Engineering, but even after a year placement was hard to find. So they decided to take the challenge head-on, came down to Kolkata, rented a garage space on Park Street, and having an affinity towards making people enjoy good food, started their firm named 'B.Tech Bread-Omlette Wala'. 

They started with three items on the menu. One was the French Toast which could be prepared in 3 minutes. The second was the Egg Tortillas which took 15 minutes to prepare. Any one of Moloy and Niloy could prepare any one of them at a time. The third was the Egg Bhurji with French Fries. This however was prepared on an automated fryer which could prepare 3 servings at a time and took 5 minutes irrespective of the number of servings equal to or below 3. The fryer did not need anyone to attend to it, and the time to put in the raw ingredients could be neglected. So one could tend to the preparation of other items while the Egg Bhurji with French Fries were being prepared. 

They wanted to serve the orders as early as possible after the order was given. The individual items in any order were served as and when all the items were ready, and the order was then considered closed. None of the items on the menu were prepared in advance in anticipation of future orders. 

On the first day, 3 groups of customers came in and ordered at 6.00 pm, 6.10 pm, and 6.13 pm. The first order was for a plate of Egg Tortillas, two plates of French Toast, and three plates of Egg Bhurji with French Fries. The second order was for a plate of French Toast and two plates of Egg Bhurji with French Fries. The third order was for a plate of Egg Tortilla and a plate of Egg Bhurji with French Fries. 

On the backdrop of the above information answer the questions given :

Question: 

A fourth customer comes in and orders two plates of French Toast at 6.24 pm. Suppose Moloy and Niloy had decided to process multiple orders at the same time, however strictly prioritising a first come first serve basis. For exactly how many minutes would one of the friends be idle from 6.00 pm till serving the last customer, assuming that the four customers were the only ones to have come in within the period being discussed ?

Option: 1

9

 


Option: 2

13


Option: 3

18


Option: 4

21


Directions for question:

Two friends Moloy and Niloy passed out from the Purulia Institute of Science and Technology with B.Tech degrees in Mechanical Engineering, but even after a year placement was hard to find. So they decided to take the challenge head-on, came down to Kolkata, rented a garage space on Park Street, and having an affinity towards making people enjoy good food, started their firm named 'B.Tech Bread-Omlette Wala'. 

They started with three items on the menu. One was the French Toast which could be prepared in 3 minutes. The second was the Egg Tortillas which took 15 minutes to prepare. Any one of Moloy and Niloy could prepare any one of them at a time. The third was the Egg Bhurji with French Fries. This however was prepared on an automated fryer which could prepare 3 servings at a time and took 5 minutes irrespective of the number of servings equal to or below 3. The fryer did not need anyone to attend to it, and the time to put in the raw ingredients could be neglected. So one could tend to the preparation of other items while the Egg Bhurji with French Fries were being prepared. 

They wanted to serve the orders as early as possible after the order was given. The individual items in any order were served as and when all the items were ready, and the order was then considered closed. None of the items on the menu were prepared in advance in anticipation of future orders. 

On the first day, 3 groups of customers came in and ordered at 6.00 pm, 6.10 pm, and 6.13 pm. The first order was for a plate of Egg Tortillas, two plates of French Toast, and three plates of Egg Bhurji with French Fries. The second order was for a plate of French Toast and two plates of Egg Bhurji with French Fries. The third order was for a plate of Egg Tortilla and a plate of Egg Bhurji with French Fries. 

On the backdrop of the above information answer the questions given :

Question: 

Had Niloy been absent on that day, and assuming that the next customer's order could only be attended to when the previous customer's order was closed, at what time would the fourth customer's order (refer to the previous question) be considered closed ?

Option: 1

6:38 pm

 


Option: 2

6:42 pm

 


Option: 3

6:47 pm


Option: 4

6:49 pm


Directions for question:

The bar-graph given below shows the foreign exchange reserves of Nepal (in million Rupees) from 2014 to 2021. Answer the following questions based on the graph :

Question:

What was the percentage increase (rounded to the nearest integer, if deemed necessary) in the foreign exchange reserves in 2020 over 2016 ?

Option: 1 None

Option: 2 None

Option: 3 None

Option: 4 None

Directions for question:

The Jadavpur University’s Prince Anwar Shah Road hostel consists of two large separate buildings, one for the ladies and the other for the gents, while having a common kitchen and dining hall. It is the hostel of the CS and the EEC department of engineering students of the university.

In recognition of the growing dissatisfaction and hence complaints among the inmates of the hostel regarding the menu served for dinner, the Dean of the engineering department, Dr Aparesh Sanyal, personally decided to investigate the matter. He set about collecting information about the preference of dinner among the inmates, separately from the gents and the ladies wing of the hostel.

Dr Sanyal was able to gather the following partial information :  

 

Hostel inmates

Menu preference for dinner

Total

Egg Meal

Fish Meal

Chicken Meal

Gents

   

20

 

Ladies

     

64

Total

 

60

   

 

The Warden of the hostel was consulted, who after investigation declared that the following facts were clear :

1. Forty percent of the hostel inmates were ladies

2. One-third of the gentlemen inmates preferred an egg meal for dinner

3. Half the hostel inmates preferred either fish meal or chicken meal

Question:

What proportion of the lady hostel inmates preferred a fish meal for dinner ?

 

 

Option: 1

0.25

 

 


Option: 2

0.50

 


Option: 3

0.75

 


Option: 4

1.00 


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