Chapter 11: Molecular Basis of Inheritance

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Introduction

Overview

• At Mendel's time, the nature of inheritance factors was unknown.


• DNA and RNA are nucleic acids made of nucleotide polymers.


• DNA is the genetic material in most organisms.


• RNA acts as genetic material in some viruses and mainly functions as a messenger, adapter, structural, or catalytic molecule.

Memory Aids

• DNA = Genetic blueprint in most life forms.


• RNA = Messenger and sometimes genetic material.

Quick Checkpoints

• What are the two main nucleic acids?


• Which nucleic acid acts as genetic material in most organisms?

The DNA

Structure and Length

• DNA is a long polymer of deoxyribonucleotides forming two complementary strands running antiparallel.


• Strands are held by hydrogen bonds between opposite nitrogen bases.


• Four types of nucleotides form DNA.


• DNA length is measured by number of nucleotides or base pairs (bp), characteristic to each organism.


• Examples: Φ x174 phage - 5386 nucleotides; λ phage - 48502 bp; Escherichia coli - 4.6 million bp; human haploid DNA - 3.3 billion bp.

Memory Aids

• DNA strands run antiparallel and complement each other.


• Base pairs define DNA length and organism specificity.

Quick Checkpoints

• What is the polarity of DNA strands?


• How is DNA length defined?

Structure of Polynucleotide Chain

Components and Linkages

• Nucleotide = nitrogenous base + pentose sugar + phosphate group.


• Purines: adenine (A), guanine (G); Pyrimidines: cytosine (C), thymine (T, DNA), uracil (U, RNA).


• Cytosine common to DNA and RNA; thymine replaced by uracil in RNA.


• Base linked to 1′C sugar via N-glycosidic bond forming nucleoside.


• Phosphate linked to 5′-OH of nucleoside via phosphodiester bond forms nucleotide.


• Nucleotides linked 3′–5′ via phosphodiester bonds form dinucleotides and polynucleotide chains.


• RNA nucleotides have 2′-OH group; DNA lacks this.


• DNA first identified by Friedrich Miescher in 1869 as 'Nuclein'.

Memory Aids

• Nucleotide = Base + Sugar + Phosphate.


• RNA has uracil instead of thymine and 2′-OH group.


• Phosphodiester bonds link nucleotides 3′ to 5′.

Quick Checkpoints

• What are the three parts of a nucleotide?


• Which base replaces thymine in RNA?


• What type of bond links nucleotides?

Structure of DNA

Double Helix Model

• Proposed by Watson and Crick (1953) based on X-ray data from Wilkins and Franklin.


• Chargaff's rules: A + G = T + C; A = T; G = C; A+T/G+C ratio varies by species but constant within species.


• Each DNA strand acts as template for new strand, ensuring identical daughter DNA.


• Features:




• Two polynucleotide chains with sugar-phosphate backbone outside, bases inside.


• Strands are antiparallel (5′→3′ and 3′→5′).


• Bases paired by hydrogen bonds: A-T (2 bonds), G-C (3 bonds); purine pairs with pyrimidine.


• Right-handed double helix with 3.4 nm pitch and ~10 bp per turn.


• Base pairs stack to stabilize helix.

Memory Aids

• Watson-Crick model = antiparallel double helix.


• A pairs with T; G pairs with C.


• 10 base pairs per helical turn.

Quick Checkpoints

• What is the base pairing rule in DNA?


• Describe the polarity of DNA strands.


• How many base pairs per turn in DNA helix?

Central Dogma

Flow of Genetic Information

• Proposed by Francis Crick.


• Genetic information flows: DNA → RNA → Protein.


• Processes involved: DNA replication, transcription (DNA to mRNA), translation (mRNA to protein).


• In some viruses, reverse flow occurs: RNA → DNA (reverse transcription).

Memory Aids

• Central dogma: DNA makes RNA makes Protein.


• Reverse transcription occurs in some viruses.

