when does chromosome replication occur in eukaryotic cell
Cell cycle Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei.[7] During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell.[8]
HIV Shortly after the viral capsid enters the cell, an enzyme called reverse transcriptase liberates the single-stranded (+)RNA genome from the attached viral proteins and copies it into a complementary DNA (cDNA) molecule.[60] The process of reverse transcription is extremely error-prone, and the resulting mutations may cause drug resistance or allow the virus to evade the body's immune system. The reverse transcriptase also has ribonuclease activity that degrades the viral RNA during the synthesis of cDNA, as well as DNA-dependent DNA polymerase activity that creates a sense DNA from the antisense cDNA.[61] Together, the cDNA and its complement form a double-stranded viral DNA that is then transported into the cell nucleus. The integration of the viral DNA into the host cell's genome is carried out by another viral enzyme called integrase.[60]
Eukaryotic transcription The initiation of gene transcription in eukaryotes occurs in specific steps.[1] First, an RNA polymerase along with general transcription factors binds to the promoter region of the gene to form a closed complex called the preinitiation complex. The subsequent transition of the complex from the closed state to the open state results in the melting or separation of the two DNA strands and the positioning of the template strand to the active site of the RNA polymerase. Without the need of a primer, RNA polymerase can initiate the synthesis of a new RNA chain using the template DNA strand to guide ribonucleotide selection and polymerization chemistry.[1] However, many of the initiated syntheses are aborted before the transcripts reach a significant length (~10 nucleotides). During these abortive cycles, the polymerase keeps making and releasing short transcripts until it is able to produce a transcript that surpasses ten nucleotides in length. Once this threshold is attained, RNA polymerase passes the promoter and transcription proceeds to the elongation phase.[1]
Telomere Telomeres are repetitive nucleotide sequences located at the termini of linear chromosomes of most eukaryotic organisms. For vertebrates, the sequence of nucleotides in telomeres is TTAGGG.[19] Most prokaryotes, having circular chromosomes rather than linear, do not have telomeres.[20] Telomeres compensate for incomplete semi-conservative DNA replication at chromosomal ends.[21] A protein complex known as shelterin serves to protect the ends of telomeres from being recognised as double-strand breaks by inhibiting homologous recombination (HR) and non-homologous end joining (NHEJ).[22][23]