Distinguish chromosomal mutations (deletion, duplication, inversion, translocation)

3.5.5 · Biology › Mutations & Gene Regulation

Isliye chromosomal mutations syndromes cause karte hain (Down syndrome, Cri-du-chat) jabki point mutations usually single proteins ko affect karte hain.

The Four Major Types

1. Deletion: Missing Chromosomal Segments

HOW it happens:

• Ek hi chromosome mein do breaks
• DNA repair machinery beech ka fragment recover karne mein fail ho jaati hai
• Toote hue ends ek saath ligate ho jaate hain

WHAT happens to genes:

• Hemizygosity: Agar deletion heterozygous hai, to ab tumhare paas un genes ki sirf EK copy hai (do nahi)
• Normal homolog pe recessive alleles bhi ab express ho jaate hain
• Agar dono copies delete ho jayein to gene function ka complete loss

Why this step? Delete hua region brain development ke liye critical genes contain karta hai. Do ki jagah sirf EK copy hone se haploinsufficiency hoti hai—normal function ke liye kaafi gene product nahi banta.

Molecular detail: CTND2 gene (catenin delta-2, jo neural development mein involve hai) critical region mein hai. Ek copy kho dene se production us threshold se neeche aa jaati hai jo normal cortical development ke liye zaruri hai.

2. Duplication: Extra Copies of Segments

HOW it happens:

• Meiosis ke dauran unequal crossing over (homologous chromosomes ka misalignment)
• DNA replication errors
• Breakage-fusion-bridge cycles

WHAT happens to genes:

• Gene dosage imbalance: Do ki jagah teen copies (ya dono homologs affect hon to char vs do)
• Gene products ka overexpression
• Evolution ka stepping stone ban sakta hai (duplicate genes mutate hone ke liye free hote hain)

Bahut se cellular processes dosage-sensitive hote hain—inhe proteins ke precise stoichiometric ratios chahiye hote hain. Agar gene A 100 units banata hai aur normally gene B se bind karta hai jo 100 units banata hai, lekin ab gene A 150 units banata hai, to tumhe milega:

• 100 functional A-B complexes
• 50 free A proteins (jo toxic aggregates ban sakte hain ya dusre pathways se compete kar sakte hain)

$\text{Functional complex}=\min (\text{protein A},\text{protein B})$

Isliye trisomies (poore chromosome duplications) aksar lethal hote hain—ek saath hundreds of genes out of balance ho jaate hain.

Why this step? PMP22 ek myelin protein code karta hai. TEEN copies → 1.5× normal protein → myelin sheath instability. Excess PMP22 myelin assembly ke liye cellular machinery ko overwhelm kar deta hai.

Derivation of dosage effect:

• Normal 2 PMP22 alleles → 100% myelin stability
• CMT1A: 3 PMP22 alleles → 150% protein → myelin aggregates form hote hain → demyelination
• Note: PMP22 ka DELETION ek alag disease (HNPP) cause karta hai—jo dosage sensitivity prove karta hai

3. Inversion: Flipped Segments

Centromere inclusion ke basis pe DO types:

Paracentric inversion: Centromere INCLUDE nahi karta (dono breaks ek arm mein)

• Para = "beside"
• Meiotic problems: dicentric (do centromeres) aur acentric (koi centromere nahi) products

Pericentric inversion: Centromere INCLUDE karta hai (dono arms mein breaks)

• Peri = "around"
• Meiotic problems: crossover products mein duplications aur deletions

HOW it causes problems: Meiosis ke dauran, inverted chromosome ko apne normal homolog ke saath pair karne ke liye ek loop banana padta hai. Agar loop ke andar crossing over ho:

Paracentric ke liye:

1. Loop mein crossover
2. Products include:
   • Ek normal chromosome
   • Ek inverted chromosome dicentric bridge (poles ke beech stretch hota hai → randomly toot ta hai)
   • Ek acentric fragment (kho jaata hai, spindle se attach hone ke liye koi centromere nahi)

Pericentric ke liye:

1. Loop mein crossover
2. Products mein ek arm ka duplication, dusre ka deletion hota hai
3. Result: unbalanced gametes → miscarriage ya malformations

Clinical significance:

• Usually benign (carriers phenotypically normal hote hain)
• Agar crossover ho to miscarriage ka thoda increased risk
• Carriers ke liye genetic counseling recommended hai

Why this matters: Yeh dikhata hai ki inversions hamesha pathogenic nahi hote—size, gene content, aur crossover frequency pe depend karta hai.