Quick Checkpoints

• What is the central dogma of molecular biology?


• Which process converts RNA back to DNA?

Packaging of DNA Helix

DNA Length and Compaction

• DNA length = number of base pairs × distance between base pairs.


• Typical mammalian cell DNA length ~2.2 meters.


• DNA is compacted to fit in nucleus.

Memory Aids

• DNA is very long but tightly packed.

Quick Checkpoints

• How is DNA length calculated?


• Why is DNA packaging necessary?

DNA Packaging in Prokaryotes

Supercoiling and Nucleoid

• DNA is supercoiled with help of RNAs and non-histone basic proteins like polyamines.


• Compacted DNA mass is called nucleoid.

Memory Aids

• Prokaryotic DNA forms nucleoid via supercoiling.

Quick Checkpoints

• What is nucleoid?


• How is prokaryotic DNA compacted?

DNA Packaging in Eukaryotes

Histones and Chromatin

• DNA wraps around histones (lysine and arginine rich proteins).


• Histone octamer = 8 histone molecules (pairs of H2A, H2B, H3, H4).


• DNA + histone octamer = nucleosome (~200 bp DNA).


• Nucleosomes form chromatin, visible as thread-like structures in nucleus.


• Chromatin appears as 'beads-on-string' under electron microscope.


• Chromatin fibers coil and condense to form chromosomes during metaphase.


• Non-histone chromosomal proteins assist higher-level packaging.


• Chromatin types: euchromatin (loosely packed, transcriptionally active), heterochromatin (densely packed, inactive).


• Linker DNA (interbead DNA) connects nucleosomes, bears H1 histone (plugging/marker protein).


• Nucleosome + linker DNA = chromatosome.

Memory Aids

• Nucleosome = DNA wrapped around histone octamer.


• H1 histone stabilizes linker DNA.


• Euchromatin = active; heterochromatin = inactive.

Quick Checkpoints

• What proteins form the histone octamer?


• Define nucleosome and chromatosome.


• Difference between euchromatin and heterochromatin?

Transforming Principle

Subtopics

• In 1928, Frederick Griffith studied Streptococcus pneumoniae with two strains: virulent S-type (smooth colonies, polysaccharide coat) and non-virulent R-type (rough colonies).


• Heat-killed S strain alone did not kill mice; live R strain did not cause disease.


• Mix of heat-killed S strain and live R strain killed mice, indicating a 'transforming principle' transferred from S to R strain, enabling R to become virulent.

Memory Aids

• Griffith’s experiment: Dead S + Live R = Dead mice (transformation).

Quick Checkpoints

• What are the two strains of S. pneumoniae?


• What did the heat-killed S strain demonstrate?


• What is the significance of the transforming principle?

Biochemical Characterisation of Transforming Principle (Avery Experiment)

Subtopics

• Avery, MacLeod, and McCarty (1933-44) purified proteins, DNA, and RNA from heat-killed S cells.


• Only DNA caused transformation of live R cells into S cells.


• Protease and RNase treatments did not stop transformation; DNase treatment did.


• Conclusion: DNA is the hereditary material responsible for transformation.

Memory Aids

• DNase stops transformation; DNA is genetic material.

Quick Checkpoints

• Which enzyme digestion inhibited transformation?


• What was the role of proteins and RNA in transformation?


• What conclusion was drawn from Avery’s experiment?

DNA is the Genetic Material (Hershey Chase Experiment)

Subtopics

• Hershey and Chase (1952) used bacteriophages labeled with radioactive isotopes: ³²P in DNA and ³⁵S in protein.


• Phages infected Escherichia coli separately.


• After infection and blending, radioactivity from ³²P was found inside bacteria; ³⁵S remained outside.


• Conclusion: DNA enters bacteria and carries genetic information, not protein.

Memory Aids

• ³²P in DNA enters bacteria; ³⁵S in protein does not.

