Structure of the Human Genome: Organization, Components, and Complexity

Structure of the Human Genome

Lecture Objectives

• Understand the structure of the human nuclear genome.

• Analyze the complexity of the human genome.

• Discuss mutagenic outcomes of non-coding DNA elements.

• Compare and contrast nuclear versus mitochondrial genomes.

DNA Structure and Organization

Watson and Crick Model of DNA

The structure of DNA was elucidated by James Watson, Francis Crick, and Rosalind Franklin in 1953. Their model described DNA as:

• Double helix: Two strands twisted around each other.

• Anti-parallel strands: The two DNA strands run in opposite directions.

• Base pairing: Strands are held together by hydrogen bonds between complementary bases (A-T, G-C).

• Stacked nitrogenous bases: Bases are stacked perpendicular to the helix axis, contributing to DNA stability.

Chromosomal Organization

Chromosome Structure

• Chromatid: One of two identical halves of a replicated chromosome.

• Centromere: The constricted region of the chromosome, consisting of thousands of repetitive A-T rich sequences, essential for proper segregation during cell division.

• Telomere: The terminal region of the chromosome, composed of hundreds to thousands of repeats of the sequence TTAGGG, protecting chromosome ends from degradation.

Human Karyotype

• The human genome consists of 23 pairs of chromosomes:

  • 22 pairs of autosomes

  • 1 pair of sex chromosomes (XX or XY)

DNA Compaction: Chromatin Structure

Chromatin and Nucleosomes

• Chromatin: The complex of DNA and proteins (mainly histones) that compacts DNA within the nucleus.

• Nucleosome: The fundamental unit of chromatin, consisting of ~146-160 base pairs of DNA wrapped around a histone octamer (called DNA superhelix).

• Histone octamer: Composed of two copies each of the core histones: H2A, H2B, H3, and H4.

• Histone H1: The linker histone, binds outside the nucleosome and stabilizes the DNA (20-60 bp).

Histones

• The human genome contains 86 histone genes, organized in clusters across 10 chromosomes.

• Histones are positively charged (rich in lysine and arginine), allowing them to bind tightly to negatively charged DNA (phosphate backbone).

Nucleosome Tails and Modifications

• Histone tails: Unstructured N-terminal regions that protrude from the nucleosome core.

• These tails are targets for post-translational modifications such as phosphorylation, acetylation, and methylation, which regulate chromatin structure and gene expression.

Chromatin Types: Heterochromatin vs. Euchromatin

• Heterochromatin: Highly condensed, transcriptionally inactive, contains fewer genes, and remains condensed during interphase.

• Euchromatin: Less condensed, transcriptionally active, contains more genes, and its condensation state fluctuates.

Organization of the Human Genome

Overview

The human genome is organized into various functional and structural categories, including gene-related sequences and repetitive DNA elements.

Gene-Related Sequences

• Typical gene structure: Includes both coding (exons) and non-coding (introns, regulatory elements) sequences.

• Regulatory sequences: Promoters, enhancers, and silencers control gene expression.

• Exons: Coding regions that are transcribed and translated into protein.

• Introns: Non-coding regions within genes, removed during RNA splicing.

Example: Gene Structure

Region	Function
Promoter	Initiates transcription
Exons	Coding sequences
Introns	Non-coding, spliced out
Enhancer/Silencer	Regulate transcription

Gene Size and Content

• Largest gene: Dystrophin (DMD gene), 2.4 Mb, 79 exons.

• Largest coding sequence: Titin (connectin), 281,434 bp, 363 exons (largest exon is 17,000 bp).

• There is an inverse correlation between gene length and exon content: larger genes tend to have a lower percentage of exonic sequence.

Gene Distribution

• Gene density varies across chromosomes:

  • Chromosome 19: highest gene density

  • Chromosomes 13 and Y: lowest gene density

• Genes can overlap or be nested within other genes.

Summary Table: Chromatin and Genome Organization

Component	Description	Function
Chromosome	DNA molecule with part or all of the genetic material	Genetic inheritance
Chromatin	DNA + histone proteins	DNA packaging, regulation
Nucleosome	DNA wrapped around histone octamer	Basic unit of chromatin
Heterochromatin	Condensed, gene-poor	Transcriptional repression
Euchromatin	Less condensed, gene-rich	Transcriptional activity

Key Equations and Concepts

• Base Pairing Rule: $A = T$, $G = C$

• Telomere Repeat Sequence: $(TTAGGG)_n$

• Gene Size Calculation: $\text{Gene size (bp)} = \text{Number of exons} \times \text{Average exon length (bp)}$

Additional info:

• Further topics such as non-coding DNA elements, repetitive DNA, and mutagenic outcomes are likely covered in subsequent slides or lectures.

• Comparisons between nuclear and mitochondrial genomes are part of the objectives but not detailed in the provided slides.