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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BIO
Eukaryotic Chromosome Organization

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The hierarchal structure of tightly packaged DNA in eukaryotes is that of the DNA's double helix, nucleosomes, supercoils, chromatin, and the chromosome.

Chromosomal proteins

Each chromosome is a highly organized structure of DNA and associated proteins. Histones are proteins around which portions of DNA are wrapped to form a string of nucleosomes, which are complexes of eight histones plus the encircling DNA.

Single copy vs. repetitive DNA

On the level of nucleotides within eukaryotic DNA, there is variation in repetitive sequences. Single copy DNA is characterized as only having one copy of its sequence and encoding for a protein product, making it a specific target for transcription (associated with euchromatin). Repetitive DNA is more generally found in non-coding regions of DNA that are kept more tightly packaged and not transcribed (associated with heterochromatin).

Supercoiling

The histone-bound DNA organized into nucleosomes is further wrapped into coils called supercoils. Nucleosomes that are so tightly organized generally block transcription of their associated DNA.

Heterochromatin vs. euchromatin

The compact organization of protein and DNA (with small amounts of RNA present) is called chromatin. The more compact the structure of chromatin, the greater the degree of impedance to transcription machinery. Tightly coiled regions of chromatin that are not available for transcription are called heterochromatin. Portions of chromatin that are uncoiled and available to be read by transcription machinery are called euchromatin.

Telomeres, centromeres

Telomeres and centromeres are special regions of a chromosome that perform structural functions. Telomeres are capping regions that exist on the ends of chromosomes that protect the chromosome from degradation during replication. A centromere is the single point region on a chromosome that aids organization during replication by connecting two sister chromatids (the original chromosome and its replicated partner). DNA around telomeres and centromeres tends to be more repetitive and non-coding (not transcribed).

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