Quick Checkpoints

• What isotopes were used to label DNA and protein?


• Where was radioactivity found after infection?


• What does this experiment prove about genetic material?

Properties of Genetic Material (DNA versus RNA)

Subtopics

• Some viruses use RNA as genetic material (e.g., Tobacco Mosaic virus, QB bacteriophage).


• Genetic material must: replicate, be chemically and structurally stable, allow mutations, and express Mendelian characters.


• DNA and RNA can replicate via base pairing; proteins cannot.


• DNA is more stable due to lack of reactive 2'-OH group present in RNA.


• RNA is catalytic and degrades easily; DNA is chemically less reactive and more stable.


• Thymine in DNA adds stability compared to uracil in RNA.


• RNA mutates faster; RNA viruses evolve rapidly.


• RNA directly codes proteins; DNA depends on RNA for protein synthesis.


• DNA is preferred for genetic information storage; RNA for transmission.

Memory Aids

• DNA: stable storage; RNA: fast transmission.


• 2'-OH in RNA = reactive and unstable.


• Thymine stabilizes DNA.

Quick Checkpoints

• Which nucleic acid is more stable and why?


• What roles do DNA and RNA play in genetic information?


• Why do RNA viruses mutate faster?

RNA World

RNA as First Genetic Material

• RNA was the original genetic material supporting early life processes like metabolism, translation, and splicing.


• It functioned both as genetic material and as a catalyst (ribozymes).


• RNA's catalytic nature made it reactive and chemically unstable.


• DNA evolved from RNA with chemical changes enhancing stability.


• DNA's double-stranded structure and complementary strands enable repair mechanisms, increasing genetic fidelity.

Types of Non-Genetic RNA

• Based on size and function, non-genetic RNA includes messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).


• mRNA: ~3.5% of cellular RNA; carries genetic code from DNA to ribosomes for protein synthesis.


• tRNA: ~15% of cellular RNA; also called soluble, acceptor, or adapter RNA; has amino acid binding site opposite anticodon.


• rRNA: ~80% of cellular RNA; structural and functional component of ribosomes; synthesized in nucleolus.

Memory Aids

• RNA = First genetic and catalytic molecule.


• DNA = Stable, double-stranded, repair-capable descendant of RNA.


• mRNA = Messenger; carries code.


• tRNA = Transfer; carries amino acids.


• rRNA = Ribosome; protein factory component.

Quick Checkpoints

• What dual roles did RNA play in early life?


• How does DNA's structure contribute to its stability?


• What are the main types of non-genetic RNA and their functions?


• Where is rRNA synthesized?


• What is the significance of the amino acid binding site in tRNA?

Replication

Process and Mechanism

• DNA replication is the process of making an identical copy of double-stranded DNA using the original strands as templates.


• Watson and Crick proposed semiconservative replication: each new DNA molecule has one parental and one newly synthesized strand.


• Replication involves unwinding the double helix and synthesizing complementary strands following base pairing rules (A-T, G-C).

Memory Aids

• \"Semi\" means half: one old strand + one new strand per DNA molecule.


• Base pairing rules guide new strand synthesis.

Quick Checkpoints

• What does semiconservative replication mean?


• Which base pairs with adenine during replication?

The Experimental Proof (Meselson and Stahl Experiment)

Experiment and Findings

• Meselson and Stahl used Escherichia coli grown in heavy nitrogen (¹⁵N) then shifted to light nitrogen (¹⁴N) medium.


• After replication, DNA showed intermediate density, proving one old and one new strand per DNA molecule.


• Further replication produced both hybrid and light DNA, confirming semiconservative replication.


• Taylor et al. confirmed semiconservative replication in chromosomes of Vicia faba using radioactive thymidine.

Memory Aids

• Meselson-Stahl: \"Heavy to light\" nitrogen shift shows DNA density changes.


• Hybrid DNA = one old + one new strand.

