Human Genome Structure and Function

The Human Genome: A Cheatsheet to DNA, RNA, and Chromosomal Organization

This cheatsheet provides a condensed overview of key concepts related to the human genome, covering DNA and RNA structure, historical context, and the complex organization of genetic material within the cell.

I. Deoxyribonucleic Acid (DNA)

1. Historical Milestones & Key Discoveries

• 1953: Watson and Crick elucidated the double helix structure of DNA, building on work by Rosalind Franklin (X-ray diffraction) and Maurice Wilkins.

• 1988: Launch of the Human Genome Project to sequence the entire human genome.

• 2001: Initial drafts of the human genome published by Celera Genomics and the International Human Genome Sequencing Consortium (approximately 92% complete).

• 2022: Telomere-to-Telomere (T2T) Consortium produced the first truly complete human genome (3.055 billion bp). This corrected previous errors and sequenced nearly 200 million bp of new DNA.

2. Chemical Composition and Structural Features of DNA

• Basic Structure: DNA is a polymer made of nucleotides, each containing a pentose sugar (deoxyribose), a nitrogenous base (A, T, C, G), and a phosphate group.

• Double Helix: DNA forms a right-handed double helix with two strands.

• Complementary Base Pairing (Chargaff's Rules):

  • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

  • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

  • Thus, . The GC content is species-specific.

• Strand Properties:

  • Strands are held together by weak hydrogen bonds.

  • They can be separated by heat or alkaline conditions (denaturation), which is reversible (renaturation or hybridization).

  • The sequence of one strand allows deduction of the other.

• Helix Dimensions (B-DNA - most common):

  • One turn = 10 base pairs = 3.4 nm length.

  • Distance between two consecutive nucleotides = 0.34 nm.

• Conformational Forms:

  • B-DNA: Most common under physiological conditions; right-handed; ~2.0 nm diameter; 10 bp/turn.

  • A-DNA: Favored in dehydrated conditions or RNA-DNA hybrids; right-handed; ~2.3 nm diameter; 11 bp/turn.

  • Z-DNA: Rare; left-handed; ~1.8 nm diameter; 12 bp/turn; associated with high salt or alternating purine-pyrimidine sequences.

II. Ribonucleic Acid (RNA)

1. Types and Functions

• Messenger RNA (mRNA):

  • ~2.3% of genome, ~5% of cellular RNA.

  • Function: Carries genetic code from DNA to ribosomes for protein synthesis.

• Transfer RNA (tRNA):

  • ~15% of total cellular RNA.

  • Function: Carries specific amino acids to ribosomes during protein translation.

• Ribosomal RNA (rRNA):

  • ~80% of total cellular RNA.

  • Function: Forms the structural and catalytic core of ribosomes, where protein synthesis occurs.

• Other Non-coding RNAs:

  • miRNA (~22 nt): Regulates gene expression post-transcriptionally.

  • snoRNA (~100 nt): Involved in rRNA modification and processing.

  • siRNA (variable): Guides degradation of complementary RNAs.

  • snRNA (100–190 nt): Involved in pre-mRNA splicing and rRNA maturation.

  • piRNA (24–31 nt): Silences transposable elements in germ cells.

Comparison with DNA:

III. Human Genome Organization

1. Genome Composition

• Total Length: Approximately 3.055 billion bp per haploid genome (~2.3 meters).

• Coding vs. Non-coding:

  • Coding sequences (~1-5%): Transcribed into mRNA and translated into proteins (19,969 genes).

  • Non-coding sequences (~95-99%): Includes introns, regulatory sequences, and repetitive DNA. Often called "junk DNA", but many have regulatory or structural roles.

• Gene Categories:

  • Single-copy/Monocopy Genes (~15% of human genome): Most protein-coding genes, present in two copies (one from each parent) in autosomes. e.g., actin, GAPDH, collagen.

  • Gene Families (~15% of human genome): Genes grouped by sequence similarity, often duplicated from an ancestral gene. e.g., alpha- and beta-globin gene clusters.

