Chromosomes and Viral Genomes

Chromosomes

• Threadlike structures made of protein and a single DNA molecule
• Carry genomic information from cell to cell
• Reside in the nucleus of cells in plants and animals, including humans
• Humans possess 22 pairs of numbered chromosomes (autosomes) and 1 pair of sex chromosomes (XX or XY), totaling 46 chromosomes
• Each pair comprises two chromosomes, one inherited from each parent
• Visible through a microscope when the nucleus dissolves during cell division
• The term "chromosome" originates from the Greek words "Chroma" (color) and "Soma" (body)

Viral Chromosomes

• Viruses are infectious microorganisms made of a nucleic acid segment (DNA or RNA) wrapped in a protein coat
• They cannot multiply independently, requiring host cells for replication
• Viruses do not possess chromosomes like eukaryotic and prokaryotic cells, instead having a segmented genome
• A genome segment constitutes a single, distinct fragment of nucleic acid within a viral genome
• Influenza A virus, has a segmented genome with eight single-stranded RNA segments

Prokaryotic Chromosomes

• Prokaryotic chromosomes (e.g., bacteria) consist of a single, circular, two-stranded DNA molecule without a membrane envelope
• They lack proteins and directly contact the cytoplasm
• Bacterial chromosomes are packed into the nucleoid by RNA that forms a core
• Attachment to the plasma membrane is permanent at one point
• Extra-chromosomal DNA molecules, or plasmids, may be present
• They are double-stranded and circular, but smaller
• Plasmids may occur independently in the cytoplasm or in association with the main chromosomal DNA, termed an episome

Eukaryotic Chromosomes

• Eukaryotic chromosomes are located in the nucleus and certain other organelles, such as mitochondria and plastids
• Nuclear chromosomes are double-stranded, long DNA molecules in a linear form, associated with proteins and surrounded by a nuclear envelope
• Extra nuclear chromosomes reside in mitochondria and plastids
• They comprise double-stranded, short DNA molecules of circular form, lacking protein association
• Offspring inherit mitochondria and mitochondrial DNA from their mother

Morphology of Chromosomes

• In interphase, eukaryotic chromosomes extend into long, thin chromatin fibers, forming the chromatin reticulum
• They replicate during the S-phase, becoming double and consisting of two chromatids held together at the centromere
• During cell division, chromosomes condense, tightly coil, and become distinct at metaphase
• Eukaryotic chromosomes vary in number, size, shape, and position

Chromosome Number

• Eukaryotic chromosomes vary in number from two to hundreds across different species
• Number of chromosomes is constant within a species (except for gametes)
• Chromosome number is helpful in determining the phylogeny and taxonomic position of a species

Chromosome Size

• Chromosomes within a species vary in size
• Organisms with fewer chromosomes tend to have larger chromosomes
• Plant chromosomes are generally larger than animal chromosomes
• Monocots possess larger chromosomes than dicots

Chromosome Structure

• At metaphase, chromosomes contain two coiled sister chromatids
• Held together at the centromere, also known as the primary constriction
• Each chromatid has a kinetochore, a structure where spindle microtubules attach during cell division
• Kinetochores pull chromatids towards the poles
• Secondary constrictions may be present, separating a part of the chromosome termed the satellite
• Chromosomes with a satellite are called sat chromosomes
• Size and shape of the satellite remain constant for a species
• Secondary constrictions associate with the nucleoli

Telomeres

• Ends of chromosomes are called telomeres
• Telomeres consist of repetitive DNA sequences
• They protect chromosome ends from fraying or tangling
• Telomeres shorten with each cell division
• When telomeres become too short, the cell can no longer divide successfully and dies

Types of Chromosomes

• Chromosomes are classified based on the position of the centromere:
• Telocentric
• Acrocentric
• Submetacentric
• Metacentric

Functions of Chromosomes

• They carry hereditary characters from parents to offspring
• Direct synthesis of structural proteins to facilitate cell growth, division, and maintenance
• Direct the formation of necessary enzymes to control metabolism
• Guide cell differentiation during development
• Form nucleoli at nucleolar organizer sites in daughter cells
• Generate variations through changes in genes, contributing to the evolution of organisms
• Play a role in sex determination
• Maintain the continuity of life through replication

DNA Packaging

• A chromatid contains a chromonema
• This is a single, long, double-stranded DNA molecule
• DNA is wrapped around histones to form nucleosomes
• Nucleosomes and non-histone proteins make up the chromatin fiber
• Chromatin fiber has reactive groups and H1 histone molecules
• These crosslink the chromatin fiber into a coiled, compact metaphase chromatid

Requirement for DNA Packaging

• DNA is approximately 3 meters long in a nucleus only micrometers in diameter
• Compaction into chromatin is necessary for accommodating DNA within the nucleus

Orders of DNA Packaging

• First order: Nucleosome formation
• Positively charged proteins adhere to negatively charged DNA, forming nucleosomes
• Each nucleosome is composed of DNA wound 1.65 times around eight histone proteins
• Second order: Solenoid fiber formation
• Nucleosomes fold up to form a 30-nanometer chromatin fiber with loops averaging 300 nanometers in length
• Third order: Scaffold loop chromatids chromosome
• The 300 nm fibers are compressed and folded into a 250 nm-wide fiber, tightly coiled into the chromatid of a chromosome

Histone Proteins

• Histone proteins associate with DNA in the nucleus
• Each histone octamer comprises two copies each of H2A, H2B, H3, and H4 (core histones)
• Histone H1 exists outside the core, binding to a linker DNA (20-60 bp) between nucleosomes

Non-Histone Proteins

• Non-histone proteins are a diverse group that organize and compact chromosomes into higher-order structures
• They regulate nucleosome remodeling, DNA replication, RNA synthesis, and processing