Quick Checkpoints

• What isotope was used to label DNA in the experiment?


• How does DNA density change after one replication cycle?

The Machinery and the Enzymes

Enzymes and Process

• DNA replication is energy-intensive, requiring enzymes called DNA-dependent DNA polymerases.


• Helicase unwinds DNA at the origin of replication (Ori site).


• Single-stranded binding proteins stabilize separated strands.


• Topoisomerases relieve supercoiling by nicking and resealing DNA strands.


• RNA primer synthesized at 5' end initiates DNA synthesis.


• DNA polymerase synthesizes new strands only in 5' to 3' direction.


• Leading strand is synthesized continuously; lagging strand discontinuously as Okazaki fragments.


• DNA ligase joins Okazaki fragments to form a continuous strand.


• Replication initiates at specific origin sites; in eukaryotes, replication occurs during S-phase of the cell cycle.

Memory Aids

• Helicase = \"unzips\" DNA.


• Polymerase adds nucleotides 5' → 3' only.


• Leading = continuous; Lagging = Okazaki fragments + ligase.


• Topoisomerase prevents supercoil tension.

Quick Checkpoints

• Which enzyme unwinds the DNA helix?


• Direction of DNA polymerase activity?


• Role of RNA primer in replication?


• What joins Okazaki fragments?

Transcription

Process and Mechanism

• Transcription is copying genetic information from one DNA strand into RNA.


• Complementarity governs transcription; adenine pairs with uracil (U) instead of thymine (T).


• Only one DNA strand (template strand) is transcribed; the other is the coding strand.


• Both DNA strands are not copied because:
  
  
  
  • Each strand codes for different RNA sequences; coding both would produce conflicting proteins.
  
  
  • Simultaneous RNA from both strands would be complementary, forming double-stranded RNA, blocking translation.
  
  
  
  


• A transcription unit has three regions: promoter, structural gene, and terminator.


• Structural gene can be monocistronic (eukaryotes) or polycistronic (prokaryotes).


• Coding strand sequence matches RNA (T replaced by U).

Memory Aids

• \"One strand, one RNA\" to remember only one DNA strand is transcribed.


• \"A pairs with U\" in RNA instead of T.


• Transcription unit = Promoter + Gene + Terminator.

Quick Checkpoints

• Why is only one DNA strand transcribed?


• What defines a transcription unit?


• How does the coding strand relate to RNA?

Types of RNA and Process of Transcription

RNA Types and Enzyme

• Bacteria have three main RNAs: mRNA (template), tRNA (amino acid carrier), and rRNA (structural and catalytic).


• Transcription requires DNA-dependent RNA polymerase holoenzyme (core enzyme + sigma factor).


• Sigma factor directs RNA polymerase to the promoter, enhancing specificity.

Transcription Steps in Bacteria

• Initiation: RNA polymerase binds promoter at -35 and -10 regions; DNA locally unwinds.


• Elongation: Sigma factor leaves; RNA polymerase synthesizes RNA 5'→3', unwinding DNA ahead.


• Termination: Rho factor binds RNA, catches polymerase at pause site, unwinds DNA-RNA hybrid, releasing polymerase.


• In bacteria, transcription and translation occur in cytoplasm; mRNA requires no processing.

Memory Aids

• \"Sigma starts, Rho stops\" to recall initiation and termination factors.


• RNA polymerase synthesizes RNA 5' to 3'.


• Bacterial mRNA is ready for translation immediately.

Quick Checkpoints

• What roles do sigma and rho factors play?


• Where does bacterial transcription occur?


• How is bacterial mRNA different from eukaryotic?

Transcription in Eukaryotes

RNA Polymerases and Processing

• Three RNA polymerases in nucleus:
  
  
  
  • RNA polymerase I: transcribes rRNAs (28S, 18S, 5.8S)
  
  
  • RNA polymerase II: transcribes precursor mRNA (hnRNA)
  
  
  • RNA polymerase III: transcribes tRNA, 5S rRNA, snRNA
  
  
  
  


• Transcription occurs in nucleus; mature mRNA moves to cytoplasm for translation.