4. Translocation: Segments Swapped Between Chromosomes

DO major types:

Reciprocal translocation: Do chromosomes segments exchange karte hain

• Dono chromosomes toot te hain
• Segments positions swap karte hain
• Balanced carriers mein genetic material ka net gain/loss nahi

Robertsonian translocation: Do acrocentric chromosomes centromeres pe fuse ho jaate hain

• Acrocentric = centromere end ke paas (chromosomes 13, 14, 15, 21, 22)
• Short arms kho jaate hain (sirf ribosomal RNA genes contain karte hain—redundant)
• Do chromosomes ek ban jaate hain

Balanced translocation carrier:

• Saara genetic material present hai, bas rearranged
• Usually phenotypically normal
• Problem: Meiosis ke dauran, unbalanced gametes produce karta hai

Balanced reciprocal translocation ke baad gamete possibilities: Maano chromosomes A, B (normal) aur A-B', A'-B (translocated) hain

Possible gamete combinations:

1. A, B (normal) → 1/6 chance
2. A-B', A'-B (balanced translocation) → 1/6 chance
3. A, A'-B (unbalanced) → 1/6 chance
4. B, A-B' (unbalanced) → 1/6 chance
5. A, A-B' (unbalanced) → 1/6 chance
6. B, A'-B (unbalanced) → 1/6 chance

Sirf 2/6 gametes viable → translocation carriers mein high miscarriage rate explain karta hai

Why it causes disease (Chronic Myeloid Leukemia):

Step 1: Normal ABL1 ek tyrosine kinase hai jo cell division control karne ke liye proteins phosphorylate karta hai

• Regulatory domain hai jo ise "off" rakhta hai jab zarurat na ho

Step 2: Translocation breakpoint regulatory domain hata deta hai

• BCR-ABL1 fusion mein "off switch" nahi hota

Step 3: Constitutive kinase activity

• Continuous proliferation signals
• Proper checkpoints ke bina cell division
• White blood cells ka accumulation → leukemia

Step 4: Treatment insight

• Imatinib (Gleevec) specifically BCR-ABL1 kinase inhibit karta hai
• CML ko fatal se manageable chronic disease mein badal diya
• KYU? Kyunki yeh ek single driver mutation hai

How it causes Down syndrome:

1. Parent mein balanced rob(14;21) translocation hai

   • Ek chromosome jisme 14 + 21 material hai
   • Ek normal 14 aur ek normal 21 missing
   • Total gene count abhi bhi correct → phenotypically normal

2. Meiosis ke dauran, gamete possibilities:

   • Normal 14 + normal 21 → viable
   • rob(14;21) + normal 21 → DOWN SYNDROME (chr21 ki teen copies)
   • Sirf rob(14;21) → monosomy 21 (lethal)
   • Sirf normal 14 → monosomy 21 (lethal)

Why this matters clinically:

• Recurrence risk sporadic trisomy 21 se BAHUT zyada hai
• Agar mother translocation carry karti hai: ~10-15% recurrence
• Agar father carry karta hai: ~2-3% recurrence
• Down syndrome diagnosis ke baad parents ki karyotyping zaruri hai

Comparison Table & Detection

| Type | Net DNA change | Gene order | Meiotic pairing | Clinical examples | |------|------------|-----------------|----------------| | Deletion | Loss | A-B-C-D-E → A-B-E | Deletion loop | Cri-du-chat, Wolf-Hirschhorn | | Duplication | Gain | A-B-C-D-E → A-B-C-C-D-E | Duplication loop | CMT1A, kuch autism cases | | Inversion | No change | A-B-C-D-E → A-D-C-B-E | Inversion loop | Chr 9 inversion (benign) | | Translocation | No change (balanced) | Chr1: A-B-C + Chr2: X-Y-Z → Chr1: A-B-Z + Chr2: X-Y-C | Quadrivalent | Philadelphia, rob(14;21) |

Detection methods:

• Karyotype (G-banding): Bade changes dekho (>5-10 Mb)
• FISH (fluorescence in situ hybridization): Fluorescent probes se specific regions target karo
• Microarray (aCGH): Genome-wide copy number changes detect karo
• Sequencing: Base-pair resolution pe breakpoints detect karo

Why it's WRONG:

1. Position effects: Heterochromatin ke paas move hue genes silence ho sakte hain

   • Heterochromatin = tightly packed, transcriptionally inactive DNA
   • Euchromatin se heterochromatin ke paas move hua gene apna expression kho deta hai

2. Breakpoint disruptions: Breaks GENES KE ANDAR bhi ho sakti hain

   • Agar break ek exon mein ho, to wo gene ab non-functional hai
   • Intronic breaks bhi regulatory elements disrupt kar sakte hain

3. Meiotic problems: Inversion loops mein crossovers → unbalanced gametes

   • Inversion carriers mein high miscarriage rate
   • Duplications/deletions wale offspring

4. Gene fusion: Breakpoints fusion proteins bana sakte hain

   • Unpredictable function wala naya protein
   • Agar growth control affect ho to oncogenic ho sakta hai

The fix: Inversions benign ho sakti hain AGAR:

• Breakpoints pe koi gene na toote
• Gene expression pe koi position effects na hon
• Inversion region chhota ho ya rarely crossing over kare Lekin yeh automatically harmless NAHI hain.