  • Pseudogenes (12,000–20,000): DNA sequences resembling functional genes but non-functional due to mutations.

• Gene Density:

  • Varies: humans ~11 genes/100,000 bp; yeast ~479 genes/100,000 bp.

  • Within chromosomes: Generally gene-poor near centromeres, gene-rich near telomeres/chromosome arms.

• Repetitive DNA: Up to ~45% of human DNA; crucial for homologous recombination.

  • Highly Repeated (Simple DNA Sequences) (~3%): Long fragments (5-500 bp) repeated millions of times.

    • Minisatellites: Variable number of repeats, used in DNA fingerprinting.

    • Microsatellites (STRs): Very short repeats (1-6 bp) with high variability, useful as genetic markers (e.g., paternity tests).

    • Satellite DNA: Mostly structural (centromeres, heterochromatin); repeat number generally conserved.

  • Dispersed Repetitive DNA (Interspersed Repeats): Not in tandem.

    • LINEs (~21%): Long interspersed nuclear elements (e.g., L1); autonomous, encode reverse transcriptase.

    • SINEs (~13%): Short interspersed nuclear elements (e.g., Alu); non-autonomous, rely on LINE machinery.

    • LTR retrotransposons (~8%): Flanked by Long Terminal Repeats; retrovirus-like mechanism.

    • DNA Transposons (~3%): "Cut-and-paste" mechanism via transposase; mostly inactive in humans.

2. Chromosome Structure and Packaging

• Chromatin: DNA-protein complex (DNA, histones, non-histone proteins, RNA) making up chromosomes.

• Chromatin States:

  • Euchromatin: Active, loosely packed, accessible for transcription; stains lightly; replicates early S-phase.

  • Heterochromatin: Inactive, tightly packed, structural/regulatory regions; stains darkly; replicates late S-phase; found near centromeres and telomeres.

• Levels of DNA Compaction:

  1. DNA Double Helix (2 nm): Basic structure.

  2. Nucleosomes (11 nm fiber, "beads-on-a-string"): DNA (approx. 146 bp) wrapped around an octameric histone core (2x each of H2A, H2B, H3, H4); stabilized by histone H1. First level of compaction.

  3. Solenoid (30 nm fiber): Condensed nucleosomes arranged in a spiral (approx. 6 nucleosomes per turn), further organized by histone H1 and non-histone proteins (e.g., topoisomerase II).

  4. Looped Domains (300 nm fiber): Solenoid fibers attach to a protein scaffold.

  5. Condensed Section of Chromosome (700 nm): Further coiling of looped domains.

  6. Entire Mitotic Chromosome (1400 nm): Fully condensed, visible during cell division.

• Chromosome Components:

  • Telomeres: Specialized ends of chromosomes (5'-TTAGGG repeats); ensure stability, prevent degradation, shorten after replication cycles leading to apoptosis.

  • Centromeres: Constricted regions where sister chromatids attach and kinetochores form; essential for proper chromosome segregation during cell division. Composed of satellite DNA.

  • Chromosome Types (based on centromere position): Metacentric, Submetacentric, Acrocentric (no Telocentric in humans).

• Human Karyotype: 23 pairs of chromosomes, arranged by size into 7 groups (A-G). Chromosome 1 is largest, 21 is smallest.

IV. Mitochondrial DNA (mtDNA)

• Location: Mitochondria.

• Structure: Naked, circular, double-stranded DNA; not associated with proteins.

• Inheritance: Maternal inheritance only.

• Mutation Rate: ~10x higher than nuclear DNA (lacks robust repair system and histones).

• Content: Encodes 13 essential proteins for oxidative phosphorylation, 22 tRNAs, and 2 rRNAs.

• Genetic Code: Slightly different from the standard nuclear code.

• Clinical Relevance: Mutations can cause mitochondrial diseases (e.g., Leber hereditary optic neuropathy, MELAS syndrome).