• mRNA maturation includes:
  
  
  
  • 5' cap addition (7-methyl guanosine)
  
  
  • 3' poly-A tail addition (200-300 adenines)
  
  
  • RNA splicing removes introns; ligase joins exons.
  
  
  
  


• Primary transcript (hnRNA) contains exons and introns.


• Splicing reflects RNA world dominance.

Memory Aids

• \"Pol I for rRNA, Pol II for mRNA, Pol III for tRNA and others.\"


• \"Cap and tail protect and guide mRNA.\"


• \"Introns out, exons join\" for splicing.

Quick Checkpoints

• Which RNA polymerase transcribes mRNA?


• What modifications occur during mRNA maturation?


• Why is RNA splicing important?

Genetic Code

Characteristics

• Genetic code is the codon representation decoding RNA into polypeptides.


• Each amino acid is coded by a sequence of three nucleotides (triplet code).


• There are 64 codons (4³) coding for 20 amino acids.


• Code is universal: same codon codes same amino acid in all organisms.


• Degeneracy: multiple codons can code for the same amino acid.


• Non-overlapping: each nucleotide is part of only one codon.


• Non-ambiguous: one codon codes for only one amino acid.


• Initiator codon (mostly AUG) starts protein synthesis and codes methionine.


• Stop codons (UAG, UAA, UGA) terminate protein synthesis; called nonsense codons.


• Wobble hypothesis: third base of codon pairs loosely, allowing tRNA to recognize multiple codons.

Memory Aids

• Triplet code = 3 nucleotides per codon.


• Universal code: same in all life forms.


• Degeneracy = multiple codons for one amino acid.


• Start codon = AUG (Methionine).


• Stop codons = UAG, UAA, UGA (amber, ochre, opal).


• Wobble at 3rd base allows flexibility.

Quick Checkpoints

• What is the length of a codon?


• Which codon initiates protein synthesis?


• Define degeneracy of the genetic code.


• What are stop codons?


• Explain the wobble hypothesis.

Mutations and Genetic Code

Types and Effects

• Point mutation: single base change; e.g., glutamate to valine causes sickle cell anemia.


• Frameshift mutation: insertion/deletion of 1 or 2 bases shifts reading frame.


• Insertion/deletion of multiples of 3 bases does not shift reading frame.


• Non-sense mutation: codon changes to stop codon, halting protein synthesis.


• Mis-sense mutation: codon changes to code different amino acid.


• Same sense (silent) mutation: codon changes but amino acid remains same due to degeneracy.

Memory Aids

• Frameshift = insertion/deletion not in multiples of 3.


• Non-sense = premature stop.


• Mis-sense = amino acid change.


• Silent mutation = no amino acid change.

Quick Checkpoints

• What causes sickle cell anemia at genetic level?


• How does frameshift mutation affect protein synthesis?


• Difference between non-sense and mis-sense mutations?


• What is silent mutation?

tRNA – the Adapter Molecule

Structure and Function

• tRNA reads mRNA codons and carries specific amino acids.


• Secondary structure resembles clover-leaf; actual shape is inverted L.


• Contains anticodon loop with bases complementary to mRNA codon.


• Amino acid acceptor end binds specific amino acid.


• Each tRNA is specific for one amino acid.


• Initiator tRNA starts translation; no tRNA for stop codons.

Memory Aids

• tRNA = adapter between mRNA codon and amino acid.


• Anticodon pairs with mRNA codon.


• Inverted L shape fits ribosome during translation.


• Initiator tRNA carries methionine.

Quick Checkpoints

• What is the role of tRNA in protein synthesis?


• Describe the structure of tRNA.


• What is an anticodon?


• Is there tRNA for stop codons?

