Chapter 10 - Cytogenetics

Virus: Noncellular infectious particle with a small nucleic acid genome and limited genes.
Nucleic Acid Types: Can be single or double-stranded DNA or RNA.
Viruses must infect a host to express genetic material and replicate using host cell machinery.
Viral genome composition can vary significantly in size and structure.
- Ranges from a few thousand bases to over 200,000 bases.
- Number of genes can vary from 5 to nearly 300 genes.

Parvovirus (ssDNA): 5,176 bases, Linear, involved with Animals.
Bacteriophage T4 (dsDNA): 168,903 bases, Linear, infects Bacteria.
Human Immunodeficiency Virus (HIV, ssRNA): 9,700 bases, Linear, impacts Animals.

Bacterial genomes are typically haploid and consist of a single chromosome of double-stranded DNA.
Most bacterial chromosomes are circular and densely packed in a region known as the nucleoid.
The organization maximizes the compactness of the DNA within the limited space.

Escherichia coli: ~4.6 Mb, containing 4,688 protein-coding genes, as well as rRNA and tRNA genes.

Composed of multiple (usually) linear chromosomes undergoing condensation during cell division.
Eukaryotic genomes contain significantly more DNA than bacterial genomes.
DNA and associated proteins are known as chromatin, with histones being the primary proteins.
Histones help compact DNA and are vital for DNA organization and function.

Histones Types: H1, H2A, H2B, H3, H4. Highly conserved across eukaryotes.
Histones form nucleosomes, which are composed of octamer structures consisting of dimers of H2A-H2B and tetramers of H3-H4.
DNA wraps around histone octamers creating nucleosomes, which condense the DNA effectively.
The structure and arrangement of DNA and histones result in a fibers' resting length of compacted 10nm and further into a 30nm fiber under specific cellular conditions.

Types of Chromatin:
- Euchromatin: Loosely packed, actively transcribed.
- Heterochromatin: Tightly packed, often transcriptionally inactive.
Chromosomes can vary in morphology based on centromere position (metacentric, submetacentric, acrocentric, telocentric).
Karyotype Analysis: Visual display of chromosomes used to identify abnormalities, employing staining techniques to visualize distinctive banding patterns.

Nondisjunction: Failure of chromosome segregation during cell division, potentially leading to aneuploidy (abnormal chromosome number).
Common types of human aneuploidies include Trisomy 21 (Down syndrome), Trisomy 18, and Turner syndrome (45, XO).
Translocations: Structural changes where segments of chromosomes are rearranged, influencing genetic stability and fertility.

Impact of gene dosage due to aneuploidy can disrupt chemical balance within cells.
Plants demonstrate more resilience to gene dosage imbalances than animals.
Even small chromosomal alterations can have significant phenotypic impacts.
Uniparental Disomy: Occurs when both copies of a chromosome come from one parent, impacting development based on the extent of genomic imbalances.
Translocational effects can influence inheritance patterns, creating risks for trisomic offspring depending on the rearrangement mechanism involved.
Overall fitness can often be reduced with incorrect recombination or structural rearrangements impacting fertility or increasing the likelihood of miscarriage.

Understanding chromosomes, including structure, function, and the effects of aberrations, is crucial for studying genetics and related disorders including cancer, hereditary conditions, and reproductive health.