Chromosome Structure and Number

Structure and Number of Chromosomes

Part #1: Chromosome Structure and Number
- Key Questions
- What are the types of changes in chromosome structure?
- What changes are more likely to be harmful? Less likely?
- What changes can result in driving evolution?

Definition: Chromosomes are cellular structures that contain genetic material (DNA) with associated proteins.
Human X & Y Chromosomes: Different configurations indicating sex chromosomes (XX for female, XY for male).

Chromosome structure can change due to DNA replication, leading to mutations.
Karyotyping allows for identification of chromosomal changes across species and populations as well as within individuals.
Applications include characterizing disease and phenotypic differences.

Definition: A segment of chromosomal material is deleted (missing).
Synonym: “Deficiency” - a chromosome is missing a region and is deficient in genetic material.
Implications: Tend to be detrimental as they disrupt genetic expression patterns (haploinsufficiency).
- Types of Deletions:
  - Terminal Deletion: Involves loss of chromosome end due to DNA double-strand breaks.
  - Interstitial Deletion: Results from chromosome crossing over with itself.

Definition: An increase in genetic material typically deemed less harmful compared to deletions.
Can occur during crossing over, particularly if repetitive sequences cause homologues to misalign, resulting in one shorter chromosome and one longer chromosome (one duplication, one deletion).
Gene Duplication: Leads to the evolution of gene families over generations due to gradual accumulation of mutations, potentially leading to neofunctionalization (evolution of new function) or subfunctionalization (gene splits responsibilities).

Definition: Structural variation where a DNA segment (>1000 basepairs) exists in different numbers among individuals or populations.
Detection via karyotype for large changes or DNA sequencing for small variations.

Definition: A segment of DNA is flipped and reinserted into the chromosome; total genetic material remains unchanged.
Classification: By their relationship to centromeres:
- Pericentric: Includes the centromere.
- Paracentric: Excludes the centromere.
- Inversions can affect phenotype if breakpoints affect vital genes or alter gene expression (position effect).

Definition: A segment of a chromosome attaches to a non-homologous chromosome, which can be simple or reciprocal.
Environmental Considerations: Damage can lead to chromosome breaks; however, most repairs occur through DNA repair mechanisms.
During meiosis, translocation can lead to unbalanced genetic segregation, resulting in gametes with extra or missing genetic material.

Ploidy: The number of sets of chromosomes; organisms may be diploid (2n), triploid (3n), tetraploid (4n), etc.
*Examples:
- Tetraploid: Organisms like Hyla versicolor (gray treefrog).
Consequences of Aneuploidy: May disrupt gene expression, typically detrimental to organisms.
Nondisjunction: Failure of chromosomes to separate properly during meiosis, potentially resulting in abnormal chromosome numbers in gametes and contributing to various diseases, including Turner syndrome, Down syndrome, Klinefelter syndrome.

Definition of Genomes: The complete set of genetic material in a cellular compartment, including nuclear and mitochondrial genomes in eukaryotes.
Viral Genomes: May consist of RNA or DNA, protected by a protein coat and, in many cases, a membranous envelope for host infection.
Bacterial Genomes: Typically singular circular chromosomes located in nucleoid; often compacted; plasmids present as extrachromosomal DNA.

Eukaryotes have more complex genomic structures, requiring compacting methods to fit within the cell nucleus, which includes levels and mechanisms of DNA packing utilizing histones and other proteins.

Definition: Segments of DNA that can move within a genome, also referred to as “jumping genes.”
Tandem Arrays: Clusters of repetitive sequences, can evolve rapidly through mutations.
Discovered by: Barbara McClintock through studies on corn strains.
Transposition Types:
- Simple Transposition: TE is excised and inserted at a new site.
- Retrotransposition: TE is transcribed into RNA and then reverse transcribed back into DNA for insertion, always increasing copy number.

Eukaryotic DNA is compacted through nucleosomes, creating chromatin. Additional levels of compaction include the organization of chromatin into chromosomal territories, with structures like heterochromatin and euchromatin dictating gene expression and genomic organization through sequential levels of folding and condensing.

The number of chromosomes does not correlate with organism complexity (C-value paradox).
Variability in genome size and organization might not reflect phenotypic complexity; instead, genome efficiency and regulation play significant roles in evolutionary adaptation and diversity.