Translation

Process Overview

• Translation is the polymerisation of amino acids into a polypeptide chain.


• Amino acid sequence is determined by the mRNA base sequence.


• Amino acids are linked by peptide bonds, requiring energy.


• Amino acids are activated with ATP and attached to their specific tRNA (charging/aminoacylation).


• Ribosomes, composed of rRNAs and ~80 proteins, synthesize proteins.


• Ribosome has two subunits: small and large; translation begins when small subunit binds mRNA.


• Large subunit has two sites for amino acid binding; ribosome acts as a catalyst (ribozyme) for peptide bond formation.


• mRNA contains untranslated regions (UTRs) at 5' and 3' ends for efficient translation.


• Translation initiation: ribosome binds mRNA at start codon (AUG) recognized by initiator tRNA.


• Elongation: aminoacyl-tRNAs bind codons via anticodons; ribosome moves codon to codon adding amino acids.


• Termination: release factor binds stop codon, releasing the polypeptide.

Memory Aids

• \"AUG\" = Start codon, recognized by initiator tRNA.


• Peptide bond formation requires energy and ribosomal catalysis.


• Charging of tRNA = amino acid + ATP + tRNA.


• UTRs at mRNA ends aid translation efficiency.

Quick Checkpoints

• What defines the amino acid sequence in translation?


• What is the role of charging tRNA?


• Which ribosomal subunit binds mRNA first?


• What catalyzes peptide bond formation?


• What signals translation termination?

Protein Biosynthesis

Post-Translational Modifications

• Proteins synthesized by ribosomes undergo modifications to become mature.


• Modifications are enzyme-catalyzed and target specific protein sequences.

Memory Aids

• PTMs = Protein maturation after synthesis.


• Enzymes recognize specific sequences for modification.

Quick Checkpoints

• What happens to proteins after synthesis?


• Who catalyzes post-translational modifications?

Polyribosomes

Simultaneous Translation

• Multiple ribosomes translate a single mRNA simultaneously, forming polyribosomes.


• First ribosome moves past start codon, allowing others to attach.


• Polyribosomes increase protein production efficiency.


• Present in both prokaryotes and eukaryotes.

Memory Aids

• Polyribosome = multiple ribosomes on one mRNA.


• Allows rapid protein synthesis from one mRNA.

Quick Checkpoints

• What is a polyribosome?


• How does polyribosome formation affect protein synthesis?


• Are polyribosomes found in eukaryotes?

Regulation of Gene Expression

Gene Expression Control

• Genes express proteins for specific functions, e.g., E. coli produces beta-galactosidase to hydrolyze lactose into glucose and galactose.


• Gene expression depends on metabolic, physiological, or environmental conditions.


• Two types of gene regulation:
  
  
  
  • Negative regulation: Genes are expressed until repressed by regulatory gene products (repressible).
  
  
  • Positive regulation: Genes are inactive until induced (inducible).
  
  
  
  


• Eukaryotic regulation occurs at:
  
  
  
  • Transcriptional level (primary transcript formation)
  
  
  • Processing level (splicing regulation)
  
  
  • mRNA transport from nucleus to cytoplasm
  
  
  • Translational level
  
  
  
  


• In prokaryotes, gene expression is mainly controlled at transcription initiation.
  
  
  
  • RNA polymerase activity at promoters is regulated by accessory proteins.
  
  
  • Operators adjacent to promoters bind specific repressors to control transcription.
  
  
  • Each operon has a specific operator and repressor, e.g., lac operator binds only lac repressor.
  
  
  
  

Memory Aids

• Negative = Repressible; Positive = Inducible


• Eukaryotes regulate at 4 levels: Transcription, Processing, Transport, Translation


• Prokaryotes regulate mainly at transcription initiation via operators and repressors

Quick Checkpoints

• What triggers gene expression changes in cells?


• Difference between positive and negative gene regulation?


• Levels of gene regulation in eukaryotes?


