Chromosomes can have 1 or 2 chromatids, depending on whether they have replicated. A chromatid is a condensed DNA subunit of a chromosome. The two chromatids of a duplicated chromosome are held together at a region of DNA called the centromere see figure below. Centromeres are the attachment points for microtubules , which are responsible for the guiding the movement of chromosomes during mitosis and meiosis.
Most eukaryotic cells contain two sets of chromosomes, with one set originating from the father and the other from the mother. For example, every human cell has 23 pairs of chromosomes: one chromosome from each pair is inherited from the father via the sperm , and the other is inherited from the mother via the egg.
During interphase , the cell grows and DNA is replicated. During the mitotic phase , the replicated DNA and cytoplasmic contents are separated and the cell divides. During interphase, the cell undergoes normal processes while also preparing for cell division. For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G 1 , S, and G 2. The first stage of interphase is called the G 1 phase , or first gap, because little change is visible.
However, during the G 1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins, as well as accumulating enough energy reserves to complete the task of replicating each chromosome in the nucleus.
Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase synthesis phase , DNA replication results in the formation of two identical copies of each chromosome—sister chromatids—that are firmly attached at the centromere region. At this stage, each chromosome is made of two sister chromatids and is a duplicated chromosome. 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.
The centrosome consists of a pair of rod-like centrioles at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of many eukaryotic species, such as plants and most fungi.
In the G 2 phase , or second gap, the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic spindle.
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. To make two daughter cells, the contents of the nucleus and the cytoplasm must be divided. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and moved to opposite poles of the cell, and then the cell is divided into two new identical daughter cells.
The first portion of the mitotic phase, mitosis , is composed of five stages, which accomplish nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into two daughter cells.
Mitosis is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus [Figure 2].
The nuclear envelope starts to break into small vesicles, and the Golgi apparatus and endoplasmic reticulum fragment and disperse to the periphery of the cell. The nucleolus disappears. The centrosomes begin to move to opposite poles of the cell.
The microtubules that form the basis of the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen.
The sister chromatids begin to coil more tightly and become visible under a light microscope. During prometaphase , many processes that were begun in prophase continue to advance and culminate in the formation of a connection between the chromosomes and cytoskeleton.
The remnants of the nuclear envelope disappear. 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 visually discrete. Each sister chromatid attaches to spindle microtubules at the centromere via a protein complex called the kinetochore. During metaphase , all of the chromosomes are aligned in a plane called the metaphase plate , or the equatorial plane, midway between the two poles of the cell.
The sister chromatids are still tightly attached to each other. At this time, the chromosomes are maximally condensed. During anaphase , the sister chromatids at the equatorial plane are split apart at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule was attached.
The cell becomes visibly elongated as the non-kinetochore microtubules slide against each other at the metaphase plate where they overlap. During telophase , all of the events that set up the duplicated chromosomes for mitosis during the first three phases are reversed. Copy Number Variation and Human Disease. Genetic Recombination.
Human Chromosome Number. Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Developing the Chromosome Theory.
Meiosis, Genetic Recombination, and Sexual Reproduction. Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. Citation: O'Connor, C.
Nature Education 1 1 Without centromeres, cells cannot divide properly and the overall process of mitosis fails. Why are these small chromosomal regions so essential to such a major cellular process? Aa Aa Aa.
Figure 1. Centromeres Are Regions of Specialized Chromatin. Figure 3: Kinetochore organization. The kinetochore can be thought of as three sets of subcomponents: a The chromosomal DNA-inner kinetochore protein interface further details of this interface are shown in Figure 3 , b the inner kinetochore-mitotic spindle interface; and c the kinetochore protein-cell cycle machinery interface.
Figure Detail. Kinetochores Assemble at Centromeres. Figure 4. Figure 5. Centromeres Still Hold Many Secrets. References and Recommended Reading Cheeseman, I. Chromosoma 91 , — Kitagawa, K. Determining centromere identity: Cyclical stories and forking paths. Nature Reviews Genetics 2 , — doi Article History Close. Share Cancel. Revoke Cancel.
Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Chromosome Analysis. Chromosome Structure. Mutations and Alterations in Chromosomes. Chromosome Number. Chromosome Theory and Cell Division.
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