Eukaryotic genetics encompasses the study of heredity in multicellular and unicellular organisms the have a true nucleus with a defined nuclear membrane. Accordingly, eukaryotic genetics excludes the study of prokaryotes (e.g., bacteria).
The cells of eukaryotes range in size from 10-50 micrometers in diameter and contain a complex internal organization comprised of membrane bound organelles that carry out specialized cellular functions. The nucleus of eukaryotic cells separates the genetic material from the cytoplasm by a nuclear envelope. In sharp contrast to eukaryotic organisms, single-celled prokaryotes range in size from 1-5 micrometers in diameter and have a simple internal organization. Prokaryotes do not have membrane bound organelles or a nucleus to house its genetic material. Instead, bacteria have a nucleoid, which has no surrounding membrane to contain the single molecule of circular DNA. Because there is no nuclear membrane, translation and transcription may occur simultaneously in prokaryotic cells.
The genetic information within a eukaryotic cell consists of DNA and protein packaged in linear chromosomes that vary in their structure throughout the cell cycle. During the process of cell division, chromosomes are condensed and tightly coiled but remain dispersed and loosely coiled during interphase. Each chromosome is made up of two genetically identical sister chromatids attached by a special DNA region called a centromere. The centromere is the site of attachment for spindle microtubules that pull the sister chromatids to either pole during mitosis and meiosis. A telomere is also a special DNA region located at the ends of chromosomes in eukaryotic cells that completes the nucleotide sequence of the newly synthesized DNA. Without telomeres, nucleotides would be omitted at one end of the chromosome because of a gap left by the removal of RNA primer during DNA replication.
Although the DNA of eukaryotic cells is divided into chromosomes, the amount of DNA in a typical eukaryotic chromosome is still much larger than the amount of DNA in any prokaryotic cell. The DNA in eukaryotic chromosomes is combined with and wrapped around histones that form repeating units called nucleosomes. These nucleosomes are coiled into chromatin fibers that configure into looped domains forming supercoils. The supercoils are further arranged and packaged into a chromosome.
DNA replication in eukaryotes includes many of the same features as in prokaryotes. For example, eukaryotes and prokaryotes replicate semiconservatively, that is the DNA strands separate and serve as a template for the newly synthesized strands. Both eukaryotic and prokaryotic organisms require many different enzymes such as primase, ligase, helicases, and topoisomerases for replication of DNA. Like that of prokaryotes, DNA synthesis in eukaryotes occurs in the 5' to 3' direction, is discontinuous on one strand, and is initiated from an RNA primer.
Although eukaryotes and prokaryotes have many similarities, complexities exist within eukaryotic genetics that are not found in the prokaryotes. Unlike the single sequence of prokaryotic DNA, eukaryotic DNA consists of repeated sequences that range in length and base composition. In addition, eukaryotes have non-coding DNA segments called introns interspersed throughout its coding regions. A major distinction between DNA replication in eukaryotes and prokaryotes involves the origin of replication called a replicon. Eukaryotes have many sites where DNA replication is initiated as opposed to a single replicon in prokaryotes. Another difference between eukaryotes and prokaryotes involves their timing of DNA synthesis during the cell cycle. In prokaryotes, the synthesis of DNA continues throughout the cell cycle, whereas DNA synthesis in eukaryotes is characterised by a specific period known as the S phase. There is an intermittent time period before (G1 phase) and after (G2 phase) the S phase characterised by metabolic activity and growth of the cell. Together, G1, S, and G2 are known as interphase. Following interphase, nuclear division of the cell called mitosis (M phase) occurs. These four phases (G1, S, G2, and M) make up the cell cycle.
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