• Role of operator and repressor in prokaryotic gene control?

The Lac Operon

Structure and Function

• Discovered by Francois Jacob and Jacques Monod; a model of transcriptional regulation in bacteria.


• Operon: cluster of genes under a single promoter; common in bacteria (e.g., lac, trp, ara operons).


• Lac operon components:
  
  
  
  • Regulatory gene: i gene codes for lac repressor protein.
  
  
  • Structural genes:
    
    
    
    • z gene: codes beta-galactosidase (hydrolyzes lactose)
    
    
    • y gene: codes permease (increases lactose uptake)
    
    
    • a gene: codes transacetylase
    
    
    
    
  
  
  
  


• Lactose acts as an inducer by binding repressor and inactivating it.


• In absence of glucose and presence of lactose:
  
  
  
  • Lactose enters cell via permease.
  
  
  • Inducer binds repressor, preventing it from binding operator.
  
  
  • RNA polymerase transcribes structural genes producing enzymes for lactose metabolism.
  
  
  
  


• Repressor binding to operator blocks transcription; inducer presence causes negative regulation relief.

Memory Aids

• i gene = inhibitor (repressor producer)


• z = zero (beta-galactosidase breaks lactose to glucose + galactose)


• y = yard (permease helps lactose enter)


• a = acetylase (transacetylase enzyme)


• Lactose = inducer that inactivates repressor

Quick Checkpoints

• What genes constitute the lac operon?


• Role of i gene in lac operon?


• How does lactose induce lac operon expression?


• What is negative regulation in lac operon context?

Human Genome Project

Overview

• Launched in 1990 as a mega project to identify all human genes and their functions.


• Known as International Human Genome Sequencing Consortium.


• Coordinated by U.S. Department of Energy and National Institute of Health; completed in 2003.


• Led to development of bioinformatics for managing genetic data.


• Goals include gene identification, sequencing 3 billion base pairs, data storage, tool improvement, technology transfer, ethical issue management, genetic linkage and physical mapping.


• Enabled by genetic engineering and fast DNA sequencing techniques.


• Sequencing also done for model organisms like bacteria, yeast, Caenorhabditis elegans, Drosophila, rice, and Arabidopsis.

Memory Aids

• HGP = Human genes + sequencing + bioinformatics + ethics.


• 13 years (1990-2003) to map entire human genome.


• Key tools: Genetic engineering + automated sequencing.

Quick Checkpoints

• What are the main goals of HGP?


• Which organizations coordinated HGP?


• What role does bioinformatics play in HGP?

Methodologies of HGP

Techniques Used

• Expressed Sequence Tags (ESTs) to identify genes expressed as RNA.


• Sequence Annotation to assign functions to coding and non-coding regions.


• Automated DNA sequencers based on Sanger's method used for sequencing fragments.


• Overlapping DNA fragments sequenced and aligned using computer programs.


• Sequences annotated and assigned to chromosomes; chromosome 1 sequencing completed in 2006.

Memory Aids

• ESTs focus on expressed genes (RNA).


• Sanger sequencing + computer alignment = complete genome map.

Quick Checkpoints

• What is the purpose of ESTs in HGP?


• Why are computer programs essential in sequence alignment?


• When was chromosome 1 sequencing completed?

Salient Features of HGP

Key Observations

• Human genome contains approximately 3,164.7 million base pairs.


• Average gene length is about 3,000 bases; largest gene (dystrophin) is 2.4 million bases.


• Estimated total genes: ~30,000; 99.9% nucleotide bases identical in all humans.


• Functions of over 50% of genes remain unknown.


• Less than 2% of genome codes for proteins.


• Large portion consists of repeated DNA sequences.


• Chromosome 1 has the most genes (2,968); Y chromosome has the fewest (231).


• About 1.4 million single nucleotide polymorphisms (SNPs) identified in humans.

Memory Aids

• ~3 billion bases, ~30,000 genes, mostly identical across humans.


