Describe nondisjunction and its consequences

2.8.17 · Biology › Cell Division

WHAT is nondisjunction?

WHY does this matter? Kyunki chromosome number gene dosage ko control karta hai. Kisi chromosome ki bahut zyada ya bahut kam copies ka matlab hai ki hundreds of gene products bahut zyada ya bahut kam honge — jo usually fatal ya disease-causing hota hai.

HOW it happens — derived from the rules of division

Normal cell division do counting rules follow karta hai. Chaliye main consequences ko scratch se build karta hoon.

Rule 1 (Meiosis I): Ek diploid cell ( $2 n$ ) mein $n$ homologous pairs hote hain. Anaphase I mein, har pair split hota hai — ek homolog har pole par jaata hai. Result: do cells, har ek $n$ (lekin har chromosome mein abhi bhi 2 chromatids hain).

Rule 2 (Meiosis II): Sister chromatids split hote hain. Result: chaar cells, har ek truly $n$ (ek chromatid each).

Ab ek rule tod do:

Case A — Nondisjunction in Meiosis I

Ek pair ke dono homologs same pole par jaate hain.

$One pole: (n + 1) Other pole: (n - 1)$

Meiosis II ke baad, charon gametes abnormal hote hain: do $n + 1$ ke saath, do $n - 1$ ke saath.

Charon kyun? Kyunki error tab hua pehle jab cells chaar mein split hui — har daughter cell yeh mistake inherit karti hai.

Case B — Nondisjunction in Meiosis II

Homologs MI mein theek se separate ho gaye, lekin ek do cells mein se ek mein, sister chromatids split nahi ho paate.

$Gametes: (n + 1), (n - 1), n, n$

Sirf do abnormal kyun? Doosri secondary cell sahi se divide hui, jisse do normal $n$ gametes mile.

Recall Forecast-then-Verify: How many abnormal gametes?

Forecast: MI error → ? MII error → ? Verify: MI nondisjunction → 4 abnormal (2× $n + 1$ , 2× $n - 1$ ). MII nondisjunction → 2 abnormal + 2 normal.

What happens at fertilization — the consequences

Ek normal gamete $n$ hota hai. Agar woh ek abnormal gamete ke saath fuse ho:

Figure — Describe nondisjunction and its consequences

Common Mistakes (Steel-manned)

Active Recall

Recall Quick self-test

Nondisjunction ko ek line mein define karo.
MI vs MII mein kya galat tarike se separate hota hai?
Trisomy 21 chromosome count?
Turner (45,X) monosomy kyun hai?
MII nondisjunction → kitne normal gametes?

Answers: failure of chromosomes/chromatids to separate; homologs (MI) vs sister chromatids (MII); 47; one X only; 2 normal.

Nondisjunction is defined as

the failure of homologous chromosomes or sister chromatids to separate during nuclear division

In meiosis I nondisjunction, what fails to separate

homologous chromosomes

In meiosis II nondisjunction, what fails to separate

sister chromatids

Aneuploidy means

having an abnormal number of chromosomes (not an exact multiple of n)

Trisomy chromosome formula

2n + 1 (one chromosome in three copies)

Monosomy chromosome formula

2n - 1 (one chromosome in a single copy)

Human Down syndrome chromosome count

47 (trisomy 21)

Turner syndrome genotype and type

45, X — a monosomy

Klinefelter syndrome genotype

47, XXY

MI nondisjunction produces how many abnormal gametes

all four (2 with n+1, 2 with n-1)

MII nondisjunction produces

2 abnormal (n+1, n-1) and 2 normal (n) gametes

Why is maternal age a risk factor

eggs are arrested in meiosis I for years, cohesin proteins degrade, increasing separation failure

Triploidy differs from trisomy because

triploidy is 3n (whole extra set); trisomy is one extra chromosome

Recall Feynman: explain to a 12-year-old

Jab ek cell baby cells banata hai, toh use apne chromosomes fairly share karne hote hain — ek ek side ko, jaise cards evenly deal karna. Kabhi kabhi cell galti karta hai aur ek side ko do cards ek jaisi ki deta hai aur doosri side ko zero. Ab woh baby cells mein cards ka galat number hai. Jab aisi ek card-cell kisi partner ke saath milti hai ek naya insaan start karne ke liye, toh naya insaan 47 chromosomes ke saath aata hai 46 ki jagah (bahut zyada) ya 45 (bahut kam). Woh extra ya missing card yeh change karta hai ki body kaise grow karti hai — isi tarah Down syndrome jaisi conditions hoti hain.

Connections

Meiosis I and II — jahan separation steps hoti hain
Anaphase and Spindle Fibres — woh machinery jo fail hoti hai
Cohesin and Sister Chromatid Cohesion — age-related errors ki molecular cause
Karyotyping — aneuploidy kaise detect hoti hai
Mutation and Genetic Disorders — consequences ki broader category
Mitosis — somatic nondisjunction → mosaicism/cancer