Chapter 11 – Chromosome Structure & DNA Sequence Organization

Chapter 11 - Chromosome Structure & DNA Sequence Organization

11.1 Overview of Chromosome Structures

  • Genetic information in viruses and bacteria is mostly held in simple, circular DNA molecules that are largely devoid of associated proteins. In contrast, eukaryotic cells contain a significant amount of DNA organized into nucleosomes and predominantly existing as chromatin fibers during most of the cell cycle.

  • During eukaryotic cell division, the uncoiled chromatin fibers characteristic of interphase condense into well-defined chromosomes.

  • Eukaryotic genomes exhibit a mix of unique and repetitive DNA sequences, with a substantial portion composed of noncoding DNA sequences, which do not encode functional genes.

11.2 Nature of Viral and Bacterial Chromosomes

  • Viral chromosomes may contain either DNA or RNA, and can be single or double stranded, as well as circular (closed loops) or linear molecules. For example, Phage lambda has linear dsDNA and ΦX174 has circular ssDNA.

  • Viral genetic material remains inert until it enters a host cell where it can become active. Common themes among viruses, bacteria, and eukaryotic cells include the ability to package DNA into small volumes during cell division.

  • Bacterial chromosomes, primarily found in the structure called a nucleoid, are mostly composed of circular, double-stranded DNA, with E. coli being the most extensively studied example. Bacterial DNA has proteins such as HU and H-NS (historical-like nucleoid structuring protein), which fold and bend the DNA to assist in its compaction.

11.3 Mitochondria and Chloroplasts

  • Both mitochondria and chloroplasts contain their own DNA, which is inherited maternally in most organisms and exhibits properties similar to those found in viruses and bacteria.

  • Mitochondrial DNA (mtDNA) is circular, double-stranded, and devoid of chromosomal proteins. Typically, mtDNA lacks introns and displays little gene redundancy. Its replication relies on nuclear DNA-encoded enzymes.

  • Mitochondrial DNA plays a key role in tracing lineage as it is passed down without recombination. It’s a focus in studies of certain diseases, including MERRF Syndrome

  • Chloroplast DNA (cpDNA) is larger compared to mtDNA, featuring a higher gene count and presence of introns and duplications, serving mainly photosynthetic functions in plants.

11.4 Organization of DNA into Chromatin in Eukaryotes

Chromatin Structure

  • During the interphase, chromosomes decondense into chromatin, which is dispersed across the nucleus and replicated prior to cell division. Upon division, chromatin coils back, leading to the formation of visible chromosomes.

  • Histones, which are positively charged proteins, are crucial to the organization and expression of eukaryotic chromatin. The five main types of histones include H2A, H2B, H3, H4, and H1. These proteins contain positively charged amino acids like lysine and arginine, necessary for forming electrostatic bonds with the negatively charged phosphates in DNA.

Nucleosomes

  • Chromatin comprises nucleosomes, which are defined as DNA particles wrapped around a histone octamer. This structure consists of two H3-H4 dimers and two H2A-H2B dimers, measuring approximately 11 nm in length and containing 147 base pairs (bp) of DNA.

  • The additional histone H1 binds to spacer DNA between nucleosomes, facilitating further structural organization.

  • Nucleosomes form a filamentous structure referred to as the 10 nm fiber, further condensing through coils to form a 30 nm solenoid, which ultimately compacts into structures of 300 nm and 700 nm chromosomes during cell division.

Chromatin Remodeling and Modifications

  • Chromatin remodeling plays a vital role in epigenetics, which involves changes in gene expression that do not modify the DNA sequence itself. Usually, DNA is inaccessible for interactions due to complex chromatin fiber formations with histones. To enable DNA-protein interactions for gene expression, chromatin must undergo relaxation, which entails a reversible mechanism.

  • Studies utilizing X-ray diffraction have established that histone tails protrude from their nucleosome structures, allowing for interaction with the minor groove of DNA. The tails of the protein contribute significantly to the potential sites for modification, creating a “histone code” critical for functional gene expression.

Histone Modifications

  1. Acetylation: Enzymes known as Histone Acetyltransferases (HATs) add acetyl groups to positively charged lysine residues found on histone tails. This modification neutralizes the charge, typically relaxing the histone's binding to DNA, associated with active genes — often referred to as euchromatin.

  2. Methylation: The process of adding methyl groups to histone tails is carried out by Histone Methyltransferases. Methylation can either enhance or inhibit gene transcription depending on whether methyl groups are added to arginine or lysine residues.

  3. Phosphorylation: Introduced by kinases, phosphate groups are added primarily to serine, threonine, histidine, and tyrosine residues on histone tails. This modification imposes a negative charge and is significant during the cell cycle and DNA replication as it impacts chromatin structure.

CpG Islands and Gene Regulation

  • The methylation of cytosine bases, particularly those adjacent to guanines, forms 5-methylcytosine, often leading to gene silencing. These CpG islands are crucial in regulating gene expression through epigenetic modifications.

Euchromatin vs. Heterochromatin

  • Chromosomes are not uniformly structured; areas classified as euchromatin are typically active, less condensed, and appear less stained. Heterochromatin, on the other hand, is condensed, generally inactive, and stains darker under cytological techniques. Common examples include centromeres, telomeres, the Y chromosome, and Barr bodies.

Chromosome Banding Patterns

  • Characteristic banding patterns of mitotic chromosomes visible through differential staining techniques (e.g., D-banding, G-banding, and C-banding) allow for the identification and analysis of chromosomal structures and abnormalities. Banding techniques highlight regions of DNA, allowing researchers to pinpoint genetic phenomena such as translocations and chromosomal aberrations.

11.5 Eukaryotic Genomes and Noncoding DNA

  • The vast majority of eukaryotic genomes consist of non-coding DNA, including various repetitive DNA sequences. Functional genes constitute a small fraction, only about 2-10% of the genome, with approximately 20,000 protein-coding genes in humans.

  • Pseudogenes are noncoding DNA remnants of duplicated genes that are typically not transcribed due to accumulated mutations, underscoring evolutionary history.

11.6 Specialized Chromosome Structures

Polytene Chromosomes

  • Polytene chromosomes, observable in Drosophila and certain plant tissues, are characterized by their multiple chromatid strands formed through numerous rounds of DNA replication without cell division, presenting areas of localized uncoiling (puffs) indicative of high gene expression activity.

Lampbrush Chromosomes

  • Lampbrush chromosomes are distinguished by extensive DNA looping, predominantly found in meiotic oocytes of vertebrates and some insect spermatocytes. These chromosomes exhibit profound looping and chromomeres, facilitating genetic activity during meiosis.

Conclusion

  • Understanding the diverse structures and organizations of chromosomes and DNA not only provides insights into genetic function but also emphasizes the intricate regulatory mechanisms underlying gene expression across different organisms. This knowledge is indispensable in fields ranging from genetic research to medical applications related to genetic disorders and disease tracking.