• Eukaryotic chromosomes contain both DNA and protein, tightly packed together to form a substance called chromatin.
  • Chromatin consists of DNA that is tightly coiled around proteins called


  • Together, the DNA and histone molecules form a beadlike structure called a nucleosome.

  • Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils.

  • DNA is a long molecule made up of units called nucleotides.
  • As the figure above shows, each nucleotide is made up of three basic components:
  • a 5-carbon sugar called deoxyribosea phosphate group
  • a nitrogenous (nitrogen-containing) base

There are four kinds of nitrogenous bases in DNA

  1. The nitrogenous bases, adenine (AD-uh-neen) and guanine (GWAH-neen), belong to a group of compounds known as purines.
  2. The remaining two bases, cytosine (SY-tuh-zeen) and thymine (THY-meen), are known as pyrimidines. Purines have two rings in their structures, whereas pyrimidines have one ring.
  • In the eukaryotic cell cycle, chromosome duplication occurs during "S phase" (the phase of DNA synthesis).  
  • During S phase, any problems with DNA replication trigger a ‘'checkpoint" — a cascade of signaling events that puts the phase on hold until the problem is resolved.
  • The S phase checkpoint operates like a surveillance camera.  
  • chromosomes are made of double-stranded (ds) DNA.
  • The goal of replication is to produce a second and identical double strand.
  • Each of the two strands in the dsDNA molecule serves as a template for a new DNA strand, semi- comnservative replication
  • The first step in DNA replication is to separate the dsDNA. This is accomplished by a DNA helicase.
  • Once the DNA template is single-stranded (ss), a DNA polymerase (an enzyme that creates DNA molecules) reads the template and incorporates the correct nucleoside-triphosphate in the opposite position.
  • Because of the characteristic y-shape of the replicating DNA, it is often referred to as a "replication fork."
  • DNA replication of the leading and lagging strand
  • The primase generates short strands of RNA that bind to the single-stranded DNA to initiate DNA synthesis by the DNA polymerase.
  • This enzyme can work only in the 5' to 3' direction, so it replicates the leading strand continuously.
  • Lagging-strand replication is discontinuous, with short Okazaki fragments being formed and later linked together.
  • The telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration from fusion with neighboring chromosomes.      
  • During chromosome replication, the enzymes that duplicate DNA cannot continue their duplication all the way to the end of the chromosome so in each duplication the end of the chromosome is shortened ( this is because the synthesis of Okazaki fragments requires RNA primers attaching ahead on the lagging strand
  • The telemeres are disposible buffers at the ends of chromsomes which are truncated during cell division
  • Their presence protects the genes before them on the chromosome form being truncated instead
  • The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres.

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