Question

Genetics Ch 11

Describe eukaryotic chromosome (components, packaging, etc..)?

Answer 1

Both prokaryotes and eukaryotes have chromosome as a specialized structure to organize their genetic materials, even though the typical chromosome is absent in prokaryotes. Chromosomes have 2 functions as to maintain the integrity of genetic materials and to ensure genome is replicated accurately during each cell cycle.

Eukaryotic chromosomes are more complex and larger when compare with that of prokaryotes. Eukaryotes have multiple chromosomes in a cell. While prokaryotes have a single covalently closed circular DNA, eukaryotic chromosomes contain linear DNA with two ends. Eukaryotic chromosomes codes for larger number of proteins and are packed in the nucleus.

Chromatin is DNA plus a protein (as in a nucleosome). In eukaryotes, DNA is coiled and condensed around a protein called histone to form structures called nucleosomes.  This allows the nucleus to store a large number of DNA. Genes are regions of DNA that can be transcribed to RNA.

Eukaryotic chromosomes are usually represented as X- shaped. During cell division, chromosomes replicate to form two copies called Chromatids which will still be attached to each other at a point called Centromere to get the X-shape. But each of these sister chromatids will be pulled apart further during cell division to form independent chromosomes. This allows proper Segregation of genetic materials during cell division. Chromosomes can be divided into 4 types based on the position of their centromeres as Metacentric, Submetacentric, Acrocentric and Telocentric.

In eukaryotes, linear DNA are packaged in three levels. in the primary level, the negatively charged DNA is tightly coiled around histone core. The DNA which is directly associated with histone core is called Core DNA.  Linker DNA connects the core DNA to the next nucleosome to form a beads and string structure.  The next level of compaction is by creating a 30nm fibre by the folding of first level nucleosomes with additional proteins.The 30 nm fibre can be organized in a solenoid model or in a Zig-zag model. In the third level of packing, many proteins are involved in the coiling of Chromatin.

Centromeres and Telomeres are two structures important in maintaining the stability of chromosomes in eukaryotes. Centromeres are the points where chromatids attached. They help in accurate segregation of genome during cell division. Telomeres are the structures at the end of linear chromosomes which block fraying or sticking of adjacent chromosomes.