Why it's WRONG for reproductive health:

1. Unbalanced gametes: Meiosis ke dauran, segregation patterns mostly unbalanced gametes produce karte hain

   • 2:1 segregation → duplications/deletions
   • Result: recurrent miscarriages, infertility

2. Adjacent-1 vs adjacent-2 vs alternate segregation: Meiosis I ke dauran, quadrivalent (char-chromosome structure) teen tarike se segregate ho sakta hai:

   • Alternate (2:2 balanced): Dono normal ya dono translocated chromosomes same pole pe jaate hain → OK
   • Adjacent-1: Homologs alag hote hain, lekin galat combination → unbalanced
   • Adjacent-2: Sister centromeres alag hote hain → unbalanced

   Sirf alternate segregation (1/3 chance) balanced gametes produce karta hai

3. Position effects: Balanced hone ke baad bhi, breakpoints ke paas ke genes ka expression alter ho sakta hai

The fix: Balanced translocation carriers ko chahiye:

• Genetic counseling
• Prenatal diagnosis options
• Awareness ki ~50-70% conceptions miscarriage mein lost ho sakte hain

Deletion aise hai jaise kisi ne book 5 ke beech se pages faar diye. Ab tumhare paas apna left hand theek se banane ke instructions nahi hain. Cri-du-chat syndrome tab hota hai jab book 5 ke pages faar diye jaate hain.

Duplication aise hai jaise photocopier ne galti kar di aur pages 50-60 do baar print kar diye, to ab wo book mein do baar stuck hain. Tumhari cells instructions do baar follow karne ki koshish karti hain aur confuse ho jaati hain—jaise agar ek recipe mein "1 cup sugar daalo" likha ho lekin wo do baar printed ho aur tumne accidentally 2 cups daal diye.

Inversion aise hai jaise kisi ne pages 100-120 liye, unhe ulta kar diya, aur ulte wapas glue kar diye. Instructions saare hain, lekin reverse order mein hain! Jaise "bake at 350°F for 20 minutes" ko ulta padhna. Kabhi problems cause karta hai (khaaskar jab tum ise normal copy ke saath line up karo kids banane ke liye), kabhi nahi karta.

Translocation aise hai jaise Book 9 ka Chapter 5 lena aur usse Book 22 ke Chapter 3 se swap karna. Ab Book 9 mein Book 22 ka chapter hai aur vice versa. Philadelphia chromosome yahi karta hai aur mixed-up chapter tumhare white blood cells ko continuously divide karne ko kehta hai, leukemia cause karta hai. Scientists ne ek medicine (Gleevec) banayi jo specifically mixed-up chapter ki instruction block karti hai!

Wild part kya hai? Chahe instructions wahan hain, unhe galat order mein ya galat amount mein rakhna problems cause karne ke liye kaafi hai. Tumhari cells instructions exactly sahi follow karne ke baare mein SUPER picky hain.

Visual mnemonic: Chromosomes ko BEADED STRINGS ki tarah draw karo

• Deletion: beads missing wali string
• Duplication: extra beads wali string
• Inversion: beads ulte order mein rearranged
• Translocation: do alag strings ke beads swap hue

Connections

• Point Mutations vs Chromosomal Mutations: Scale aur impact differences
• Meiotic Nondisjunction: Chromosomal abnormalities ka ek aur source (poore chromosome errors)
• Karyotyping and FISH: Chromosomal mutations ke detection techniques
• Position Effect Variegation: Gene position expression ke liye kyun matter karta hai
• Gene Dosage Compensation: Dosage balance karne ke mechanism ke roop mein X-inactivation
• Cancer Cytogenetics: Oncogenic drivers ke roop mein translocations (BCR-ABL, MYC translocations)
• Evolutionary Role of Gene Duplication: Duplications nayi genes ke liye raw material kaise create karte hain
• Meiotic Recombination: Unequal crossing over ke underlying mechanism
• Prenatal Genetic Screening: Chromosomal mutations detect karne ke clinical applications
• Comparative Genomics: Evolutionary relationships trace karne ke liye chromosomal rearrangements ka use

#flashcards/biology

Chromosomal mutations ke char major types kya hain? :: Deletion (segment ka loss), Duplication (segment ki extra copy), Inversion (segment reversed), Translocation (segment alag chromosome pe move hota hai)

Chromosomal deletion define karo aur example do :: Ek chromosomal segment ka loss jisme genes hote hain; hemizygosity create karta hai. Example: Cri-du-chat syndrome (5p ka deletion) intellectual disability aur distinctive cry cause karta hai.

Philadelphia chromosome aur uska mechanism explain karo :: t(9;22) translocation jo BCR gene ko ABL1 gene ke saath fuse karta hai, constitutively active tyrosine kinase BCR-ABL1 create karta hai. Continuously cell division signal deke chronic myeloid leukemia cause karta hai. Imatinib (Gleevec) se treat hota hai.

Hemizygosity define karo :: Do ki jagah gene ki sirf EK copy hona. Deletions ke saath ya males mein X chromosome pe hota hai. Iska matlab hai ki recessive alleles bhi express ho jaate hain kyunki unhe mask karne ke liye koi doosri copy nahi hoti.

Concept Map