Chromosomes, Genomes, and Karyotypes

Chromosomes and the Genome

• Chromosomes are condensed DNA molecules wrapped around histone proteins; they carry genetic material to daughter cells during mitosis and meiosis.

• Nucleosomes: Formed when linear eukaryotic DNA coils around histones to prevent tangling and enzymatic degradation.

• The Genome is the total DNA content of an organism measured in base pairs within a haploid ($n$) set.

• The human genome contains approximately $3234$ million base pairs per set.

Chromosome Structure and Types

• Centromere: The connection point on a chromosome used during replication.

• Unreplicated Chromosome: Refereed to as "I-shaped"; typically not condensed to allow for RNA polymerase to perform transcription.

• Duplicated Chromosome: Seen as an "X-shape," occurring just before cell division.

Classification Based on Centromere Position

• Metacentric: Centromere is in the center; arms are of equal length.

• Submetacentric: Centromere is towards one end; the long arms are typically twice the size of the short arms.

• Acrocentric: Centromere is located very close to one end.

• Telocentric: Centromere is at the very tip; only long arms exist.

Homologous Chromosomes and Ploidy

• Homologous Chromosomes: Matching pairs that carry the same genes at the same loci; of these, humans have 22 pairs of Autosomes.

• Allosomes: Sex chromosomes that determine biological sex; can be homogametic ($XX$) or heterogametic ($XY$).

• Diploid ($2n$): The total number of chromosomes in an organism (e.g., Humans = $46$, Cat = $36$, Chimpanzee = $38$, Cabbage = $18$, Fern = $1260$).

• Haploid ($n$): Half the total chromosome number (e.g., Humans = $23$).

Karyotypes

• Definition: A visual representation of a full set of chromosomes isolated from a single cell, ordered from largest to smallest.

• Utility: Used to identify the species, determine biological sex, and detect genetic abnormalities such as Down syndrome.

Questions & Discussion

• Q: Which chromosome structure is typically visible just before mitosis or meiosis?

• A: X-shaped, duplicated chromosome.

• Q: Which of the following chromosome types has the centromere at the very end?

• A: Telocentric.

• Q: What makes homologous chromosomes homologous?

• A: They are the same length and carry genes at the same loci.

• Q: What would a karyotype most likely be used for?

• A: Identifying chromosome number and structure.