When is dna condensed during the cell cycle

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The synthesis phase of interphase takes the longest because of the complexity of the genetic material being duplicated. Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration.

In the S phase, DNA replication results in the formation of identical pairs of DNA molecules, sister chromatids, that are firmly attached to the centromeric region. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles, which are at right angles to each other.

Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G 2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis.

During the multistep mitotic phase, the cell nucleus divides, and the cell components split into two identical daughter cells. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis or nuclear division.

The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells. Karyokinesis, also known as mitosis, is divided into a series of phases prophase, prometaphase, metaphase, anaphase, and telophase that result in the division of the cell nucleus. Stages of the Cell Cycle : Karyokinesis or mitosis is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. The images at the bottom were taken by fluorescence microscopy hence, the black background of cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA chromosomes and green fluorescence indicates microtubules spindle apparatus.

The membranous organelles such as the Golgi apparatus and endoplasmic reticulum fragment and disperse toward the periphery of the cell. The nucleolus disappears and the centrosomes begin to move to opposite poles of the cell. Microtubules that will eventually form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen.

The sister chromatids begin to coil more tightly with the aid of condensin proteins and become visible under a light microscope. The remnants of the nuclear envelope fragment. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in the centromeric region. The proteins of the kinetochore attract and bind mitotic spindle microtubules.

Kinetochore and Mitotic Spindle : During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore. In anaphase, the connection between the sister chromatids breaks down and the microtubules pull the chromosomes toward opposite poles. The exact length of the DNA segment associated with each histone core varies from species to species, but most such segments are approximately base pairs in length.

Furthermore, each histone molecule within the core particle has one end that sticks out from the particle. These ends are called N-terminal tails , and they play an important role in higher-order chromatin structure and gene expression. Figure 4: The nucleosome structure within chromatin.

Each nucleosome contains eight histone proteins blue , and DNA wraps around these histone structures to achieve a more condensed coiled form. Figure 5: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes.

Although nucleosomes may look like extended "beads on a string" under an electron microscope, they appear differently in living cells. In such cells, nucleosomes stack up against one another in organized arrays with multiple levels of packing. The first level of packing is thought to produce a fiber about 30 nanometers nm wide. These 30 nm fibers then form a series of loops, which fold back on themselves for additional compacting Figure 5. The multiple levels of packing that exist within eukaryotic chromosomes not only permit a large amount of DNA to occupy a very small space, but they also serve several functional roles.

For example, the looping of nucleosome-containing fibers brings specific regions of chromatin together, thereby influencing gene expression. In fact, the organized packing of DNA is malleable and appears to be highly regulated in cells. Chromatin packing also offers an additional mechanism for controlling gene expression. Specifically, cells can control access to their DNA by modifying the structure of their chromatin. Highly compacted chromatin simply isn't accessible to the enzymes involved in DNA transcription , replication , or repair.

Thus, regions of chromatin where active transcription is taking place called euchromatin are less condensed than regions where transcription is inactive or is being actively inhibited or repressed called heterochromatin Figure 6. Figure 6: The structure of chromatin in interphase Heterochromatin is more condensed than euchromatin. Typically, the more condensed chromatin is, the less accessible it is by transcription factors and polymerases.

The dynamic nature of chromatin is regulated by enzymes. For example, chromatin can be loosened by changing the position of the DNA strands within a nucleosome. This loosening occurs because of chromatin remodeling enzymes, which function to slide nucleosomes along the DNA strand so that other enzymes can access the strand. This process is closely regulated and allows specific genes to be accessed in response to metabolic signals within the cell.

Another way cells control gene expression is by modifying their histones with small chemical groups, such as methyl and acetyl groups in the N-terminal tails that extend from the core particle. Different enzymes catalyze each kind of N-terminal modification. Scientists occasionally refer to the complex pattern of histone modification in cells as a "histone code.

In electron micrographs, eukaryotic interphase chromatin appears much like a plate of spaghetti — in other words, there is no obvious pattern of organization. In recent years, however, investigators have begun using fluorescent probes for each of the different interphase chromosomes.

In doing so, they have discovered that these chromosomes have functional and decidedly nonrandom arrangements. One of the first things these scientists noted was that uncondensed chromosomes occupy characteristic regions of the nucleus, which they termed chromosome territories. The spatial localization of these territories is thought to be important for gene expression.

In fact, with the advent of gene-specific probes, researchers are beginning to understand how the arrangement of chromosome territories can bring particular genes closer together. A second major observation related to chromosome territories is that the position of chromosomes relative to one another differs from cell to cell. Such differences reflect variation in gene expression patterns. This page appears in the following eBook.

Aa Aa Aa. What Are Chromosomes? How Are Eukaryotic Chromosomes Structured? Figure 3. Figure 6: The structure of chromatin in interphase.