Study guide: Human Genome Molecular Biology

📋 Course Outline

1. DNA Discovery and Chargaff Rules
2. Nucleic Acid Structure
3. DNA Helical Forms and Size
4. DNA Compaction and Chromatin
5. Chromosome Structure and Classification
6. Human Genome Composition
7. Genomic Variation and Repetitive DNA
8. RNA Types and Functions

📅 Key Dates

1. 1952Rosalind Franklin produced Photograph 51, an important image associated with the study of DNA structure.
2. 1953Watson and Crick published “A Structure for Deoxyribose Nucleic Acid” in Nature, volume 171, pages 737–738. - WATSON, J. D. & CRICK, F. H. C., 1953
3. Late 1988The Human Genome Project was launched to sequence the entire human genome.
4. 2001Celera Genomics and the International Human Genome Sequencing Consortium published first drafts covering approximately 92% of the human genome.
5. 2022The T2T-CHM13 Consortium produced the first complete human genome sequence and added previously unresolved DNA regions.

📖 1. DNA Discovery and Chargaff Rules

📝 Essential Points

📌 Chargaff’s rules state that DNA has complementary base pairing with A equal to T and G equal to C, so $(A+G)/(T+C)=1$, whereas $(C+G)/(A+T)\ne1$.

💡 Memory Hook

Franklin’s image → Chargaff’s ratios → Watson and Crick’s helix

📖 2. Nucleic Acid Structure

🔑 Key Concepts & Definitions

• Nucleotide: a nucleic-acid unit composed of a pentose sugar, a nitrogenous base, and a phosphate group

★ Must-know

📌 DNA generally contains deoxyribose, adenine, thymine, cytosine, and guanine, whereas RNA contains ribose and uracil instead of thymine.

Further detail

• DNA polymerization links nucleotides into a nucleic-acid chain through the sugar, phosphate, and base components.

💡 Memory Hook

A pairs with T, whereas C pairs with G

📖 3. DNA Helical Forms and Size

★ Must-know

📌 B-DNA is a right-handed helix with approximately 10 base pairs per turn and a diameter of about 2.0 nm, whereas A-DNA has approximately 11 base pairs per turn and a diameter of about 2.3 nm.

• One turn of the B-DNA double helix contains 10 base pairs and measures 3.4 nm, while adjacent nucleotides on the same strand are separated by 0.34 nm.

Further detail

• Z-DNA is a rare left-handed helix with approximately 12 base pairs per turn, a diameter of about 1.8 nm, and a zigzag backbone.

💡 Memory Hook

B-DNA is physiological and right-handed; Z-DNA is rare and left-handed

📖 4. DNA Compaction and Chromatin

🔑 Key Concepts & Definitions

• Nucleosome: approximately 146 base pairs of DNA wrapped around a histone octamer containing two copies each of H2A, H2B, H3, and H4

★ Must-know

• 🔄 DNA compaction proceeds through four levels:
  1. primary nucleosomes
  2. secondary solenoid
  3. tertiary looped domains
  4. quaternary metaphase chromosome

📌 Euchromatin is loosely packed, transcriptionally active, and accessible, whereas heterochromatin is tightly packed, transcriptionally inactive, and associated with structural or regulatory regions.

Further detail

• Linker DNA of approximately 55 base pairs connects nucleosomes, and histone H1 stabilizes this linker DNA.

💡 Memory Hook

Nucleosome → solenoid → looped domains → metaphase chromosome

📖 5. Chromosome Structure and Classification

🔑 Key Concepts & Definitions

• Telomere: a specialized region at each chromosome end that stabilizes the chromosome by forming a loop and contains approximately 1000 tandem repeats of 5′-TTAGGG
• Centromere: a chromosome region where kinetochores attach spindle microtubules, allowing chromosome alignment and equal segregation during cell division

📝 Essential Points

📌 After approximately 40–60 replication cycles, telomere shortening can lead to apoptosis.

• A human nucleated cell contains 23 pairs of chromosomes, and chromosome 1 is nearly five times larger than chromosome 21.

💡 Memory Hook

Telomeres protect chromosome ends, whereas centromeres ensure segregation

📖 6. Human Genome Composition

🔑 Key Concepts & Definitions

• Human genome: the complete set of genetic material of an individual or species encoded in DNA and contains approximately 3.055 billion base pairs

• Gene: a DNA sequence that encodes a functional product, either RNA or protein, and can produce multiple protein isoforms through alternative splicing

📝 Essential Points

• Coding DNA represents approximately 1–5% of the human genome and includes 19,969 genes, whereas repeated sequences represent approximately 54%.

📌 Exons remain in mature RNA, introns are removed during RNA processing, promoters regulate transcription upstream of genes, and enhancers or silencers regulate transcription from distant sites.

📖 7. Genomic Variation and Repetitive DNA

🔑 Key Concepts & Definitions

• Microsatellite: a short tandem repeat of 1–6 base pairs whose repeat number is highly variable and can serve as a genetic marker

★ Must-know

📌 Single-nucleotide variants change one nucleotide, whereas short tandem repeat variants change the number of repeated DNA units at a locus.

• LINEs are autonomous retrotransposons of approximately 6–7 kb that encode reverse transcriptase and represent about 20% of the human genome, whereas SINEs are non-autonomous elements shorter than 500 bp that depend on LINE machinery.

📌 Mitochondrial DNA is circular, lacks histones and a DNA repair system, has a mutation rate approximately ten times higher than nuclear DNA, and is inherited maternally, whereas nuclear DNA is linear, chromosomal, and biparentally inherited.

Further detail

• Highly repeated DNA sequences include:
  • minisatellite DNA
  • microsatellite DNA
  • satellite DNA

💡 Memory Hook

Tandem repeats sit side by side, whereas dispersed repeats are scattered

📖 8. RNA Types and Functions

★ Must-know

• Messenger RNA is transcribed from DNA and serves as the template for protein synthesis during translation, representing approximately 5% of total cellular RNA.

• Transfer RNA carries amino acids to ribosomes during translation and constitutes approximately 15% of total cellular RNA.

• Ribosomal RNA combines with proteins to form ribosomes and represents approximately 80% of total cellular RNA.

📌 DNA is usually double-stranded, contains deoxyribose and thymine, and stores hereditary information, whereas RNA is usually single-stranded, contains ribose and uracil, and performs messenger, transfer, ribosomal, and regulatory functions.

Further detail

• Important non-coding RNAs include:
  • miRNAs
  • snoRNAs
  • siRNAs
  • snRNAs
  • piRNAs

💡 Memory Hook

mRNA carries the message, tRNA carries amino acids, and rRNA builds the ribosome

📊 Synthesis Tables

DNA Helical Forms

Feature	B-DNA	A-DNA	Z-DNA
Handedness	Right-handed	Right-handed	Left-handed
Base pairs per turn	Approximately 10	Approximately 11	Approximately 12
Diameter	Approximately 2.0 nm	Approximately 2.3 nm	Approximately 1.8 nm
Typical conditions	Physiological conditions	Dehydrated conditions or RNA–DNA hybrids	High salt, negative supercoiling, or alternating purine–pyrimidine sequences

Nuclear and Mitochondrial DNA

Feature	Nuclear DNA	Mitochondrial DNA
Structure	Linear chromosomes	Circular DNA
Inheritance	Biparental	Maternal
Protein organization	Associated with histones	Lacks histones
Genetic code	Standard nuclear code	Slightly different mitochondrial code