Eukaryotic Chromosomes

Most human cells other than gametes contain two full sets of genetic information, one from the mother and the other from the father. As in prokaryotes, this genetic information consists of DNA molecules. However, unlike prokaryotes, humans and other eukaryotes have more than one chromosome, and during interphase those chromosomes reside within a membrane-enclosed organelle, the nucleus.

The basic unit of the eukaryotic chromosome is a gigantic, linear, double-stranded molecule of DNA complexed with many proteins to form a dense material called chromatin (Figure 9.5). Before the S phase, each chromosome contains only one such double-stranded DNA molecule. However,

Chromatin fibers consist of DNA and proteins.

Chromatin fibers consist of DNA and proteins.

Chromatin Chromosome Chromatid
9.5 Chromosomes, Chromatids, and Chromatin A human chromosome, shown as the cell prepares to divide.

after the DNA molecule replicates during the S phase, the two resulting DNA molecules, now called chromatids, are held together along most of their length by a protein called cohesin. They stay this way until mitosis, when most of the cohesin is removed, except in a region called the centromere at which the chromatids are still held together (see Figure 9.9). A second group of proteins called condensins coats the DNAmol-ecules at this time and makes them more compact.

The DNA in a typical human cell has a total length of 2 meters. Yet the nucleus is only 5 |im (0.000005 meters) in diameter. So, although the DNA in an interphase nucleus is "unwound," it is still impressively packed! This packing is achieved largely by proteins associated closely with the chromosomal DNA (Figure 9.6).

Chromosomes contain large quantities of proteins called histones (from the Greek, "web"). There are five classes of histones. All of them have a positive charge at cellular pH

levels because of their high content of the basic amino acids lysine and arginine. These positive charges electrostatically attract the negative phosphate groups on DNA. These interactions, as well as interactions among the histones themselves, form beadlike units called nucleosomes. Each nucle-osome contains the following components:

► Eight histone molecules, two each of four of the histone classes, united to form a core or spool.

► 146 base pairs of DNA, 1.65 turns of it wound around the histone core.

► Histone H1 (the remaining histone class) on the outside of the DNA, which may clamp it to the histone core.

During interphase, a chromosome is made up of a single DNA molecule running around vast numbers of nucleo-somes like beads on a string. Between the nucleosomes stretches a variable amount of non-nucleosomal "linker"

9.6 DNA Packs into a Mitotic Chromosome The nucleosome, formed by DNA and histones, is the essential building block in this highly packed structure.

DNA double helix

9.6 DNA Packs into a Mitotic Chromosome The nucleosome, formed by DNA and histones, is the essential building block in this highly packed structure.

DNA double helix

Dna Packs Into Mitotic Chromosome
Metaphase chromosomes

DNA. Since this DNA is exposed to the nuclear environment, it is accessible to proteins involved in its duplication and the regulation of its expression, as we will see in Chapter 14.

During both mitosis and meiosis, the chromatin becomes ever more coiled and condensed as its nucleosomes pack together and coil, with further folding of the chromatin continuing up to the time at which the chromosomes begin to move apart.

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Responses

  • taija hyt
    What are the components of eukaryotic chromosome Many DNA molecules and many proteins?
    6 years ago
  • THEODORE
    How dna is packed into a chromosome?
    5 years ago
  • Chica
    How is dna packaged into chromosomes?
    5 years ago
  • Hamish Johnston
    How dna turns into a chromosome?
    5 years ago
  • Robinia
    How chromosomes are formed?
    5 years ago
  • riku
    How are nucleosomes formed?
    5 years ago
  • franziska
    How chromosome is formed?
    5 years ago
  • petra
    How is dna packaged into a chromosome?
    5 years ago
  • rayyan
    Are eukaryotic chromosome histones positive?
    5 years ago
  • Dora
    How does a chromosome look like in metaphase?
    4 years ago
  • charlie
    How many DNA double helices would be in the metaphase of a mitotic eukaryotic cell with N=10?
    4 years ago
  • battista
    Does eukaryotic chromosome contain only dna with membrane?
    1 year ago
  • MELISSA
    Does eukaryotic chromosome contain only dna with membrane or dna and protein?
    1 year ago
  • enza
    How nuclear DNA is packaged into a metaphase chromosome structure in eukaryotes?
    9 months ago
  • dominik
    How many ars are in a single eukaryotic chromosome?
    7 months ago
  • semira
    How dna arranged in chromosome?
    4 months ago

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