• Protein-coding genes <2% of genome.


• SNPs = single base variations, important for diversity.

Quick Checkpoints

• What percentage of the genome codes for proteins?


• Which chromosome has the highest gene count?


• What are SNPs and their significance?

Applications of HGP

Uses

• Enables systematic, large-scale biological research.


• Allows study of all genes or transcripts in specific tissues or tumors.


• Helps understand gene and protein networks in life processes.

Memory Aids

• HGP = foundation for genomics and personalized medicine.


• From gene mapping to understanding complex biological systems.

Quick Checkpoints

• How does HGP facilitate biological research?


• What can be studied using HGP data in tissues or tumors?


• Why is understanding gene networks important?

DNA Fingerprinting

Principles and Process

• DNA fingerprinting detects variations in repetitive DNA regions called Variable Number of Tandem Repeats (VNTRs).


• VNTR sizes range from 0.1 to 20 kb; repetitive DNA forms satellite DNA peaks distinct from bulk genomic DNA.


• High polymorphism in VNTRs allows individual identification.


• DNA from any tissue (blood, hair, skin, saliva, sperm) shows the same polymorphic pattern.


• Useful in forensic identification and paternity testing.


• Polymorphism arises from mutations; multiple alleles at a locus occur with frequency >0.01.


• Technique developed by Alec Jeffreys using satellite DNA probes and Southern blot hybridization.


• Steps include DNA extraction, PCR amplification, restriction enzyme digestion, electrophoresis, denaturation, Southern blotting, probe hybridization, and autoradiography.

Memory Aids

• VNTR = Variable Number of Tandem Repeats, key to unique DNA patterns.


• Polymorphism = multiple alleles due to mutations.


• Jeffreys = Father of DNA fingerprinting.


• Southern blot = DNA transfer to membrane for probe detection.

Quick Checkpoints

• What is the role of VNTRs in DNA fingerprinting?


• How does polymorphism aid individual identification?


• What are the main steps in DNA fingerprinting?


• Who developed DNA fingerprinting?

Southern Blotting

Technique Overview

• Transfers DNA fragments from gel electrophoresis onto a nylon or nitrocellulose membrane.


• Preserves DNA banding pattern for hybridization with labeled probes.


• Named after Edwin Southern (1975).


• Steps: DNA digestion by restriction enzymes, gel electrophoresis, transfer to membrane, prehybridization, probe hybridization, detection by autoradiography.


• Used to detect specific DNA sequences within complex mixtures.

Memory Aids

• Southern blot = DNA transfer and detection.


• Restriction enzymes cut DNA into fragments.


• Probe binds only to complementary DNA sequences.


• Autoradiography reveals probe-bound DNA as dark bands.

Quick Checkpoints

• What is the purpose of Southern blotting?


• Which enzyme cuts DNA before blotting?


• How is the target DNA detected on the membrane?


• Who invented Southern blotting?

DNA Sequencing by Sanger's Method

Method Summary

• Determines exact nucleotide order (A, T, C, G) in DNA.


• Developed by F. Sanger in 1977 using chain termination.


• Steps: DNA isolation, fragmentation, PCR amplification, denaturation, primer binding, DNA polymerase extension with normal and dideoxynucleotides (ddNTPs).


• ddNTPs terminate DNA chain randomly, producing fragments of varying lengths.


• Fragments separated by gel electrophoresis; sequence deduced from fragment sizes.


• Essential for genome analysis and understanding biological processes.

Memory Aids

• ddNTPs lack 3' OH group, causing chain termination.


• Gel electrophoresis separates DNA fragments by size.


• Sequence read from smallest to largest fragment.


• Sanger = chain termination sequencing.

Quick Checkpoints

• What causes chain termination in Sanger sequencing?


• Why is PCR used before sequencing?


• How are DNA fragments separated for analysis?


• What is the role of primers in sequencing?

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